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Sample records for turbulent flame structures

  1. The turbulence structure in an unconfined swirling diffusion flame

    International Nuclear Information System (INIS)

    Finzenhagen, F.; Doherty, T.O.; Bates, C.; Wirtz, S.; Kremer, H.

    1999-01-01

    Turbulent swirling flows are used in many practical combustion systems. The swirl improves the flame stability as a result of the formation of a central recirculation zone combined with fast mixing at the boundaries of this zone. Knowledge about swirl flames has increased over the last few decades as a result of practical experience and fundamental research. Some important questions concerning the influence of the turbulence structure on the flame stability and chemical kinetics of the combustion process remain unresolved. The structure of turbulence, especially turbulent scales and time dependent effects, at the outlet zone controls the mixing process and therefore the flame properties. Understanding of these complex phenomena is far from complete. The present work describes the results of an experimental study of the turbulence structure of a swirled diffusion flame using laser-optical measurement techniques, e.g. Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PW). All the processed information available from the burst-mode Laser Doppler Anemometry (LDA) measurements has been combined and compared with high spatial resolution PIV measurements of the flow. The extensive statistical post processing of the data has enabled the turbulent microstructure to be characterised. (author)

  2. Turbulent structure and dynamics of swirled, strongly pulsed jet diffusion flames

    KAUST Repository

    Liao, Ying-Hao

    2013-11-02

    The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree of turbulent flame puff interaction, and the turbulent flame puff celerity were determined from high-speed imaging of the luminous flame. All of the tests presented here were conducted with a fixed fuel injection velocity at a Reynolds number of 5000. The flame dimensions were generally found to be more impacted by swirl for the cases of longer injection time and faster co-flow flow rate. Flames with swirl exhibited a flame length up to 34% shorter compared to nonswirled flames. Both the turbulent flame puff separation and the flame puff celerity generally decreased when swirl was imposed. The decreased flame length, flame puff separation, and flame puff celerity are consistent with a greater momentum exchange between the flame and the surrounding co-flow, resulting from an increased rate of air entrainment due to swirl. Three scaling relations were developed to account for the impact of the injection time, the volumetric fuel-to-air flow rate ratio, and the jet-on fraction on the visible flame length. © 2013 Copyright Taylor and Francis Group, LLC.

  3. Turbulent structure and dynamics of swirled, strongly pulsed jet diffusion flames

    KAUST Repository

    Liao, Ying-Hao; Hermanson, James C.

    2013-01-01

    The structure and dynamics of swirled, strongly pulsed, turbulent jet diffusion flames were examined experimentally in a co-flow swirl combustor. The dynamics of the large-scale flame structures, including variations in flame dimensions, the degree

  4. Flame Structure and Emissions of Strongly-Pulsed Turbulent Diffusion Flames with Swirl

    Science.gov (United States)

    Liao, Ying-Hao

    This work studies the turbulent flame structure, the reaction-zone structure and the exhaust emissions of strongly-pulsed, non-premixed flames with co-flow swirl. The fuel injection is controlled by strongly-pulsing the fuel flow by a fast-response solenoid valve such that the fuel flow is completely shut off between pulses. This control strategy allows the fuel injection to be controlled over a wide range of operating conditions, allowing the flame structure to range from isolated fully-modulated puffs to interacting puffs to steady flames. The swirl level is controlled by varying the ratio of the volumetric flow rate of the tangential air to that of the axial air. For strongly-pulsed flames, both with and without swirl, the flame geometry is strongly impacted by the injection time. Flames appear to exhibit compact, puff-like structures for short injection times, while elongated flames, similar in behaviors to steady flames, occur for long injection times. The flames with swirl are found to be shorter for the same fuel injection conditions. The separation/interaction level between flame puffs in these flames is essentially governed by the jet-off time. The separation between flame puffs decreases as swirl is imposed, consistent with the decrease in flame puff celerity due to swirl. The decreased flame length and flame puff celerity are consistent with an increased rate of air entrainment due to swirl. The highest levels of CO emissions are generally found for compact, isolated flame puffs, consistent with the rapid quenching due to rapid dilution with excess air. The imposition of swirl generally results in a decrease in CO levels, suggesting more rapid and complete fuel/air mixing by imposing swirl in the co-flow stream. The levels of NO emissions for most cases are generally below the steady-flame value. The NO levels become comparable to the steady-flame value for sufficiently short jet-off time. The swirled co-flow air can, in some cases, increase the NO

  5. TURBULENT OXYGEN FLAMES IN TYPE Ia SUPERNOVAE

    International Nuclear Information System (INIS)

    Aspden, A. J.; Bell, J. B.; Woosley, S. E.

    2011-01-01

    In previous studies, we examined turbulence-flame interactions in carbon-burning thermonuclear flames in Type Ia supernovae. In this study, we consider turbulence-flame interactions in the trailing oxygen flames. The two aims of the paper are to examine the response of the inductive oxygen flame to intense levels of turbulence, and to explore the possibility of transition to detonation in the oxygen flame. Scaling arguments analogous to the carbon flames are presented and then compared against three-dimensional simulations for a range of Damkoehler numbers (Da 16 ) at a fixed Karlovitz number. The simulations suggest that turbulence does not significantly affect the oxygen flame when Da 16 16 >1, turbulence enhances heat transfer and drives the propagation of a flame that is narrower than the corresponding inductive flame would be. Furthermore, burning under these conditions appears to occur as part of a combined carbon-oxygen turbulent flame with complex compound structure. The simulations do not appear to support the possibility of a transition to detonation in the oxygen flame, but do not preclude it either.

  6. Turbulent structure and emissions of strongly-pulsed jet diffusion flames

    Science.gov (United States)

    Fregeau, Mathieu

    This current research project studied the turbulent flame structure, the fuel/air mixing, the combustion characteristics of a nonpremixed pulsed (unsteady) and unpulsed (steady) flame configuration for both normal- and microgravity conditions, as well as the flame emissions in normal gravity. The unsteady flames were fully-modulated, with the fuel flow completely shut off between injection pulses using an externally controlled valve, resulting in the generation of compact puff-like flame structures. Conducting experiments in normal and microgravity environments enabled separate control over the relevant Richardson and Reynolds numbers to clarify the influence of buoyancy on the flame behavior, mixing, and structure. Experiments were performed in normal gravity in the laboratory at the University of Washington and in microgravity using the NASA GRC 2.2-second Drop Tower facility. High-speed imaging, as well as temperature and emissions probes were used to determine the large-scale structure dynamics, the details of the flame structure and oxidizer entrainment, the combustion temperatures, and the exhaust emissions of the pulsed and steady flames. Of particular interest was the impact of changes in flame structure due to pulsing on the combustion characteristics of this system. The turbulent flame puff celerity (i.e., the bulk velocity of the puffs) was strongly impacted by the jet-off time, increasing markedly as the time between pulses was decreased, which caused the degree of puff interaction to increase and the strongly-pulsed flame to more closely resemble a steady flame. This increase occurred for all values of injection time as well as for constant fuelling rate and in both the presence and absence of buoyancy. The removal of positive buoyancy in microgravity resulted in a decrease in the flame puff celerity in all cases, amounting to as much as 40%, for both constant jet injection velocity and constant fuelling rate. The mean flame length of the strongly

  7. Turbulent premixed flames on fractal-grid-generated turbulence

    Energy Technology Data Exchange (ETDEWEB)

    Soulopoulos, N; Kerl, J; Sponfeldner, T; Beyrau, F; Hardalupas, Y; Taylor, A M K P [Mechanical Engineering Department, Imperial College London, London SW7 2AZ (United Kingdom); Vassilicos, J C, E-mail: ns6@ic.ac.uk [Department of Aeronautics, Imperial College London, London SW7 2AZ (United Kingdom)

    2013-12-15

    A space-filling, low blockage fractal grid is used as a novel turbulence generator in a premixed turbulent flame stabilized by a rod. The study compares the flame behaviour with a fractal grid to the behaviour when a standard square mesh grid with the same effective mesh size and solidity as the fractal grid is used. The isothermal gas flow turbulence characteristics, including mean flow velocity and rms of velocity fluctuations and Taylor length, were evaluated from hot-wire measurements. The behaviour of the flames was assessed with direct chemiluminescence emission from the flame and high-speed OH-laser-induced fluorescence. The characteristics of the two flames are considered in terms of turbulent flame thickness, local flame curvature and turbulent flame speed. It is found that, for the same flow rate and stoichiometry and at the same distance downstream of the location of the grid, fractal-grid-generated turbulence leads to a more turbulent flame with enhanced burning rate and increased flame surface area. (paper)

  8. Turbulence-flame interactions in DNS of a laboratory high Karlovitz premixed turbulent jet flame

    Science.gov (United States)

    Wang, Haiou; Hawkes, Evatt R.; Chen, Jacqueline H.

    2016-09-01

    In the present work, direct numerical simulation (DNS) of a laboratory premixed turbulent jet flame was performed to study turbulence-flame interactions. The turbulent flame features moderate Reynolds number and high Karlovitz number (Ka). The orientations of the flame normal vector n, the vorticity vector ω and the principal strain rate eigenvectors ei are examined. The in-plane and out-of-plane angles are introduced to quantify the vector orientations, which also measure the flame geometry and the vortical structures. A general observation is that the distributions of these angles are more isotropic downstream as the flame and the flow become more developed. The out-of-plane angle of the flame normal vector, β, is a key parameter in developing the correction of 2D measurements to estimate the corresponding 3D quantities. The DNS results show that the correction factor is unity at the inlet and approaches its theoretical value of an isotropic distribution downstream. The alignment characteristics of n, ω and ei, which reflect the interactions of turbulence and flame, are also studied. Similar to a passive scalar gradient in non-reacting flows, the flame normal has a tendency to align with the most compressive strain rate, e3, in the flame, indicating that turbulence contributes to the production of scalar gradient. The vorticity dynamics are examined via the vortex stretching term, which was found to be the predominant source of vorticity generation balanced by dissipation, in the enstrophy transport equation. It is found that although the vorticity preferentially aligns with the intermediate strain rate, e2, the contribution of the most extensive strain rate, e1, to vortex stretching is comparable with that of the intermediate strain rate, e2. This is because the eigenvalue of the most extensive strain rate, λ1, is always large and positive. It is confirmed that the vorticity vector is preferentially positioned along the flame tangential plane, contributing

  9. Characteristics of transitional and turbulent jet diffusion flames in microgravity

    Science.gov (United States)

    Bahadori, Yousef M.; Small, James F., Jr.; Hegde, Uday G.; Zhou, Liming; Stocker, Dennis P.

    1995-01-01

    This paper presents the ground-based results obtained to date in preparation of a proposed space experiment to study the role of large-scale structures in microgravity transitional and turbulent gas-jet diffusion flames by investigating the dynamics of vortex/flame interactions and their influence on flame characteristics. The overall objective is to gain an understanding of the fundamental characteristics of transitional and turbulent gas-jet diffusion flames. Understanding of the role of large-scale structures on the characteristics of microgravity transitional and turbulent flames will ultimately lead to improved understanding of normal-gravity turbulent combustion.

  10. Flames in fractal grid generated turbulence

    Energy Technology Data Exchange (ETDEWEB)

    Goh, K H H; Hampp, F; Lindstedt, R P [Department of Mechanical Engineering, Imperial College, London SW7 2AZ (United Kingdom); Geipel, P, E-mail: p.lindstedt@imperial.ac.uk [Siemens Industrial Turbomachinery AB, SE-612 83 Finspong (Sweden)

    2013-12-15

    Twin premixed turbulent opposed jet flames were stabilized for lean mixtures of air with methane and propane in fractal grid generated turbulence. A density segregation method was applied alongside particle image velocimetry to obtain velocity and scalar statistics. It is shown that the current fractal grids increase the turbulence levels by around a factor of 2. Proper orthogonal decomposition (POD) was applied to show that the fractal grids produce slightly larger turbulent structures that decay at a slower rate as compared to conventional perforated plates. Conditional POD (CPOD) was also implemented using the density segregation technique and the results show that CPOD is essential to segregate the relative structures and turbulent kinetic energy distributions in each stream. The Kolmogorov length scales were also estimated providing values {approx}0.1 and {approx}0.5 mm in the reactants and products, respectively. Resolved profiles of flame surface density indicate that a thin flame assumption leading to bimodal statistics is not perfectly valid under the current conditions and it is expected that the data obtained will be of significant value to the development of computational methods that can provide information on the conditional structure of turbulence. It is concluded that the increase in the turbulent Reynolds number is without any negative impact on other parameters and that fractal grids provide a route towards removing the classical problem of a relatively low ratio of turbulent to bulk strain associated with the opposed jet configuration. (paper)

  11. Flame Speed and Self-Similar Propagation of Expanding Turbulent Premixed Flames

    Science.gov (United States)

    Chaudhuri, Swetaprovo; Wu, Fujia; Zhu, Delin; Law, Chung K.

    2012-01-01

    In this Letter we present turbulent flame speeds and their scaling from experimental measurements on constant-pressure, unity Lewis number expanding turbulent flames, propagating in nearly homogeneous isotropic turbulence in a dual-chamber, fan-stirred vessel. It is found that the normalized turbulent flame speed as a function of the average radius scales as a turbulent Reynolds number to the one-half power, where the average radius is the length scale and the thermal diffusivity is the transport property, thus showing self-similar propagation. Utilizing this dependence it is found that the turbulent flame speeds from the present expanding flames and those from the Bunsen geometry in the literature can be unified by a turbulent Reynolds number based on flame length scales using recent theoretical results obtained by spectral closure of the transformed G equation.

  12. Stratified turbulent Bunsen flames : flame surface analysis and flame surface density modelling

    NARCIS (Netherlands)

    Ramaekers, W.J.S.; Oijen, van J.A.; Goey, de L.P.H.

    2012-01-01

    In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold

  13. Simulation of flame surface density and burning rate of a premixed turbulent flame using contour advection

    Energy Technology Data Exchange (ETDEWEB)

    Tang, B.H.Y.; Chan, C.K. [Department of Applied Mathematics, The Hong Kong Polytechnic University, Hung Hom, Kowloon (Hong Kong)

    2006-10-15

    In this paper, a 2-dimensional rod-stabilized V-shaped flame is simulated using contour advection with surgery as well as the random vortex method. Effects of turbulence on various quantities, such as flame brush thickness and flame surface density, are investigated. The flame surface density S is estimated using the Bray-Moss-Libby formulation, which involves the use of a mean orientation factor {sigma}{sub c}. As a comparison, values of S are also obtained using Shepherd's model, which employs the values of mean flame surface area and mean flame length. Local flame structure is characterized in terms of turbulent flame brush, orientation factor, and flame surface density. Profiles of S obtained using the two different models are compared and show that discrepancy is more evident with increasing turbulence intensity. (author)

  14. Turbulent Flame Speed Scaling for Positive Markstein Number Expanding Flames in Near Isotropic Turbulence

    Science.gov (United States)

    Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung

    2012-11-01

    In this work we clarify the role of Markstein diffusivity on turbulent flame speed and it's scaling, from analysis and experimental measurements on constant-pressure expanding flames propagating in near isotropic turbulence. For all C0-C4 hydrocarbon-air mixtures presented in this work and recently published C8 data from Leeds, the normalized turbulent flame speed data of individual mixtures approximately follows the recent theoretical and experimental ReT, f 0 . 5 scaling, where the average radius is the length scale and thermal diffusivity is the transport property. We observe that for a constant ReT, f 0 . 5 , the normalized turbulent flame speed decreases with increasing Mk. This could be explained by considering Markstein diffusivity as the large wavenumber, flame surface fluctuation dissipation mechanism. As originally suggested by the theory, replacing thermal diffusivity with Markstein diffusivity in the turbulence Reynolds number definition above, the present and Leeds dataset could be scaled by the new ReT, f 0 . 5 irrespective of the fuel considered, equivalence ratio, pressure and turbulence intensity for positive Mk flames. This work was supported by the Combustion Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001198 and by the Air Force Office of Scientific Research.

  15. Straining and wrinkling processes during turbulence-premixed flame interaction measured using temporally-resolved diagnostics

    Energy Technology Data Exchange (ETDEWEB)

    Steinberg, Adam M.; Driscoll, James F. [Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI 48109 (United States)

    2009-12-15

    The dynamical processes of flame surface straining and wrinkling that occur as turbulence interacts with a premixed flame were measured using cinema-stereoscopic PIV (CS-PIV) and orthogonal-plane cinema-stereoscopic PIV (OPCS-PIV). These diagnostics provided temporally resolved measurements of turbulence-flame interaction at frame rates of up to 3 kHz and spatial resolutions as small as 280{mu} m. Previous descriptions of flame straining and wrinkling have typically been derived based on a canonical interaction between a pair of counter-rotating vortices and a planar flame surface. However, it was found that this configuration did not properly represent real turbulence-flame interaction. Interactions resembling the canonical configuration were observed in less than 10% of the recorded frames. Instead, straining and wrinkling were generally caused more geometrically complex turbulence, consisting of large groups of structures that could be multiply curved and intertwined. The effect of the interaction was highly dependent on the interaction geometry. Furthermore, even when the turbulence did exist in the canonical geometry, the straining and wrinkling of the flame surface were not well characterized by the vortical structures. A new mechanistic description of the turbulence-flame interaction was therefore identified and confirmed by the measurements. In this description, flame surface straining is caused by coherent structures of fluid-dynamic strain-rate (strain-rate structures). The role of vortical structures is to curve existing flame surface, creating wrinkles. By simultaneously considering both forms of turbulent structure, turbulence-flame interactions in both the canonical configuration and more complex geometries could be understood. (author)

  16. Pulsating Instability of Turbulent Thermonuclear Flames in Type Ia Supernovae

    Science.gov (United States)

    Poludnenko, Alexei Y.

    2014-01-01

    Presently, one of the main explosion scenarios of type Ia supernovae (SNIa), aimed at explaining both "normal" and subluminous events, is the thermonuclear incineration of a white-dwarf in a single-degenerate system. The underlying engine of such explosions is the turbulent thermonuclear flame. Modern, large-scale, multidimensional simulations of SNIa cannot resolve the internal flame structure, and instead must include a subgrid-scale prescription for the turbulent-flame properties. As a result, development of robust, parameter-free, large-scale models of SNIa crucially relies on the detailed understanding of the turbulent flame properties during each stage of the flame evolution. Due to the complexity of the flame dynamics, such understanding must be validated by the first-principles direct numerical simulations (DNS). In our previous work, we showed that sufficiently fast turbulent flames are inherently susceptible to the development of detonations, which may provide the mechanism for the deflagration-to-detonation transition (DDT) in the delayed-detonation model of SNIa. Here we extend this study by performing detailed analysis of the turbulent flame properties at turbulent intensities below the critical threshold for DDT. We carried out a suite of 3D DNS of turbulent flames for a broad range of turbulent intensities and system sizes using a simplified, single-step, Arrhenius-type reaction kinetics. Our results show that at the later stages of the explosion, as the turbulence intensity increases prior to the possible onset of DDT, the flame front will become violently unstable. We find that the burning rate exhibits periodic pulsations with the energy release rate varying by almost an order of magnitude. Furthermore, such flame pulsations can produce pressure waves and shocks as the flame speed approaches the critical Chapman-Jouguet deflagration speed. Finally, in contrast with the current theoretical understanding, such fast turbulent flames can propagate at

  17. Sooting turbulent jet flame: characterization and quantitative soot measurements

    Science.gov (United States)

    Köhler, M.; Geigle, K. P.; Meier, W.; Crosland, B. M.; Thomson, K. A.; Smallwood, G. J.

    2011-08-01

    Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C2H4/air turbulent jet flame to fill this role and presents an extensive database for soot model validation. The sooting turbulent jet flame has a total visible flame length of approximately 400 mm and a fuel-jet Reynolds number of 10,000. The flame has a measured lift-off height of 26 mm which acts as a sensitive marker for CFD model validation, while this novel compiled experimental database of soot properties, temperature and velocity maps are useful for the validation of kinetic soot models and numerical flame simulations. Due to the relatively simple burner design which produces a flame with sufficient soot concentration while meeting modelers' needs with respect to boundary conditions and flame specifications as well as the present lack of a sooting "standard flame", this flame is suggested as a new reference turbulent sooting flame. The flame characterization presented here involved a variety of optical diagnostics including quantitative 2D laser-induced incandescence (2D-LII), shifted-vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS), and particle image velocimetry (PIV). Producing an accurate and comprehensive characterization of a transient sooting flame was challenging and required optimization of these diagnostics. In this respect, we present the first simultaneous, instantaneous PIV, and LII measurements in a heavily sooting flame environment. Simultaneous soot and flow field measurements can provide new insights into the interaction between a turbulent vortex and flame chemistry, especially since soot structures in turbulent flames are known to be small and often treated in a statistical manner.

  18. Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

    Science.gov (United States)

    Hualei, ZHANG; Liming, HE; Jinlu, YU; Wentao, QI; Gaocheng, CHEN

    2018-02-01

    The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge). OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame, and the experimental apparatus consists of dump burner, plasma-generating system, gas supply system and OH-PLIF system. Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes: regime I for voltage lower than 6.6 kV; regime II for voltage between 6.6 and 11.1 kV; and regime III for voltage between 11.1 and 12.5 kV. In regime I, aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role, while in regime III, the temperature rising effect will probably superimpose on the chemical effect and amplify it. For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field, the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape. With regard to in situ plasma discharge in flames, the discharge pattern changes from streamer type to glow type. Compared with the case of reactants pretreatment, the flame propagates further in the upstream direction. In the discharge region, the OH intensity is highest for in situ plasma assisted combustion, indicating that the plasma energy is coupled into flame reaction zone.

  19. On the theory of turbulent flame velocity

    OpenAIRE

    Bychkov, Vitaly; Akkerman, Vyacheslav; Petchenko, Arkady

    2012-01-01

    The renormalization ideas of self-similar dynamics of a strongly turbulent flame front are applied to the case of a flame with realistically large thermal expansion of the burning matter. In that case a flame front is corrugated both by external turbulence and the intrinsic flame instability. The analytical formulas for the velocity of flame propagation are obtained. It is demonstrated that the flame instability is of principal importance when the integral turbulent length scale is much large...

  20. Experimental Investigation of Turbulent Flames in Hypersonic Flows

    Science.gov (United States)

    2015-09-01

    the flow direction and (b) typical flame length scales seen in the OH-PLIF image with Mach 4.5 freestream (high turbulence) at P0 = 0.65 bar, T0...flame structures (3 mm) are observed at the upstream location of area 1 where the combustion localization first appears. The typical flame length scale

  1. Turbulent Premixed Flame Propagation in Microgravity

    Science.gov (United States)

    Menon, S.; Disseau, M.; Chakravarthy, V. K.; Jagoda, J.

    1997-01-01

    A facility in which turbulent Couette flow could be generated in a microgravity environment was designed and built. To fit into the NASA Lewis drop tower the device had to be very compact. This means that edge effects and flow re-circulation were expected to affect the flow. The flow was thoroughly investigated using LDV and was found to be largely two dimensional away from the edges with constant turbulence intensities in the core. Slight flow asymmetries are introduced by the non symmetric re-circulation of the fluid outside the test region. Belt flutter problems were remedied by adding a pair of guide plates to the belt. In general, the flow field was found to be quite similar to previously investigated Couette flows. However, turbulence levels and associated shear stresses were higher. This is probably due to the confined re-circulation zone reintroducing turbulence into the test section. An estimate of the length scales in the flow showed that the measurements were able to resolve nearly all the length scales of interest. Using a new LES method for subgrid combustion it has been demonstrated that the new procedure is computational feasible even on workstation type environment. It is found that this model is capable of capturing the propagation of the premixed names by resolving the flame in the LES grid within 2-3 grid points. In contrast, conventional LES results in numerical smearing of the flame and completely inaccurate estimate of the turbulent propagation speed. Preliminary study suggests that there is observable effect of buoyancy in the 1g environment suggesting the need for microgravity experiments of the upcoming experimental combustion studies. With the cold flow properties characterized, an identical hot flow facility is under construction. It is assumed that the turbulence properties ahead of the flame in this new device will closely match the results obtained here. This is required since the hot facility will not enable LDV measurements. The

  2. NO concentration imaging in turbulent nonpremixed flames

    Energy Technology Data Exchange (ETDEWEB)

    Schefer, R.W. [Sandia National Laboratories, Livermore, CA (United States)

    1993-12-01

    The importance of NO as a pollutant species is well known. An understanding of the formation characteristics of NO in turbulent hydrocarbon flames is important to both the desired reduction of pollutant emissions and the validation of proposed models for turbulent reacting flows. Of particular interest is the relationship between NO formation and the local flame zone, in which the fuel is oxidized and primary heat release occurs. Planar imaging of NO provides the multipoint statistics needed to relate NO formation to the both the flame zone and the local turbulence characteristics. Planar imaging of NO has been demonstrated in turbulent flames where NO was seeded into the flow at high concentrations (2000 ppm) to determine the gas temperature distribution. The NO concentrations in these experiments were significantly higher than those expected in typical hydrocarbon-air flames, which require a much lower detectability limit for NO measurements. An imaging technique based on laser-induced fluorescence with sufficient sensitivity to study the NO formation mechanism in the stabilization region of turbulent lifted-jet methane flames.

  3. Hydrogen-hydrocarbon turbulent non-premixed flame structure

    Energy Technology Data Exchange (ETDEWEB)

    Tabet, F. [ANSYS-Benelux, 4 Avenue Pasteur, B-1300 Wavre (Belgium); Sarh, B.; Goekalp, I. [Institut de Combustion, Aerothermique, Reactivite et Environnement (ICARE), Centre National de la Recherche Scientifique (CNRS), 1 C avenue de la recherche scientifique, Orleans 45071 Cedex 2 (France)

    2009-06-15

    In this study, the structure of turbulent non-premixed CH{sub 4}-H{sub 2}/air flames is analyzed with a special emphasis on mixing and air entrainment. The amount of H{sub 2} in the fuel mixture varies under constant volumetric fuel flow. Mixing is described by mixture fraction and its variance while air entrainment is characterized by the ratio of gas mass flow to fuel mass flow at the inlet section. The flow field and the chemistry are coupled by the flamelet assumption. Mixture fraction and its variance are transported by the computational fluid dynamics (CFD) code. The slow chemistry aspect of NO{sub x} is handled by solving an additional transport equation with a source term derived from flamelet library. The results obtained show an improvement of mixing with hydrogen addition leading to a strong consumption of CH{sub 4} and a high air entrainment into the centerline region. As a global effect of this, the composite fuels burn faster and thereby reduce the residence time which ultimately shortens the flame length and thickness. On the other hand, hydrogen is found to increase NO{sub x} level. (author)

  4. Chaotic radiation/turbulence interactions in flames

    Energy Technology Data Exchange (ETDEWEB)

    Menguec, M.P.; McDonough, J.M.

    1998-11-01

    In this paper, the authors present a review of their recent efforts to model chaotic radiation-turbulence interactions in flames. The main focus is to characterize soot volume fraction fluctuations in turbulent diffusion flames, as they strongly contribute to these interaction. The approach is based on the hypothesis that the fluctuations of properties in turbulent flames are deterministic in nature, rather than random. The authors first discuss the theoretical details and then they briefly outline the experiments conducted to measure the scattered light signals from fluctuating soot particles along the axis of an ethylene-air diffusion flame. They compare the power spectra and time series obtained from experiments against the ad-hoc and rigorous models derived using a series of logistic maps. These logistic maps can be used in simulation of the fluctuations in these type of flames, without extensive computational effort or sacrifice of physical detail. Availability of accurate models of these kinds allows investigation of radiation-turbulence interactions at a more fundamental level than it was previously possible.

  5. The Effects of Buoyancy on Characteristics of Turbulent Nonpremixed Jet Flames

    Science.gov (United States)

    Idicheria, Cherian; Boxx, Isaac; Clemens, Noel

    2002-11-01

    This work addresses the influence of buoyant forces on the underlying structure of turbulent nonpremixed jet flames. Buoyancy effects are investigated by studying transitional and turbulent propane and ethylene flames (Re_D=2500-10500) at normal, low and microgravity conditions. The reduced gravity experiments are conducted by dropping a combustion rig in the University of Texas 1.25-second drop tower and the NASA Glenn 2.2-second drop tower. The diagnostic employed is high-speed luminosity imaging using a CCD camera. The images obtained are used to compare flame length, mean, RMS and flame tip oscillation characteristics The results showed that, in contrast to previous studies, the high Reynolds number flames at all gravity levels were essentially identical. Furthermore, the parameter ξL (Becker and Yamazaki, 1978) is sufficient for quantifying the effects of buoyancy on the flame characteristics. The large-scale structure and flame tip dynamics are essentially identical to those of purely momentum driven flames provided ξL is less than approximately 3.

  6. PDF modelling and particle-turbulence interaction of turbulent spray flames

    NARCIS (Netherlands)

    Beishuizen, N.A.

    2008-01-01

    Turbulent spray flames can be found in many applications, such as Diesel engines, rocket engines and power plants. The many practical applications are a motivation to investigate the physical phenomena occurring in turbulent spray flames in detail in order to be able to understand, predict and

  7. Flow Field Measurements of Methane-Oxygen Turbulent Nonpremixed Flames at High Pressure

    Science.gov (United States)

    Iino, Kimio; Kikkawa, Hoshitaka; Akamatsu, Fumiteru; Katsuki, Masashi

    We carried out the flow field measurement of methane-oxygen turbulent nonpremixed flame in non-combusting and combusting situations at high pressures using LDV. The main objectives are to study the influences of combustion on the turbulence structure at high pressures and to provide detailed data on which numerical predictions on such flows can rely. Direct observation and CH* chemiluminescence detection are conducted at high pressures up to 1.0MPa. It was found that the flame length at elevated pressures became constant. From flow field measurements, the following features of flames at elevated pressure were found: (1) the existence of flame suppressed turbulence in the upstream region of the jet and enhanced it in the downstream region with increasing pressure; (2) Turbulence in the flame was more anisotropic than in the corresponding cold jet in all regions of the flow with increasing pressure; (3) Reynolds shear stresses did not change at elevated pressure; (4) Combustion processes had a marked influence on the turbulence macroscale under high pressures, however, the turbulence macroscale was not changed even with the increase in pressure.

  8. Effects of premixed flames on turbulence and turbulent scalar transport

    Energy Technology Data Exchange (ETDEWEB)

    Lipatnikov, A.N.; Chomiak, J. [Department of Applied Mechanics, Chalmers University of Technology, 412 75 Goeteborg (Sweden)

    2010-02-15

    Experimental data and results of direct numerical simulations are reviewed in order to show that premixed combustion can change the basic characteristics of a fluctuating velocity field (the so-called flame-generated turbulence) and the direction of scalar fluxes (the so-called countergradient or pressure-driven transport) in a turbulent flow. Various approaches to modeling these phenomena are discussed and the lack of a well-elaborated and widely validated predictive approach is emphasized. Relevant basic issues (the transition from gradient to countergradient scalar transport, the role played by flame-generated turbulence in the combustion rate, the characterization of turbulence in premixed flames, etc.) are critically considered and certain widely accepted concepts are disputed. Despite the substantial progress made in understanding the discussed effects over the past decades, these basic issues strongly need further research. (author)

  9. Turbulent Flame Propagation Characteristics of High Hydrogen Content Fuels

    Energy Technology Data Exchange (ETDEWEB)

    Seitzman, Jerry [Georgia Inst. of Technology, Atlanta, GA (United States); Lieuwen, Timothy [Georgia Inst. of Technology, Atlanta, GA (United States)

    2014-09-30

    This final report describes the results of an effort to better understand turbulent flame propagation, especially at conditions relevant to gas turbines employing fuels with syngas or hydrogen mixtures. Turbulent flame speeds were measured for a variety of hydrogen/carbon monoxide (H2/CO) and hydrogen/methane (H2/CH4) fuel mixtures with air as the oxidizer. The measurements include global consumption speeds (ST,GC) acquired in a turbulent jet flame at pressures of 1-10 atm and local displacement speeds (ST,LD) acquired in a low-swirl burner at atmospheric pressure. The results verify the importance of fuel composition in determining turbulent flame speeds. For example, different fuel-air mixtures having the same unstretched laminar flame speed (SL,0) but different fuel compositions resulted in significantly different ST,GC for the same turbulence levels (u'). This demonstrates the weakness of turbulent flame speed correlations based simply on u'/SL,0. The results were analyzed using a steady-steady leading points concept to explain the sensitivity of turbulent burning rates to fuel (and oxidizer) composition. Leading point theories suggest that the premixed turbulent flame speed is controlled by the flame front characteristics at the flame brush leading edge, or, in other words, by the flamelets that advance farthest into the unburned mixture (the so-called leading points). For negative Markstein length mixtures, this is assumed to be close to the maximum stretched laminar flame speed (SL,max) for the given fuel-oxidizer mixture. For the ST,GC measurements, the data at a given pressure were well-correlated with an SL,max scaling. However the variation with pressure was not captured, which may be due to non-quasi-steady effects that are not included in the current model. For the ST,LD data, the leading points model again faithfully captured the variation of turbulent flame speed over a wide range of fuel-compositions and turbulence intensities. These

  10. Strained flamelets for turbulent premixed flames II: Laboratory flame results

    Energy Technology Data Exchange (ETDEWEB)

    Kolla, H.; Swaminathan, N. [Department of Engineering, Cambridge University, Cambridge CB2 1PZ (United Kingdom)

    2010-07-15

    The predictive ability of strained flamelets model for turbulent premixed flames is assessed using Reynolds Averaged Navier Stokes (RANS) calculations of laboratory flames covering a wide range of conditions. Reactant-to-product (RtP) opposed flow laminar flames parametrised using the scalar dissipation rate of reaction progress variable are used as strained flamelets. Two turbulent flames: a rod stabilised V-flame studied by Robin et al. [Combust. Flame 153 (2008) 288-315] and a set of pilot stabilised Bunsen flames studied by Chen et al. [Combust. Flame 107 (1996) 223-244] are calculated using a single set of model parameters. The V-flame corresponds to the corrugated flamelets regime. The strained flamelet model and an unstrained flamelet model yield similar predictions which are in good agreement with experimental measurements for this flame. On the other hand, for the Bunsen flames which are in the thin reaction zones regime, the unstrained flamelet model predicts a smaller flame brush compared to experiment. The predictions of the strained flamelets model allowing for fluid-dynamics stretch induced attenuation of the chemical reaction are in good agreement with the experimental data. This model predictions of major and minor species are also in good agreement with experimental data. The results demonstrate that the strained flamelets model using the scalar dissipation rate can be used across the combustion regimes. (author)

  11. Visualization of the heat release zone of highly turbulent premixed jet flames

    Science.gov (United States)

    Lv, Liang; Tan, Jianguo; Zhu, Jiajian

    2017-10-01

    Visualization of the heat release zone (HRZ) of highly turbulent flames is significantly important to understand the interaction between turbulence and chemical reactions, which is the foundation to design and optimize engines. Simultaneous measurements of OH and CH2O using planar laser-induced fluorescence (PLIF) were performed to characterize the HRZ. A well-designed piloted premixed jet burner was employed to generate four turbulent premixed CH4/air jet flames, with different jet Reynolds numbers (Rejet) ranging from 4900 to 39200. The HRZ was visualized by both the gradient of OH and the pixel-by-pixel product of OH and CH2O. It is shown that turbulence has an increasing effect on the spatial structure of the flame front with an increasing height above the jet exit for the premixed jet flames, which results in the broadening of the HRZ and the increase of the wrinkling. The HRZ remains thin as the Rejet increases, whereas the preheat zone is significantly broadened and thickened. This indicates that the smallest turbulent eddies can only be able to enter the flame front rather than the HRZ in the present flame conditions. The flame quenching is observed with Rejet = 39200, which may be due to the strong entrainment of the cold air from outside of the burned gas region.

  12. Effect of the mixing fields on the stability and structure of turbulent partially premixed flames in a concentric flow conical nozzle burner

    KAUST Repository

    Mansour, Mohy S.

    2016-10-22

    The mixing field is known to be one of the key parameters that affect the stability and structure of partially premixed flames. Data in these flames are now available covering the effects of turbulence, combustion system geometry, level of partially premixing and fuel type. However, quantitative analyses of the flame structure based on the mixing field are not yet available. The aim of this work is to present a comprehensive study of the effects of the mixing fields on the structure and stability of partially premixed methane flames. The mixing field in a concentric flow conical nozzle (CFCN) burner with well-controlled mechanism of the mixing is investigated using Rayleigh scattering technique. The flame stability, structure and flow field of some selected cases are presented using LIF of OH and PIV. The experimental data of the mixing field cover wide ranges of Reynolds number, equivalence ratio and mixing length. The data show that the mixing field is significantly affected by the mixing length and the ratio of the air-to-fuel velocities. The Reynolds number has a minimum effect on the mixing field in high turbulent flow regime and the stability is significantly affected by the turbulence level. The temporal fluctuations of the range of mixture fraction within the mixing field correlate with the flame stability. The highest point of stability occurs at recess distances where fluid mixtures near the jet exit plane are mostly within the flammability limits. This paper provides some correlations between the stability range in mixture fraction space and the turbulence level for different equivalence ratios.

  13. An Experimental Study of the Structure of Turbulent Non-Premixed Jet Flames in Microgravity

    Science.gov (United States)

    Boxx, Isaac; Idicheria, Cherian; Clemens, Noel

    2000-11-01

    The aim of this work is to investigate the structure of transitional and turbulent non-premixed jet flames under microgravity conditions. The microgravity experiments are being conducted using a newly developed drop rig and the University of Texas 1.5 second drop tower. The rig itself measures 16”x33”x38” and contains a co-flowing round jet flame facility, flow control system, CCD camera, and data/image acquisition computer. These experiments are the first phase of a larger study being conducted at the NASA Glenn Research Center 2.2 second drop tower facility. The flames being studied include methane and propane round jet flames at jet exit Reynolds numbers as high as 10,000. The primary diagnostic technique employed is emission imaging of flame luminosity using a relatively high-speed (350 fps) CCD camera. The high-speed images are used to study flame height, flame tip dynamics and burnout characteristics. Results are compared to normal gravity experimental results obtained in the same apparatus.

  14. Unsteady Flame Embedding (UFE) Subgrid Model for Turbulent Premixed Combustion Simulations

    KAUST Repository

    El-Asrag, Hossam

    2010-01-04

    We present a formulation for an unsteady subgrid model for premixed combustion in the flamelet regime. Since chemistry occurs at the unresolvable scales, it is necessary to introduce a subgrid model that accounts for the multi-scale nature of the problem using the information available on the resolved scales. Most of the current models are based on the laminar flamelet concept, and often neglect the unsteady effects. The proposed model\\'s primary objective is to encompass many of the flame/turbulence interactions unsteady features and history effects. In addition it provides a dynamic and accurate approach for computing the subgrid flame propagation velocity. The unsteady flame embedding approach (UFE) treats the flame as an ensemble of locally one-dimensional flames. A set of elemental one dimensional flames is used to describe the turbulent flame structure at the subgrid level. The stretched flame calculations are performed on the stagnation line of a strained flame using the unsteady filtered strain rate computed from the resolved- grid. The flame iso-surface is tracked using an accurate high-order level set formulation to propagate the flame interface at the coarse resolution with minimum numerical diffusion. In this paper the solver and the model components are introduced and used to investigate two unsteady flames with different Lewis numbers in the thin reaction zone regime. The results show that the UFE model captures the unsteady flame-turbulence interactions and the flame propagation speed reasonably well. Higher propagation speed is observed for the lower than unity Lewis number flame because of the impact of differential diffusion.

  15. Modelling of Turbulent Lifted Jet Flames using flamelets: a priori assessment and a posteriori validation

    OpenAIRE

    Ruan, S; Swaminathan, Nedunchezhian; Darbyshire, O

    2014-01-01

    This study focuses on the modelling of turbulent lifted jet flames using flamelets and presumed PDF approach with interests on both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes to the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction, Z, and progress ...

  16. Three-dimensional temporally resolved measurements of turbulence-flame interactions using orthogonal-plane cinema-stereoscopic PIV

    Energy Technology Data Exchange (ETDEWEB)

    Steinberg, Adam Michael; Driscoll, James F. [University of Michigan, Department of Aerospace Engineering, Ann Arbor, MI (United States); Ceccio, Steven L. [University of Michigan, Department of Mechanical Engineering, Ann Arbor, MI (United States)

    2009-09-15

    A new orthogonal-plane cinema-stereoscopic particle image velocimetry (OPCS-PIV) diagnostic has been used to measure the dynamics of three-dimensional turbulence-flame interactions. The diagnostic employed two orthogonal PIV planes, with one aligned perpendicular and one aligned parallel to the streamwise flow direction. In the plane normal to the flow, temporally resolved slices of the nine-component velocity gradient tensor were determined using Taylor's hypothesis. Volumetric reconstruction of the 3D turbulence was performed using these slices. The PIV plane parallel to the streamwise flow direction was then used to measure the evolution of the turbulence; the path and strength of 3D turbulent structures as they interacted with the flame were determined from their image in this second plane. Structures of both vorticity and strain-rate magnitude were extracted from the flow. The geometry of these structures agreed well with predictions from direct numerical simulations. The interaction of turbulent structures with the flame also was observed. In three dimensions, these interactions had complex geometries that could not be reflected in either planar measurements or simple flame-vortex configurations. (orig.)

  17. Investigation of Turbulent Hydrogen Premixed Flame Topologies at Different Combustion Regimes Using Computational Singular Perturbation

    Science.gov (United States)

    Tingas, Efstathios-Alexandros; Hernandez Perez, Francisco; Im, Hong

    2017-11-01

    The investigation of turbulent flames at higher Reynolds and Karlovitz numbers has been gaining research interest, due to the advances in the computational power that has facilitated the use of direct numerical simulations (DNS). One of the additional challenges associated with highly turbulent premixed flames is the difficulties in identifying the turbulent flame topologies as the flame structures become severely corrugated or even disrupted by the small scale turbulent eddies. In these conditions, the conventional methods using a scalar iso-surface may lead to uncertainties in describing the flame front dynamics. In this study, the computational singular perturbation (CSP) is utilized as an automated tool to identify the flame front topologies based on the dynamical time scales and eigenvalues. In particular, the tangential stretch rate (TSR) approach, an extended generalized method to depict the dynamics of chemical and transport processes, is used for the flame front identification. The CSP/TSR approach and tools are used to compare the flame fronts of two turbulent H2/air premixed flames and to identify their similarities/differences, from a dynamical point of view. The results for two different combustion regimes are analyzed and compared.

  18. Tomographic reconstruction of OH* chemiluminescence in two interacting turbulent flames

    International Nuclear Information System (INIS)

    Worth, Nicholas A; Dawson, James R

    2013-01-01

    The tomographic reconstruction of OH* chemiluminescence was performed on two interacting turbulent premixed bluff-body stabilized flames under steady flow conditions and acoustic excitation. These measurements elucidate the complex three-dimensional (3D) vortex–flame interactions which have previously not been accessible. The experiment was performed using a single camera and intensifier, with multiple views acquired by repositioning the camera, permitting calculation of the mean and phase-averaged volumetric OH* distributions. The reconstructed flame structure and phase-averaged dynamics are compared with OH planar laser-induced fluorescence and flame surface density measurements for the first time. The volumetric data revealed that the large-scale vortex–flame structures formed along the shear layers of each flame collide when the two flames meet, resulting in complex 3D flame structures in between the two flames. With a fairly simple experimental setup, it is shown that the tomographic reconstruction of OH* chemiluminescence in forced flames is a powerful tool that can yield important physical insights into large-scale 3D flame dynamics that are important in combustion instability. (paper)

  19. GRAVITATIONALLY UNSTABLE FLAMES: RAYLEIGH-TAYLOR STRETCHING VERSUS TURBULENT WRINKLING

    International Nuclear Information System (INIS)

    Hicks, E. P.; Rosner, R.

    2013-01-01

    In this paper, we provide support for the Rayleigh-Taylor-(RT)-based subgrid model used in full-star simulations of deflagrations in Type Ia supernovae explosions. We use the results of a parameter study of two-dimensional direct numerical simulations of an RT unstable model flame to distinguish between the two main types of subgrid models (RT or turbulence dominated) in the flamelet regime. First, we give scalings for the turbulent flame speed, the Reynolds number, the viscous scale, and the size of the burning region as the non-dimensional gravity (G) is varied. The flame speed is well predicted by an RT-based flame speed model. Next, the above scalings are used to calculate the Karlovitz number (Ka) and to discuss appropriate combustion regimes. No transition to thin reaction zones is seen at Ka = 1, although such a transition is expected by turbulence-dominated subgrid models. Finally, we confirm a basic physical premise of the RT subgrid model, namely, that the flame is fractal, and thus self-similar. By modeling the turbulent flame speed, we demonstrate that it is affected more by large-scale RT stretching than by small-scale turbulent wrinkling. In this way, the RT instability controls the flame directly from the large scales. Overall, these results support the RT subgrid model.

  20. Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)

    International Nuclear Information System (INIS)

    Pitsch, H.; Steiner, H.

    2000-01-01

    The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel. (c) 2000 American Institute of Physics

  1. Effect of pressure on high Karlovitz number lean turbulent premixed hydrogen-enriched methane-air flames using LES

    Science.gov (United States)

    Cicoria, David; Chan, C. K.

    2017-07-01

    Large eddy simulation (LES) is employed to investigate the effect of pressure on lean CH4-H2-air turbulent premixed flames at high Karlovitz number for mixtures up to 60% of hydrogen in volume. The subfilter combustion term representing the interaction between turbulence and chemistry is modelled using the PaSR model, along with complex chemistry using a skeletal mechanism based on GRI-MECH3.0. The influence of pressure at high turbulence levels is studied by means of the local flame structure, and the assessment of species formation inside the flame. Results show that the ratio of turbulent flame thickness to laminar flame thickness δt/δu increases faster with pressure, and increases with the fraction of hydrogen in the mixture, leading to higher ratio of turbulent to laminar flame speed. The flame displays smaller structures and higher degree of wrinkling at higher pressure. Final species of CO2 and H2O formation is almost independent of pressure. For intermediate species CO and OH, an increase in pressure at constant volume fraction of hydrogen β leads to a decrease of emission of these species.

  2. Analysis of flame shapes in turbulent hydrogen jet flames with coaxial air

    International Nuclear Information System (INIS)

    Moon, Hee Jang

    2009-01-01

    This paper addresses the characteristics of flame shapes and flame length in three types of coaxial air flames realizable by varying coaxial air and/or fuel velocity. Forcing coaxial air into turbulent jet flames induces substantial changes in flame shapes and NOx emissions through the complex flow interferences that exist within the mixing region. Mixing enhancement driven by coaxial air results in flame volume decrease, and such a diminished flame volume finally reduces NOx emissions significantly by decreasing NOx formation zone where a fuel/air mixture burns. It is found that mixing in the vicinity of high temperature zone mainly results from the increase of diffusive flux than the convective flux, and that the increase of mass diffusion is amplified as coaxial air is increased. Besides, it is reaffirmed that nonequilibrium chemistry including HO 2 /H 2 O 2 should be taken into account for NOx prediction and scaling analysis by comparing turbulent combustion models. In addition, it is found that coaxial air can break down the self-similarity law of flames by changing mixing mechanism, and that EINOx scaling parameters based on the self-similarity law of simple jet flames may not be eligible in coaxial air flames

  3. Large scale Direct Numerical Simulation of premixed turbulent jet flames at high Reynolds number

    Science.gov (United States)

    Attili, Antonio; Luca, Stefano; Lo Schiavo, Ermanno; Bisetti, Fabrizio; Creta, Francesco

    2016-11-01

    A set of direct numerical simulations of turbulent premixed jet flames at different Reynolds and Karlovitz numbers is presented. The simulations feature finite rate chemistry with 16 species and 73 reactions and up to 22 Billion grid points. The jet consists of a methane/air mixture with equivalence ratio ϕ = 0 . 7 and temperature varying between 500 and 800 K. The temperature and species concentrations in the coflow correspond to the equilibrium state of the burnt mixture. All the simulations are performed at 4 atm. The flame length, normalized by the jet width, decreases significantly as the Reynolds number increases. This is consistent with an increase of the turbulent flame speed due to the increased integral scale of turbulence. This behavior is typical of flames in the thin-reaction zone regime, which are affected by turbulent transport in the preheat layer. Fractal dimension and topology of the flame surface, statistics of temperature gradients, and flame structure are investigated and the dependence of these quantities on the Reynolds number is assessed.

  4. Structural aspects of coaxial oxy-fuel flames

    Science.gov (United States)

    Ditaranto, M.; Sautet, J. C.; Samaniego, J. M.

    Oxy-fuel combustion has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. The study of 25-kW turbulent diffusion flames of natural gas with pure oxygen is undertaken on a coaxial burner with quarl. The structural properties are analysed by imaging the instantaneous reaction zone by OH* chemiluminescence and measuring scalar and velocity profiles. The interaction between the flame front and the shear layers present in the coaxial jets depends on the momentum ratio which dictates the turbulent structure development. Flame length and NOx emission sensitivity to air leaks in the combustion chamber are also investigated.

  5. Direct Numerical Simulations of Statistically Stationary Turbulent Premixed Flames

    KAUST Repository

    Im, Hong G.

    2016-07-15

    Direct numerical simulations (DNS) of turbulent combustion have evolved tremendously in the past decades, thanks to the rapid advances in high performance computing technology. Today’s DNS is capable of incorporating detailed reaction mechanisms and transport properties of hydrocarbon fuels, with physical parameter ranges approaching laboratory scale flames, thereby allowing direct comparison and cross-validation against laser diagnostic measurements. While these developments have led to significantly improved understanding of fundamental turbulent flame characteristics, there are increasing demands to explore combustion regimes at higher levels of turbulent Reynolds (Re) and Karlovitz (Ka) numbers, with a practical interest in new combustion engines driving towards higher efficiencies and lower emissions. The article attempts to provide a brief overview of the state-of-the-art DNS of turbulent premixed flames at high Re/Ka conditions, with an emphasis on homogeneous and isotropic turbulent flow configurations. Some important qualitative findings from numerical studies are summarized, new analytical approaches to investigate intensely turbulent premixed flame dynamics are discussed, and topics for future research are suggested. © 2016 Taylor & Francis.

  6. Simultaneous planar measurements of soot structure and velocity fields in a turbulent lifted jet flame at 3 kHz

    Science.gov (United States)

    Köhler, M.; Boxx, I.; Geigle, K. P.; Meier, W.

    2011-05-01

    We describe a newly developed combustion diagnostic for the simultaneous planar imaging of soot structure and velocity fields in a highly sooting, lifted turbulent jet flame at 3000 frames per second, or two orders of magnitude faster than "conventional" laser imaging systems. This diagnostic uses short pulse duration (8 ns), frequency-doubled, diode-pumped solid state (DPSS) lasers to excite laser-induced incandescence (LII) at 3 kHz, which is then imaged onto a high framerate CMOS camera. A second (dual-cavity) DPSS laser and CMOS camera form the basis of a particle image velocity (PIV) system used to acquire 2-component velocity field in the flame. The LII response curve (measured in a laminar propane diffusion flame) is presented and the combined diagnostics then applied in a heavily sooting lifted turbulent jet flame. The potential challenges and rewards of application of this combined imaging technique at high speeds are discussed.

  7. Analysis of flame shapes in turbulent hydrogen jet flames with coaxial air

    Energy Technology Data Exchange (ETDEWEB)

    Moon, Hee Jang [Korea Aerospace University, Goyang (Korea, Republic of)

    2009-06-15

    This paper addresses the characteristics of flame shapes and flame length in three types of coaxial air flames realizable by varying coaxial air and/or fuel velocity. Forcing coaxial air into turbulent jet flames induces substantial changes in flame shapes and NOx emissions through the complex flow interferences that exist within the mixing region. Mixing enhancement driven by coaxial air results in flame volume decrease, and such a diminished flame volume finally reduces NOx emissions significantly by decreasing NOx formation zone where a fuel/air mixture burns. It is found that mixing in the vicinity of high temperature zone mainly results from the increase of diffusive flux than the convective flux, and that the increase of mass diffusion is amplified as coaxial air is increased. Besides, it is reaffirmed that nonequilibrium chemistry including HO{sub 2}/H{sub 2}O{sub 2} should be taken into account for NOx prediction and scaling analysis by comparing turbulent combustion models. In addition, it is found that coaxial air can break down the self-similarity law of flames by changing mixing mechanism, and that EINOx scaling parameters based on the self-similarity law of simple jet flames may not be eligible in coaxial air flames

  8. Flame kernel generation and propagation in turbulent partially premixed hydrocarbon jet

    KAUST Repository

    Mansour, Mohy S.

    2014-04-23

    Flame development, propagation, stability, combustion efficiency, pollution formation, and overall system efficiency are affected by the early stage of flame generation defined as flame kernel. Studying the effects of turbulence and chemistry on the flame kernel propagation is the main aim of this work for natural gas (NG) and liquid petroleum gas (LPG). In addition the minimum ignition laser energy (MILE) has been investigated for both fuels. Moreover, the flame stability maps for both fuels are also investigated and analyzed. The flame kernels are generated using Nd:YAG pulsed laser and propagated in a partially premixed turbulent jet. The flow field is measured using 2-D PIV technique. Five cases have been selected for each fuel covering different values of Reynolds number within a range of 6100-14400, at a mean equivalence ratio of 2 and a certain level of partial premixing. The MILE increases by increasing the equivalence ratio. Near stoichiometric the energy density is independent on the jet velocity while in rich conditions it increases by increasing the jet velocity. The stability curves show four distinct regions as lifted, attached, blowout, and a fourth region either an attached flame if ignition occurs near the nozzle or lifted if ignition occurs downstream. LPG flames are more stable than NG flames. This is consistent with the higher values of the laminar flame speed of LPG. The flame kernel propagation speed is affected by both turbulence and chemistry. However, at low turbulence level chemistry effects are more pronounced while at high turbulence level the turbulence becomes dominant. LPG flame kernels propagate faster than those for NG flame. In addition, flame kernel extinguished faster in LPG fuel as compared to NG fuel. The propagation speed is likely to be consistent with the local mean equivalence ratio and its corresponding laminar flame speed. Copyright © Taylor & Francis Group, LLC.

  9. Stratified turbulent Bunsen flames: flame surface analysis and flame surface density modelling

    Science.gov (United States)

    Ramaekers, W. J. S.; van Oijen, J. A.; de Goey, L. P. H.

    2012-12-01

    In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold (FGM) reduction method for reaction kinetics. Before examining the suitability of the FSD model, flame surfaces are characterized in terms of thickness, curvature and stratification. All flames are in the Thin Reaction Zones regime, and the maximum equivalence ratio range covers 0.1⩽φ⩽1.3. For all flames, local flame thicknesses correspond very well to those observed in stretchless, steady premixed flamelets. Extracted curvature radii and mixing length scales are significantly larger than the flame thickness, implying that the stratified flames all burn in a premixed mode. The remaining challenge is accounting for the large variation in (subfilter) mass burning rate. In this contribution, the FSD model is proven to be applicable for Large Eddy Simulations (LES) of stratified flames for the equivalence ratio range 0.1⩽φ⩽1.3. Subfilter mass burning rate variations are taken into account by a subfilter Probability Density Function (PDF) for the mixture fraction, on which the mass burning rate directly depends. A priori analysis point out that for small stratifications (0.4⩽φ⩽1.0), the replacement of the subfilter PDF (obtained from DNS data) by the corresponding Dirac function is appropriate. Integration of the Dirac function with the mass burning rate m=m(φ), can then adequately model the filtered mass burning rate obtained from filtered DNS data. For a larger stratification (0.1⩽φ⩽1.3), and filter widths up to ten flame thicknesses, a β-function for the subfilter PDF yields substantially better predictions than a Dirac function. Finally, inclusion of a simple algebraic model for the FSD resulted only in small additional deviations from DNS data

  10. Radiation turbulence interactions in pulverized coal flames: Chaotic map models of soot fluctuations in turbulent diffusion flames. Quarterly report, October 1995--December 1995

    Energy Technology Data Exchange (ETDEWEB)

    McDonough, J.M.; Menguc, M.P.; Mukerji, S.; Swabb, S.; Manickavasagam, S.; Ghosal, S.

    1995-12-31

    In this paper, we introduce a methodology to characterize soot volume fraction fluctuations in turbulent diffusion flames via chaotic maps. The approach is based on the hypothesis that the fluctuations of properties in turbulent flames is deterministic in nature, rather than statistical. Out objective is to develop models to mimic these fluctuations. The models will be used eventually in comprehensive algorithms to study the true physics of turbulent flames and the interaction of turbulence with radiation. To this extent, we measured the time series of soot scattering coefficient in an ethylene diffusion flame from light scattering experiments. Following this, corresponding power spectra and delay maps were calculated. It was shown that if the data were averaged, the characteristics of the fluctuations were almost completely washed out. The psds from experiments were successfully modeled using a series of logistic maps.

  11. LES of a laboratory-scale turbulent premixed bunsen flame using FSD, PCM-FPI and thickened flame models

    NARCIS (Netherlands)

    Hernandez Perez, F.E.; Yuen, F.T.C.; Groth, C.P.T.; Gülder, O.L.

    2011-01-01

    Large-eddy simulations (LES) of a turbulent premixed Bunsen flame were carried out with three subfilter-scale (SFS) modelling approaches for turbulent premixed combustion. One approach is based on the artificially thickened flame and power-law flame wrinkling models, the second approach is based on

  12. Effects of Buoyancy on Laminar and Turbulent Premixed V-Flame

    Science.gov (United States)

    Cheng, Robert K.; Bedat, Benoit

    1997-01-01

    Turbulent combustion occurs naturally in almost all combustion systems and involves complex dynamic coupling of chemical and fluid mechanical processes. It is considered as one of the most challenging combustion research problems today. Though buoyancy has little effect on power generating systems operating under high pressures (e.g., IC engines and turbines), flames in atmospheric burners and the operation of small to medium furnaces and boilers are profoundly affected by buoyancy. Changes in burner orientation impacts on their blow-off, flash-back and extinction limits, and their range of operation, burning rate, heat transfer, and emissions. Theoretically, buoyancy is often neglected in turbulent combustion models. Yet the modeling results are routinely compared with experiments of open laboratory flames that are obviously affected by buoyancy. This inconsistency is an obstacle to reconciling experiments and theories. Consequently, a fundamental understanding of the coupling between turbulent flames and buoyancy is significant to both turbulent combustion science and applications. The overall effect of buoyancy relates to the dynamic interaction between the flame and its surrounding, i.e., the so-called elliptical problem. The overall flame shape, its flowfield, stability, and mean and local burning rates are dictated by both upstream and downstream boundary conditions. In steady propagating premixed flames, buoyancy affects the products region downstream of the flame zone. These effects are manifested upstream through the mean and fluctuating pressure fields to influence flame stretch and flame wrinkling. Intuitively, the effects buoyancy should diminish with increasing flow momentum. This is the justification for excluding buoyancy in turbulent combustion models that treats high Reynolds number flows. The objectives of our experimental research program is to elucidate flame-buoyancy coupling processes in laminar and turbulent premixed flames, and to

  13. Measurements of Turbulent Flame Speed and Integral Length Scales in a Lean Stationary Premixed Flame

    OpenAIRE

    Klingmann, Jens; Johansson, Bengt

    1998-01-01

    Turbulent premixed natural gas - air flame velocities have been measured in a stationary axi-symmetric burner using LDA. The flame was stabilized by letting the flow retard toward a stagnation plate downstream of the burner exit. Turbulence was generated by letting the flow pass through a plate with drilled holes. Three different hole diameters were used, 3, 6 and 10 mm, in order to achieve different turbulent length scales. Turbulent integral length scales were measured using two-point LD...

  14. Gas concentration and temperature in acoustically excited Delft turbulent jet flames

    Energy Technology Data Exchange (ETDEWEB)

    Ana Maura A. Rocha; Joao A. Carvalho Jr.; Pedro T. Lacava [Sao Paulo State University, Guaratingueta (Brazil)

    2008-11-15

    This paper shows the experimental results for changes in the flame structure when acoustic fields are applied in natural gas Delft turbulent diffusion flames. The acoustic field (pulsating combustion) generates zones of intense mixture of reactants in the flame region, promoting a more complete combustion and, consequently, lower pollutant emissions, increase in convective heat transfer rates, and lower fuel consumption. The results show that the presence of the acoustic field changes drastically the flame structure, mainly in the burner natural frequencies. However, for higher acoustic amplitudes, or acoustic pressures, a hydrogen pilot flame is necessary in order to keep the main flame anchored. In the flame regions where the acoustic field is more intense, premixed flame characteristics were observed. Besides, the pulsating regime modifies the axial and radial combustion structure, which could be verified by the radial distribution of concentrations of O{sub 2}, CO, CO{sub 2}, and NOx, and by the temperature profile. The experiments also presented the reduction of flame length with the increase of acoustic amplitude. 30 refs., 15 figs., 3 tabs.

  15. Numerical modeling of turbulent combustion and flame spread

    Energy Technology Data Exchange (ETDEWEB)

    Yan Zhenghua

    1999-01-01

    Theoretical models have been developed to address several important aspects of numerical modeling of turbulent combustion and flame spread. The developed models include a pyrolysis model for charring and non-charring solid materials, a fast narrow band radiation property evaluation model (FASTNB) and a turbulence model for buoyant flow and flame. In the pyrolysis model, a completely new algorithm has been proposed, where a moving dual mesh concept was developed and implemented. With this new concept, it provides proper spatial resolution for both temperature and density and automatically considers the regression of the surface of the non-charring solid material during its pyrolysis. It is simple, very efficient and applicable to both charring and non-charring materials. FASTNB speeds up significantly the evaluation of narrow band spectral radiation properties and thus provides a potential of applying narrow band model in numerical simulations of practical turbulent combustion. The turbulence model was developed to improve the consideration of buoyancy effect on turbulence and turbulent transport. It was found to be simple, promising and numerically stable. It has been tested against both plane and axisymmetric thermal plumes and an axisymmetric buoyant diffusion flame. When compared with the widely used standard buoyancy-modified {kappa} - {epsilon} model, it gives significant improvement on numerical results. These developed models have been fully incorporated into CFD (Computational Fluid Dynamics) code and coupled with other CFD sub-models, including the DT (Discrete Transfer) radiation model, EDC (Eddy Dissipation Concept) combustion model, flamelet combustion model, various soot models and transpired wall function. Comprehensive numerical simulations have been carried out to study soot formation and oxidation in turbulent buoyant diffusion flames, flame heat transfer and flame spread in fires. The gas temperature and velocity, soot volume fraction, wall

  16. A study of transient flow turbulence generation during flame/wall interactions in explosions

    Science.gov (United States)

    Hargrave, G. K.; Jarvis, S.; Williams, T. C.

    2002-07-01

    Experimental data are presented for the turbulent velocity field generated during flame/solid wall interactions in explosions. The presence of turbulence in a flammable gas mixture can wrinkle a flame front, increasing the flame surface area and enhancing the burning rate. In congested process plant, any flame propagating through an accidental release of flammable mixture will encounter obstructions in the form of walls, pipe-work or storage vessels. The interaction between the gas movement and the obstacle creates turbulence by vortex shedding and local wake/recirculation, whereby the flame can be wrapped in on itself, increasing the surface area available for combustion. Particle image velocimetry (PIV) was used to characterize the turbulent flow field in the wake of the obstacles placed in the path of propagating flames. This allowed the quantification of the interaction of the propagating flame and the generated turbulent flow field. Due to the accelerating nature of the explosion flow field, the wake flows develop `transient' turbulent fields and PIV provided data to define the spatial and temporal variation of the velocity field ahead of the propagating flame, providing an understanding of the direct interaction between flow and flame.

  17. Flame surface statistics of constant-pressure turbulent expanding premixed flames

    Science.gov (United States)

    Saha, Abhishek; Chaudhuri, Swetaprovo; Law, Chung K.

    2014-04-01

    In this paper we investigate the local flame surface statistics of constant-pressure turbulent expanding flames. First the statistics of local length ratio is experimentally determined from high-speed planar Mie scattering images of spherically expanding flames, with the length ratio on the measurement plane, at predefined equiangular sectors, defined as the ratio of the actual flame length to the length of a circular-arc of radius equal to the average radius of the flame. Assuming isotropic distribution of such flame segments we then convolute suitable forms of the length-ratio probability distribution functions (pdfs) to arrive at the corresponding area-ratio pdfs. It is found that both the length ratio and area ratio pdfs are near log-normally distributed and shows self-similar behavior with increasing radius. Near log-normality and rather intermittent behavior of the flame-length ratio suggests similarity with dissipation rate quantities which stimulates multifractal analysis.

  18. Modelling thermal radiation in buoyant turbulent diffusion flames

    Science.gov (United States)

    Consalvi, J. L.; Demarco, R.; Fuentes, A.

    2012-10-01

    This work focuses on the numerical modelling of radiative heat transfer in laboratory-scale buoyant turbulent diffusion flames. Spectral gas and soot radiation is modelled by using the Full-Spectrum Correlated-k (FSCK) method. Turbulence-Radiation Interactions (TRI) are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA), the resulting time-averaged Radiative Transfer Equation (RTE) being solved by the Finite Volume Method (FVM). Emission TRIs and the mean absorption coefficient are then closed by using a presumed probability density function (pdf) of the mixture fraction. The mean gas flow field is modelled by the Favre-averaged Navier-Stokes (FANS) equation set closed by a buoyancy-modified k-ɛ model with algebraic stress/flux models (ASM/AFM), the Steady Laminar Flamelet (SLF) model coupled with a presumed pdf approach to account for Turbulence-Chemistry Interactions, and an acetylene-based semi-empirical two-equation soot model. Two sets of experimental pool fire data are used for validation: propane pool fires 0.3 m in diameter with Heat Release Rates (HRR) of 15, 22 and 37 kW and methane pool fires 0.38 m in diameter with HRRs of 34 and 176 kW. Predicted flame structures, radiant fractions, and radiative heat fluxes on surrounding surfaces are found in satisfactory agreement with available experimental data across all the flames. In addition further computations indicate that, for the present flames, the gray approximation can be applied for soot with a minor influence on the results, resulting in a substantial gain in Computer Processing Unit (CPU) time when the FSCK is used to treat gas radiation.

  19. Fast Hydrogen-Air Flames for Turbulence Driven Deflagration to Detonation Transition

    Science.gov (United States)

    Chambers, Jessica; Ahmed, Kareem

    2016-11-01

    Flame acceleration to Detonation produces several combustion modes as the Deflagration-to-Detonation Transition (DDT) is initiated, including fast deflagration, auto-ignition, and quasi-detonation. Shock flame interactions and turbulence levels in the reactant mixture drive rapid flame expansion, formation of a leading shockwave and post-shock conditions. An experimental study to characterize the developing shock and flame front behavior of propagating premixed hydrogen-air flames in a square channel is presented. To produce each flame regime, turbulence levels and flame propagation velocity are controlled using perforated plates in several configurations within the experimental facility. High speed optical diagnostics including Schlieren and Particle Image Velocimetry are used to capture the flow field. In-flow pressure measurements acquired post-shock, detail the dynamic changes that occur in the compressed gas directly ahead of the propagating flame. Emphasis on characterizing the turbulent post-shock environment of the various flame regimes helps identify the optimum conditions to initiate the DDT process. The study aims to further the understanding of complex physical mechanisms that drive transient flame conditions for detonation initiation. American Chemical Society.

  20. Characteristics of sound radiation from turbulent premixed flames

    Science.gov (United States)

    Rajaram, Rajesh

    Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation, which occurs due to the unsteady expansion of reacting gases. While prior studies have extensively characterized the total sound power radiated by turbulent flames, their spectral characteristics are not well understood. The objective of this research work is to measure the flow and acoustic properties of an open turbulent premixed jet flame and explain the spectral trends of combustion noise. The flame dynamics were characterized using high speed chemiluminescence images of the flame. A model based on the solution of the wave equation with unsteady heat release as the source was developed and was used to relate the measured chemiluminescence fluctuations to its acoustic emission. Acoustic measurements were performed in an anechoic environment for several burner diameters, flow velocities, turbulence intensities, fuels, and equivalence ratios. The acoustic emissions are shown to be characterized by four parameters: peak frequency (Fpeak), low frequency slope (beta), high frequency slope (alpha) and Overall Sound Pressure Level (OASPL). The peak frequency (Fpeak) is characterized by a Strouhal number based on the mean velocity and a flame length. The transfer function between the acoustic spectrum and the spectrum of heat release fluctuations has an f2 dependence at low frequencies, while it converged to a constant value at high frequencies. Furthermore, the OASPL was found to be characterized by (Fpeak mfH)2, which resembles the source term in the wave equation.

  1. Strained flamelets for turbulent premixed flames, I: Formulation and planar flame results

    Energy Technology Data Exchange (ETDEWEB)

    Kolla, H.; Swaminathan, N. [Department of Engineering, Cambridge University, Cambridge CB2 1PZ (United Kingdom)

    2010-05-15

    A strained flamelet model is proposed for turbulent premixed flames using scalar dissipation rate as a parameter. The scalar dissipation rate of reaction progress variable is a suitable quantity to describe the flamelet structure since it is governed by convection-diffusion-reaction balance and it is defined at every location in the flamelets, which are represented by laminar flames in reactant-to-product opposed flow configuration. The mean reaction rate is obtained by using the flamelets reaction rate and the joint pdf of the progress variable and its dissipation rate. The marginal pdf of the progress variable is presumed to be {beta}-pdf and the pdf of the conditional dissipation rate is taken to be log-normal. The conditional mean dissipation rate is obtained from modelled mean dissipation rate. This reaction rate closure is assessed using RANS calculations of statistically planar flames in the corrugated flamelets and thin reaction zones regimes. The flame speeds calculated using this closure are close to the experimental data of Abdel-Gayed et al. (1987) for flames in both the regimes. Comparisons with other reaction rate closures showed the benefits of the strained flamelets approach. (author)

  2. Blow-off characteristics of turbulent premixed flames in curved-wall Jet Burner

    KAUST Repository

    Mansour, Morkous S.

    2015-08-02

    This study concerns the flame dynamics of a curved-wall jet (CWJ) stabilized turbulent premixed flame as it approaches blow-off conditions. Time resolved OH planar laser-induced fluorescence (PLIF) delineated reaction zone contours and simultaneously stereoscopic particle image velocimetry (SPIV) quantified the turbulent flow field features. Ethylene/air flames were stabilized in CWJ burner to determine the sequence of events leading to blowoff. For stably burning flames far from blowoff, flames are characterized with a recirculation zone (RZ) upstream for flame stabilization followed by an intense turbulent interaction jet (IJ) and merged-jet regions downstream; the flame front counterparts the shear layer vortices. Near blowoff, as the velocity of reactants increases, high local stretch rates exceed the extinction stretch rates instantaneously resulting in localized flame extinction along the IJ region. As Reynolds number (Re) increases, flames become shorter and are entrained by larger amounts of cold reactants. The increased strain rates together with heat loss effects result in further fragmentation of the flame, eventually leading to the complete quenching of the flame. This is explained in terms of local turbulent Karlovitz stretch factor (K) and principal flow strain rates associated with C contours. Hydrogen addition and increasing the RZ size lessen the tendency of flames to be locally extinguished.

  3. Buoyancy Effects in Turbulent Jet Flames in Crossflow

    Science.gov (United States)

    Boxx, Isaac; Idicheria, Cherian; Clemens, Noel

    2003-11-01

    The aim of this study is to investigate the effects of buoyancy on the structure of turbulent, non-premixed hydrocarbon jet-flames in crossflow (JFICF). This was accomplished using a small jet-in-crossflow facility which can be oriented at a variety of angles with respect to the gravity vector. This facility enables us to alter the relative influence of buoyancy on the JFICF without altering the jet-exit Reynolds number, momentum flux ratio or the geometry of the system. Results are compared to similar, but non-buoyant, JFICF studied in microgravity. Departures of jet-centerline trajectory from the well-known power-law scaling of turbulent JFICF were used to explore the transition from a buoyancy-influenced regime to a momentum dominated one. The primary diagnostic was CCD imaging of soot-luminosity. We present results on ethylene jet flames with jet-exit Reynolds numbers of 1770 to 8000 and momentum flux ratios of 5 to 13.

  4. Influence of Turbulent Scalar Mixing Physics on Premixed Flame Propagation

    Directory of Open Access Journals (Sweden)

    H. Kolla

    2011-01-01

    Full Text Available The influence of reactive scalar mixing physics on turbulent premixed flame propagation is studied, within the framework of turbulent flame speed modelling, by comparing predictive ability of two algebraic flame speed models: one that includes all relevant physics and the other ignoring dilatation effects on reactive scalar mixing. This study is an extension of a previous work analysing and validating the former model. The latter is obtained by neglecting modelling terms that include dilatation effects: a direct effect because of density change across the flame front and an indirect effect due to dilatation on turbulence-scalar interaction. An analysis of the limiting behaviour shows that neglecting the indirect effect alters the flame speed scaling considerably when / is small and the scaling remains unaffected when / is large. This is evident from comparisons of the two models with experimental data which show that the quantitative difference between the two models is as high as 66% at /=0.3 but only 4% at /=52.4. Furthermore, neglecting the direct effect results in a poor prediction of turbulent flame speed for all values of /, and both effects are important for practically relevant values of this velocity ratio.

  5. Early structure of LPG partially premixed conically stabilized flames

    KAUST Repository

    Elbaz, Ayman M.

    2013-01-01

    This paper presents experimental investigation of LPG partially premixed turbulent flames stabilized within a conical nozzle burner under constant degree of partial premixing. The stability limits and mean flame structure are presented based

  6. Dynamics and structure of stretched flames

    Energy Technology Data Exchange (ETDEWEB)

    Law, C.K. [Princeton Univ., NJ (United States)

    1993-12-01

    This program aims to gain fundamental understanding on the structure, geometry, and dynamics of laminar premixed flames, and relate these understanding to the practical issues of flame extinction and stabilization. The underlying fundamental interest here is the recent recognition that the response of premixed flames can be profoundly affected by flame stretch, as manifested by flow nonuniformity, flame curvature, and flame/flow unsteadiness. As such, many of the existing understanding on the behavior of premixed flames need to be qualitatively revised. The research program consists of three major thrusts: (1) detailed experimental and computational mapping of the structure of aerodynamically-strained planar flames, with emphasis on the effects of heat loss, nonequidiffusion, and finite residence time on the flame thickness, extent of incomplete reaction, and the state of extinction. (2) Analytical study of the geometry and dynamics of stretch-affected wrinkled flame sheets in simple configurations, as exemplified by the Bunsen flame and the spatially-periodic flame, with emphasis on the effects of nonlinear stretch, the phenomena of flame cusping, smoothing, and tip opening, and their implications on the structure and burning rate of turbulent flames. (3) Stabilization and blowoff of two-dimensional inverted premixed and stabilization and determining the criteria governing flame blowoff. The research is synergistically conducted through the use of laser-based diagnostics, computational simulation of the flame structure with detailed chemistry and transport, and mathematical analysis of the flame dynamics.

  7. Large-scale vortex structures and local heat release in lean turbulent swirling jet-flames under vortex breakdown conditions

    Science.gov (United States)

    Chikishev, Leonid; Lobasov, Aleksei; Sharaborin, Dmitriy; Markovich, Dmitriy; Dulin, Vladimir; Hanjalic, Kemal

    2017-11-01

    We investigate flame-flow interactions in an atmospheric turbulent high-swirl methane/air lean jet-flame at Re from 5,000 to 10,000 and equivalence ratio below 0.75 at the conditions of vortex breakdown. The focus is on the spatial correlation between the propagation of large-scale vortex structures, including precessing vortex core, and the variations of the local heat release. The measurements are performed by planar laser-induced fluorescence of hydroxyl and formaldehyde, applied simultaneously with the stereoscopic particle image velocimetry technique. The data are processed by the proper orthogonal decomposition. The swirl rate exceeded critical value for the vortex breakdown resulting in the formation of a processing vortex core and secondary helical vortex filaments that dominate the unsteady flow dynamics both of the non-reacting and reacting jet flows. The flame front is located in the inner mixing layer between the recirculation zone and the annular swirling jet. A pair of helical vortex structures, surrounding the flame, stretch it and cause local flame extinction before the flame is blown away. This work is supported by Russian Science Foundation (Grant No 16-19-10566).

  8. Beam steering effects in turbulent high pressure flames

    Energy Technology Data Exchange (ETDEWEB)

    Hemmerling, B; Kaeppeli, B [Paul Scherrer Inst. (PSI), Villigen (Switzerland)

    1997-06-01

    The propagation of a laser beam through a flame is influenced by variations of the optical density. Especially in turbulent high pressure flames this may seriously limit the use of laser diagnostic methods. (author) 1 fig., 2 refs.

  9. Effect of hydrogen on hydrogen-methane turbulent non-premixed flame under MILD condition

    Energy Technology Data Exchange (ETDEWEB)

    Mardani, Amir; Tabejamaat, Sadegh [Department of Aerospace engineering, Amirkabir university of technology (Tehran polytechnic), Hafez Ave., PO. Box: 15875-4413, Tehran (Iran)

    2010-10-15

    Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH{sub 4}+H{sub 2} jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147-1154]. In general, the modelling is carried out using the EDC model, to describe turbulence-chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H{sub 2}/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H{sub 2}+95%CH{sub 4}, 10%H{sub 2}+90%CH{sub 4} and 20% H{sub 2}+80%CH{sub 4}(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release. (author)

  10. Experimental analysis of an oblique turbulent flame front propagating in a stratified flow

    Energy Technology Data Exchange (ETDEWEB)

    Galizzi, C.; Escudie, D. [Universite de Lyon, CNRS, CETHIL, INSA-Lyon, UMR5008, F-69621 Cedex (France)

    2010-12-15

    This paper details the experimental study of a turbulent V-shaped flame expanding in a nonhomogeneous premixed flow. Its aim is to characterize the effects of stratification on turbulent flame characteristics. The setup consists of a stationary V-shaped flame stabilized on a rod and expanding freely in a lean premixed methane-air flow. One of the two oblique fronts interacts with a stratified slice, which has an equivalence ratio close to one and a thickness greater than that of the flame front. Several techniques such as PIV and CH{sup *} chemiluminescence are used to investigate the instantaneous fields, while laser Doppler anemometry and thermocouples are combined with a concentration probe to provide information on the mean fields. First, in order to provide a reference, the homogeneous turbulent case is studied. Next, the stratified turbulent premixed flame is investigated. Results show significant modifications of the whole flame and of the velocity field upstream of the flame front. The analysis of the geometric properties of the stratified flame indicates an increase in flame brush thickness, closely related to the local equivalence ratio. (author)

  11. Investigation of mass transfer in swirling turbulent flames

    International Nuclear Information System (INIS)

    Sharaborin, D; Abdurakipov, S; Dulin, V

    2016-01-01

    The present paper reports on analysis of flow structure and turbulent transport in swirling flames. The particle image velocimetry and spontaneous Raman scattering techniques were used for the measurements of 2D velocity and density distributions. The focus was placed on comparison between low- and high-swirl flows. A pronounced bubble-type vortex breakdown with strong flow precession took place in the latter case. (paper)

  12. Computational Enhancements for Direct Numerical Simulations of Statistically Stationary Turbulent Premixed Flames

    KAUST Repository

    Mukhadiyev, Nurzhan

    2017-05-01

    Combustion at extreme conditions, such as a turbulent flame at high Karlovitz and Reynolds numbers, is still a vast and an uncertain field for researchers. Direct numerical simulation of a turbulent flame is a superior tool to unravel detailed information that is not accessible to most sophisticated state-of-the-art experiments. However, the computational cost of such simulations remains a challenge even for modern supercomputers, as the physical size, the level of turbulence intensity, and chemical complexities of the problems continue to increase. As a result, there is a strong demand for computational cost reduction methods as well as in acceleration of existing methods. The main scope of this work was the development of computational and numerical tools for high-fidelity direct numerical simulations of premixed planar flames interacting with turbulence. The first part of this work was KAUST Adaptive Reacting Flow Solver (KARFS) development. KARFS is a high order compressible reacting flow solver using detailed chemical kinetics mechanism; it is capable to run on various types of heterogeneous computational architectures. In this work, it was shown that KARFS is capable of running efficiently on both CPU and GPU. The second part of this work was numerical tools for direct numerical simulations of planar premixed flames: such as linear turbulence forcing and dynamic inlet control. DNS of premixed turbulent flames conducted previously injected velocity fluctuations at an inlet. Turbulence injected at the inlet decayed significantly while reaching the flame, which created a necessity to inject higher than needed fluctuations. A solution for this issue was to maintain turbulence strength on the way to the flame using turbulence forcing. Therefore, a linear turbulence forcing was implemented into KARFS to enhance turbulence intensity. Linear turbulence forcing developed previously by other groups was corrected with net added momentum removal mechanism to prevent mean

  13. Combustion characteristics of natural gas-hydrogen hybrid fuel turbulent diffusion flame

    Energy Technology Data Exchange (ETDEWEB)

    El-Ghafour, S.A.A.; El-dein, A.H.E.; Aref, A.A.R. [Mechanical Power Engineering Department, Faculty of Engineering, Suez Canal University, Port-Said (Egypt)

    2010-03-15

    Combustion characteristics of natural gas - hydrogen hybrid fuel were investigated experimentally in a free jet turbulent diffusion flame flowing into a slow co-flowing air stream. Experiments were carried out at a constant jet exit Reynolds number of 4000 and with a wide range of NG-H{sub 2} mixture concentrations, varied from 100%NG to 50%NG-50% H{sub 2} by volume. The effect of hydrogen addition on flame stability, flame length, flame structure, exhaust species concentration and pollutant emissions was conducted. Results showed that, hydrogen addition sustains a progressive improvement in flame stability and reduction in flame length, especially for relatively high hydrogen concentrations. Hydrogen-enriched flames found to have a higher combustion temperatures and reactivity than natural gas flame. Also, it was found that hydrogen addition to natural gas is an ineffective strategy for NO and CO reduction in the studied range, while a significant reduction in the %CO{sub 2} molar concentration by about 30% was achieved. (author)

  14. An Experimental Study of Turbulent Nonpremixed Jet Flames in Crossflow Under Low-Gravity Conditions

    Science.gov (United States)

    Boxx, Isaac G.; Idicheria, Cherian A.; Clemens, Noel T.

    2002-11-01

    We will present results of a study of turbulent nonpremixed jet flames in crossflow under normal and low gravity conditions. This enables us to experimentally separate the competing influences of initial jet-to-crossflow momentum ratio and buoyancy effects on the flame structure. The low gravity conditions (10-30 milli-g) are achieved by dropping a self-contained jet flame rig in the University of Texas 1.25-second drop tower facility. This rig uses a small blow-through wind tunnel to create the crossflow. The jet flames issue from an orifice that is flush with the wall. High-speed CCD imaging of jet flame luminosity is the primary diagnostic. We present results for hydrocarbon jet flames with initial jet-to-crossflow momentum ratios of 10-20. Results such as flame trajectory, flame length, large scale structure and flame tip dynamics will be presented.

  15. Analysis of Flame Characteristics in a Laboratory-Scale Turbulent Lifted Jet Flame via DNS

    Directory of Open Access Journals (Sweden)

    Haiou Wang

    2013-09-01

    Full Text Available A fully compressible 3D solver for reacting flows has been developed and applied to investigate a turbulent lifted jet flame in a vitiated coflow by means of direct numerical simulation (DNS to validate the solver and analyze the flame characteristics. An eighth-order central differencing scheme is used for spatial discretization and a fourth-order Runge-Kutta method is employed for time integration. The DNS results agree well with the experimental measurements for the conditional means of reactive scalars. However, the lift-off height is under predicted. The mean axial velocity develops into a self-similar profile after x/D = 6. The normalized flame index is employed to characterize the combustion regime. It is found that at the flame base the gradients of the reactants are opposed and diffusion combustion is dominant. Further downstream, the contribution of premixed combustion increases and peaks at x/D = 8. Finally, the stabilization process is examined. The turbulent lifted flame is proved to stabilize in the lean mixtures and low scalar dissipation rate regions.

  16. Understanding and predicting soot generation in turbulent non-premixed jet flames.

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Hai (University of Southern California, Los Angeles, CA); Kook, Sanghoon; Doom, Jeffrey; Oefelein, Joseph Charles; Zhang, Jiayao; Shaddix, Christopher R.; Schefer, Robert W.; Pickett, Lyle M.

    2010-10-01

    This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation

  17. Flame kernel generation and propagation in turbulent partially premixed hydrocarbon jet

    KAUST Repository

    Mansour, Mohy S.; Elbaz, Ayman M.; Zayed, M. F.

    2014-01-01

    Flame development, propagation, stability, combustion efficiency, pollution formation, and overall system efficiency are affected by the early stage of flame generation defined as flame kernel. Studying the effects of turbulence and chemistry

  18. Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa

    KAUST Repository

    Bagdanavicius, Audrius

    2015-11-01

    Independent research at two centres using a burner and an explosion bomb has revealed important aspects of turbulent premixed flame structure. Measurements at pressures and temperatures up to 1.25MPa and 673K in the two rigs were aimed at quantifying the influences of flame stretch rate and strain rate Markstein number, Masr , on both turbulent burning velocity and flame surface density. That on burning velocity is expressed through the stretch rate factor, Io , or probability of burning, Pb 0.5. These depend on Masr , but they grow in importance as the Karlovitz stretch factor, K, increases, and are evaluated from the associated burning velocity data. Planar laser tomography was employed to identify contours of reaction progress variable in both rigs. These enabled both an appropriate flame front for the measurement of the turbulent burning velocity to be identified, and flame surface densities, with the associated factors, to be evaluated. In the explosion measurements, these parameters were derived also from the flame surface area, the derived Pb 0.5 factor and the measured turbulent burning velocities. In the burner measurement they were calculated directly from the flame surface density, which was derived from the flame contours.A new overall correlation is derived for the Pb 0.5 factor, in terms of Masr at different K and this is discussed in the light of previous theoretical studies. The wrinkled flame surface area normalised by the area associated with the turbulent burning velocity measurement, and the ratio of turbulent to laminar burning velocity, ut /ul , are also evaluated. The higher the value of Pb0.5, the more effective is an increased flame wrinkling in increasing ut /ul A correlation of the product of k and the laminar flame thickness with Karlovitz stretch factor and Markstein number is explored using the present data and those of other workers. Some generality is revealed, enabling the wave length associated with the spatial change in mean

  19. Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa

    KAUST Repository

    Bagdanavicius, Audrius; Bowen, Phil J.; Bradley, Derek; Lawes, Malcolm; Mansour, Morkous S.

    2015-01-01

    Independent research at two centres using a burner and an explosion bomb has revealed important aspects of turbulent premixed flame structure. Measurements at pressures and temperatures up to 1.25MPa and 673K in the two rigs were aimed at quantifying the influences of flame stretch rate and strain rate Markstein number, Masr , on both turbulent burning velocity and flame surface density. That on burning velocity is expressed through the stretch rate factor, Io , or probability of burning, Pb 0.5. These depend on Masr , but they grow in importance as the Karlovitz stretch factor, K, increases, and are evaluated from the associated burning velocity data. Planar laser tomography was employed to identify contours of reaction progress variable in both rigs. These enabled both an appropriate flame front for the measurement of the turbulent burning velocity to be identified, and flame surface densities, with the associated factors, to be evaluated. In the explosion measurements, these parameters were derived also from the flame surface area, the derived Pb 0.5 factor and the measured turbulent burning velocities. In the burner measurement they were calculated directly from the flame surface density, which was derived from the flame contours.A new overall correlation is derived for the Pb 0.5 factor, in terms of Masr at different K and this is discussed in the light of previous theoretical studies. The wrinkled flame surface area normalised by the area associated with the turbulent burning velocity measurement, and the ratio of turbulent to laminar burning velocity, ut /ul , are also evaluated. The higher the value of Pb0.5, the more effective is an increased flame wrinkling in increasing ut /ul A correlation of the product of k and the laminar flame thickness with Karlovitz stretch factor and Markstein number is explored using the present data and those of other workers. Some generality is revealed, enabling the wave length associated with the spatial change in mean

  20. Structure and combustion characteristics of turbulent, pre-mixed high-pressure flames; Projekt 'Struktur und Brenneigenschaften von turbulenten, vorgemischten Hochdruckflammen'

    Energy Technology Data Exchange (ETDEWEB)

    Griebel, P.; Boschek, E.; Erne, D.; Siewert, P.

    2005-12-15

    This illustrated annual report for 2005 for the Swiss Federal Office of Energy (SFOE) reports on the work done in 2005 at the Paul Scherrer Institute (PSI) on the structure and combustion characteristics of turbulent, pre-mixed high-pressure flames. The aims of the project are described in detail, which include, among other things, the completion of previous work, the validation of simulations and the influence of turbulence on the flame front. Work done on the project in 2005 is described and commented on. Experimental installations are described and the results obtained are presented. Also, the influence of adding hydrogen to the methane fuel is commented on. National and international co-operation is reviewed and future work to be carried out is noted.

  1. A parametric study of microjet assisted methane/air turbulent flames

    International Nuclear Information System (INIS)

    Chouaieb, Sirine; Kriaa, Wassim; Mhiri, Hatem; Bournot, Philippe

    2017-01-01

    Highlights: • Microjet assisted methane/air turbulent flames are numerically investigated. • A parametric study concerning the microjet velocity and diameter is carried out. • Previous validation of temperature, mixture fraction and soot is enhanced. • Mixing and soot emission are controlled for higher velocities and lower diameters. • Soot production is reduced by 94% for a microjet velocity equal to 1 m/s. - Abstract: A parametric study of microjet assisted methane/air turbulent flames characteristics is numerically investigated. The Presumed Probability Density Function model and the Discrete Ordinates model are respectively considered for combustion and radiation modeling. The k-epsilon Standard model with Pope Correction is adopted as a turbulence closure model. The two step Tesner model is used to quantify the soot particle production in the flame configuration. Comparison with our previous work using the k-epsilon Realizable model shows that the k-epsilon Standard model with Pope Correction ensures better predictions. The microjet velocity and diameter effects on thermal field, mixing process and soot emission are then discussed. Numerical findings show that the microjet can be used as an efficient tool controlling methane/air turbulent flames. On the one hand, it is shown that the microjet creates an inner flame in the vicinity of the central nozzle exit but does not globally alter the methane/air flame shape. On the other hand, mixing process can be enhanced for high microjet Reynolds number either by increasing the microjet velocity or by decreasing its nozzle diameter for a constant microjet mass flow rate. Soot production can be consequently reduced for low microjet diameter and high velocity values.

  2. Measurements of turbulent premixed flame dynamics using cinema stereoscopic PIV

    Energy Technology Data Exchange (ETDEWEB)

    Steinberg, Adam M.; Driscoll, James F. [University of Michigan, Department of Aerospace Engineering, Ann Arbor, MI (United States); Ceccio, Steven L. [University of Michigan, Department of Mechanical Engineering, Ann Arbor, MI (United States)

    2008-06-15

    A new experimental method is described that provides high-speed movies of turbulent premixed flame wrinkling dynamics and the associated vorticity fields. This method employs cinema stereoscopic particle image velocimetry and has been applied to a turbulent slot Bunsen flame. Three-component velocity fields were measured with high temporal and spatial resolutions of 0.9 ms and 140{mu}m, respectively. The flame-front location was determined using a new multi-step method based on particle image gradients, which is described. Comparisons are made between flame fronts found with this method and simultaneous CH-PLIF images. These show that the flame contour determined corresponds well to the true location of maximum gas density gradient. Time histories of typical eddy-flame interactions are reported and several important phenomena identified. Outwardly rotating eddy pairs wrinkle the flame and are attenuated at they pass through the flamelet. Significant flame-generated vorticity is produced downstream of the wrinkled tip. Similar wrinkles are caused by larger groups of outwardly rotating eddies. Inwardly rotating pairs cause significant convex wrinkles that grow as the flame propagates. These wrinkles encounter other eddies that alter their behavior. The effects of the hydrodynamic and diffusive instabilities are observed and found to be significant contributors to the formation and propagation of wrinkles. (orig.)

  3. Numerical study of flame structure in the mild combustion regime

    Directory of Open Access Journals (Sweden)

    Mardani Amir

    2015-01-01

    Full Text Available In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution (MILD combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in MILD regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot Co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions.

  4. Direct numerical simulation of two-phases turbulent combustion: application to study of propagation and structure of flames; Simulation numerique directe de la combustion turbulente diphasique: application a l'etude de la propagation et de la structure des flammes

    Energy Technology Data Exchange (ETDEWEB)

    Canneviere, K.

    2003-12-15

    This work is devoted to the study of the propagation and the structure of two-phases turbulent flames. To this end, Direct Numerical Simulations (DNS) are used. First, numerical systems for two-phases flow simulations is presented along with a specific chemical model. Then, a study of laminar spray flames is carried out. An analytical study related to the dynamics of evaporation of droplets is first proposed where the influence on the equivalence ratio of the ratio between the heating delay of the droplet and the evaporation delay is detailed. The simulation of a propagating flame through a cloud of droplets is carried out and a pulsating behavior is highlighted. A study of these flames according to the topology of liquid fuel enabled us to characterize a double flame structure composed of a premixed flame and a diffusion flame. Our last study is devoted to spray turbulent flames. Two-phase combustion of turbulent jets has been simulated. By varying the spray injection parameters (density, equivalence ratio), a database has been generated. This database allowed us to describe local and global flame regimes appearing in the combustion of sprays. They have been categorized in four main structures: open and closed external regime, group combustion and mixed combustion. Eventually, a combustion diagram has been developed. It involves the spray vaporization time, the mean inter-space between droplets or group of droplets and eventually the injected equivalence ratio. (author)

  5. Simulation and analysis of the soot particle size distribution in a turbulent nonpremixed flame

    KAUST Repository

    Lucchesi, Marco

    2017-02-05

    A modeling framework based on Direct Simulation Monte Carlo (DSMC) is employed to simulate the evolution of the soot particle size distribution in turbulent sooting flames. The stochastic reactor describes the evolution of soot in fluid parcels following Lagrangian trajectories in a turbulent flow field. The trajectories are sampled from a Direct Numerical Simulation (DNS) of a n-heptane turbulent nonpremixed flame. The DSMC method is validated against experimentally measured size distributions in laminar premixed flames and found to reproduce quantitatively the experimental results, including the appearance of the second mode at large aggregate sizes and the presence of a trough at mobility diameters in the range 3–8 nm. The model is then applied to the simulation of soot formation and growth in simplified configurations featuring a constant concentration of soot precursors and the evolution of the size distribution in time is found to depend on the intensity of the nucleation rate. Higher nucleation rates lead to a higher peak in number density and to the size distribution attaining its second mode sooner. The ensemble-averaged PSDF in the turbulent flame is computed from individual samples of the PSDF from large sets of Lagrangian trajectories. This statistical measure is equivalent to time-averaged, scanning mobility particle size (SMPS) measurements in turbulent flames. Although individual trajectories display strong bimodality as in laminar flames, the ensemble-average PSDF possesses only one mode and a long, broad tail, which implies significant polydispersity induced by turbulence. Our results agree very well with SMPS measurements available in the literature. Conditioning on key features of the trajectory, such as mixture fraction or radial locations does not reduce the scatter in the size distributions and the ensemble-averaged PSDF remains broad. The results highlight and explain the important role of turbulence in broadening the size distribution of

  6. Investigation of buoyancy effects on turbulent nonpremixed jet flames by using normal and low-gravity conditions

    Science.gov (United States)

    Idicheria, Cherian Alex

    An experimental study was performed with the aim of investigating the structure of transitional and turbulent nonpremixed jet flames under different gravity conditions. In particular, the focus was to determine the effect of buoyancy on the mean and fluctuating characteristics of the jet flames. Experiments were conducted under three gravity levels, viz. 1 g, 20 mg and 100 mug. The milligravity and microgravity conditions were achieved by dropping a jet-flame rig in the UT-Austin 1.25-second and the NASA-Glenn Research Center 2.2-second drop towers, respectively. The principal diagnostics employed were time-resolved, cinematographic imaging of the visible soot luminosity and planar laser Mie scattering (PLMS). For the cinematographic flame luminosity imaging experiments, the flames studied were piloted nonpremixed propane, ethylene and methane jet flames at source Reynolds numbers ranging from 2000 to 10500. From the soot luminosity images, mean and root-mean square (RMS) images were computed, and volume rendering of the image sequences was used to investigate the large-scale structure evolution and flame tip dynamics. The relative importance of buoyancy was quantified with the parameter, xL , as defined by Becker and Yamazaki [1978]. The results show, in contrast to previous microgravity studies, that the high Reynolds number flames have the same flame length irrespective of the gravity level. The RMS fluctuations and volume renderings indicate that the large-scale structure and flame tip dynamics are essentially identical to those of purely momentum driven flames provided xL is approximately less than 2. The volume-renderings show that the luminous structure celerities (normalized by jet exit velocity) are approximately constant for xL 8. The celerity values for xL > 8 are seen to follow a x3/2L scaling, which can be predicted with a simplified momentum equation analysis for the buoyancy-dominated regime. The underlying turbulent structure and mean mixture

  7. Comprehensive Validation of Skeletal Mechanism for Turbulent Premixed Methane–Air Flame Simulations

    KAUST Repository

    Luca, Stefano

    2017-08-01

    A new skeletal mechanism, consisting of 16 species and 72 reactions, has been developed for lean methane–air premixed combustion from the GRI-Mech 3.0. The skeletal mechanism is validated for elevated unburnt temperatures (800 K) and pressures up to 4 atm, thereby addressing realistic gas turbine conditions. The skeletal mechanism is obtained by applying the directed relation graph method and performing sensitivity analysis on the detailed mechanism. The mechanism has been validated for flame speed and flame structure in a wide range of conditions and configurations. A good agreement between the skeletal mechanism and GRI-3.0 was obtained. The configurations considered include one-dimension laminar premixed flames, laminar non-premixed counterflow burners, and two- and three-dimensional unsteady configurations with variations of temperature, pressure, and composition. The skeletal mechanism allows for the inclusion of accurate finite rate chemistry in large-scale direct numerical simulations of lean turbulent premixed flames. In a large-scale direct numerical simulation, the use of the skeletal mechanism reduces the memory requirements by more than a factor of 3 and accelerates the simulation by a factor of 7 compared with the detailed mechanism. The skeletal mechanism is suitable for unsteady three-dimensional simulations of methane turbulent premixed, non-premixed, and globally lean partially premixed flames and is available as supplementary material.

  8. DNS of turbulent premixed slot flames with mixture inhomogeneity: a study of NOx formation

    Science.gov (United States)

    Luca, Stefano; Attili, Antonio; Bisetti, Fabrizio

    2016-11-01

    A set of Direct Numerical Simulations of three-dimensional methane/air lean flames in a spatially developing turbulent slot burner are performed. The flames are in the thin-reaction zone regimes and the jet Reynolds number is 5600. This configuration is of interest since it displays turbulent production by mean shear as in real devices. The gas phase hydrodynamics are modeled with the reactive, unsteady Navier-Stokes equations in the low Mach number limit. Combustion is treated with finite-rate chemistry. The jet is characterized by a non-uniform equivalence ratio at the inlet and varying levels of incomplete premixing for the methane/air mixture are considered. The global equivalence ratio is 0.7 and temperature is 800 K. All simulations are performed at 4 atm. The instantaneous profiles of the mass fractions of methane and air at the inlet are sampled from a set of turbulent channel simulations that provide realistic, fully turbulent fields. The data are analyzed to study the influence of partial premixing on the flame structure. Particular focus is devoted to the assessment of heat release rate fluctuations and NOx formation. In particular, the effects of partial premixing on the production rates for the various pathways to NOx formation are investigated.

  9. A NEW DOUBLE-SLIT CURVED WALL-JET (CWJ) BURNER FOR STABILIZING TURBULENT PREMIXED AND NON-PREMIXED FLAMES

    KAUST Repository

    Mansour, Morkous S.

    2015-06-30

    A novel double-slit curved wall-jet (CWJ) burner was proposed and employed, which utilizes the Coanda effect by supplying fuel and air as annular-inward jets over a curved surface. We investigated the stabilization characteristics and structure of methane/air, and propane/air turbulent premixed and non-premixed flames with varying global equivalence ratio, , and Reynolds number, Re. Simultaneous time-resolved measurements of particle image velocimetry and planar laser-induced fluorescence of OH radicals were conducted. The burner showed potential for stable operation for methane flames with relatively large fuel loading and overall rich conditions. These have a non-sooting nature. However, propane flames exhibit stable mode for a wider range of equivalence ratio and Re. Mixing characteristics in the cold flow of non-premixed cases were first examined using acetone fluorescence technique, indicating substantial transport between the fuel and air by exhibiting appreciable premixing conditions.PIV measurements revealed that velocity gradients in the shear layers at the boundaries of the annularjets generate the turbulence, enhanced with the collisions in the interaction jet, IJ,region. Turbulent mean and rms velocities were influenced significantly by Re and high rms turbulent velocities are generated within the recirculation zone improving the flame stabilization in this burner.Premixed and non-premixed flames with high equivalence ratio were found to be more resistant to local extinction and exhibited a more corrugated and folded nature, particularly at high Re. For flames with low equivalence ratio, the processes of local quenching at IJ region and of re-ignition within merged jet region maintained these flames further downstream particularly for non-premixed methane flame, revealing a strong intermittency.

  10. Turbulent Flame Speeds and NOx Kinetics of HHC Fuels with Contaminants and High Dilution Levels

    Energy Technology Data Exchange (ETDEWEB)

    Peterson, Eric [Texas A & M Univ., College Station, TX (United States); Krejci, Michael [Texas A & M Univ., College Station, TX (United States); Mathieu, Olivier [Texas A & M Univ., College Station, TX (United States); Vissotski, Andrew [Texas A & M Univ., College Station, TX (United States); Ravi, Sankat [Texas A & M Univ., College Station, TX (United States); Plichta, Drew [Texas A & M Univ., College Station, TX (United States); Sikes, Travis [Texas A & M Univ., College Station, TX (United States); Levacque, Anthony [Texas A & M Univ., College Station, TX (United States); Camou, Alejandro [Texas A & M Univ., College Station, TX (United States); Aul, Christopher [Texas A & M Univ., College Station, TX (United States)

    2014-01-24

    This final report documents the technical results of the 3-year project entitled, “Turbulent Flame Speeds and NOx Kinetics of HHC Fuels with Contaminants and High Dilution Levels,” funded under the NETL of DOE. The research was conducted under six main tasks: 1) program management and planning; 2) turbulent flame speed measurements of syngas mixtures; 3) laminar flame speed measurements with diluents; 4) NOx mechanism validation experiments; 5) fundamental NOx kinetics; and 6) the effect of impurities on NOx kinetics. Experiments were performed using primary constant-volume vessels for laminar and turbulent flame speeds and shock tubes for ignition delay times and species concentrations. In addition to the existing shock- tube and flame speed facilities, a new capability in measuring turbulent flame speeds was developed under this grant. Other highlights include an improved NOx kinetics mechanism; a database on syngas blends for real fuel mixtures with and without impurities; an improved hydrogen sulfide mechanism; an improved ammonia kintics mechanism; laminar flame speed data at high pressures with water addition; and the development of an inexpensive absorption spectroscopy diagnostic for shock-tube measurements of OH time histories. The Project Results for this work can be divided into 13 major sections, which form the basis of this report. These 13 topics are divided into the five areas: 1) laminar flame speeds; 2) Nitrogen Oxide and Ammonia chemical kinetics; 3) syngas impurities chemical kinetics; 4) turbulent flame speeds; and 5) OH absorption measurements for chemical kinetics.

  11. Soot emissions from turbulent diffusion flames burning simple alkane fuels

    Energy Technology Data Exchange (ETDEWEB)

    Canteenwalla, P.M.; Johnson, M.R. [Carleton Univ., Ottawa, ON (Canada). Dept. of Mechanical and Aerospace Engineering; Thomson, K.A.; Smallwood, G.J. [National Research Council of Canada, Ottawa, ON (Canada). Inst. for Chemical Process and Environmental Technology

    2007-07-01

    A classic problem in combustion involves measurement and prediction of soot emissions from turbulent diffusion flames. Very high-sensitivity measurements of particulate matter (PM) from very low-sooting diffusion flames burning methane and other simple alkane fuels have been enabled from recent advances in laser-induced incandescence (LII). In order to quantify soot emissions from a lab-scale turbulent diffusion flame burner, this paper presented a study that used LII to develop a sampling protocol. The purpose of the study was to develop an experimentally based model to predict PM emissions from flares used in industry using soot emissions from lab-scale flares. Quantitative results of mass of soot emitted per mass of fuel burned were presented across a range of flow conditions and fuels. The experiment used digital imaging to measure flame lengths and estimate flame residence times. Comparisons were also made between current measurements and results of previous researchers for soot in the overfire region. The study also considered the validity applicability of buoyancy based models for predicting and scaling soot emissions. The paper described the experimental setup including sampling system and flame length imaging. Background information on soot yield and a comparison of flame residence time definitions were provided. The results and discussion of results were also presented. It was concluded that the results highlighted the subjective nature of flame length measurements. 10 refs., 4 figs.

  12. Turbulent Jet Flames Into a Vitiated Coflow. PhD Thesis awarded Spring 2003

    Science.gov (United States)

    Holdeman, James D. (Technical Monitor); Cabra, Ricardo

    2004-01-01

    Examined is the vitiated coflow flame, an experimental condition that decouples the combustion processes of flows found in practical combustors from the associated recirculating fluid mechanics. The configuration consists of a 4.57 mm diameter fuel jet into a coaxial flow of hot combustion products from a lean premixed flame. The 210 mm diameter coflow isolates the jet flame from the cool ambient, providing a hot environment similar to the operating conditions of advanced combustors; this important high temperature element is lacking in the traditional laboratory experiments of jet flames into cool (room) air. A family of flows of increasing complexity is presented: 1) nonreacting flow, 2) all hydrogen flame (fuel jet and premixed coflow), and 3) set of methane flames. This sequence of experiments provides a convenient ordering of validation data for combustion models. Laser Raman-Rayleigh-LIF diagnostics at the Turbulent Diffusion Flame laboratory of Sandia National Laboratories produced instantaneous multiscalar point measurements. These results attest to the attractive features of the vitiated coflow burner and the well-defined boundary conditions provided by the coflow. The coflow is uniform and steady, isolating the jet flame from the laboratory air for a downstream distance ranging from z/d = 50-70. The statistical results show that differential diffusion effects in this highly turbulent flow are negligible. Complementing the comprehensive set of multiscalar measurements is a parametric study of lifted methane flames that was conducted to analyze flame sensitivity to jet and coflow velocity, as well as coflow temperature. The linear relationship found between the lift-off height and the jet velocity is consistent with previous experiments. New linear sensitivities were found correlating the lift-off height to coflow velocity and temperature. A blow-off study revealed that the methane flame blows off at a common coflow temperature (1260 K), regardless of

  13. Inertial particles in a turbulent premixed Bunsen flame

    International Nuclear Information System (INIS)

    Battista, F.; Picano, F.; Casciola, C.M.

    2012-01-01

    Many fields of engineering and physics are characterized by reacting flows seeded with particles of different inertia and dimensions, e.g. solid-propellant rockets, reciprocating engines where carbon particles form due to combustion, vulcano eruptions. Particles are also used as velocity transducers in Particle Image Velocimetry (PIV) of turbulent flames. The effects of combustion on inertial particle dynamics is still poorly understood, despite its relevance for its effects on particle collisions and coalescence, phenomena which have a large influence in soot formation and growth. As a matter of fact, the flame front induces abrupt accelerations of the fluid in a very thin region which particles follow with different lags depending on their inertia. This phenomenon has a large impact on the particle spatial arrangement. The issuing clustering is here analyzed by a DNS of Bunsen turbulent flame coupled with particle Lagrangian tracking with the aim of evaluating the effect of inertia on particle spatial localization in combustion applications. The Eulerian algorith is based on Low-Mach number expansion of Navier-Stokes equations that allow arbitrary density variations neglecting acoustics waves. (orig.)

  14. Inertial particles in a turbulent premixed Bunsen flame

    Energy Technology Data Exchange (ETDEWEB)

    Battista, F.; Picano, F.; Casciola, C.M. [Sapienza Univ., Rome (Italy). Dipt. di Meccanica e Aeronautica; Troiani, G. [ENEA C.R. Casaccia, Rome (Italy)

    2012-07-01

    Many fields of engineering and physics are characterized by reacting flows seeded with particles of different inertia and dimensions, e.g. solid-propellant rockets, reciprocating engines where carbon particles form due to combustion, vulcano eruptions. Particles are also used as velocity transducers in Particle Image Velocimetry (PIV) of turbulent flames. The effects of combustion on inertial particle dynamics is still poorly understood, despite its relevance for its effects on particle collisions and coalescence, phenomena which have a large influence in soot formation and growth. As a matter of fact, the flame front induces abrupt accelerations of the fluid in a very thin region which particles follow with different lags depending on their inertia. This phenomenon has a large impact on the particle spatial arrangement. The issuing clustering is here analyzed by a DNS of Bunsen turbulent flame coupled with particle Lagrangian tracking with the aim of evaluating the effect of inertia on particle spatial localization in combustion applications. The Eulerian algorith is based on Low-Mach number expansion of Navier-Stokes equations that allow arbitrary density variations neglecting acoustics waves. (orig.)

  15. An Investigation of a Hybrid Mixing Model for PDF Simulations of Turbulent Premixed Flames

    Science.gov (United States)

    Zhou, Hua; Li, Shan; Wang, Hu; Ren, Zhuyin

    2015-11-01

    Predictive simulations of turbulent premixed flames over a wide range of Damköhler numbers in the framework of Probability Density Function (PDF) method still remain challenging due to the deficiency in current micro-mixing models. In this work, a hybrid micro-mixing model, valid in both the flamelet regime and broken reaction zone regime, is proposed. A priori testing of this model is first performed by examining the conditional scalar dissipation rate and conditional scalar diffusion in a 3-D direct numerical simulation dataset of a temporally evolving turbulent slot jet flame of lean premixed H2-air in the thin reaction zone regime. Then, this new model is applied to PDF simulations of the Piloted Premixed Jet Burner (PPJB) flames, which are a set of highly shear turbulent premixed flames and feature strong turbulence-chemistry interaction at high Reynolds and Karlovitz numbers. Supported by NSFC 51476087 and NSFC 91441202.

  16. The development of kilohertz planar laser diagnostics for applications in high power turbulent flames

    Science.gov (United States)

    Slabaugh, Carson Daniel

    In modern gas-turbine combustors, flame stabilization is achieved by inducing exhaust gas circulation within the flame zone through swirl-induced vortex breakdown. Swirling flows exhibit strong shear regions resulting in high turbulence and effective mixing. In combustion, these flows are characterized by complex unsteady interactions between turbulent flow structures and chemical reactions. Developments in high-resolution, quantitative, experimental measurement techniques must continue to improve fundamental understanding and support modeling efforts. This work describes the development of a gas turbine combustion experiment to support the application of advanced optical measurement techniques in flames operating at realistic engine conditions. Facility requirements are addressed, including instrumentation and control needs for remote operation when working with high energy flows. The methodology employed in the design of the optically-accessible combustion chamber is elucidated, including window considerations and thermal management of the experimental hardware under extremely high heat loads. Experimental uncertainties are also quantified. The stable operation of the experiment is validated using multiple techniques and the boundary conditions are verified. The successful prediction of operating conditions by the design analysis is documented and preliminary data is shown to demonstrate the capability of the experiment to produce high-fidelity datasets for advanced combustion research. Building on this experimental infrastructure, simultaneous measurements of velocity and scalar fields were performed in turbulent nonpremixed flames at gas turbine engine operating conditions using 5 kHz Particle-Image Velocimetry (PIV) and OH Planar Laser Induced Fluorescence (OH-PLIF). The experimental systems and the challenges associated with acquiring useful data at high pressures and high thermal powers are discussed. The quality of the particle scattering images used in the

  17. Numerical study of turbulent normal diffusion flame CH4-air stabilized by coaxial burner

    Directory of Open Access Journals (Sweden)

    Riahi Zouhair

    2013-01-01

    Full Text Available The practical combustion systems such as combustion furnaces, gas turbine, engines, etc. employ non-premixed combustion due to its better flame stability, safety, and wide operating range as compared to premixed combustion. The present numerical study characterizes the turbulent flame of methane-air in a coaxial burner in order to determine the effect of airflow on the distribution of temperature, on gas consumption and on the emission of NOx. The results in this study are obtained by simulation on FLUENT code. The results demonstrate the influence of different parameters on the flame structure, temperature distribution and gas emissions, such as turbulence, fuel jet velocity, air jet velocity, equivalence ratio and mixture fraction. The lift-off height for a fixed fuel jet velocity is observed to increase monotonically with air jet velocity. Temperature and NOx emission decrease of important values with the equivalence ratio, it is maximum about the unity.

  18. Large-eddy simulation of a bluff-body stabilised turbulent premixed flame using the transported flame surface density approach

    Science.gov (United States)

    Lee, Chin Yik; Cant, Stewart

    2017-07-01

    A premixed propane-air flame stabilised on a triangular bluff body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported flame surface density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out. For the non-reacting flow, good agreement for both the time-averaged and root-mean-square velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and

  19. Analysis of Hydrogen/Air Turbulent Premixed Flames at Different Karlovitz Numbers Using Computational Singular Perturbation

    KAUST Repository

    Manias, Dimitrios; Tingas, Alexandros-Efstathios; Hernandez Perez, Francisco E.; Im, Hong G.; Galassi, Riccardo Malpica; Ciottoli, Pietro Paolo; Valorani, Mauro

    2018-01-01

    The dynamics and structure of two turbulent H2/air premixed flames, representative of the corrugated flamelet (Case 1) and thin reaction zone (Case 2) regimes, are analyzed and compared, using the computational singular perturbation (CSP) tools

  20. Premixed Turbulent Flames and Spectral Approach Flammes turbulentes de prémélange Approche spectrale

    Directory of Open Access Journals (Sweden)

    Mathieu J.

    2006-11-01

    Full Text Available Scientific and technical approach concerning the behaviour of flames developing in a turbulent medium are related in many recent papers. On the whole the problem is very complex!The chemical reaction develops inside a turbulent flow which requires a double scaling. Characteristic times and characteristic lengths have to be defined for both flame and the turbulent fields. With a view to enlarging these comparisons a spectral analyses of the turbulent field is proposed. It is widely supported by previous experimental data. The flame can be acted upon by an external turbulent fields. That supposes the flame to be thicker that the smallest turbulent structures in connection with the Kolmogorov scale. With increasing Reynolds numbers turbulent structures penetrate the flame front, they can disturb the preheat zone or event the chemical zone. The passage of a flame front regime to the case of a chemical reaction developing in a volume is thereby emphasized. As the reaction rate is decreasing, the domain affected by the reaction is increased chemical reactions generate a segregation process whereas the chemical species are mixed by the turbulent motion. In the premixed combustion engine a large range of operating points can be defined. The diagram usually used is that of Barrere Borghi. Several modeling methods should probably be developed according to the positions of the operating points in the diagram. Modeling methods are not presented herein. However the existence of typical structures in connection with the architecture of the combustion chamber could be examined in subsequent paper. The flame front can be subjected to distorting effects due to isolated rolling or to a sequence of vortices. Previously this last case has been touched upon. Using a spectral approach no discremination has to be made as for the sizes of these rollings: that could lead to new modeling methods if restricted shapes of vortices are accepted. By using a spectral method

  1. Cellular burning in lean premixed turbulent hydrogen-air flames: Coupling experimental and computational analysis at the laboratory scale

    International Nuclear Information System (INIS)

    Day, M S; Bell, J B; Beckner, V E; Lijewski, M J; Cheng, R K; Tachibana, S

    2009-01-01

    One strategy for reducing US dependence on petroleum is to develop new combustion technologies for burning the fuel-lean mixtures of hydrogen or hydrogen-rich syngas fuels obtained from the gasification of coal and biomass. Fuel-flexible combustion systems based on lean premixed combustion have the potential for dramatically reducing pollutant emissions in transportation systems, heat and stationary power generation. However, lean premixed flames are highly susceptible to fluid-dynamical combustion instabilities making robust and reliable systems difficult to design. Low swirl burners are emerging as an important technology for meeting design requirements in terms of both reliability and emissions for next generation combustion devices. In this paper, we present simulations of a lean, premixed hydrogen flame stabilized on a laboratory-scale low swirl burner. The simulations use detailed chemistry and transport without incorporating explicit models for turbulence or turbulence/chemistry interaction. Here we discuss the overall structure of the flame and compare with experimental data. We also use the simulation data to elucidate the characteristics of the turbulent flame interaction and how this impacts the analysis of experimental measurements.

  2. Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Munki; Choi, Youngil; Oh, Jeongseog; Yoon, Youngbin [School of Mechanical and Aerospace Engineering, Seoul National University, Seoul (Korea)

    2009-12-15

    This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO{sub x} emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO{sub x} is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO{sub x} emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface. (author)

  3. DNS of spark ignition and edge flame propagation in turbulent droplet-laden mixing layers

    Energy Technology Data Exchange (ETDEWEB)

    Neophytou, A.; Mastorakos, E.; Cant, R.S. [Hopkinson Laboratory, Department of Engineering, University of Cambridge (United Kingdom)

    2010-06-15

    A parametric study of forced ignition at the mixing layer between air and air carrying fine monosized fuel droplets is done through one-step chemistry direct numerical simulations to determine the influence of the size and volatility of the droplets, the spark location, the droplet-air mixing layer initial thickness and the turbulence intensity on the ignition success and the subsequent flame propagation. The propagation is analyzed in terms of edge flame displacement speed, which has not been studied before for turbulent edge spray flames. Spark ignition successfully resulted in a tribrachial flame if enough fuel vapour was available at the spark location, which occurred when the local droplet number density was high. Ignition was achieved even when the spark was offset from the spray, on the air side, due to the diffusion of heat from the spark, provided droplets evaporated rapidly. Large kernels were obtained by sparking close to the spray, since fuel was more readily available. At long times after the spark, for all flames studied, the probability density function of the displacement speed was wide, with a mean value in the range 0.55-0.75S{sub L}, with S{sub L} the laminar burning velocity of a stoichiometric gaseous premixed flame. This value is close to the mean displacement speed in turbulent edge flames with gaseous fuel. The displacement speed was negatively correlated with curvature. The detrimental effect of curvature was attenuated with a large initial kernel and by increasing the thickness of the mixing layer. The mixing layer was thicker when evaporation was slow and the turbulence intensity higher. However, high turbulence intensity also distorted the kernel which could lead to high values of curvature. The edge flame reaction component increased when the maximum temperature coincided with the stoichiometric contour. The results are consistent with the limited available experimental evidence and provide insights into the processes associated with

  4. Blow-off characteristics of turbulent premixed flames in curved-wall Jet Burner

    KAUST Repository

    Mansour, Morkous S.; Mannaa, O.; Chung, Suk-Ho

    2015-01-01

    and simultaneously stereoscopic particle image velocimetry (SPIV) quantified the turbulent flow field features. Ethylene/air flames were stabilized in CWJ burner to determine the sequence of events leading to blowoff. For stably burning flames far from blowoff

  5. Direct Numerical Simulations of NOx formation in spatially developing turbulent premixed Bunsen flames with mixture inhomogeneity

    KAUST Repository

    Luca, Stefano

    2017-01-05

    Direct Numerical Simulation of three-dimensional spatially developing turbulent methane/air flames are performed. Four flames are simulated; they differ for the level of premixing of the fuel inlet: one has a fully premixed inlet, the other three have a partially premixed inlet that mimic a common injection strategy in stationary gas turbines. The jet consist of a methane/air mixture with global equivalence ratio ɸ = 0.7 and temperature of 800 K. The simulations are performed at 4 atm. The inlet velocity field and the fuel/air fields were extracted from a fully developed turbulent channel simulation. Chemistry is treated with a new skeletal chemical mechanism consisting of 33 species developed specifically for the DNS. The data are analyzed to study possible influences of partial premixing on the flame structure and the combustion efficiency. The results show that increasing the level of partial premixing, the fluctuations of heat release rate increase, due to the richer and leaner pockets of mixture in the flame, while the conditional mean decreases. Increasing the level of partial premixing, the peak of NO and the range of NO values for a given temperature increase. An analysis of NO production is performed categorizing the different initiation steps in the Ndecomposition through four pathways: thermal, prompt, NNH and NO. Different behaviour with respect to laminar flames is found for the NNH pathway suggesting that turbulence influences this pathway of formation of NO.

  6. Formation, growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

    KAUST Repository

    Attili, Antonio; Bisetti, Fabrizio; Mü eller, Michael E.; Pitsch, Heinz G.

    2014-01-01

    The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of

  7. Formation, growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

    KAUST Repository

    Attili, Antonio

    2014-07-01

    The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of

  8. Gaseous diffusion flames: simple structures and their interaction

    Energy Technology Data Exchange (ETDEWEB)

    Cavaliere, A. [Universita degli Studi Federico II, Naples (Italy). Dip. di Ingegneria Chimica; Ragucci, R. [Istituto di Ricerche sulla Combustione C,N.R., Naples (Italy)

    2001-07-01

    This is a synoptic overview of a selection of works dealing with single diffusive structures, with their mutual interaction in simple flows and their statistical modeling in complex flows. The focus is on reacting conditions pertaining to gaseous diffusion flames, but isothermal structures are also described when they are of some conceptual interest. This paper considers only few representative works for each subject, which are functional in explaining the key characteristics of the diffusive structures. The extension, given to single subjects, is not weighed according to the number of related publications but on the relevance to the basic understanding of the general framework concerning diffusion flames. One-dimensional structures are first discussed. They are ordered according to the number of balance equation terms needed for their description. Two-dimensional (2D) structures are then introduced following an order based on their convolution level. Some pioneering work on three-dimensional structures is further quoted. The temporal evolution of simple structures in quiescent or simple flowing 2D systems is considered. The latter case is exploited to present classification of diffusion-controlled mixing regimes. Modeling characterization approach of turbulent diffusion flames is also described in order to yield a self-sufficient didactic presentation. The approach based on the flame surface density model is specifically discussed because of its potential use in the determination of qualitative and quantitative features of simple diffusion flames. (author)

  9. Stabilization and structure of N-heptane flame on CWJ-spray burner with kHZ SPIV and OH-PLIF

    KAUST Repository

    Mansour, Morkous S.

    2015-08-31

    A curved wall-jet (CWJ) burner was employed to stabilize turbulent spray flames that utilized a Coanda effect by supplying air as annular-inward jet over a curved surface, surrounding an axisymmetric solid cone fuel spray. The stabilization characteristics and structure of n-heptane/air turbulent flames were investigated with varying fuel and air flow rates and the position of pressure atomizer (L). High-speed planar laser-induced fluorescence (PLIF) of OH radicals delineated reaction zone contours and simultaneously stereoscopic particle image velocimetry (SPIV) quantified the flow field features, involving turbulent mixing within spray, ambient air entrainment and flame-turbulence interaction. High turbulent rms velocities were generated within the recirculation zone, which improved the flame stabilization. OH fluorescence signals revealed a double flame structure near the stabilization edge of lifted flame that consisted of inner partially premixed flame and outer diffusion flame front. The inner reaction zone is highly wrinkled and folded due to significant turbulent mixing between the annular-air jet and the fuel vapor generated from droplets along the contact interface of this air jet with the fuel spray. Larger droplets, having higher momentum are able to penetrate the inner reaction zone and then vaporized in the low-speed hot region bounded by these reaction zones; this supports the outer diffusion flame. Frequent local extinctions in the inner reaction zone were observed at low air flow rate. As flow rate increases, the inner zone is more resistant to local extinction despite of its high wrinkling and corrugation degree. However, the outer reaction zone exhibits stable and mildly wrinkled features irrespective of air flow rate. The liftoff height increases with the air mass flow rate but decreases with L.

  10. Phenomenology and numerical calculations of lean hydrogen-air premixed flame propagation in a turbulent flow

    International Nuclear Information System (INIS)

    Faix-Gantier, A.

    2001-12-01

    This thesis concerns the study of flame propagation in a turbulent flow of lean hydrogen-air mixtures. The aim is to precise the characteristics of propagation as well as combustion and turbulence models able to take into account the peculiarities of these mixtures. This research work is related to the prevention of fire hazards associated with accidental release of hydrogen within the reactor of a nuclear power plant. In a first part, the scales (the flame velocity and thickness) associated with the laminar flame propagation in hydrogen-air mixtures are studied. A specific attention is devoted to the intrinsic instability properties of such flames. Then, the turbulence scales potentially present within a reactor are estimated in order to allow for the determination of the regimes of combustion that might be present within the reactor and among which the flamelet regime appears to be conceivable. In a second part, starting with the analysis of the propagation properties of a mean reaction zone calculated with a flamelet model, we show that, with an adequate tuning of the parameter appearing in the mean reaction rate expression, it is possible to predict numerically the turbulent flame speeds available with the literature. (author)

  11. Soot and Spectral Radiation Modeling for a High-Pressure Turbulent Spray Flame

    Energy Technology Data Exchange (ETDEWEB)

    Ferreryo-Fernandez, Sebastian [Pennsylvania State Univ., University Park, PA (United States); Paul, Chandan [Pennsylvania State Univ., University Park, PA (United States); Sircar, Arpan [Pennsylvania State Univ., University Park, PA (United States); Imren, Abdurrahman [Pennsylvania State Univ., University Park, PA (United States); Haworth, Daniel C [Pennsylvania State Univ., University Park, PA (United States); Roy, Somesh P [Marquette University (United States); Modest, Michael F [University of California Merced (United States)

    2017-04-26

    Simulations are performed of a transient high-pressure turbulent n-dodecane spray flame under engine-relevant conditions. An unsteady RANS formulation is used, with detailed chemistry, a semi-empirical two-equation soot model, and a particle-based transported composition probability density function (PDF) method to account for unresolved turbulent fluctuations in composition and temperature. Results from the PDF model are compared with those from a locally well-stirred reactor (WSR) model to quantify the effects of turbulence-chemistry-soot interactions. Computed liquid and vapor penetration versus time, ignition delay, and flame lift-off height are in good agreement with experiment, and relatively small differences are seen between the WSR and PDF models for these global quantities. Computed soot levels and spatial soot distributions from the WSR and PDF models show large differences, with PDF results being in better agreement with experimental measurements. An uncoupled photon Monte Carlo method with line-by-line spectral resolution is used to compute the spectral intensity distribution of the radiation leaving the flame. This provides new insight into the relative importance of molecular gas radiation versus soot radiation, and the importance of turbulent fluctuations on radiative heat transfer.

  12. Direct numerical simulations of premixed turbulent flames with flamelet-generated manifolds

    NARCIS (Netherlands)

    Oijen, van J.A.; Bastiaans, R.J.M.; Goey, de L.P.H.

    2005-01-01

    Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the thin reaction zone regime and in the broken reaction zone regime are

  13. Effect of hydrogen addition on burning rate and surface density of turbulent lean premixed methane-air flames

    International Nuclear Information System (INIS)

    Guo, H.; Tayebi, B.; Galizzi, C.; Escudie, D.

    2009-01-01

    Hydrogen (H 2 ) is a clean burning component, but relatively expensive. Mixing a small amount of hydrogen with other fuels is an effective way to use H 2 . H 2 enriched combustion significantly improves fuel efficiency and reduces pollutant (nitrogen oxide and particulate matter) emissions. This presentation discussed the effect of hydrogen addition on burning rate and surface density of turbulent lean premixed methane-air flames. The presentation discussed flame configuration; the experimental methodology using laser tomography; and results for typical images, burning velocity, ratio of turbulent to laminar burning velocities, flame surface density, curvature, flame brush thickness, and integrated flame surface area. It was concluded that the increase of turbulent burning velocity was faster than that of laminar burning velocity, which contradicted traditional theory. figs.

  14. Blowoff dynamics of bluff body stabilized turbulent premixed flames

    Energy Technology Data Exchange (ETDEWEB)

    Chaudhuri, Swetaprovo; Kostka, Stanislav; Renfro, Michael W.; Cetegen, Baki M. [Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, U-3139, Storrs, CT 06269 (United States)

    2010-04-15

    This article concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric bluff body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment. (author)

  15. Early structure of LPG partially premixed conically stabilized flames

    KAUST Repository

    Elbaz, Ayman M.

    2013-01-01

    This paper presents experimental investigation of LPG partially premixed turbulent flames stabilized within a conical nozzle burner under constant degree of partial premixing. The stability limits and mean flame structure are presented based on the mean gas temperature and the concentration of CO, O 2, NO, and HC at the flame early region of reaction. The investigation covered the influence of the nozzle cone angle, the jet exit velocity and the jet equivalence ratio. The stability results show that the flames with cone are more stable than those without cone. For conical stabilized flames, the stability results exhibit three different sensitivity regions between the jet velocity and equivalence ratio. The inflame measurements prove that the flame stability could be attributed to the triple flame structure at the flame leading edge. The data show that the triple flame structure is influenced by cone angle, the jet velocity and the equivalence ratio. The flame is believed to be controlled by the recirculation flow inside the cone. Increasing the cone angle induced higher air entrainment to the reaction zone as depicted by a higher O 2 concentration within the flame leading edge. Increasing the jet velocity to a certain limit enhances the intensity of combustion at the flame leading edge, while excessive increase in jet velocity reduces this intensity. At a fixed jet velocity the higher the equivalence ratio, the higher the amount of fuel diffused and engulfed to the reaction zone, the more delay of the combustion completion and the higher the emission concentrations of the flame. © 2012 Elsevier Inc.

  16. Combined Influence of Strain and Heat Loss on Turbulent Premixed Flame Stabilization

    KAUST Repository

    Tay-Wo-Chong, Luis

    2015-11-16

    The present paper argues that the prediction of turbulent premixed flames under non-adiabatic conditions can be improved by considering the combined effects of strain and heat loss on reaction rates. The effect of strain in the presence of heat loss on the consumption speed of laminar premixed flames was quantified by calculations of asymmetric counterflow configurations (“fresh-to-burnt”) with detailed chemistry. Heat losses were introduced by setting the temperature of the incoming stream of products on the “burnt” side to values below those corresponding to adiabatic conditions. The consumption speed decreased in a roughly exponential manner with increasing strain rate, and this tendency became more pronounced in the presence of heat losses. An empirical relation in terms of Markstein number, Karlovitz Number and a non-dimensional heat loss parameter was proposed for the combined influence of strain and heat losses on the consumption speed. Combining this empirical relation with a presumed probability density function for strain in turbulent flows, an attenuation factor that accounts for the effect of strain and heat loss on the reaction rate in turbulent flows was deduced and implemented into a turbulent combustion model. URANS simulations of a premixed swirl burner were carried out and validated against flow field and OH chemiluminescence measurements. Introducing the effects of strain and heat loss into the combustion model, the flame topology observed experimentally was correctly reproduced, with good agreement between experiment and simulation for flow field and flame length.

  17. The effects of buoyancy on turbulent nonpremixed jet flames in crossflow

    Science.gov (United States)

    Boxx, Isaac G.

    An experimental research study was conducted to investigate what effect buoyancy had on the mean and instantaneous flow-field characteristics of turbulent jet-flames in crossflow (JFICF). The study used an experimental technique wherein a series of normal-gravity, hydrogen-diluted propane JFICF were compared with otherwise identical ones in low-gravity. Experiments were conducted at the University of Texas Drop Tower Facility, a new microgravity science laboratory built for this study at the University of Texas at Austin. Two different diagnostic techniques were employed, high frame-rate digital cinematographic imaging and planar laser Mie scattering (PLMS). The flame-luminosity imaging revealed significant elongation and distortion of the large-scale luminous structure of the JFICF. This was seen to affect the flametip oscillation and burnout characteristics. Mean and root-mean-square (RMS) images of flame-luminosity were computed from the flame-luminosity image sequences. These were used to compare visible flame-shapes, flame chord-lengths and jet centerline-trajectories of the normal- and low-gravity flames. In all cases the jet-centerline penetration and mean luminous flame-width were seen to increase with decreasing buoyancy. The jet-centerline trajectories for the normal-gravity flames were seen to behave differently to those of the low-gravity flames. This difference led to the conclusion that the jet transitions from a momentum-dominated forced convection limit to a buoyancy-influenced regime when it reaches xiC ≈ 3, where xiC is the Becker and Yamazaki (1978) buoyancy parameter based on local flame chord-length. The mean luminous flame-lengths showed little sensitivity to buoyancy or momentum flux ratio. Consistent with the flame-luminosity imaging experiments, comparison of the instantaneous PLMS flow-visualization images revealed substantial buoyancy-induced elongation and distortion of the large-scale shear-layer vortices in the flow. This effect

  18. Structure of diffusion flames from a vertical burner

    Science.gov (United States)

    Mark A. Finney; Dan Jimenez; Jack D. Cohen; Isaac C. Grenfell; Cyle Wold

    2010-01-01

    Non-steady and turbulent flames are commonly observed to produce flame contacts with adjacent fuels during fire spread in a wide range of fuel bed depths. A stationary gas-fired burner (flame wall) was developed to begin study of flame edge variability along an analagous vertical fuel source. This flame wall is surrogate for a combustion interface at the edge of a deep...

  19. Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

    Energy Technology Data Exchange (ETDEWEB)

    S. Mahesh; D.P. Mishra [Indian Institute of Technology, Kanpur (India). Combustion Laboratory, Department of Aerospace Engineering

    2008-09-15

    The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H{sub p}, to visible flame length, H{sub f}, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINOX happens to occur closer to stoichiometric overall equivalence ratio. 16 refs., 9 figs.

  20. An Investigation of a Hybrid Mixing Timescale Model for PDF Simulations of Turbulent Premixed Flames

    Science.gov (United States)

    Zhou, Hua; Kuron, Mike; Ren, Zhuyin; Lu, Tianfeng; Chen, Jacqueline H.

    2016-11-01

    Transported probability density function (TPDF) method features the generality for all combustion regimes, which is attractive for turbulent combustion simulations. However, the modeling of micromixing due to molecular diffusion is still considered to be a primary challenge for TPDF method, especially in turbulent premixed flames. Recently, a hybrid mixing rate model for TPDF simulations of turbulent premixed flames has been proposed, which recovers the correct mixing rates in the limits of flamelet regime and broken reaction zone regime while at the same time aims to properly account for the transition in between. In this work, this model is employed in TPDF simulations of turbulent premixed methane-air slot burner flames. The model performance is assessed by comparing the results from both direct numerical simulation (DNS) and conventional constant mechanical-to-scalar mixing rate model. This work is Granted by NSFC 51476087 and 91441202.

  1. Thermal Radiation Properties of Turbulent Lean Premixed Methane Air Flames

    National Research Council Canada - National Science Library

    Ji, Jun; Sivathanu, Y. R; Gore, J. P

    2000-01-01

    ... of turbulent premixed flames. Reduced cooling airflows in lean premixed combustors, miniaturization of combustors, and the possible use of radiation sensors in combustion control schemes are some of the practical reasons...

  2. On the formation and early evolution of soot in turbulent nonpremixed flames

    KAUST Repository

    Bisetti, Fabrizio

    2012-01-01

    A Direct Numerical Simulation (DNS) of soot formation in an n-heptane/air turbulent nonpremixed flame has been performed to investigate unsteady strain effects on soot growth and transport. For the first time in a DNS of turbulent combustion, Polycyclic Aromatic Hydrocarbons (PAH) are included via a validated, reduced chemical mechanism. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments is used [M.E. Mueller, G. Blanquart, H. Pitsch, Combust. Flame 156 (2009) 1143-1155], which allows for an accurate state-of-the-art description of soot number density, volume fraction, and morphology of the aggregates. In agreement with previous experimental studies in laminar flames, Damköhler number effects are found to be significant for PAH. Soot nucleation and growth from PAH are locally inhibited by high scalar dissipation rate, thus providing a possible explanation for the experimentally observed reduction of soot yields at increasing levels of mixing in turbulent sooting flames. Furthermore, our data indicate that soot growth models that rely on smaller hydrocarbon species such as acetylene as a proxy for large PAH molecules ignore or misrepresent the effects of turbulent mixing and hydrodynamic strain on soot formation due to differences in the species Damköhler number. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction iso-surfaces due to differential diffusion effects between soot and the gas-phase. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow primarily by condensation of PAH on the particle surface. In contrast to previous DNS studies based on simplified soot and chemistry models, surface reactions are found to contribute barely to the growth of soot, for nucleation and condensation processes occurring in the fuel stream are responsible for the most of soot mass generation. Furthermore, the morphology of the soot aggregates is

  3. Presumed PDF modeling of microjet assisted CH4–H2/air turbulent flames

    International Nuclear Information System (INIS)

    Chouaieb, Sirine; Kriaa, Wassim; Mhiri, Hatem; Bournot, Philippe

    2016-01-01

    Highlights: • Microjet assisted CH 4 –H 2 /air turbulent flames are numerically investigated. • Temperature, species and soot are well predicted by the Presumed PDF model. • An inner flame is identified due to the microjet presence. • The addition of hydrogen to the microjet assisted flames enhances mixing. • Soot emission is reduced by 36% for a 10% enriched microjet assisted flame. - Abstract: The characteristics of microjet assisted CH 4 –H 2 /air flames in a turbulent mode are numerically investigated. Simulations are performed using the Computational Fluid Dynamics code Fluent. The Presumed PDF and the Discrete Ordinates models are considered respectively for combustion and radiation modeling. The k–ε Realizable model is adopted as a turbulence closure model. The Tesner model is used to calculate soot particle quantities. In the first part of this paper, the Presumed PDF model is compared to the Eddy Dissipation model and to slow chemistry combustion models from literature. Results show that the Presumed PDF model predicts correctly thermal and species fields, as well as soot formation. The effect of hydrogen enrichment on CH 4 /air confined flames under the addition of an air microjet is investigated in the second part of this work. The found results show that an inner flame was identified due to the air microjet for the CH 4 –H 2 /air flames. Moreover, the increase of hydrogen percentage in the fuel mixture leads to mixing enhancement and consequently to considerable soot emission reduction.

  4. Influence of Pilot Flame Parameters on the Stability of Turbulent Jet Flames

    KAUST Repository

    Guiberti, Thibault F.

    2016-11-08

    This paper presents a comprehensive study of the effects of pilot parameters on flame stability in a turbulent jet flame. The Sydney inhomogeneous piloted burner is employed as the experimental platform with two main fuels, namely, compressed natural gas and liquefied petroleum gas. Various concentrations of five gases are used in the pilot stream, hydrogen, acetylene, oxygen, nitrogen, and argon, to enable a sufficient range in exploring the following parameters: pilot heat release, temperature, burnt gas velocity, equivalence ratio, and H/C ratio. The experimental results are mainly presented in the form of blow-off limits and supported by simple calculations, which simulate various conditions of the pilot–mixture interface. It is found that increasing the pilot adiabatic flame temperature benefits the flame stability and has an even greater influence than the heat release, which is also known to enhance the blow-off limits. Conversely, increasing the pilot burnt gas velocity reduces the blow-off velocity, except for the limiting case when the jet is fully non-premixed. The H/C ratio has negligible effects, while resorting to lean pilots significantly increases the stability of globally rich partially premixed and premixed jets. Such findings are consistent with trends obtained from laminar flame calculations for rich fuel/air mixtures issuing against hot combustion products to simulate the pilot stream.

  5. Geometrical properties of turbulent premixed flames and other corrugated interfaces

    Science.gov (United States)

    Thiesset, F.; Maurice, G.; Halter, F.; Mazellier, N.; Chauveau, C.; Gökalp, I.

    2016-01-01

    This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar

  6. A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

    Science.gov (United States)

    Hu, Yong; Olguin, Hernan; Gutheil, Eva

    2017-05-01

    A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new

  7. Radiation/turbulence interactions in pulverized-coal flames. Second year technical progress report, September 30, 1994--September 30, 1995

    Energy Technology Data Exchange (ETDEWEB)

    Menguec, M.P.; McDonough, J.M.; Manickavsagam, S.; Mukerji, S.; Wang, D.; Ghosal, S.; Swabb, S.

    1995-12-31

    Our goal in this project is to investigate the interaction of radiation and turbulence in coalfired laboratory scale flames and attempt to determine the boundaries of the ``uncertainty domain`` in Figure 3 more rigorously. We have three distinct objectives: (1) To determine from experiments the effect of turbulent fluctuations on the devolatilization/pyrolysis of coal particles and soot yield, and to measure the change in the ``effective`` radiative properties of particulates due to turbulence interactions; (2) To perform local small-scale simulations to investigate the radiation-turbulence interactions in coal-fired flames starting from first principles; and (3) To develop a thorough and rigorous, but computationally practical, turbulence model for coal flames, starting from the experimental observations and small scale simulations.

  8. A NEW DOUBLE-SLIT CURVED WALL-JET (CWJ) BURNER FOR STABILIZING TURBULENT PREMIXED AND NON-PREMIXED FLAMES

    KAUST Repository

    Mansour, Morkous S.; Chung, Suk-Ho

    2015-01-01

    Mixing characteristics in the cold flow of non-premixed cases were first examined using acetone fluorescence technique, indicating substantial transport between the fuel and air by exhibiting appreciable premixing conditions.PIV measurements revealed that velocity gradients in the shear layers at the boundaries of the annularjets generate the turbulence, enhanced with the collisions in the interaction jet, IJ,region. Turbulent mean and rms velocities were influenced significantly by Re and high rms turbulent velocities are generated within the recirculation zone improving the flame stabilization in this burner.Premixed and non-premixed flames with high equivalence ratio were found to be more resistant to local extinction and exhibited a more corrugated and folded nature, particularly at high Re. For flames with low equivalence ratio, the processes of local quenching at IJ region and of re-ignition within merged jet region maintained these flames further downstream particularly for non-premixed methane flame, revealing a strong intermittency.

  9. The evolution of the flame surface in turbulent premixed jet flames at high Reynolds number

    Science.gov (United States)

    Luca, Stefano; Attili, Antonio; Bisetti, Fabrizio

    2017-11-01

    A set of direct numerical simulations of turbulent premixed flames in a spatially developing turbulent slot burner at four Reynolds number is presented. This configuration is of interest since it displays turbulent production by mean shear as in real combustion devices. The gas phase hydrodynamics are modeled with the reactive, unsteady Navier-Stokes equations in the low Mach number limit, with finite-rate chemistry consisting of 16 species and 73 reactions. For the highest jet Reynolds number of 22 ×103, 22 Billion grid points are employed. The jet consists of a lean methane/air mixture at 4 atm and preheated to 800 K. The analysis of stretch statistics shows that the mean total stretch is close to zero. Mean stretch decreases moving downstream from positive to negative values, suggesting a formation of surface area in the near field and destruction at the tip of the flame; the mean contribution of the tangential strain term is positive, while the mean contribution of the propagative term is always negative. Positive values of stretch are due to the tangential strain rate term, while large negative values are associated with the propagative term. Increasing Reynolds number is found to decrease the correlation between stretch and the single contributions.

  10. Correlation of optical emission and turbulent length scale in a coaxial jet diffusion flame

    OpenAIRE

    松山, 新吾; Matsuyama, Shingo

    2014-01-01

    This article investigates the correlation between optical emission and turbulent length scale in a coaxial jet diffusion flame. To simulate the H2O emission from an H2/O2 diffusion flame, radiative transfer is calculated on flame data obtained by numerical simulation. H2O emission characteristics are examined for a one-dimensional opposed-flow diffusion flame. The results indicate that H2O emission intensity is linearly dependent on flame thickness. The simulation of H2O emission is then exte...

  11. Evaluation of turbulent transport and flame surface dissipation using direct numerical simulation of turbulent combustion; Evaluation des termes de transport et de dissipation de surface de flamme par simulation numerique directe de la combustion turbulente

    Energy Technology Data Exchange (ETDEWEB)

    Boughanem, H.

    1998-03-24

    The assumption of gradient transport for the mean reaction progress variable has a limited domain of validity in premixed turbulent combustion. The existence of two turbulent transport regimes, gradient and counter-gradient, is demonstrated in the present work using Direct Numerical Simulations (DNS) of plane flame configurations. The DNS data base describes the influence of the heat release factor, of the turbulence-to-flame velocity ratio, and of an external pressure gradient. The simulations reveal a strong correlation between the regime of turbulent transport and the turbulent flame speed and turbulent flame thickness. These effects re not well described by current turbulent combustion models. A conditional approach `fresh gases / burnt gases` is proposed to overcome these difficulties. Furthermore, he development of flame instabilities in turbulent configurations is also observed in the simulations. A criterion is derived that determines the domain of occurrence of these instabilities (Darrieus- Landau instabilities, Rayleigh- Taylor instabilities, thermo-diffusive instabilities). This criterion suggests that the domain of occurrence of flame instabilities is not limited to small Reynolds numbers. (author) 98 refs.

  12. Simultaneous identification of transfer functions and combustion noise of a turbulent flame

    Science.gov (United States)

    Merk, M.; Jaensch, S.; Silva, C.; Polifke, W.

    2018-05-01

    The Large Eddy Simulation/System Identification (LES/SI) approach allows to deduce a flame transfer function (FTF) from LES of turbulent reacting flow: Time series of fluctuations of reference velocity and global heat release rate resulting from broad-band excitation of a simulated turbulent flame are post-processed via SI techniques to derive a low order model of the flame dynamics, from which the FTF is readily deduced. The current work investigates an extension of the established LES/SI approach: In addition to estimation of the FTF, a low order model for the combustion noise source is deduced from the same time series data. By incorporating such a noise model into a linear thermoacoustic model, it is possible to predict the overall level as well as the spectral distribution of sound pressure in confined combustion systems that do not exhibit self-excited thermoacoustic instability. A variety of model structures for estimation of a noise model are tested in the present study. The suitability and quality of these model structures are compared against each other, their sensitivity regarding certain time series properties is studied. The influence of time series length, signal-to-noise ratio as well as acoustic reflection coefficient of the boundary conditions on the identification are examined. It is shown that the Box-Jenkins model structure is superior to simpler approaches for the simultaneous identification of models that describe the FTF as well as the combustion noise source. Subsequent to the question of the most adequate model structure, the choice of optimal model order is addressed, as in particular the optimal parametrization of the noise model is not obvious. Akaike's Information Criterion and a model residual analysis are applied to draw qualitative and quantitative conclusions on the most suitable model order. All investigations are based on a surrogate data model, which allows a Monte Carlo study across a large parameter space with modest

  13. Experimental study of laminar and turbulent flame speed of a spherical flame in a fan-stirred closed vessel for hydrogen safety application

    Energy Technology Data Exchange (ETDEWEB)

    Goulier, J. [Institut de Combustion, Aérothermique, Réactivité et Environnement, CNRS-ICARE (France); Institut de Radioprotection et de Sûreté Nucléaire (IRSN) (France); Chaumeix, N., E-mail: chaumeix@cnrs-orleans.fr [Institut de Combustion, Aérothermique, Réactivité et Environnement, CNRS-ICARE (France); Halter, F. [Institut de Combustion, Aérothermique, Réactivité et Environnement, CNRS-ICARE (France); Meynet, N.; Bentaïb, A. [Institut de Radioprotection et de Sûreté Nucléaire (IRSN) (France)

    2017-02-15

    The aim of this paper is to report new experimental results on the effect of turbulence on the propagation speed of hydrogen/air flames. To do so, a new experimental setup, called the spherical bomb, has been designed and built at CNRS-ICARE laboratory. With this new setup, the effect of a given and well-characterized turbulence intensity on the increase of hydrogen/air flame speed can be investigated. This new facility consists of a spherical vessel equipped (563 mm internal diameter) equipped with 8 motors which are linked to fans inside the bomb. Fan actuation induces the generation of a turbulent flow inside the vessel prior to any ignition. The spherical bomb is equipped with 4 quartz windows (200 mm optical diameter) that allow the use of a Particle Image Velocimetry diagnostic in order to characterize the turbulence level inside the bomb. The flame propagation was recorded using a high speed camera at 19,002 frames per second. These experiments were performed for lean to stoichiometric hydrogen/air mixtures (16–20% of H{sub 2} in air), initially at ambient temperature and pressure, and for a rotation speed from 1000 to 5000 rpm. The PIV measurements showed that a homogeneous and isotropic turbulence is created with a fluctuation speed that can reach 4 m/s at 5000 rpm.

  14. Damköhler number effects on soot formation and growth in turbulent nonpremixed flames

    KAUST Repository

    Attili, Antonio

    2015-01-01

    The effect of Damköhler number on turbulent nonpremixed sooting flames is investigated via large scale direct numerical simulation in three-dimensional n-heptane/air jet flames at a jet Reynolds number of 15,000 and at three different Damköhler numbers. A reduced chemical mechanism, which includes the soot precursor naphthalene, and a high-order method of moments are employed. At the highest Damköhler number, local extinction is negligible, while flames holes are observed in the two lowest Damköhler number cases. Compared to temperature and other species controlled by fuel oxidation chemistry, naphthalene is found to be affected more significantly by the Damköhler number. Consequently, the overall soot mass fraction decreases by more than one order of magnitude for a fourfold decrease of the Damköhler number. On the contrary, the overall number density of soot particles is approximately the same, but its distribution in mixture fraction space is different in the three cases. The total soot mass growth rate is found to be proportional to the Damköhler number. In the two lowest Da number cases, soot leakage across the flame is observed. Leveraging Lagrangian statistics, it is concluded that soot leakage is due to patches of soot that cross the stoichiometric surface through flame holes. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

  15. Investigation of Scalar Filtered Density Function in Turbulent Partially Premixed Flames

    National Research Council Canada - National Science Library

    Tong, Chenning

    2006-01-01

    ... using measurement data obtained in turbulent partially premixed methane/air (Sandia) flames. For SGS scalar variance small compared to its mean, the FMDF is not far from Gaussian and the SGS scalar is well mixed...

  16. High-Pressure Turbulent Flame Speeds and Chemical Kinetics of Syngas Blends with and without Impurities

    Energy Technology Data Exchange (ETDEWEB)

    Peterson, Eric; Mathieu, Olivier; Morones, Anibal; Ravi, Sankar; Keesee, Charles; Hargis, Joshua; Vivanco, Jose

    2014-12-01

    This Topical Report documents the first year of the project, from October 1, 2013 through September 30, 2014. Efforts for this project included experiments to characterize the atmospheric-pressure turbulent flame speed vessel over a range of operating conditions (fan speeds and turbulent length scales). To this end, a new LDV system was acquired and set up for the detailed characterization of the turbulence field. Much progress was made in the area of impurity kinetics, which included a numerical study of the effect of impurities such as NO2, NO, H2S, and NH3 on ignition delay times and laminar flame speeds of syngas blends at engine conditions. Experiments included a series of laminar flame speed measurements for syngas (CO/H2) blends with various levels of CH4 and C2H6 addition, and the results were compared to the chemical kinetics model of NUI Galway. Also, a final NOx kinetics mechanism including ammonia was assembled, and a journal paper was written and is now in press. Overall, three journal papers and six conference papers related to this project were published this year. Finally, much progress was made on the design of the new high-pressure turbulent flame speed facility. An overall design that includes a venting system was decided upon, and the detailed design is in progress.

  17. The near-field region behaviour of hydrogen-air turbulent non-premixed flame

    Energy Technology Data Exchange (ETDEWEB)

    Tabet, F. [EDF R and D, EIFER (European Institute for Energy Research), Karlsruhe (Germany); Sarh, B. [Centre National de la Recherche Scientifique (CNRS), Institut de Combustion, Aerothermique, Reactivite et Environnement (ICARE), Orleans (France); Universite d' Orleans, Institut Universitaire de Technologie d' Orleans (France); Birouk, M. [University of Manitoba, Department of Mechanical and Manufacturing Engineering, Winnipeg, MB (Canada); Goekalp, I. [Centre National de la Recherche Scientifique (CNRS), Institut de Combustion, Aerothermique, Reactivite et Environnement (ICARE), Orleans (France)

    2012-02-15

    A computational study of mixing process and air entrainment in hydrogen turbulent non-premixed flame characterized by strong gradients of velocity and density at the inlet section is presented. Different approaches for turbulence-combustion interactions are evaluated in the framework of RSM (Reynolds Stress Model) turbulence model and the computational results are compared to experimental data. The combustion models investigated are SLFM (Steady Laminar Flamelet Model) and EDC (Eddy Dissipation Concept). Mixing is described by oxygen atom mixture fraction and air entrainment is characterized by gas mass flow rate. Computational results are compared to measurements in physical space at two locations (the first one represent the near-field region and the second one the far-field region). At the first station, the results showed an overestimation of mixing and air entrainment and an inaccurate consumption of O{sub 2} and H{sub 2}. In addition, the predictions are found to be sensitive to combustion modelling. At the second station, the description of mixing and air entrainment is improved and the predictions are in reasonably agreement with experimental data. Less dependency to combustion modelling is noticed in this location. Further analysis of the near-field region based on the turbulence time scales revealed that turbulence is not well developed in this region of the flame. (orig.)

  18. Numerical simulation of premixed turbulent methane combustion

    International Nuclear Information System (INIS)

    Bell, John B.; Day, Marcus S.; Grcar, Joseph F.

    2001-01-01

    In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame

  19. A scale-entropy diffusion equation to describe the multi-scale features of turbulent flames near a wall

    Science.gov (United States)

    Queiros-Conde, D.; Foucher, F.; Mounaïm-Rousselle, C.; Kassem, H.; Feidt, M.

    2008-12-01

    Multi-scale features of turbulent flames near a wall display two kinds of scale-dependent fractal features. In scale-space, an unique fractal dimension cannot be defined and the fractal dimension of the front is scale-dependent. Moreover, when the front approaches the wall, this dependency changes: fractal dimension also depends on the wall-distance. Our aim here is to propose a general geometrical framework that provides the possibility to integrate these two cases, in order to describe the multi-scale structure of turbulent flames interacting with a wall. Based on the scale-entropy quantity, which is simply linked to the roughness of the front, we thus introduce a general scale-entropy diffusion equation. We define the notion of “scale-evolutivity” which characterises the deviation of a multi-scale system from the pure fractal behaviour. The specific case of a constant “scale-evolutivity” over the scale-range is studied. In this case, called “parabolic scaling”, the fractal dimension is a linear function of the logarithm of scale. The case of a constant scale-evolutivity in the wall-distance space implies that the fractal dimension depends linearly on the logarithm of the wall-distance. We then verified experimentally, that parabolic scaling represents a good approximation of the real multi-scale features of turbulent flames near a wall.

  20. Numerical study of influences of crosswind and additional steam on the flow field and temperature of propane non-premixed turbulence flame

    Science.gov (United States)

    Wusnah; Bindar, Y.; Yunardi; Nur, F. M.; Syam, A. M.

    2018-03-01

    This paper presents results the process of combustion propane using computational fluid dynamics (CFD) to simulate the turbulent non-premixed flame under the influences of crosswinds and the ratio of fuel (propane) to steam, S. Configuration, discretization and boundary conditions of the flame are described using GambitTM software and integrated with FluentTM software for calculations of flow and reactive fields. This work focuses on the influence of various crosswind speeds (0–10 m/s) and values of S (0.14–2.35) while the velocity of fuel issued from the nozzle was kept constant at 20 m/s. A turbulence model, k-ɛ standard and combustion model, Eddy Dissipation model were employed for the calculation of velocity and temperature fields, respectively. The results are displayed in the form of predictive terrain profile of the propane flame at different crosswind speeds. The results of the propane flame profile demonstrated that the crosswind significantly affect the structure velocity and position of the flame which was off-center moving towards the direction of crosswind, eventually affect the temperature along the flame. As the values of S is increasing, the flame contour temperature decreases, until the flame was extinguished at S equals to 2.35. The combustion efficiency for a variety of crosswind speeds decreases with increasing values of S.

  1. TRAJECTORY AND INCINERATION OF ROGUE DROPLETS IN A TURBULENT DIFFUSION FLAME

    Science.gov (United States)

    The trajectory and incineration efficiency of individual droplet streams of a fuel mixture injected into a swirling gas turbulent diffusion flame were measured as a function of droplet size, droplet velocity, interdroplet spacing, and droplet injection angle. Additional experimen...

  2. Effect of CH4–Air Ratios on Gas Explosion Flame Microstructure and Propagation Behaviors

    Directory of Open Access Journals (Sweden)

    Ying Zhang

    2012-10-01

    Full Text Available To reveal the inner mechanism of gas explosion dynamic behavior affected by gas equivalent concentration, a high speed Schlieren image system and flow field measurement technology was applied to record the gas explosion flame propagation and flame structure transition. The results show that a flame front structure transition occurs, followed by a flame accelerating propagation process. The laminar to turbulence transition was the essential cause of the flame structure changes. The laminar flame propagation behavior was influenced mainly by gas expansion and fore-compressive wave effect, while the turbulent flame speed mostly depended on turbulence intensity, which also played an important role in peak value of the explosive pressure and flame speed. On the condition that the laminar-turbulent transition was easier to form, the conclusion was drawn that, the lowest CH4 concentration for maximum overpressure can be obtained, which was the essential reason why the ideal explosive concentration differs under different test conditions.

  3. Transport of inertial particles in a turbulent premixed jet flame

    International Nuclear Information System (INIS)

    Battista, F; Picano, F; Casciola, C M; Troiani, G

    2011-01-01

    The heat release, occurring in reacting flows, induces a sudden fluid acceleration which particles follow with a certain lag, due to their finite inertia. Actually, the coupling between particle inertia and the flame front expansion strongly biases the spatial distribution of the particles, by inducing the formation of localized clouds with different dimensions downstream the thin flame front. A possible indicator of this preferential localization is the so-called Clustering Index, quantifying the departure of the actual particle distribution from the Poissonian, which would correspond to a purely random spatial arrangement. Most of the clustering is found in the flame brush region, which is spanned by the fluctuating instantaneous flame front. The effect is significant also for very light particles. In this case a simple model based on the Bray-Moss-Libby formalism is able to account for most of the deviation from the Poissonian. When the particle inertia increases, the effect is found to increases and persist well within the region of burned gases. The effect is maximum when the particle relaxation time is of the order of the flame front time scale. The evidence of this peculiar source of clustering is here provided by data from a direct numerical simulation of a turbulent premixed jet flame and confirmed by experimental data.

  4. Scalar flux modeling in turbulent flames using iterative deconvolution

    Science.gov (United States)

    Nikolaou, Z. M.; Cant, R. S.; Vervisch, L.

    2018-04-01

    In the context of large eddy simulations, deconvolution is an attractive alternative for modeling the unclosed terms appearing in the filtered governing equations. Such methods have been used in a number of studies for non-reacting and incompressible flows; however, their application in reacting flows is limited in comparison. Deconvolution methods originate from clearly defined operations, and in theory they can be used in order to model any unclosed term in the filtered equations including the scalar flux. In this study, an iterative deconvolution algorithm is used in order to provide a closure for the scalar flux term in a turbulent premixed flame by explicitly filtering the deconvoluted fields. The assessment of the method is conducted a priori using a three-dimensional direct numerical simulation database of a turbulent freely propagating premixed flame in a canonical configuration. In contrast to most classical a priori studies, the assessment is more stringent as it is performed on a much coarser mesh which is constructed using the filtered fields as obtained from the direct simulations. For the conditions tested in this study, deconvolution is found to provide good estimates both of the scalar flux and of its divergence.

  5. Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

    Science.gov (United States)

    Huang, Rong Fung; Kivindu, Reuben Mwanza; Hsu, Ching Min

    2018-06-01

    The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

  6. Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

    Science.gov (United States)

    Huang, Rong Fung; Kivindu, Reuben Mwanza; Hsu, Ching Min

    2017-12-01

    The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

  7. A flame particle tracking analysis of turbulence–chemistry interaction in hydrogen–air premixed flames

    KAUST Repository

    Uranakara, Harshavardhana A.; Chaudhuri, Swetaprovo; Dave, Himanshu L.; Arias, Paul G.; Im, Hong G.

    2015-01-01

    Interactions of turbulence, molecular transport, and energy transport, coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation, and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen–air premixed turbulent flames. Direct numerical simulations (DNSs) are conducted at different parametric conditions with a detailed reaction mechanism and transport properties for hydrogen–air flames. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (Sd) of a temperature isosurface is considered, and the contributions of transport, chemistry, and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Statistics of principal curvature evolving in time, obtained using FPT, suggest that these islands are ellipsoidal on average enclosing fresh reactants. Further examinations show that the increase in Sd is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number are found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.

  8. A flame particle tracking analysis of turbulence–chemistry interaction in hydrogen–air premixed flames

    KAUST Repository

    Uranakara, Harshavardhana A.

    2015-11-21

    Interactions of turbulence, molecular transport, and energy transport, coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation, and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen–air premixed turbulent flames. Direct numerical simulations (DNSs) are conducted at different parametric conditions with a detailed reaction mechanism and transport properties for hydrogen–air flames. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (Sd) of a temperature isosurface is considered, and the contributions of transport, chemistry, and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Statistics of principal curvature evolving in time, obtained using FPT, suggest that these islands are ellipsoidal on average enclosing fresh reactants. Further examinations show that the increase in Sd is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number are found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.

  9. Asymptotic analysis, direct numerical simulation and modeling of premixed turbulent flame-wall interaction; Etude asymptotique, simulation numerique directe et modelisation de l`interaction flamme turbulente premelangee-paroi

    Energy Technology Data Exchange (ETDEWEB)

    Bruneaux, G.

    1996-05-20

    Premixed turbulent flame-wall interaction is studied using theoretical and numerical analysis. Laminar interactions are first investigated through a literature review. This gives a characterization of the different configurations of interaction and justifies the use of simplified kinetic schemes to study the interaction. Calculations are then performed using Direct Numerical Simulation with a one-step chemistry model, and are compared with good agreements to asymptotic analysis. Flame-wall distances and wall heat fluxes obtained are compared successfully with those of the literature. Heat losses decrease the consumption rate, leading to extinction at the maximum of wall heat flux. It is followed by a flame retreat, when the fuel diffuses into the reaction zone, resulting in low unburnt hydrocarbon levels. Then, turbulent regime is investigated, using two types of Direct Numerical Simulations: 2D variable density and 3D constant density. Similar results are obtained: the local turbulent flame behavior is identical to a laminar interaction, and tongues of fresh gases are expelled from the wall region, near zones of quenching. In the 2D simulations, minimal flame-wall distances and maximum wall heat fluxes are similar to laminar values. However, the structure of the turbulence in the 3D calculations induces smaller flame-wall distances and higher wall heat fluxes. Finally, a flame-wall interaction model is built and validated. It uses the flamelet approach, where the flame is described in terms of consumption speed and flame surface density. This model is simplified to produce a law of the wall, which is then included in a averaged CFD code (Kiva2-MB). It is validated in an engine calculation. (author) 36 refs.

  10. Turbulent jet diffusion flame length evolution with cross flows in a sub-pressure atmosphere

    International Nuclear Information System (INIS)

    Wang, Qiang; Hu, Longhua; Zhang, Xiaozheng; Zhang, Xiaolei; Lu, Shouxiang; Ding, Hang

    2015-01-01

    Highlights: • Quantifying turbulent jet diffusion flame length with cross flows. • Unique data revealed for a sub-atmospheric pressure. • Non-dimensional global correlation proposed for flame trajectory-line length. - Abstract: This paper investigates the evolution characteristics of turbulent jet diffusion flame (flame trajectory-line length, flame height in vertical jet direction) with increasing cross flows in a sub-pressure (64 kPa) atmosphere. The combined effect of cross flow and a special sub-pressure atmosphere condition is revealed, where no data is available in the literatures. Experiments are carried out with a wind tunnel built specially in Lhasa city (altitude: 3650 m; pressure: 64 kPa) and in Hefei city (altitude: 50 m; pressure: 100 kPa), using nozzles with diameter of 3 mm, 4 mm and 5 mm and propane as fuel. It is found that, as cross flow air speed increases from zero, the flame trajectory-line length firstly decreases and then becomes almost stable (for relative small nozzle, 3 mm in this study) or increases (for relative large nozzle, 4 mm and 5 mm in this study) beyond a transitional critical cross flow air speed in normal pressure, however decreases monotonically until being blown-out in the sub-pressure atmosphere. The flame height in jet direction decreases monotonically with cross air flow speed and then reaches a steady value in both pressures. For the transitional state of flame trajectory-line length with increasing cross air flow speed, the corresponding critical cross flow air speed is found to be proportional to the fuel jet velocity, meanwhile independent of nozzle diameter. Correlation models are proposed for the flame height in jet direction and the flame trajectory-line length for both ambient pressures, which are shown to be in good agreement with the experimental results.

  11. Turbulence-chemistry interactions in reacting flows

    Energy Technology Data Exchange (ETDEWEB)

    Barlow, R.S.; Carter, C.D. [Sandia National Laboratories, Livermore, CA (United States)

    1993-12-01

    Interactions between turbulence and chemistry in nonpremixed flames are investigated through multiscalar measurements. Simultaneous point measurements of major species, NO, OH, temperature, and mixture fraction are obtained by combining spontaneous Raman scattering, Rayleigh scattering, and laser-induced fluorescence (LIF). NO and OH fluorescence signals are converted to quantitative concentrations by applying shot-to-shot corrections for local variations of the Boltzmann fraction and collisional quenching rate. These measurements of instantaneous thermochemical states in turbulent flames provide insights into the fundamental nature of turbulence-chemistry interactions. The measurements also constitute a unique data base for evaluation and refinement of turbulent combustion models. Experimental work during the past year has focused on three areas: (1) investigation of the effects of differential molecular diffusion in turbulent combustion: (2) experiments on the effects of Halon CF{sub 3}Br, a fire retardant, on the structure of turbulent flames of CH{sub 4} and CO/H{sub 2}/N{sub 2}; and (3) experiments on NO formation in turbulent hydrogen jet flames.

  12. Investigation on Effect of Air Velocity in Turbulent Non-Premixed Flames

    Directory of Open Access Journals (Sweden)

    Namazian Zafar

    2016-09-01

    Full Text Available In this study, the turbulent non-premixed methane-air flame is simulated to determine the effect of air velocity on the length of flame, temperature distribution and mole fraction of species. The computational fluid dynamics (CFD technique is used to perform this simulation. To solve the turbulence flow, k-ε model is used. In contrast to the previous works, in this study, in each one of simulations the properties of materials are taken variable and then the results are compared. The results show that at a certain flow rate of fuel, by increasing the air velocity, similar to when the properties are constant, the width of the flame becomes thinner and the maximum temperature is higher; the penetration of oxygen into the fuel as well as fuel consumption is also increased. It is noteworthy that most of the pollutants produced are NOx, which are strongly temperature dependent. The amount of these pollutants rises when the temperature is increased. As a solution, decreasing the air velocity can decrease the amount of these pollutants. Finally, comparing the result of this study and the other work, which considers constant properties, shows that the variable properties assumption leads to obtaining more exact solution but the trends of both results are similar.

  13. Modeling of Dissipation Element Statistics in Turbulent Non-Premixed Jet Flames

    Science.gov (United States)

    Denker, Dominik; Attili, Antonio; Boschung, Jonas; Hennig, Fabian; Pitsch, Heinz

    2017-11-01

    The dissipation element (DE) analysis is a method for analyzing and compartmentalizing turbulent scalar fields. DEs can be described by two parameters, namely the Euclidean distance l between their extremal points and the scalar difference in the respective points Δϕ . The joint probability density function (jPDF) of these two parameters P(Δϕ , l) is expected to suffice for a statistical reconstruction of the scalar field. In addition, reacting scalars show a strong correlation with these DE parameters in both premixed and non-premixed flames. Normalized DE statistics show a remarkable invariance towards changes in Reynolds numbers. This feature of DE statistics was exploited in a Boltzmann-type evolution equation based model for the probability density function (PDF) of the distance between the extremal points P(l) in isotropic turbulence. Later, this model was extended for the jPDF P(Δϕ , l) and then adapted for the use in free shear flows. The effect of heat release on the scalar scales and DE statistics is investigated and an extended model for non-premixed jet flames is introduced, which accounts for the presence of chemical reactions. This new model is validated against a series of DNS of temporally evolving jet flames. European Research Council Project ``Milestone''.

  14. Flame Acceleration and Transition to Detonation in High Speed Turbulent Combustion

    Science.gov (United States)

    2016-12-21

    Contract number N000141410177 for the time period December 31, 2013- December 31 , 2016 Principal Investigator: Elaine S. Oran Co-Investigator: Norman ...channel shown. (a) Rough walls generate turbulence. Flame ( green ) accelerates as it propagates down the channel. (b) DDT in the boundary layer near t

  15. Fuel effects on the stability of turbulent flames with compositionally inhomogeneous inlets

    KAUST Repository

    Guiberti, T. F.; Juddoo, M.; Lacoste, Deanna; Dunn, M. J.; Roberts, William L.; Masri, A. R.

    2016-01-01

    This paper reports an analysis of the influence of fuels on the stabilization of turbulent piloted jet flames with inhomogeneous inlets. The burner is identical to that used earlier by the Sydney Group and employs two concentric tubes within

  16. An experimental study on turbulent lifted flames of methane in coflow jets at elevated temperatures

    KAUST Repository

    Choi, Byungchul

    2013-01-01

    An experimental study was conducted on the effects of initial temperature variation on the stabilization characteristics of turbulent nonpremixed flames in coflow jets of methane fuel diluted by nitrogen. The typical behavior seen in the study showed that the liftoff height increased linearly with the jet velocity regardless of the initial temperature in the turbulent regime. Two models were investigated for predicting liftoff heights in the methane jets: the premixed flame model and the large-scale mixing model. For the premixed flame model, the liftoff heights in the methane jets were accurately predicted using the thermal diffusivity of the unburned gas temperature αst,0, instead of that of the burned gas temperature αst,b. For the large-scale mixing model, however, the prediction of liftoff heights differed slightly for the various fuel mole fractions. However, when considering the initial fuel mass fraction YF,0, the liftoff heights were successfully predicted. This result implies that the characteristics of the unburned fuel-air mixture play a crucial role for flame stabilization in coflow jets for a variety of initial conditions. In the turbulent regime, the blowout velocity and the liftoff height at blowout could be accurately predicted by the two models based on a consideration of the physical properties and the buoyancy effect of the initial temperature variation. © 2012 Elsevier Ltd. All rights reserved.

  17. Effect of Low Co-flow Air Velocity on Hydrogen-air Non-premixed Turbulent Flame Model

    Directory of Open Access Journals (Sweden)

    Noor Mohsin Jasim

    2017-08-01

    Full Text Available The aim of this paper is to provide information concerning the effect of low co-flow velocity on the turbulent diffusion flame for a simple type of combustor, a numerical simulated cases of turbulent diffusion hydrogen-air flame are performed. The combustion model used in this investigation is based on chemical equilibrium and kinetics to simplify the complexity of the chemical mechanism. Effects of increased co-flowing air velocity on temperature, velocity components (axial and radial, and reactants have been investigated numerically and examined. Numerical results for temperature are compared with the experimental data. The comparison offers a good agreement. All numerical simulations have been performed using the Computational Fluid Dynamics (CFD commercial code FLUENT. A comparison among the various co-flow air velocities, and their effects on flame behavior and temperature fields are presented.

  18. Turbulent flame acceleration and detonation quenching and reinitiation - modelling and validation

    International Nuclear Information System (INIS)

    Fischer, M.; Kratzel, T.; Pantow, E.

    1997-01-01

    For both, the reactor safety in an accidental release of hydrogen into containment compartments and also for the industrial safety of the production, storage and transport of combustibles like hydrogen, propane, methane and others in the Petroleum, Petrochemical and Pharmaceutical Industries, it is of great interest to know how the pressure forces of fast hydrogen combustion processes can be reduced. The numerical study of highly turbulent or detonation driven flame propagation processes is relatively recent because it depends on the availability of high performance computers and specialized numerical algorithms to solve the governing equations of reactive fluid dynamic processes. Numerical simulation can be used at a number of levels to study turbulent combustion and detonations. What is needed is both, to use modelling and numerical simulation to investigate fundamental interactions, and using modelling and numerical simulation as a tool to predict turbulent flame accelerating processes and decoupling or re-initiation of detonation waves in complex geometries of technical applications. Today, modelling and simulation show good agreement with a variety of fast combustion phenomena observed in experiments. Results of reactive computational fluid dynamics codes deliver inputs to reduce experimental parameters and provide the basis for an innovative design of arresters for deflagration and detonation processes. (author)

  19. Flame Front Detection Using Formaldehyde Laser Induced Fluorescence In Turbulent Lean Premixed Flames

    Energy Technology Data Exchange (ETDEWEB)

    Schenker, S.; Tylli, N.; Bombach, R.

    2005-03-01

    The present work aims at suggesting the excitation-detection scheme best suited for laser-induced fluorescence measurements of formaldehyde in turbulent lean premixed flames. In the literature, three different excitation schemes within the A{sup 1} X{sup 1} electronic transition have been suggested, with excitation into the 2{sup 1}{sub 0} 4{sup 1}{sub 0} , 4{sup 1}{sub 0} , and 4{sup 0}{sub 1} vibratoric bands, respectively. These excitation schemes were tested systematically and both advantages and disadvantages for each scheme are discussed. (author)

  20. Fuel effects on the stability of turbulent flames with compositionally inhomogeneous inlets

    KAUST Repository

    Guiberti, T. F.

    2016-10-11

    This paper reports an analysis of the influence of fuels on the stabilization of turbulent piloted jet flames with inhomogeneous inlets. The burner is identical to that used earlier by the Sydney Group and employs two concentric tubes within the pilot stream. The inner tube, carrying fuel, can be recessed, leading to a varying degree of inhomogeneity in mixing with the outer air stream. Three fuels are tested: dimethyl ether (DME), liquefied petroleum gas (LPG), and compressed natural gas (CNG). It is found that improvement in flame stability at the optimal compositional inhomogeneity is highest for CNG and lowest for DME. Three possible reasons for this different enhancement in stability are investigated: mixing patterns, pilot effects, and fuel chemistry. Numerical simulations realized in the injection tube highlight similarities and differences in the mixing patterns for all three fuels and demonstrate that mixing cannot explain the different stability gains. Changing the heat release rates from the pilot affects the three fuels in similar ways and this also implies that the pilot stream is unlikely to be responsible for the observed differences. Fuel reactivity is identified as a key factor in enhancing stability at some optimal compositional inhomogeneity. This is confirmed by inference from joint images of PLIF-OH and PLIF-CHO, collected at a repetition rate of 10kHz in turbulent flames of DME, and from one-dimensional calculations of laminar flames using detailed chemistry for DME, CNG, and LPG.

  1. Numerical modeling of soot formation in a turbulent C2H4/air diffusion flame

    Directory of Open Access Journals (Sweden)

    Manedhar Reddy Busupally

    2016-06-01

    Full Text Available Soot formation in a lifted C2H4-Air turbulent diffusion flame is studied using two different paths for soot nucleation and oxidation; by a 2D axisymmetric RANS simulation using ANSYS FLUENT 15.0. The turbulence-chemistry interactions are modeled using two different approaches: steady laminar flamelet approach and flamelet-generated manifold. Chemical mechanism is represented by POLIMI to study the effect of species concentration on soot formation. P1 approximation is employed to approximate the radiative transfer equation into truncated series expansion in spherical harmonics while the weighted sum of gray gases is invoked to model the absorption coefficient while the soot model accounts for nucleation, coagulation, surface growth, and oxidation. The first route for nucleation considers acetylene concentration as a linear function of soot nucleation rate, whereas the second route considers two and three ring aromatic species as function of nucleation rate. Equilibrium-based and instantaneous approach has been used to estimate the OH concentration for soot oxidation. Lee and Fenimore-Jones soot oxidation models are studied to shed light on the effect of OH on soot oxidation. Moreover, the soot-radiation interactions are also included in terms of absorption coefficient of soot. Furthermore, the soot-turbulence interactions have been invoked using a temperature/mixture fraction-based single variable PDF. Both the turbulence-chemistry interaction models are able to accurately predict the flame liftoff height, and for accurate prediction of flame length, radiative heat loss should be accounted in an accurate way. The soot-turbulence interactions are found sensitive to the PDF used in present study.

  2. Direct numerical simulations of turbulent lean premixed combustion

    International Nuclear Information System (INIS)

    Sankaran, Ramanan; Hawkes, Evatt R; Chen, Jacqueline H; Lu Tianfeng; Law, Chung K

    2006-01-01

    In recent years, due to the advent of high-performance computers and advanced numerical algorithms, direct numerical simulation (DNS) of combustion has emerged as a valuable computational research tool, in concert with experimentation. The role of DNS in delivering new Scientific insight into turbulent combustion is illustrated using results from a recent 3D turbulent premixed flame simulation. To understand the influence of turbulence on the flame structure, a 3D fully-resolved DNS of a spatially-developing lean methane-air turbulent Bunsen flame was performed in the thin reaction zones regime. A reduced chemical model for methane-air chemistry consisting of 13 resolved species, 4 quasi-steady state species and 73 elementary reactions was developed specifically for the current simulation. The data is analyzed to study possible influences of turbulence on the flame thickness. The results show that the average flame thickness increases, in qualitative agreement with several experimental results

  3. Scalar mixing in LES/PDF of a high-Ka premixed turbulent jet flame

    Science.gov (United States)

    You, Jiaping; Yang, Yue

    2016-11-01

    We report a large-eddy simulation (LES)/probability density function (PDF) study of a high-Ka premixed turbulent flame in the Lund University Piloted Jet (LUPJ) flame series, which has been investigated using direct numerical simulation (DNS) and experiments. The target flame, featuring broadened preheat and reaction zones, is categorized into the broken reaction zone regime. In the present study, three widely used mixing modes, namely the Interaction by Exchange with the Mean (IEM), Modified Curl (MC), and Euclidean Minimum Spanning Tree (EMST) models are applied to assess their performance through detailed a posteriori comparisons with DNS. A dynamic model for the time scale of scalar mixing is formulated to describe the turbulent mixing of scalars at small scales. Better quantitative agreement for the mean temperature and mean mass fractions of major and minor species are obtained with the MC and EMST models than with the IEM model. The multi-scalar mixing in composition space with the three models are analyzed to assess the modeling of the conditional molecular diffusion term. In addition, we demonstrate that the product of OH and CH2O concentrations can be a good surrogate of the local heat release rate in this flame. This work is supported by the National Natural Science Foundation of China (Grant Nos. 11521091 and 91541204).

  4. Analysis of Hydrogen/Air Turbulent Premixed Flames at Different Karlovitz Numbers Using Computational Singular Perturbation

    KAUST Repository

    Manias, Dimitrios

    2018-01-08

    The dynamics and structure of two turbulent H2/air premixed flames, representative of the corrugated flamelet (Case 1) and thin reaction zone (Case 2) regimes, are analyzed and compared, using the computational singular perturbation (CSP) tools, by incorporating the tangential stretch rate (TSR) approach. First, the analysis is applied to a laminar premixed H2/air flame for reference. Then, a two-dimensional (2D) slice of Case 1 is studied at three time steps, followed by the comparison between two representative 2D slices of Case 1 and Case 2, respectively. Last, statistical analysis is performed on the full three-dimensional domain for the two cases. The dominant reaction and transport processes are identified for each case and the overall role of kinetics/transport is determined.

  5. Comparative study of micromixing models in transported scalar PDF simulations of turbulent nonpremixed bluff body flames

    Energy Technology Data Exchange (ETDEWEB)

    Merci, Bart [Department of Flow, Heat and Combustion Mechanics, Ghent University-UGent, Ghent (Belgium); Roekaerts, Dirk [Department of Multi-Scale Physics, Delft University of Technology, Delft (Netherlands); Naud, Bertrand [CIEMAT, Madrid (Spain); Pope, Stephen B. [Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY (United States)

    2006-07-15

    Numerical simulation results are presented for turbulent jet diffusion flames with various levels of turbulence-chemistry interaction, stabilized behind a bluff body (Sydney Flames HM1-3). Interaction between turbulence and combustion is modeled with the transported joint-scalar PDF approach. The mass density function transport equation is solved in a Lagrangian manner. A second-moment-closure turbulence model is applied to obtain accurate mean flow and turbulent mixing fields. The behavior of two micromixing models is discussed: the Euclidean minimum spanning tree model and the modified Curl coalescence dispersion model. The impact of the micromixing model choice on the results in physical space is small, although some influence becomes visible as the amount of local extinction increases. Scatter plots and profiles of conditional means and variances of thermochemical quantities, conditioned on the mixture fraction, are discussed both within and downstream of the recirculation region. A distinction is made between local extinction and incomplete combustion, based on the CO species mass fraction. The differences in qualitative behavior between the micromixing models are explained and quantitative comparison to experimental data is made. (author)

  6. Turbulent Non-Premixed Flames Stabilized on Double-Slit Curved Wall-Jet Burner with Simultaneous OH-Planar Laser-Induced Fluorescence and Particle Image Velocimetry Measurements

    KAUST Repository

    Mansour, Morkous S.

    2015-04-29

    A double-slit curved wall-jet (CWJ) burner utilizing a Coanda effect by supplying fuel and air as annular-inward jets over a curved surface was employed to investigate the stabilization characteristics and structure of propane/air turbulent non-premixed flames with varying global equivalence ratio and Reynolds number. Simultaneous time-resolved measurements of particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of OH radicals were conducted. The burner showed a potential of stable and non-sooting operation for relatively large fuel loading and overall rich conditions. Mixing characteristics in cold flow were first examined using an acetone fluorescence technique, indicating substantial transport between the fuel and air by exhibiting appreciable premixing conditions. PIV measurements revealed that the flow field consisted of a wall-jet region leading to a recirculation zone through flow separation, an interaction jet region resulting from the collision of annular-inward jets, followed by a merged-jet region. The flames were stabilized in the recirculation zone and, in extreme cases, only a small flame seed remained in the recirculation zone. Together with the collision of the slit jets in the interaction jet region, the velocity gradients in the shear layers at the boundaries of the annular jets generate the turbulence. Turbulent mean and rms velocities were influenced by the presence of the flame, particularly in the recirculation zone. Flames with a high equivalence ratio were found to be more resistant to local extinction and exhibited a more corrugated and folded nature, particularly at high Reynolds numbers. For flames with a low equivalence ratio, local quenching and re-ignition processes maintained flames in the merged jet region, revealing a strong intermittency, which was substantiated by the increased principal strain rates for these flames. © 2015 Taylor & Francis Group, LLC.

  7. Numerical modeling of turbulent jet diffusion flames in the atmospheric surface layer

    NARCIS (Netherlands)

    Hernández, J.; Crespo, A.; Duijm, N.J.

    1995-01-01

    The evolution of turbulent jet diffusion flames of natural gas in air is predicted using a finite-volume procedure for solving the flow equations. The model is three dimensional, elliptic and based on the conserved-scalar approach and the laminar flamelet concept. A laminar flamelet prescription for

  8. The flow field structure of highly stabilized partially premixed flames in a concentric flow conical nozzle burner with coflow

    KAUST Repository

    Elbaz, Ayman M.

    2015-08-29

    The stability limits, the stabilization mechanism, and the flow field structure of highly stabilized partially premixed methane flames in a concentric flow conical nozzle burner with air co-flow have been investigated and presented in this work. The stability map of partial premixed flames illustrates that the flames are stable between two extinction limits. A low extinction limit when partial premixed flames approach non-premixed flame conditions, and a high extinction limit, with the partial premixed flames approach fully premixed flame conditions. These two limits showed that the most stable flame conditions are achieved at a certain degree of partial premixed. The stability is improved by adding air co-flow. As the air co-flow velocity increases the most stable flames are those that approach fully premixed. The turbulent flow field of three flames at 0, 5, 10 m/s co-flow velocity are investigated using Stereo Particle Image Velocimetry (SPIV) in order to explore the improvement of the flame stability due to the use of air co-flow. The three flames are all at a jet equivalence ratio (Φj) of 2, fixed level of partial premixing and jet Reynolds number (Rej) of 10,000. The use of co-flow results in the formation of two vortices at the cone exit. These vortices act like stabilization anchors for the flames to the nozzle tip. With these vortices in the flow field, the reaction zone shifts toward the reduced turbulence intensity at the nozzle rim of the cone. Interesting information about the structure of the flow field with and without co-flow are identified and reported in this work.

  9. Modeling of free and confined turbulent natural gas flames using an extension of CFX-F3D

    Energy Technology Data Exchange (ETDEWEB)

    Roekaerts, D [Shell Research and Technology Centre, Amsterdam (Netherlands); Hsu, A

    1998-12-31

    A general form of the fast chemistry / assumed shape probability density function model for turbulent gaseous diffusion flames has been implemented in a new combination of computer programs consisting of the commercial code CFX-F3D (formerly CFDS-FLOW3D) and the program FLAME, developed at Delft University of Technology. Also a mixedness-reactedness model with two independent variables (mixture fraction and reaction progress variable) has been implemented. The main strength of the new program is that it combines the advantages of a general purpose commercial CFD code (applicable to arbitrarily shaped domains, wide range of solvers) with the advantages of special purpose combustion subroutines (more detail in modeling of chemistry and of turbulence-chemistry interaction, flexibility). The new combination of programs has been validated by the application to the prediction of the properties of a labscale turbulent natural gas diffusion flame for which detailed measurements are available. The mixedness-reactedness model has been applied to the case of a confined natural gas diffusion flame at globally rich conditions. In contrast with fast chemistry models, the mixedness-reactedness model can be used to predict the amount of methane at the end of the reactor vessel (`methane slip`) as a function of operating conditions. (author)

  10. Modeling of free and confined turbulent natural gas flames using an extension of CFX-F3D

    Energy Technology Data Exchange (ETDEWEB)

    Roekaerts, D. [Shell Research and Technology Centre, Amsterdam (Netherlands); Hsu, A.

    1997-12-31

    A general form of the fast chemistry / assumed shape probability density function model for turbulent gaseous diffusion flames has been implemented in a new combination of computer programs consisting of the commercial code CFX-F3D (formerly CFDS-FLOW3D) and the program FLAME, developed at Delft University of Technology. Also a mixedness-reactedness model with two independent variables (mixture fraction and reaction progress variable) has been implemented. The main strength of the new program is that it combines the advantages of a general purpose commercial CFD code (applicable to arbitrarily shaped domains, wide range of solvers) with the advantages of special purpose combustion subroutines (more detail in modeling of chemistry and of turbulence-chemistry interaction, flexibility). The new combination of programs has been validated by the application to the prediction of the properties of a labscale turbulent natural gas diffusion flame for which detailed measurements are available. The mixedness-reactedness model has been applied to the case of a confined natural gas diffusion flame at globally rich conditions. In contrast with fast chemistry models, the mixedness-reactedness model can be used to predict the amount of methane at the end of the reactor vessel (`methane slip`) as a function of operating conditions. (author)

  11. Simultaneous Laser Raman-rayleigh-lif Measurements and Numerical Modeling Results of a Lifted Turbulent H2/N2 Jet Flame in a Vitiated Coflow

    Science.gov (United States)

    Cabra, R.; Chen, J. Y.; Dibble, R. W.; Myhrvold, T.; Karpetis, A. N.; Barlow, R. S.

    2002-01-01

    An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

  12. Combustion Characteristics for Turbulent Prevaporized Premixed Flame Using Commercial Light Diesel and Kerosene Fuels

    Directory of Open Access Journals (Sweden)

    Mohamed S. Shehata

    2014-01-01

    Full Text Available Experimental study has been carried out for investigating fuel type, fuel blends, equivalence ratio, Reynolds number, inlet mixture temperature, and holes diameter of perforated plate affecting combustion process for turbulent prevaporized premixed air flames for different operating conditions. CO2, CO, H2, N2, C3H8, C2H6, C2H4, flame temperature, and gas flow velocity are measured along flame axis for different operating conditions. Gas chromatographic (GC and CO/CO2 infrared gas analyzer are used for measuring different species. Temperature is measured using thermocouple technique. Gas flow velocity is measured using pitot tube technique. The effect of kerosene percentage on concentration, flame temperature, and gas flow velocity is not linearly dependent. Correlations for adiabatic flame temperature for diesel and kerosene-air flames are obtained as function of mixture strength, fuel type, and inlet mixture temperature. Effect of equivalence ratio on combustion process for light diesel-air flame is greater than for kerosene-air flame. Flame temperature increases with increased Reynolds number for different operating conditions. Effect of Reynolds number on combustion process for light diesel flame is greater than for kerosene flame and also for rich flame is greater than for lean flame. The present work contributes to design and development of lean prevaporized premixed (LPP gas turbine combustors.

  13. On the feasibility of tomographic-PIV with low pulse energy illumination in a lifted turbulent jet flame

    Science.gov (United States)

    Boxx, I.; Carter, C. D.; Meier, W.

    2014-08-01

    Tomographic particle image velocimetry (tomographic-PIV) is a recently developed measurement technique used to acquire volumetric velocity field data in liquid and gaseous flows. The technique relies on line-of-sight reconstruction of the rays between a 3D particle distribution and a multi-camera imaging system. In a turbulent flame, however, index-of-refraction variations resulting from local heat-release may inhibit reconstruction and thereby render the technique infeasible. The objective of this study was to test the efficacy of tomographic-PIV in a turbulent flame. An additional goal was to determine the feasibility of acquiring usable tomographic-PIV measurements in a turbulent flame at multi-kHz acquisition rates with current-generation laser and camera technology. To this end, a setup consisting of four complementary metal oxide semiconductor cameras and a dual-cavity Nd:YAG laser was implemented to test the technique in a lifted turbulent jet flame. While the cameras were capable of kHz-rate image acquisition, the laser operated at a pulse repetition rate of only 10 Hz. However, use of this laser allowed exploration of the required pulse energy and thus power for a kHz-rate system. The imaged region was 29 × 28 × 2.7 mm in size. The tomographic reconstruction of the 3D particle distributions was accomplished using the multiplicative algebraic reconstruction technique. The results indicate that volumetric velocimetry via tomographic-PIV is feasible with pulse energies of 25 mJ, which is within the capability of current-generation kHz-rate diode-pumped solid-state lasers.

  14. Transported PDF Modeling of Nonpremixed Turbulent CO/H-2/N-2 Jet Flames

    Energy Technology Data Exchange (ETDEWEB)

    Zhao, xinyu; Haworth, D. C.; Huckaby, E. David

    2012-01-01

    Turbulent CO/H{sub 2}/N{sub 2} (“syngas”) flames are simulated using a transported composition probability density function (PDF) method. A consistent hybrid Lagrangian particle/Eulerian mesh algorithm is used to solve the modeled PDF transport equation. The model includes standard k–ϵ turbulence, gradient transport for scalars, and Euclidean minimum spanning tree (EMST) mixing. Sensitivities of model results to variations in the turbulence model, the treatment of radiation heat transfer, the choice of chemical mechanism, and the PDF mixing model are explored. A baseline model reproduces the measured mean and rms temperature, major species, and minor species profiles reasonably well, and captures the scaling that is observed in the experiments. Both our results and the literature suggest that further improvements can be realized with adjustments in the turbulence model, the radiation heat transfer model, and the chemical mechanism. Although radiation effects are relatively small in these flames, consideration of radiation is important for accurate NO prediction. Chemical mechanisms that have been developed specifically for fuels with high concentrations of CO and H{sub 2} perform better than a methane mechanism that was not designed for this purpose. It is important to account explicitly for turbulence–chemistry interactions, although the details of the mixing model do not make a large difference in the results, within reasonable limits.

  15. Computational Flame Diagnostics for Direct Numerical Simulations with Detailed Chemistry of Transportation Fuels

    Energy Technology Data Exchange (ETDEWEB)

    Lu, Tianfeng [Univ. of Connecticut, Storrs, CT (United States)

    2017-02-16

    The goal of the proposed research is to create computational flame diagnostics (CFLD) that are rigorous numerical algorithms for systematic detection of critical flame features, such as ignition, extinction, and premixed and non-premixed flamelets, and to understand the underlying physicochemical processes controlling limit flame phenomena, flame stabilization, turbulence-chemistry interactions and pollutant emissions etc. The goal has been accomplished through an integrated effort on mechanism reduction, direct numerical simulations (DNS) of flames at engine conditions and a variety of turbulent flames with transport fuels, computational diagnostics, turbulence modeling, and DNS data mining and data reduction. The computational diagnostics are primarily based on the chemical explosive mode analysis (CEMA) and a recently developed bifurcation analysis using datasets from first-principle simulations of 0-D reactors, 1-D laminar flames, and 2-D and 3-D DNS (collaboration with J.H. Chen and S. Som at Argonne, and C.S. Yoo at UNIST). Non-stiff reduced mechanisms for transportation fuels amenable for 3-D DNS are developed through graph-based methods and timescale analysis. The flame structures, stabilization mechanisms, local ignition and extinction etc., and the rate controlling chemical processes are unambiguously identified through CFLD. CEMA is further employed to segment complex turbulent flames based on the critical flame features, such as premixed reaction fronts, and to enable zone-adaptive turbulent combustion modeling.

  16. Soot volume fraction in a piloted turbulent jet non-premixed flame of natural gas

    Energy Technology Data Exchange (ETDEWEB)

    Qamar, N.H.; Alwahabi, Z.T.; King, K.D. [Fluid Mechanics, Energy and Combustion Group, University of Adelaide, Adelaide, SA 5005 (Australia); School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005 (Australia); Chan, Q.N. [Fluid Mechanics, Energy and Combustion Group, University of Adelaide, Adelaide, SA 5005 (Australia); School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005 (Australia); School of Mechanical Engineering, University of Adelaide, Adelaide, SA 5005 (Australia); Nathan, G.J. [Fluid Mechanics, Energy and Combustion Group, University of Adelaide, Adelaide, SA 5005 (Australia); School of Mechanical Engineering, University of Adelaide, Adelaide, SA 5005 (Australia); Roekaerts, D. [Department of Multi-Scale Physics, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg, 1, NL-2628 CJ Delft (Netherlands)

    2009-07-15

    Planar laser-induced incandescence (LII) has been used to measure soot volume fraction in a well-characterised, piloted, turbulent non-premixed flame known as the ''Delft Flame III''. Simulated Dutch natural gas was used as the fuel to produce a flame closely matching those in which a wide range of previous investigations, both experimental and modelling, have been performed. The LII method was calibrated using a Santoro-style burner with ethylene as the fuel. Instantaneous and time-averaged data of the axial and radial soot volume fraction distributions of the flame are presented here along with the Probability Density Functions (PDFs) and intermittency. The PDFs were found to be well-characterised by a single exponential distribution function. The distribution of soot was found to be highly intermittent, with intermittency typically exceeding 97%, which increases measurement uncertainty. The instantaneous values of volume fraction are everywhere less than the values in strained laminar flames. This is consistent with the soot being found locally in strained flame sheets that are convected and distorted by the flow. (author)

  17. Evaluation of partially premixed turbulent flame stability from mixture fraction statistics in a slot burner

    KAUST Repository

    Kruse, Stephan

    2018-04-11

    Partially premixed combustion is characterized by mixture fraction inhomogeneity upstream of the reaction zone and occurs in many applied combustion systems. The temporal and spatial fluctuations of the mixture fraction have tremendous impact on the combustion characteristics, emission formation, and flame stability. In this study, turbulent partially premixed flames are experimentally studied in a slot burner configuration. The local temperature and gas composition is determined by means of one-dimensional, simultaneous detection of Rayleigh and Raman scattering. The statistics of the mixture fraction are utilized to characterize the impact of the Reynolds number, the global equivalence ratio, the progress of mixing within the flame, as well as the mixing length on the mixing field. Furthermore, these effects are evaluated by means of a regime diagram for partially premixed flames. In this study, it is shown that the increase of the mixing length results in a significantly more stable flame. The impact of the Reynolds number on flame stability is found to be minor.

  18. Evaluation of partially premixed turbulent flame stability from mixture fraction statistics in a slot burner

    KAUST Repository

    Kruse, Stephan; Mansour, Mohy S.; Elbaz, Ayman M.; Varea, Emilien; Grü nefeld, Gerd; Beeckmann, Joachim; Pitsch, Heinz

    2018-01-01

    Partially premixed combustion is characterized by mixture fraction inhomogeneity upstream of the reaction zone and occurs in many applied combustion systems. The temporal and spatial fluctuations of the mixture fraction have tremendous impact on the combustion characteristics, emission formation, and flame stability. In this study, turbulent partially premixed flames are experimentally studied in a slot burner configuration. The local temperature and gas composition is determined by means of one-dimensional, simultaneous detection of Rayleigh and Raman scattering. The statistics of the mixture fraction are utilized to characterize the impact of the Reynolds number, the global equivalence ratio, the progress of mixing within the flame, as well as the mixing length on the mixing field. Furthermore, these effects are evaluated by means of a regime diagram for partially premixed flames. In this study, it is shown that the increase of the mixing length results in a significantly more stable flame. The impact of the Reynolds number on flame stability is found to be minor.

  19. Computational fluid dynamic on the temperature simulation of air preheat effect combustion in propane turbulent flame

    Science.gov (United States)

    Elwina; Yunardi; Bindar, Yazid

    2018-04-01

    this paper presents results obtained from the application of a computational fluid dynamics (CFD) code Fluent 6.3 to modelling of temperature in propane flames with and without air preheat. The study focuses to investigate the effect of air preheat temperature on the temperature of the flame. A standard k-ε model and Eddy Dissipation model are utilized to represent the flow field and combustion of the flame being investigated, respectively. The results of calculations are compared with experimental data of propane flame taken from literature. The results of the study show that a combination of the standard k-ε turbulence model and eddy dissipation model is capable of producing reasonable predictions of temperature, particularly in axial profile of all three flames. Both experimental works and numerical simulation showed that increasing the temperature of the combustion air significantly increases the flame temperature.

  20. Lifted Turbulent Jet Flames

    Science.gov (United States)

    1993-04-14

    flame length L simultaneously with h, and measuring the visible radiation I simultaneously with h. L(t) was found to be nearly uncorrelated with h(t...variation of 7i/2 /76 with ýh. These experiments included measuring the flame length L simultaneously with h, and measuring the visible radiation I...Measurements of Liftoff Height and Flame Length ... 66 4.5 Simultaneous Measurements of Liftoff Height and Radiation ....... 71 4.6 D scussion

  1. The structure of horizontal hydrogen-steam diffusion flames

    International Nuclear Information System (INIS)

    Chan, C.K.; Guerrero, A.

    1997-01-01

    This paper summarizes a systematic study on the stability, peak temperature and flame length of various horizontal hydrogen-steam diffusion flames in air. Results from this study are discussed in terms of their impact on hydrogen management in a nuclear containment building after a nuclear reactor accident. They show that, for a certain range of emerging hydrogen-steam compositions, a stable diffusion flame can anchor itself at the break in the primary heat transport system. The length of this flame can be up to 100 times the break diameter. This implies that creation of a stable diffusion flame at the break is a possible outcome of the deliberate ignition mitigation scheme. The high temperature and heat flux from a diffusion flame can threaten nearby equipment. However, due to the presence of steam and turbulent mixing with surrounding air, the peak temperatures of these diffusion flames are much lower than the adiabatic constant pressure combustion temperature of a stoichiometric hydrogen-air mixture. These results suggest that the threat of a diffusion flame anchored at the break may be less severe than conservative analysis would indicate. Furthermore, such a flame can remove hydrogen at the source and minimize the possibility of a global gas explosion. (author)

  2. Experimental Studies of Premixed Flame Structure and Propagation Characteristics in Compressible Flow

    Science.gov (United States)

    2015-07-14

    turbulence levels resulted in higher values for both curvature and flame length . The curvature values shown in Figure 124 are determined first finding the all...and the product the maximum FSD and brush thickness (right). Figure 133: Instantaneous flame length histograms for t = 1.842ms for M = 0.2 (left) and M...0.3 (center). Mean flame length verse time is also shown (right) with error bars for standard deviations. Figure 133 shows the instantaneous flame

  3. An experimental study on turbulent lifted flames of methane in coflow jets at elevated temperatures

    KAUST Repository

    Choi, Byungchul; Chung, Suk-Ho

    2013-01-01

    An experimental study was conducted on the effects of initial temperature variation on the stabilization characteristics of turbulent nonpremixed flames in coflow jets of methane fuel diluted by nitrogen. The typical behavior seen in the study

  4. Modelling of structural effects on chemical reactions in turbulent flows

    Energy Technology Data Exchange (ETDEWEB)

    Gammelsaeter, H.R.

    1997-12-31

    Turbulence-chemistry interactions are analysed using algebraic moment closure for the chemical reaction term. The coupling between turbulence and chemical length and time scales generate a complex interaction process. This interaction process is called structural effects in this work. The structural effects are shown to take place on all scales between the largest scale of turbulence and the scales of the molecular motions. The set of equations describing turbulent correlations involved in turbulent reacting flows are derived. Interactions are shown schematically using interaction charts. Algebraic equations for the turbulent correlations in the reaction rate are given using the interaction charts to include the most significant couplings. In the frame of fundamental combustion physics, the structural effects appearing on the small scales of turbulence are proposed modelled using a discrete spectrum of turbulent scales. The well-known problem of averaging the Arrhenius law, the specific reaction rate, is proposed solved using a presumed single variable probability density function and a sub scale model for the reaction volume. Although some uncertainties are expected, the principles are addressed. Fast chemistry modelling is shown to be consistent in the frame of algebraic moment closure when the turbulence-chemistry interaction is accounted for in the turbulent diffusion. The modelling proposed in this thesis is compared with experimental data for an laboratory methane flame and advanced probability density function modelling. The results show promising features. Finally it is shown a comparison with full scale measurements for an industrial burner. All features of the burner are captured with the model. 41 refs., 33 figs.

  5. CONDITIONAL FLOW STATISTICS AND ALIGNMENT OF PRINCIPAL STRAIN RATES, VORTICITY, AND SCALAR GRADIENTS IN A TURBULENT NONPREMIXED JET FLAME

    KAUST Repository

    Attili, Antonio

    2015-06-30

    The alignment of vorticity and gradients of conserved and reactive scalars with the eigenvectors of the strain rate tensor (i.e., the principal strains) is investigated in a direct numerical simulation of a turbulent nonpremixed flame achieving a Taylor’s scale Reynolds number in the range 100≤Reλ≤150 (Attili et al. Comb. Flame, 161, 2014). The vorticity vector displays a pronounced tendency to align with the direction of the intermediate strain. These alignment statistics are in almost perfect agreement with those in homogeneous isotropic turbulence (Ashurst et al. Physics of Fluids 30, 1987) and differ significantly from the results obtained in other nonpremixed flames in which vorticity alignment with the most extensive strain was observed (Boratavet al. Physics of Fluids 8, 1996). The gradients of conserved and reactive scalars align with the most compressive strain. It is worth noting that conditioning on the local values of the mixture fraction, or equivalently conditioning on the distance from the flame sheet, does not affect the statistics. Our results suggest that turbulence overshadows the effects of heat release and chemical reactions. This may be due to the larger Reynolds number achieved in the present study compared to that in previous works.

  6. CONDITIONAL FLOW STATISTICS AND ALIGNMENT OF PRINCIPAL STRAIN RATES, VORTICITY, AND SCALAR GRADIENTS IN A TURBULENT NONPREMIXED JET FLAME

    KAUST Repository

    Attili, Antonio; Bisetti, Fabrizio

    2015-01-01

    The alignment of vorticity and gradients of conserved and reactive scalars with the eigenvectors of the strain rate tensor (i.e., the principal strains) is investigated in a direct numerical simulation of a turbulent nonpremixed flame achieving a Taylor’s scale Reynolds number in the range 100≤Reλ≤150 (Attili et al. Comb. Flame, 161, 2014). The vorticity vector displays a pronounced tendency to align with the direction of the intermediate strain. These alignment statistics are in almost perfect agreement with those in homogeneous isotropic turbulence (Ashurst et al. Physics of Fluids 30, 1987) and differ significantly from the results obtained in other nonpremixed flames in which vorticity alignment with the most extensive strain was observed (Boratavet al. Physics of Fluids 8, 1996). The gradients of conserved and reactive scalars align with the most compressive strain. It is worth noting that conditioning on the local values of the mixture fraction, or equivalently conditioning on the distance from the flame sheet, does not affect the statistics. Our results suggest that turbulence overshadows the effects of heat release and chemical reactions. This may be due to the larger Reynolds number achieved in the present study compared to that in previous works.

  7. Modeling of turbulent chemical reaction

    Science.gov (United States)

    Chen, J.-Y.

    1995-01-01

    Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.

  8. Soot Particle Size Distribution Functions in a Turbulent Non-Premixed Ethylene-Nitrogen Flame

    KAUST Repository

    Boyette, Wesley

    2017-02-21

    A scanning mobility particle sizer with a nano differential mobility analyzer was used to measure nanoparticle size distribution functions in a turbulent non-premixed flame. The burner utilizes a premixed pilot flame which anchors a C2H4/N2 (35/65) central jet with ReD = 20,000. Nanoparticles in the flame were sampled through a N2-filled tube with a 500- μm orifice. Previous studies have shown that insufficient dilution of the nanoparticles can lead to coagulation in the sampling line and skewed particle size distribution functions. A system of mass flow controllers and valves were used to vary the dilution ratio. Single-stage and two-stage dilution systems were investigated. A parametric study on the effect of the dilution ratio on the observed particle size distribution function indicates that particle coagulation in the sampling line can be eliminated using a two-stage dilution process. Carbonaceous nanoparticle (soot) concentration particle size distribution functions along the flame centerline at multiple heights in the flame are presented. The resulting distributions reveal a pattern of increasing mean particle diameters as the distance from the nozzle along the centerline increases.

  9. Soot Particle Size Distribution Functions in a Turbulent Non-Premixed Ethylene-Nitrogen Flame

    KAUST Repository

    Boyette, Wesley; Chowdhury, Snehaunshu; Roberts, William L.

    2017-01-01

    A scanning mobility particle sizer with a nano differential mobility analyzer was used to measure nanoparticle size distribution functions in a turbulent non-premixed flame. The burner utilizes a premixed pilot flame which anchors a C2H4/N2 (35/65) central jet with ReD = 20,000. Nanoparticles in the flame were sampled through a N2-filled tube with a 500- μm orifice. Previous studies have shown that insufficient dilution of the nanoparticles can lead to coagulation in the sampling line and skewed particle size distribution functions. A system of mass flow controllers and valves were used to vary the dilution ratio. Single-stage and two-stage dilution systems were investigated. A parametric study on the effect of the dilution ratio on the observed particle size distribution function indicates that particle coagulation in the sampling line can be eliminated using a two-stage dilution process. Carbonaceous nanoparticle (soot) concentration particle size distribution functions along the flame centerline at multiple heights in the flame are presented. The resulting distributions reveal a pattern of increasing mean particle diameters as the distance from the nozzle along the centerline increases.

  10. Premixed CH4-Air Flame Structure Characteristic and Flow Behavior Induced by Obstacle in an Open Duct

    Directory of Open Access Journals (Sweden)

    DengKe Li

    2015-01-01

    Full Text Available To study the fuel gas combustion hazards, the methane/air flame structure and flow characteristic in an open duct influenced by a rectangular obstacle were explored by experiment and realizable k-∊ model (RKE. In the test, the high-speed schlieren photography technology and dynamic detection technology were applied to record the flame propagation behavior. Meanwhile, the interaction between flame front and flame flow field induced by the obstacle was disclosed. In addition, the laminar-turbulence transition was also taken into consideration. The RKE and eddy dissipation concept (EDC premixed combustion model were applied to obtain an insight into the phenomenon of flow change and wrinkle appearing, which potently explained the experimental observations. As a result, the obstacle blocked the laminar flame propagation velocity and increased pressure a little in an open duct. Some small-scale vortices began to appear near the obstacle, mainly due to Kelvin-Helmholtz instability (KHI, and gradually grew into large-scale vortices, which led to laminar-turbulent transition directly. The vortices thickened the reaction area and hastened the reaction rate; reversely, the higher reaction rate induced larger vortices. The RKE model result fitted the test data well and explained the wrinkle forming mechanism of two special vortices in the case.

  11. Fuel rich and fuel lean catalytic combustion of the stabilized confined turbulent gaseous diffusion flames over noble metal disc burners

    Directory of Open Access Journals (Sweden)

    Amal S. Zakhary

    2014-03-01

    Full Text Available Catalytic combustion of stabilized confined turbulent gaseous diffusion flames using Pt/Al2O3 and Pd/Al2O3 disc burners situated in the combustion domain under both fuel-rich and fuel-lean conditions was experimentally studied. Commercial LPG fuel having an average composition of: 23% propane, 76% butane, and 1% pentane was used. The thermal structure of these catalytic flames developed over Pt/Al2O3 and Pd/Al2O3 burners were examined via measuring the mean temperature distribution in the radial direction at different axial locations along the flames. Under-fuel-rich condition the flames operated over Pt catalytic disc attained high temperature values in order to express the progress of combustion and were found to achieve higher activity as compared to the flames developed over Pd catalytic disc. These two types of catalytic flames demonstrated an increase in the reaction rate with the downstream axial distance and hence, an increase in the flame temperatures was associated with partial oxidation towards CO due to the lack of oxygen. However, under fuel-lean conditions the catalytic flame over Pd catalyst recorded comparatively higher temperatures within the flame core in the near region of the main reaction zone than over Pt disc burner. These two catalytic flames over Pt and Pd disc burners showed complete oxidation to CO2 since the catalytic surface is covered by more rich oxygen under the fuel-lean condition.

  12. Stabilization and structure of N-heptane flame on CWJ-spray burner with kHZ SPIV and OH-PLIF

    KAUST Repository

    Mansour, Morkous S.; Al Khesho, Issam; Chung, Suk-Ho

    2015-01-01

    characteristics and structure of n-heptane/air turbulent flames were investigated with varying fuel and air flow rates and the position of pressure atomizer (L). High-speed planar laser-induced fluorescence (PLIF) of OH radicals delineated reaction zone contours

  13. Direct Numerical Simulations of Turbulent Autoigniting Hydrogen Jets

    Science.gov (United States)

    Asaithambi, Rajapandiyan

    Autoignition is an important phenomenon and a tool in the design of combustion engines. To study autoignition in a canonical form a direct numerical simulation of a turbulent autoigniting hydrogen jet in vitiated coflow conditions at a jet Reynolds number of 10,000 is performed. A detailed chemical mechanism for hydrogen-air combustion and non-unity Lewis numbers for species transport is used. Realistic inlet conditions are prescribed by obtaining the velocity eld from a fully developed turbulent pipe flow simulation. To perform this simulation a scalable modular density based method for direct numerical simulation (DNS) and large eddy simulation (LES) of compressible reacting flows is developed. The algorithm performs explicit time advancement of transport variables on structured grids. An iterative semi-implicit time advancement is developed for the chemical source terms to alleviate the chemical stiffness of detailed mechanisms. The algorithm is also extended from a Cartesian grid to a cylindrical coordinate system which introduces a singularity at the pole r = 0 where terms with a factor 1/r can be ill-defined. There are several approaches to eliminate this pole singularity and finite volume methods can bypass this issue by not storing or computing data at the pole. All methods however face a very restrictive time step when using a explicit time advancement scheme in the azimuthal direction (theta) where the cell sizes are of the order DelrDeltheta. We use a conservative finite volume based approach to remove the severe time step restriction imposed by the CFL condition by merging cells in the azimuthal direction. In addition, fluxes in the radial direction are computed with an implicit scheme to allow cells to be clustered along the jet's shear layer. This method is validated and used to perform the large scale turbulent reacting simulation. The resulting flame structure is found to be similar to a turbulent diusion flame but stabilized by autoignition at the

  14. Modeling and Simulation of Swirl Stabilized Turbulent Non-Premixed Flames

    Science.gov (United States)

    Badillo-Rios, Salvador; Karagozian, Ann

    2017-11-01

    Flame stabilization is an important design criterion for many combustion chambers, especially at lean conditions and/or high power output, where insufficient stabilization can result in dangerous oscillations and noisy or damaged combustors. At high flow rates, swirling flow can offer a suitable stabilization mechanism, although understanding the dynamics of swirl-stabilized turbulent flames remains a significant challenge. Utilizing the General Equation and Mesh Solver (GEMS) code, which solves the Navier-Stokes equations along with the energy equation and five species equations, 2D axisymmetric and full 3D parametric studies and simulations are performed to guide the design and development of an experimental swirl combustor configuration and to study the effects of swirl on statistically stationary combustion. Results show that as the momentum of air is directed into the inner air inlet rather than the outer inlet of the swirl combustor, the central recirculating region becomes stronger and more unsteady, improving mixing and burning efficiency in that region. A high temperature region is found to occur as a result of burning of the trapped fuel from the central toroidal vortex. The effects of other parameters on flowfield and flame-stabilization dynamics are explored. Supported by ERC, Inc. (PS150006) and AFOSR (Dr. Chiping Li).

  15. Investigation of a flame holder geometry effect on flame structure in non-premixed combustion

    International Nuclear Information System (INIS)

    Hashemi, S. A.; Hajialigol, N.; Fattahi, A.; Heydari, R.; Mazaheri, K.

    2013-01-01

    In this paper the effect of flame holder geometry on flame structure is studied. The obtained numerical results using realizable k-ε and β-PDF models show a good agreement with experimental data. The results show that increasing in flame holder length decreases flame length and increases flame temperature. Additionally, it is observed that flame lengths decrease by increasing in flame holder radius and increase for larger radii. Furthermore in various radii, the flame temperature is higher for smaller flame lengths. It was found that behavior of flame structure is mainly affected by the mass flow rate of hot gases that come near the reactant by the recirculation zone.

  16. Investigation of a flame holder geometry effect on flame structure in non-premixed combustion

    Energy Technology Data Exchange (ETDEWEB)

    Hashemi, S. A.; Hajialigol, N.; Fattahi, A.; Heydari, R. [University of Kashan, Kashan (Iran, Islamic Republic of); Mazaheri, K. [University of Tarbiat Moddares, Tehran (Iran, Islamic Republic of)

    2013-11-15

    In this paper the effect of flame holder geometry on flame structure is studied. The obtained numerical results using realizable k-ε and β-PDF models show a good agreement with experimental data. The results show that increasing in flame holder length decreases flame length and increases flame temperature. Additionally, it is observed that flame lengths decrease by increasing in flame holder radius and increase for larger radii. Furthermore in various radii, the flame temperature is higher for smaller flame lengths. It was found that behavior of flame structure is mainly affected by the mass flow rate of hot gases that come near the reactant by the recirculation zone.

  17. Flame structure of methane inverse diffusion flame

    KAUST Repository

    Elbaz, Ayman M.

    2014-07-01

    This paper presents high speed images of OH-PLIF at 10. kHz simultaneously with 2D PIV (particle image velocimetry) measurements collected along the entire length of an inverse diffusion flame with circumferentially arranged methane fuel jets. For a fixed fuel flow rate, the central air jet Re was varied, leading to four air to fuel velocity ratios, namely Vr = 20.7, 29, 37.4 and 49.8. A double flame structure could be observed composed of a lower fuel entrainment region and an upper mixing and intense combustion region. The entrainment region was enveloped by an early OH layer, and then merged through a very thin OH neck to an annular OH layer located at the shear layer of the air jet. The two branches of this annular OH layer broaden as they moved downstream and eventfully merged together. Three types of events were observed common to all flames: breaks, closures and growing kernels. In upstream regions of the flames, the breaks were counterbalanced by flame closures. These breaks in OH signal were found to occur at locations where locally high velocity flows were impinging on the flame. As the Vr increased to 37.4, the OH layers became discontinuous over the downstream region of the flame, and these regions of low or no OH moved upstream. With further increases in Vr, these OH pockets act as flame kernels, growing as they moved downstream, and became the main mechanism for flame re-ignition. Along the flame length, the direction of the two dimensional principle compressive strain rate axis exhibited a preferred orientation of approximately 45° with respect to the flow direction. Moreover, the OH zones were associated with elongated regions of high vorticity. © 2013 Elsevier Inc.

  18. An Experimental Investigation of Premixed Combustion in Extreme Turbulence

    Science.gov (United States)

    Wabel, Timothy Michael

    This work has explored various aspects of high Reynolds number combustion that have received much previous speculation. A new high-Reynolds number premixed Bunsen burner, called Hi-Pilot, was designed to produce turbulence intensities in the extreme range of turbulence. The burner was modified several times in order to prevent boundary layer separation in the nozzle, and a large co-flow was designed that was capable of maintaining reactions over the entire flame surface. Velocity and turbulence characteristics were measured using a combination of Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV). Flame structure was studied using a combination of formaldehyde (CH2O), hydroxyl (OH), and the CH radical. Planar Laser Induced Fluorescence (PLIF). The spatial Overlap of formaldehyde and OH PLIF qualitatively measures the reaction rate between formaldehyde molecules and OH radicals, and is a measure of the reaction layers of the flame. CH PLIF provides an alternative measure of the reaction zone, and was measured to compare with the Overlap PLIF results. Reaction layers are the full-width at half-maximum of the Overlap or CH PLIF signal, and extinction events were defined as regions where the PLIF signal drops below this threshold. Preheat structures were measured using formaldehyde PLIF, and are defined as beginning at 35% of the local maximum PLIF signal, and continue up to the leading edge of the reaction layer. Previous predictions of regime diagram boundaries were tested at the largest values of turbulent Reynolds number to date. The Overlap and CH PLIF diagnostics allowed extensive testing of the predicted broken reaction zones boundary of Peters. Measurements indicated that all run conditions are in the Broadened Preheat - Thin Reaction layers regime, but several conditions are expected to display a broken reaction zone structure. Therefore the work shows that Peters's predicted boundary is not correct, and therefore a Karlovitz number of 100 is

  19. Statistics of strain rates and surface density function in a flame-resolved high-fidelity simulation of a turbulent premixed bluff body burner

    Science.gov (United States)

    Sandeep, Anurag; Proch, Fabian; Kempf, Andreas M.; Chakraborty, Nilanjan

    2018-06-01

    The statistical behavior of the surface density function (SDF, the magnitude of the reaction progress variable gradient) and the strain rates, which govern the evolution of the SDF, have been analyzed using a three-dimensional flame-resolved simulation database of a turbulent lean premixed methane-air flame in a bluff-body configuration. It has been found that the turbulence intensity increases with the distance from the burner, changing the flame curvature distribution and increasing the probability of the negative curvature in the downstream direction. The curvature dependences of dilatation rate ∇ṡu → and displacement speed Sd give rise to variations of these quantities in the axial direction. These variations affect the nature of the alignment between the progress variable gradient and the local principal strain rates, which in turn affects the mean flame normal strain rate, which assumes positive values close to the burner but increasingly becomes negative as the effect of turbulence increases with the axial distance from the burner exit. The axial distance dependences of the curvature and displacement speed also induce a considerable variation in the mean value of the curvature stretch. The axial distance dependences of the dilatation rate and flame normal strain rate govern the behavior of the flame tangential strain rate, and its mean value increases in the downstream direction. The current analysis indicates that the statistical behaviors of different strain rates and displacement speed and their curvature dependences need to be included in the modeling of flame surface density and scalar dissipation rate in order to accurately capture their local behaviors.

  20. Premixed burner experiments: Geometry, mixing, and flame structure issues

    Energy Technology Data Exchange (ETDEWEB)

    Gupta, A.K.; Lewis, M.J.; Gupta, M. [Univ of Maryland, College Park, MD (United States)] [and others

    1995-10-01

    This research program is exploring techniques for improved fuel-air mixing, with the aim of achieving combustor operations up to stoichiometric conditions with minimal NO x and maximum efficiency. The experimental studies involve the use of a double-concentric natural gas burner that is operable in either premixed or non-premixed modes, and the system allows systematic variation of equivalence ratio, swirl strength shear length region and flow momentum in each annulus. Flame structures formed with various combinations of swirl strengths, flow throughput and equivalence ratios in premixed mode show the significant impact of swirl flow distribution on flame structure emanating from the mixedness. This impact on flame structure is expected to have a pronounced effect on the heat release rate and the emission of NO{sub x}. Thus, swirler design and configuration remains a key factor in the quest for completely optimized combustion. Parallel numerical studies of the flow and combustion phenomena were carried out, using the RSM and thek-{epsilon} turbulence models. These results have not only indicated the strengths and limitations of CFD in performance and pollutants emission predictions, but have provided guidelines on the size and strength of the recirculation produced and the spatio-temporal structure of the combustion flowfield. The first stage of parametric studies on geometry and operational parameters at Morgan State University have culminated in the completion of a one-dimensional flow code that is integrated with a solid, virtual model of the existing premixed burner. This coupling will provide the unique opportunity to study the impact of geometry on the flowfield and vice-versa, with particular emphasis on concurrent design optimization.

  1. Effects of pressure fluctuations on the combustion process in turbulent premixed flames

    Science.gov (United States)

    Beardsell, Guillaume; Lapointe, Simon; Blanquart, Guillaume

    2016-11-01

    The need for a thorough understanding of turbulence-combustion interactions in compressible flows is driven by recent technological developments in propulsion as well as renewed interest in the development of next generation supersonic and hypersonic vehicles. In such flows, pressure fluctuations displaying a wide range of length and timescales are present. These fluctuations are expected to impact the combustion process to varying degrees, depending amongst other things on the amplitude of the pressure variations and the timescales of the chemical reactions taking place in the flame. In this context, numerical simulations of these flows can provide insight into the impact of pressure fluctuations on the combustion process. In the present work, we analyze data from simulations of statistically-flat premixed n-heptane/air flames at high Karlovitz numbers. The compressible Navier-Stokes equations are solved exactly (DNS) and results obtained with both detailed kinetic modeling and one-step chemistry are considered. The effects of pressure fluctuations on the fuel burning rate are investigated. The findings are compared with results obtained from simulations of one-dimensional premixed flames subjected to various pressure waves.

  2. Unsteady Flame Embedding

    KAUST Repository

    El-Asrag, Hossam A.

    2011-01-01

    Direct simulation of all the length and time scales relevant to practical combustion processes is computationally prohibitive. When combustion processes are driven by reaction and transport phenomena occurring at the unresolved scales of a numerical simulation, one must introduce a dynamic subgrid model that accounts for the multiscale nature of the problem using information available on a resolvable grid. Here, we discuss a model that captures unsteady flow-flame interactions- including extinction, re-ignition, and history effects-via embedded simulations at the subgrid level. The model efficiently accounts for subgrid flame structure and incorporates detailed chemistry and transport, allowing more accurate prediction of the stretch effect and the heat release. In this chapter we first review the work done in the past thirty years to develop the flame embedding concept. Next we present a formulation for the same concept that is compatible with Large Eddy Simulation in the flamelet regimes. The unsteady flame embedding approach (UFE) treats the flame as an ensemble of locally one-dimensional flames, similar to the flamelet approach. However, a set of elemental one-dimensional flames is used to describe the turbulent flame structure directly at the subgrid level. The calculations employ a one-dimensional unsteady flame model that incorporates unsteady strain rate, curvature, and mixture boundary conditions imposed by the resolved scales. The model is used for closure of the subgrid terms in the context of large eddy simulation. Direct numerical simulation (DNS) data from a flame-vortex interaction problem is used for comparison. © Springer Science+Business Media B.V. 2011.

  3. Interaction of turbulent deflagrations with representative flow obstacles

    International Nuclear Information System (INIS)

    Durst, B.; Ardey, N.; Mayinger, F.

    1997-01-01

    In the case of a gradual release of hydrogen in the course of an assumed, severe accident in a light water reactor, the combustion will normally start out as a slow deflagration. Acceleration of an initially slow flame due to interactions of chemical kinetics and turbulent heat and mass transfer can result in very high flame speeds. Therefore, in order to assess hydrogen mitigation techniques, detailed knowledge about flame acceleration and interaction of flames with obstacles is required. The reported investigations are aimed at the investigation of the mechanisms responsible for turbulent flame acceleration and improving present correlations for estimates and models for numerical simulations of hydrogen combustion processes. A medium-scale square cross-section setup is employed, using flow obstacles with shapes representative for reactor containments. The global flame speed is deduced from measurements using thermocouples, pressure transducers and photodiodes. Measurements using a two-component LDA-system are being carried through in order to correlate global flame spread and local turbulence parameters. Results indicate that low blockage-ratio obstacles only marginally influence the flame, as disturbances which are induced remain local to the vicinity of the obstacle and die out very quickly downstream thereof. Flow visualizations by means of a Schlieren setup indicate very complex flow structures in the vicinity of obstacles. The results are being used to validate turbulent reaction models. A model based on probability density functions (pdf) of assumed shape has been developed and initial calculations are presented. (author)

  4. A Contribution to Turbulent Combustion: Premixed Flames and Material Surfaces Une contribution à la combustion turbulente : flammes prémélangées et surfaces des matériaux

    Directory of Open Access Journals (Sweden)

    Nicolleau F.

    2006-11-01

    Full Text Available The behavior of premixed flames has been examined by many authors. In fact the problem of combustion which develops in a turbulent medium depends on two scalings. One makes reference to the scales of the flame the other one is related to the turbulent field. Comparisons between these two scalings allow us to identified what sort of regime is expected. In this paper we first study the development of a material surface which may be identify with a flame front under rather severe conditions. An analytical approach is first used. Hereafter a numerical simulation will be introduced. The role of a fine grained turbulence is more active on the extension of the surface than large structures. To a large extent big eddies convey the surface without distorting it. The risks of extinction are generally predicted by making comparisons between the scales of the flame and the scales of the turbulent field starting from a direct simulation. Poinçot et al show that the smallest structures are not responsible for the extinction : intermediate structures are more efficient than the smallest ones. In a previous paper the role of these structures was examined : the distorting mechanism are acting in a cumulative way. The life time of the smallest structures is too short to have them playing a decive role in the extinction process. Intermediate sized structures are less active but they strain the flame during a longer period. This idea requires a detailed description of the turbulent field. That is made possible by using the ß model which accounts for the location of turbulent structures whose ranks in the whole sequence is termed n . The cumulative role of the velocity gradients is given as a function of n . The influence of the intermediate structure on the extinction process is thereby emphasized. Finally the ß model is also used to describe the domain of distributed combustion zones. Flames propagate in limited regions of space. These regions are disconnected

  5. Validation of an LES Model for Soot Evolution against DNS Data in Turbulent Jet Flames

    Science.gov (United States)

    Mueller, Michael

    2012-11-01

    An integrated modeling approach for soot evolution in turbulent reacting flows is validated against three-dimensional Direct Numerical Simulation (DNS) data in a set of n-heptane nonpremixed temporal jet flames. As in the DNS study, the evolution of the soot population is described statistically with the Hybrid Method of Moments (HMOM). The oxidation of the fuel and formation of soot precursors are described with the Radiation Flamelet/Progress Variable (RFPV) model that includes an additional transport equation for Polycyclic Aromatic Hydrocarbons (PAH) to account for the slow chemistry governing these species. In addition, the small-scale interactions between soot, chemistry, and turbulence are described with a presumed subfilter PDF approach that accounts for the very large spatial intermittency characterizing soot in turbulent reacting flows. The DNS dataset includes flames at three different Damköhler numbers to study the influence of global mixing rates on the evolution of PAH and soot. In this work, the ability of the model to capture these trends quantitatively as Damköhler number varies is investigated. In order to reliably assess the LES approach, the LES is initialized from the filtered DNS data after an initial transitional period in an effort to minimize the hydrodynamic differences between the DNS and the LES.

  6. An Investigation on Flame Shape and Size for a High-Pressure Turbulent Non-Premixed Swirl Combustion

    Directory of Open Access Journals (Sweden)

    Zhongya Xi

    2018-04-01

    Full Text Available Flame shape and size for a high-pressure turbulent non-premixed swirl combustion were experimentally investigated over a wide range of varying parameters including fuel mass flow rate, combustor pressure, primary-air mass flow rate, and nozzle exit velocity. A CFD simulation was conducted to predict the flame profile. Meanwhile, a theoretical calculation was also performed to estimate flame length. It was observed that flame length increased linearly with increasing fuel mass flow rate but decreased with the increment of combustor pressure in the power function. The flame diminished at a larger primary-air mass flow rate but remained unaffected by the increasing nozzle exit velocity. Considering the global effect of all parameters at a particular pressure, the flame length generally decreased as the primary-air to fuel ratio increased. This was attributed to the reduced air entrainment required to dilute the fuel to stoichiometric proportions. The CFD simulation offered a good prediction of the variation trends of flame length, although some deviations from experimental values were observed. The theoretical calculation estimated the trends of flame length variation particularly well. Nevertheless the difference between the theoretical and experimental results was found to be due to the swirl influence. Hence, a swirl factor was proposed to be added to the original equation for swirl flames.

  7. Experiment and Simulation of Autoignition in Jet Flames and its Relevance to Flame Stabilization and Structure

    KAUST Repository

    Al-Noman, Saeed M.

    2016-01-01

    temperature coflow air were studied numerically. Several flame configurations were investigated by varying the initial temperature and fuel mole fraction. Characteristics of chemical kinetics structures for autoignited lifted flames were discussed based on the kinetic structures of homogeneous autoignition and flame propagation of premixed mixtures. Results showed that for autoignited lifted flame with tribrachial structure, a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. Characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to a nozzle-attached flame was also investigated by increasing the fuel mole fraction.

  8. Turbulence Generation in Combustion.

    Science.gov (United States)

    1987-07-22

    flame length . This work is summarized in this section. I1.1 Model for Turbulent Burning Velocity For a range of turbulence conditions including...Variable density effects have been added in an approximation, and an expression for the length of jet flames has been developed. The flame length expression...of jet mixing and jet flame length data using fractals, College of Engineering, Energy Report E-86-02, Comell University, Ithaca, NY, 1986. Results

  9. A LES-CMC formulation for premixed flames including differential diffusion

    Science.gov (United States)

    Farrace, Daniele; Chung, Kyoungseoun; Bolla, Michele; Wright, Yuri M.; Boulouchos, Konstantinos; Mastorakos, Epaminondas

    2018-05-01

    A finite volume large eddy simulation-conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane-air flame with Leeff = 0.99 and a lean hydrogen-air flame with Leeff = 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane-air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable.

  10. An experimental and numerical study of diffusion flames in cross-flow and quiescent environment at smoke point condition

    Science.gov (United States)

    Goh, Sien Fong

    An experimental and numerical study of a turbulent smoke point diffusion flame in a quiescent and cross-flow condition was performed. The fuel mass flow rate of a turbulent smoke point flame was determined at a quiescent condition and in cross-flow with velocity ranging from 2 to 4 m/s. This fuel mass flow rate is defined as the Critical Fuel Mass Flow Rate (CFMFR). At a fuel mass flow rate below the CFMFR the flame produces smoke. In the dilution study, an amount of inert gas (nitrogen) was added to the fuel stream to achieve the smoke point condition for ten different fractions of CFMFR. From this dilution study, three regions were defined, the chemically-dominated region, transition region, and momentum-dominated region. The first objective of this study was to determine the factors behind the distinction of these three regions. The second objective was to understand the effect of cross-flow velocity on the smoke point flame structure. The flame temperature, radiation, geometrical dimension of flame, velocity, and global emissions and in-flame species concentration were measured. The third objective was to study a numerical model that can simulate the turbulent smoke point flame structure. The dilution study showed that the flames in quiescent condition and in the 3.5 and 4 m/s cross-flow condition had the chemically-dominated region at 5% to 20% CFMFR, the transition region at 20% to 40% CFMFR, and the momentum-dominated region at 40% to 100% CFMFR. On the other hand, the flame in cross-flow of 2 to 3 m/s showed the chemically-dominated region at 5% to 10% CFMFR, the transition region at 10% to 30% CFMFR, and the momentum-dominated region at 30% to 100% CFMFR. The chemically-dominated flame had a sharp dual-peak structure for the flame temperature, CO2 and NO concentration profiles at 25% and 50% flame length. However, the momentum-dominated region flame exhibited a dual peak structure only at 25% flame length. The decrease of flow rate from 30% to 10% CFMFR

  11. Characteristics of strongly-forced turbulent jets and non-premixed jet flames

    Energy Technology Data Exchange (ETDEWEB)

    Lakshminarasimhan, K.; Ezekoye, O.A. [University of Texas at Austin, Department of Mechanical Engineering, Austin, TX (United States); Clemens, N.T. [University of Texas at Austin, Department of Aerospace Engineering and Engineering Mechanics, Austin, TX (United States)

    2006-10-15

    Previous researchers have demonstrated that strong pulsations of the fuel flow rate can significantly reduce the flame length and luminosity of laminar/transitional non-premixed jet flames. The physical mechanisms responsible for these changes are investigated experimentally in acoustically-forced jet flows where the peak velocity fluctuations are up to eight times the mean flow velocity. Both reacting and non-reacting flows were studied and Reynolds numbers, based on the mean flow properties, ranged from 800 to 10,000 (corresponding to peak Reynolds numbers of 1,450-23,000), and forcing frequencies ranged from 290 to 1,140 Hz. Both the first and second organ-pipe resonance modes of the fuel delivery tube were excited to obtain these frequencies. An analysis of the acoustic forcing characteristics within the resonance tube is provided in order to understand the source of the high amplitude forcing. Flow visualization of jets with first resonant forcing confirms the presence of large-scale coherent vortices and strong reverse flow near the exit of the fuel tube. With second-resonant forcing, however, vortices are not emitted from the tube as they are drawn back into the fuel tube before they can fully form. Increased fine-scale turbulence is associated with both resonant cases, but particularly at second resonance. The power spectra of the velocity fluctuations for a resonantly pulsed jet show the presence of an inertial subrange indicating that the flow becomes fully turbulent even for mean-Reynolds-number jets that are nominally laminar. It is shown that these pulsed jet flows exhibit strong similarities to synthetic jets and that the Strouhal number, based on the maximum velocity at the fuel tube exit, is the dominant parameter for scaling these flows. The Strouhal number determines the downstream location where the coherent vortices breakdown, and is found to provide better collapse of flame length data (both current and previous) than other parameters that have

  12. Blow-out limits of nonpremixed turbulent jet flames in a cross flow at atmospheric and sub-atmospheric pressures

    KAUST Repository

    Wang, Qiang; Hu, Longhua; Yoon, Sung Hwan; Lu, Shouxiang; Delichatsios, Michael; Chung, Suk-Ho

    2015-01-01

    The blow-out limits of nonpremixed turbulent jet flames in cross flows were studied, especially concerning the effect of ambient pressure, by conducting experiments at atmospheric and sub-atmospheric pressures. The combined effects of air flow

  13. Numerical Simulation of a Laboratory-Scale Turbulent SlotFlame

    Energy Technology Data Exchange (ETDEWEB)

    Bell, John B.; Day, Marcus S.; Grcar, Joseph F.; Lijewski,Michael J.; Driscoll, James F.; Filatyev, Sergei A.

    2006-04-20

    We present three-dimensional, time-dependent simulations ofthe flowfield of a laboratory-scale slot burner. The simulations areperformed using an adaptive time-dependent low Mach number combustionalgorithm based on a second-order projection formulation that conservesboth species mass and total enthalpy. The methodology incorporatesdetailed chemical kinetics and a mixture model for differential speciesdiffusion. Methane chemistry and transport are modeled using the DRM-19mechanism along with its associated thermodynamics and transportdatabases. Adaptive mesh refinementdynamically resolves the flame andturbulent structures. Detailedcomparisons with experimental measurementsshow that the computational results provide a good prediction of theflame height, the shape of the time-averaged parabolic flame surfacearea, and the global consumption speed (the volume per second ofreactants consumed divided by the area of the time-averaged flame). Thethickness of the computed flamebrush increases in the streamwisedirection, and the flamesurface density profiles display the same generalshapes as the experiment. The structure of the simulated flame alsomatches the experiment; reaction layers are thin (typically thinner than1 mm) and the wavelengths of large wrinkles are 5--10 mm. Wrinklesamplify to become long fingers of reactants which burn through at a neckregion, forming isolated pockets of reactants. Thus both the simulatedflame and the experiment are in the "corrugated flameletregime."

  14. FLAMES IN TYPE Ia SUPERNOVA: DEFLAGRATION-DETONATION TRANSITION IN THE OXYGEN-BURNING FLAME

    International Nuclear Information System (INIS)

    Woosley, S. E.; Kerstein, A. R.; Aspden, A. J.

    2011-01-01

    The flame in a Type Ia supernova is a conglomerate structure that, depending on density, may involve separate regions of carbon, oxygen, and silicon burning, all propagating in a self-similar, subsonic front. The separation between these three burning regions increases as the density declines until eventually, below about 2 x 10 7 g cm -3 , only carbon burning remains active, the other two burning phases having 'frozen out' on stellar scales. Between 2 and 3 x 10 7 g cm -3 , however, there remains an energetic oxygen-burning region that trails the carbon burning by an amount that is sensitive to the turbulence intensity. As the carbon flame makes a transition to the distributed regime (Karlovitz number ∼> 10), the characteristic separation between the carbon- and oxygen-burning regions increases dramatically, from a fraction of a meter to many kilometers. The oxygen-rich mixture between the two flames is created at a nearly constant temperature, and turbulence helps to maintain islands of well-mixed isothermal fuel as the temperature increases. The delayed burning of these regions can be supersonic and could initiate a detonation.

  15. Aerodynamic properties of turbulent combustion fields

    Science.gov (United States)

    Hsiao, C. C.; Oppenheim, A. K.

    1985-01-01

    Flow fields involving turbulent flames in premixed gases under a variety of conditions are modeled by the use of a numerical technique based on the random vortex method to solve the Navier-Stokes equations and a flame propagation algorithm to trace the motion of the front and implement the Huygens principle, both due to Chorin. A successive over-relaxation hybrid method is applied to solve the Euler equation for flows in an arbitrarily shaped domain. The method of images, conformal transformation, and the integral-equation technique are also used to treat flows in special cases, according to their particular requirements. Salient features of turbulent flame propagation in premixed gases are interpreted by relating them to the aerodynamic properties of the flow field. Included among them is the well-known cellular structure of flames stabilized by bluff bodies, as well as the formation of the characteristic tulip shape of flames propagating in ducts. In its rudimentary form, the mechanism of propagation of a turbulent flame is shown to consist of: (1) rotary motion of eddies at the flame front, (2) self-advancement of the front at an appropriate normal burning speed, and (3) dynamic effects of expansion due to exothermicity of the combustion reaction. An idealized model is used to illustrate these fundamental mechanisms and to investigate basic aerodynamic features of flames in premixed gases. The case of a confined flame stabilized behind a rearward-facing step is given particular care and attention. Solutions are shown to be in satisfactory agreement with experimental results, especially with respect to global properties such as the average velocity profiles and reattachment length.

  16. The influence of fuel-air swirl intensity on flame structures of syngas swirl-stabilized diffusion flame

    Science.gov (United States)

    Shao, Weiwei; Xiong, Yan; Mu, Kejin; Zhang, Zhedian; Wang, Yue; Xiao, Yunhan

    2010-06-01

    Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured using the OH-PLIF method under different fuel and air swirl intensity. The flame operated under atmospheric pressure with air and a typical low heating-value syngas with a composition of 28.5% CO, 22.5% H2 and 49% N2 at a thermal power of 34 kW. Results indicate that increasing the air swirl intensity with the same fuel, swirl intensity flame structures showed little difference except a small reduction of flame length; but also, with the same air swirl intensity, fuel swirl intensity showed great influence on flame shape, length and reaction zone distribution. Therefore, compared with air swirl intensity, fuel swirl intensity appeared a key effect on the flame structure for the model combustor. Instantaneous OH-PLIF images showed that three distinct typical structures with an obvious difference of reaction zone distribution were found at low swirl intensity, while a much compacter flame structure with a single, stable and uniform reaction zone distribution was found at large fuel-air swirl intensity. It means that larger swirl intensity leads to efficient, stable combustion of the syngas diffusion flame.

  17. Turbulent deflagrations, autoignitions, and detonations

    KAUST Repository

    Bradley, Derek

    2012-09-01

    Measurements of turbulent burning velocities in fan-stirred explosion bombs show an initial linear increase with the fan speed and RMS turbulent velocity. The line then bends over to form a plateau of high values around the maximum attainable burning velocity. A further increase in fan speed leads to the eventual complete quenching of the flame due to increasing localised extinctions because of the flame stretch rate. The greater the Markstein number, the more readily does flame quenching occur. Flame propagation along a duct closed at one end, with and without baffles to increase the turbulence, is subjected to a one-dimensional analysis. The flame, initiated at the closed end of the long duct, accelerates by the turbulent feedback mechanism, creating a shock wave ahead of it, until the maximum turbulent burning velocity for the mixture is attained. With the confining walls, the mixture is compressed between the flame and the shock plane up to the point where it might autoignite. This can be followed by a deflagration to detonation transition. The maximum shock intensity occurs with the maximum attainable turbulent burning velocity, and this defines the limit for autoignition of the mixture. For more reactive mixtures, autoignition can occur at turbulent burning velocities that are less than the maximum attainable one. Autoignition can be followed by quasi-detonation or fully developed detonation. The stability of ensuing detonations is discussed, along with the conditions that may lead to their extinction. © 2012 by Pleiades Publishing, Ltd.

  18. Physical and Chemical Processes in Turbulent Flames

    Science.gov (United States)

    2015-06-23

    equiangular sectors, defined as the ratio of the actual flame length to the length of a circular-arc of radius equal to the average flame radius. Assuming... flame length ratio obtained directly from the experiments, without any assumption. As explained earlier (Eq. 2.8) the length ratio, (LR=dl(G0)/dl0) is...spherically expanding flames, with the length ratio on the measurement plane, at predefined equiangular sectors, defined as the ratio of the actual flame length to

  19. Predictions of nitrogen oxides production in diffusion turbulent flames; Predictions de la production des oxydes d`azote dans les flammes turbulentes de diffusion

    Energy Technology Data Exchange (ETDEWEB)

    Sanders, H.; Gokalp, I. [Centre National de la Recherche Scientifique (CNRS), 45 - Orleans-la-Source (France). Laboratoire de Combustion Systemes Reactifs

    1996-12-31

    The suitability of the turbulent combustion flamelets model in order to predict the index of NO{sub x} production in turbulent flames of hydrogen diffusion is analyzed. In the flamelet approach, the turbulent flame is equivalent to a group of laminar flames submitted to a mechanical stretching which generates a chemical disequilibrium. This effect can be described by the stretching or by the scalar dissipation ratio. A numerical modeling is performed in order to evaluate the advantages of both approaches and to compare the behaviour of the NO{sub x} emission index with the experiments of Chen and Driscoll. This study shows that predictions of NO{sub x} emission indexes have a correct behaviour with respect to the Damkoehler number only when the scalar dissipation ratio is used as a parameter to describe the chemical state outside equilibrium. Predictions of the flamelet models are improving when the Damkoehler number increases. On the other hand, the absolute NO{sub x} concentrations are overestimated and can be due to the effects of differential diffusion. (J.S.) 14 refs.

  20. The modelling of direct chemical kinetic effects in turbulent flames

    Energy Technology Data Exchange (ETDEWEB)

    Lindstet, R.P. [Imperial College of Science, Technology and Medicine, London (United Kingdom). Dept. of Mechanical Engineering

    2000-06-01

    Combustion chemistry-related effects have traditionally been of secondary importance in the design of gas turbine combustors. However, the need to deal with issues such as flame stability, relight and pollutant emissions has served to bring chemical kinetics and the coupling of finite rate chemistry with turbulent flow fields to the centre of combustor design. Indeed, improved cycle efficiency and more stringent environmental legislation, as defined by the ICAO, are current key motivators in combustor design. Furthermore, lean premixed prevaporized (LPP) combustion systems, increasingly used for power generation, often operate close to the lean blow-off limit and are prone to extinction/reignition type phenomena. Thus, current key design issues require that direct chemical kinetic effects be accounted for accurately in any simulation procedure. The transported probability density function (PDF) approach uniquely offers the potential of facilitating the accurate modelling of such effects. The present paper thus assesses the ability of this technique to model kinetically controlled phenomena, such as carbon monoxide emissions and flame blow-off, through the application of a transported PDF method closed at the joint scalar level. The closure for the velocity field is at the second moment level, and a key feature of the present work is the use of comprehensive chemical kinetic mechanisms. The latter are derived from recent work by Lindstedt and co-workers that has resulted in a compact 141 reactions and 28 species mechanism for LNG combustion. The systematically reduced form used here features 14 independent C/H/O scalars, with the remaining species incorporated via steady state approximations. Computations have been performed for hydrogen/carbon dioxide and methane flames. The former (high Reynolds number) flames permit an assessment of the modelling of flame blow-off, and the methane flame has been selected to obtain an indication of the influence of differential

  1. Measurements and simulation of the interaction of turbulence and premixed flames; Messungen und Simulationen zur Wechselwirkung zwischen Turbulenz und vorgemischten Flammen

    Energy Technology Data Exchange (ETDEWEB)

    Durst, B

    2000-11-01

    The interaction between turbulence and hydrogen-flames was investigated in an explosion tube. The flow velocity around single flow obstacles was measured with a laser-Doppler system and compared to the flame velocity which was recorded using photodiodes. The highest turbulence intensity (up to 10 m/s) and correspondingly the highest flame acceleration was measured in the shear layer downstream of the obstacle with the highest blockage ratio. A closure model based on probability density functions (PDF) was developed for the time averaged chemical reaction rate for the purpose of simulating turbulent combustion processes. Comparisons of the results gained from simulations using the PDF combustion modell showed good agreement with the measurements performed. [German] Die Wechselwirkung zwischen Turbulenz und Wasserstoff-Flammen wurde in einem Explosionsrohr untersucht. Die Stroemungsgeschwindigkeit wurde mit einem Laser-Doppler System an Einzelhindernissen gemessen und mit der Flammengeschwindigkeit, die mittels Photodioden erfasst wurde, verglichen. In der Scherschicht hinter dem Hindernis mit der hoechsten Blockierrate wurde die hoechste Turbulenzintensitaet (bis 10 m/s) und damit die hoechste Flammenbeschleunigung gemessen. Fuer numerische Simulationen der turbulenten Verbrennung wurde ein Schliessungsansatz fuer die zeitgemittelte chemische Reaktionsrate entwickelt, der auf Wahrscheinlichkeitsdichtefunktionen (englisch: PDF) basiert. Vergleichsrechnungen mit dem PDF-Verbrennungsmodell zeigten gute Uebereinstimmung mit den durchgefuehrten Messungen.

  2. Experiment and Simulation of Autoignition in Jet Flames and its Relevance to Flame Stabilization and Structure

    KAUST Repository

    Al-Noman, Saeed M.

    2016-06-01

    Autoignition characteristics of pre-vaporized iso-octane, primary reference fuels, gasolines, and dimethyl ether (DME) have been investigated experimentally in a coflow with elevated temperature of air. With the coflow air at relatively low initial temperatures below autoignition temperature Tauto, an external ignition source was required to stabilize the flame. Non-autoignited lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization balanced with local flow velocity. At high initial temperatures over Tauto, the autoignited flames were stabilized without requiring an external ignition source. The autoignited lifted flames exhibited either tribrachial edge structures or Mild combustion behaviors depending on the level of fuel dilution. For the iso-octane and n-heptane fuels, two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then a sudden transition to lifted Mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times for the pre-vaporized fuels. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions. For the gasoline fuels for advanced combustion engines (FACEs), and primary reference fuels (PRFs), autoignited liftoff data were correlated with Research Octane Number and Cetane Number. For the DME fuel, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and CH* chemiluminescence were visualized qualitatively. In the autoignition regime for both tribrachial structure and mild combustion, formaldehyde were found

  3. Recent measurements of flame acceleration in semiconfined geometries

    Energy Technology Data Exchange (ETDEWEB)

    Abou-Arab, T.W. (King Fahd Univ. of Petroleum and Minerals, Dhahran (SA). Mechanical Engineering Dept.); Enayet, M.M.; Kamel, M.M. (Cairo Univ., Giza (EG). Mechanical Power Engineering Dept.)

    1991-04-01

    Turbulent premixed combustion under certain conditions may lead to large flame speeds sufficient to cause significant damage to nearby structures. Experiments, both large and small scale, have confirmed that obstructions cause severe flame acceleration to occur. In these cases, flame speeds as high as 800 ms{sup -1} may be achieved. In this work experimental investigation of some factors affecting flame acceleration in a semiconfined channel has been carried out. The experimental facility and the developed ionization gap measuring technique are also described. It has been found that the presence of obstacles, degree of confinement, height of fuel-air cloud (FAC), as well as fuel concentration gradient in the FAC have profound effects on the rate at which the flame accelerates. Finally, consideration of the flame acceleration as a possible mechanism for the transition to detonation will be discussed. (author).

  4. A temporal PIV study of flame/obstacle generated vortex interactions within a semi-confined combustion chamber

    Science.gov (United States)

    Jarvis, S.; Hargrave, G. K.

    2006-01-01

    Experimental data obtained using a new multiple-camera digital particle image velocimetry (PIV) technique are presented for the interaction between a propagating flame and the turbulent recirculating velocity field generated during flame-solid obstacle interaction. The interaction between the gas movement and the obstacle creates turbulence by vortex shedding and local wake recirculations. The presence of turbulence in a flammable gas mixture can wrinkle a flame front, increasing the flame surface area and enhancing the burning rate. To investigate propagating flame/turbulence interaction, a novel multiple-camera digital PIV technique was used to provide high spatial and temporal characterization of the phenomenon for the turbulent flow field in the wake of three sequential obstacles. The technique allowed the quantification of the local flame speed and local flow velocity. Due to the accelerating nature of the explosion flow field, the wake flows develop 'transient' turbulent fields. Multiple-camera PIV provides data to define the spatial and temporal variation of both the velocity field ahead of the propagating flame and the flame front to aid the understanding of flame-vortex interaction. Experimentally obtained values for flame displacement speed and flame stretch are presented for increasing vortex complexity.

  5. Gravitational Effects on Cellular Flame Structure

    Science.gov (United States)

    Dunsky, C. M.; Fernandez-Pello, A. C.

    1991-01-01

    An experimental investigation has been conducted of the effect of gravity on the structure of downwardly propagating, cellular premixed propane-oxygen-nitrogen flames anchored on a water-cooled porous-plug burner. The flame is subjected to microgravity conditions in the NASA Lewis 2.2-second drop tower, and flame characteristics are recorded on high-speed film. These are compared to flames at normal gravity conditions with the same equivalence ratio, dilution index, mixture flow rate, and ambient pressure. The results show that the cellular instability band, which is located in the rich mixture region, changes little under the absence of gravity. Lifted normal-gravity flames near the cellular/lifted limits, however, are observed to become cellular when gravity is reduced. Observations of a transient cell growth period following ignition point to heat loss as being an important mechanism in the overall flame stability, dominating the stabilizing effect of buoyancy for these downwardly-propagating burner-anchored flames. The pulsations that are observed in the plume and diffusion flame generated downstream of the premixed flame in the fuel rich cases disappear in microgravity, verifying that these fluctuations are gravity related.

  6. Turbulent diffusion in the flame of a rotary kiln

    Energy Technology Data Exchange (ETDEWEB)

    Strekotin, V.V.; Telegin, A.A.; Lisin, F.N.; Malysheva, O.I.

    1987-09-01

    Experimental data on the distribution of velocities in the stream in the flow of air from models of a burner with a normal annular Laval nozzle and a burner with an increase in the angle of opening of the stream under supersonic conditions were obtained. The results of the work may be used in the design of burners for rotary kilns. According to the experimental data the coefficient of turbulent diffusion reaches a value of 0.0071 m/sup 2//sec for a pure flow and is reduced by 30% with an increase in the dust content from 0 to 1 kg/kg. It is desirable to use the data obtained in calculations of the flame processes and selection of means of intensification of the process of mixing of the fuel with the oxidizer in the presence of dust.

  7. Chemical Reactions in Turbulent Mixing Flows

    Science.gov (United States)

    1992-07-01

    Chemically-Reacting, Gas-Phase Turbulent Jets (Gilbrech 1991), that explored Reynolds number effects on turbulent flame length and the influence of...and asymptotes to a constant value beyond the flame tip. The main result of the work is that the flame length , as estimated from the temperature...8217. Specifically, the normalized flame length Lf/d* displays a linear dependence on the stoichiometric mixture ratio 0, with a slope that decreases from Re "• 1.0

  8. Investigation of turbulent swirling jet-flames by PIV / OH PLIF / HCHO PLIF

    Science.gov (United States)

    Lobasov, A. S.; Chikishev, L. M.

    2018-03-01

    The present paper reports on the investigation of fuel-lean and fuel-rich turbulent combustion in a high-swirl jet. Swirl rate of the flow exceeded a critical value for breakdown of the swirling jet’s vortex core and formation of the recirculation zone at the jet axis. The measurements were performed by the stereo PIV, OH PLIF and HCHO PLIF techniques, simultaneously. The Reynolds number based on the flow rate and viscosity of the air was fixed as 5 000 (the bulk velocity was U 0 = 5 m/s). Three cases of the equivalence ratio ϕ of the mixture issuing from the nozzle-burner were considered, viz., 0.7, 1.4 and 2.5. The latter case corresponded to a lifted flame of fuel-rich swirling jet flow, partially premixed with the surrounding air. In all cases the flame front was subjected to deformations due to large-scale vortices, which rolled-up in the inner (around the central recirculation zone) and outer (between the annular jet core and surrounding air) mixing layers.

  9. Effect of the mixing fields on the stability and structure of turbulent partially premixed flames in a concentric flow conical nozzle burner

    KAUST Repository

    Mansour, Mohy S.; Elbaz, Ayman M.; Roberts, William L.; Senosy, Mohamed S.; Zayed, Mohamed F.; Juddoo, Mrinal; Masri, Assaad R.

    2016-01-01

    of partially premixed methane flames. The mixing field in a concentric flow conical nozzle (CFCN) burner with well-controlled mechanism of the mixing is investigated using Rayleigh scattering technique. The flame stability, structure and flow field of some

  10. CFD predictions of wake-stabilised jet flames in a cross-flow

    International Nuclear Information System (INIS)

    Lawal, Mohammed S.; Fairweather, Michael; Gogolek, Peter; Ingham, Derek B.; Ma, Lin; Pourkashanian, Mohamed; Williams, Alan

    2013-01-01

    This study describes an investigation into predicting the major flow properties in wake-stabilised jet flames in a cross flow of air using first- and second-order turbulence models, applied within a RANS (Reynolds-averaged Navier–Stokes) modelling framework. Standard and RNG (re-normalisation group) versions of the k-ε turbulence model were employed at the first-order level and the results compared with a second-moment closure, or RSM (Reynolds stress model). The combustion process was modelled using the laminar flamelet approach together with a thermal radiation model using the discrete ordinate method. The ability of the various turbulence models to reproduce experimentally established flame appearance, profiles of velocity and turbulence intensity, as well as the combustion efficiency of such flames is reported. The results show that all the turbulence models predict similar velocity profiles over the majority of the flow domain considered, except in the wake region, where the predictions of the RSM and RNG k-ε models are in closer agreement with experimental data. In contrast, the standard k-ε model over-predicts the peak turbulence intensity. Also, it is found that the RSM provides superior predictions of the planar recirculation and flame zones attached to the release pipe in the wake region. - Highlights: ► We investigated the prediction of the major properties in wake-stabilised methane jet flames in a cross flow. ► The ability of the various turbulence models to reproduce experimentally established flame parameters is reported. ► All the turbulence models considered predict similar velocity profiles, except in the wake region

  11. Taylor dispersion in premixed combustion: Questions from turbulent combustion answered for laminar flames

    Science.gov (United States)

    Daou, Joel; Pearce, Philip; Al-Malki, Faisal

    2018-02-01

    We present a study of Taylor dispersion in premixed combustion and use it to clarify fundamental issues related to flame propagation in a flow field. In particular, simple analytical formulas are derived for variable density laminar flames with arbitrary Lewis number Le providing clear answers to important questions arising in turbulent combustion, when these questions are posed for the case of one-scale laminar parallel flows. Exploiting, in the context of a laminar Poiseuille flow model, a thick flame distinguished asymptotic limit for which the flow amplitude is large with the Reynolds number Re fixed, three main contributions are made. First, a link is established between Taylor dispersion [G. Taylor, Proc. R. Soc. London Ser. A 219, 186 (1953), 10.1098/rspa.1953.0139] and Damköhler's second hypothesis [G. Damköhler, Ber. Bunsen. Phys. Chem. 46, 601 (1940)] by describing analytically the enhancement of the effective propagation speed UT due to small flow scales. More precisely, it is shown that Damköhler's hypothesis is only partially correct for one-scale parallel laminar flows. Specifically, while the increase in UT due to the flow is shown to be directly associated with the increase in the effective diffusivity as suggested by Damköhler, our results imply that UT˜Re (for Re≫1 ) rather than UT˜√{Re} , as implied by Damköhler's hypothesis. Second, it is demonstrated analytically and confirmed numerically that, when UT is plotted versus the flow amplitude for fixed values of Re, the curve levels off to a constant value depending on Re. We may refer to this effect as the laminar bending effect as it mimics a similar bending effect known in turbulent combustion. Third, somewhat surprising implications associated with the dependence of UT and of the effective Lewis number Leeff on the flow are reported. For example, Leeff is found to vary from Le to Le-1 as Re varies from small to large values. Also, UT is found to be a monotonically increasing function

  12. Time-averaged probability density functions of soot nanoparticles along the centerline of a piloted turbulent diffusion flame using a scanning mobility particle sizer

    KAUST Repository

    Chowdhury, Snehaunshu

    2017-01-23

    In this study, we demonstrate the use of a scanning mobility particle sizer (SMPS) as an effective tool to measure the probability density functions (PDFs) of soot nanoparticles in turbulent flames. Time-averaged soot PDFs necessary for validating existing soot models are reported at intervals of ∆x/D∆x/D = 5 along the centerline of turbulent, non-premixed, C2H4/N2 flames. The jet exit Reynolds numbers of the flames investigated were 10,000 and 20,000. A simplified burner geometry based on a published design was chosen to aid modelers. Soot was sampled directly from the flame using a sampling probe with a 0.5-mm diameter orifice and diluted with N2 by a two-stage dilution process. The overall dilution ratio was not evaluated. An SMPS system was used to analyze soot particle concentrations in the diluted samples. Sampling conditions were optimized over a wide range of dilution ratios to eliminate the effect of agglomeration in the sampling probe. Two differential mobility analyzers (DMAs) with different size ranges were used separately in the SMPS measurements to characterize the entire size range of particles. In both flames, the PDFs were found to be mono-modal in nature near the jet exit. Further downstream, the profiles were flatter with a fall-off at larger particle diameters. The geometric mean of the soot size distributions was less than 10 nm for all cases and increased monotonically with axial distance in both flames.

  13. Streamline segment statistics of premixed flames with nonunity Lewis numbers

    Science.gov (United States)

    Chakraborty, Nilanjan; Wang, Lipo; Klein, Markus

    2014-03-01

    The interaction of flame and surrounding fluid motion is of central importance in the fundamental understanding of turbulent combustion. It is demonstrated here that this interaction can be represented using streamline segment analysis, which was previously applied in nonreactive turbulence. The present work focuses on the effects of the global Lewis number (Le) on streamline segment statistics in premixed flames in the thin-reaction-zones regime. A direct numerical simulation database of freely propagating thin-reaction-zones regime flames with Le ranging from 0.34 to 1.2 is used to demonstrate that Le has significant influences on the characteristic features of the streamline segment, such as the curve length, the difference in the velocity magnitude at two extremal points, and their correlations with the local flame curvature. The strengthenings of the dilatation rate, flame normal acceleration, and flame-generated turbulence with decreasing Le are principally responsible for these observed effects. An expression for the probability density function (pdf) of the streamline segment length, originally developed for nonreacting turbulent flows, captures the qualitative behavior for turbulent premixed flames in the thin-reaction-zones regime for a wide range of Le values. The joint pdfs between the streamline length and the difference in the velocity magnitude at two extremal points for both unweighted and density-weighted velocity vectors are analyzed and compared. Detailed explanations are provided for the observed differences in the topological behaviors of the streamline segment in response to the global Le.

  14. Effects of combined dimension reduction and tabulation on the simulations of a turbulent premixed flame using a large-eddy simulation/probability density function method

    Science.gov (United States)

    Kim, Jeonglae; Pope, Stephen B.

    2014-05-01

    A turbulent lean-premixed propane-air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained.

  15. Direct numerical simulations of non-premixed ethylene-air flames: Local flame extinction criterion

    KAUST Repository

    Lecoustre, Vivien R.

    2014-11-01

    Direct Numerical Simulations (DNS) of ethylene/air diffusion flame extinctions in decaying two-dimensional turbulence were performed. A Damköhler-number-based flame extinction criterion as provided by classical large activation energy asymptotic (AEA) theory is assessed for its validity in predicting flame extinction and compared to one based on Chemical Explosive Mode Analysis (CEMA) of the detailed chemistry. The DNS code solves compressible flow conservation equations using high order finite difference and explicit time integration schemes. The ethylene/air chemistry is simulated with a reduced mechanism that is generated based on the directed relation graph (DRG) based methods along with stiffness removal. The numerical configuration is an ethylene fuel strip embedded in ambient air and exposed to a prescribed decaying turbulent flow field. The emphasis of this study is on the several flame extinction events observed in contrived parametric simulations. A modified viscosity and changing pressure (MVCP) scheme was adopted in order to artificially manipulate the probability of flame extinction. Using MVCP, pressure was changed from the baseline case of 1 atm to 0.1 and 10 atm. In the high pressure MVCP case, the simulated flame is extinction-free, whereas in the low pressure MVCP case, the simulated flame features frequent extinction events and is close to global extinction. Results show that, despite its relative simplicity and provided that the global flame activation temperature is correctly calibrated, the AEA-based flame extinction criterion can accurately predict the simulated flame extinction events. It is also found that the AEA-based criterion provides predictions of flame extinction that are consistent with those provided by a CEMA-based criterion. This study supports the validity of a simple Damköhler-number-based criterion to predict flame extinction in engineering-level CFD models. © 2014 The Combustion Institute.

  16. Effects of elliptical burner geometry on partially premixed gas jet flames in quiescent surroundings

    Science.gov (United States)

    Baird, Benjamin

    This study is the investigation of the effect of elliptical nozzle burner geometry and partial premixing, both 'passive control' methods, on a hydrogen/hydrocarbon flame. Both laminar and turbulent flames for circular, 3:1, and 4:1 aspect ratio (AR) elliptical burners are considered. The amount of air mixed with the fuel is varied from fuel-lean premixed flames to fuel-rich partially premixed flames. The work includes measurements of flame stability, global pollutant emissions, flame radiation, and flame structure for the differing burner types and fuel conditions. Special emphasis is placed on the near-burner region. Experimentally, both conventional (IR absorption, chemiluminecent, and polarographic emission analysis,) and advanced (laser induced fluorescence, planar laser induced fluorescence, Laser Doppler Velocimetry (LDV), Rayleigh scattering) diagnostic techniques are used. Numerically, simulations of 3-dimensional laminar and turbulent reacting flow are conducted. These simulations are run with reduced chemical kinetics and with a Reynolds Stress Model (RSM) for the turbulence modeling. It was found that the laminar flames were similar in appearance and overall flame length for the 3:1 AR elliptical and the circular burner. The laminar 4:1 AR elliptical burner flame split into two sub-flames along the burner major axis. This splitting had the effect of greatly shortening the 4:1 AR elliptical burner flame to have an overall flame length about half of that of the circular and 3:1 AR elliptical burner flames. The length of all three burners flames increased with increasing burner exit equivalence ratio. The blowout velocity for the three burners increased with increase in hydrogen mass fraction of the hydrogen/propane fuel mixture. For the rich premixed flames, the circular burner was the most stable, the 3:1 AR elliptical burner, was the least stable, and the 4:1 AR elliptical burner was intermediate to the two other burners. This order of stability was due

  17. Flame Dynamics and Chemistry in LRE Combustion Instability

    Science.gov (United States)

    2016-12-22

    negative temperature coefficient phenomenon and engine knock. In this work, the coupling of cool flame chemistry and convective– diffusive transport...note, practical engine conditions are highly turbulent, and the autoignition phenomenon depends on both chemistry and turbulent mixing. For example...negative temperature coefficient (NTC) phenomenon and engine knock. In this work, the coupling of cool flame chemistry and convective–diffusive

  18. Numerical simulation of nitrogen oxide formation in lean premixed turbulent H2/O2/N2 flames

    DEFF Research Database (Denmark)

    Day, Marc S.; Bell, John B.; Gao, Xinfeng

    2011-01-01

    Lean premixed hydrogen flames are thermodiffusively unstable and burn in cellular structures. Within these cellular structures the flame is locally enriched by preferential diffusion of hydrogen, leading to local hotspots that burn more intensely than an idealized flat steady flame at comparable ...

  19. Flame Structure and Chemiluminescence Emissions of Inverse Diffusion Flames under Sinusoidally Driven Plasma Discharges

    Directory of Open Access Journals (Sweden)

    Maria Grazia De Giorgi

    2017-03-01

    Full Text Available Reduction of nitric oxides (NOx in aircraft engines and in gas turbines by lean combustion is of great interest in the design of novel combustion systems. However, the stabilization of the flame under lean conditions is a main issue. In this context, the present work investigates the effects of sinusoidal dielectric barrier discharge (DBD on a lean inverse diffusive methane/air flame in a Bunsen-type burner under different actuation conditions. The flame appearance was investigated with fixed methane loading (mass flux, but with varying inner airflow rate. High-speed flame imaging was done by using an intensified (charge-coupled device CCD camera equipped with different optical filters in order to selectively record signals from the chemiluminescent species OH*, CH*, or CO2* to evaluate the flame behavior in presence of plasma actuation. The electrical power consumption was less than 33 W. It was evident that the plasma flame enhancement was significantly influenced by the plasma discharges, particularly at high inner airflow rates. The flame structure changes drastically when the dissipated plasma power increases. The flame area decreases due to the enhancement of mixing and chemical reactions that lead to a more anchored flame on the quartz exit with a reduction of the flame length.

  20. Effect of von Karman Vortex Shedding on Regular and Open-slit V-gutter Stabilized Turbulent Premixed Flames

    Science.gov (United States)

    2012-04-01

    Both flame lengths shrink and large scale disruptions occur downstream with vortex shedding carrying reaction zones. Flames in both flameholders...9) the flame structure changes dramatically for both regular and open-slit V-gutter. Both flame lengths shrink and large scale disruptions occur...reduces the flame length . However, qualitatively the open-slit V-gutter appears to be more sensitive than the regular V-gutter. Both flames remain

  1. Flame-Resistant Composite Materials For Structural Members

    Science.gov (United States)

    Spears, Richard K.

    1995-01-01

    Matrix-fiber composite materials developed for structural members occasionally exposed to hot, corrosive gases. Integral ceramic fabric surface layer essential for resistance to flames and chemicals. Endures high temperature, impedes flame from penetrating to interior, inhibits diffusion of oxygen to interior where it degrades matrix resin, resists attack by chemicals, helps resist erosion, and provides additional strength. In original intended application, composite members replace steel structural members of rocket-launching structures that deteriorate under combined influences of atmosphere, spilled propellants, and rocket exhaust. Composites also attractive for other applications in which corrosion- and fire-resistant structural members needed.

  2. Models for turbulent flows with variable density and combustion

    International Nuclear Information System (INIS)

    Jones, W.P.

    1980-01-01

    Models for transport processes and combustion in turbulent flows are outlined with emphasis on the situation where the fuel and air are injected separately. Attention is restricted to relatively simple flames. The flows investigated are high Reynolds number, single-phase, turbulent high-temperature flames in which radiative heat transfer can be considered negligible. Attention is given to the lower order closure models, algebraic stress and flux models, the k-epsilon turbulence model, the diffusion flame approximation, and finite rate reaction mechanisms

  3. Computational Investigation of Soot and Radiation in Turbulent Reacting Flows

    Science.gov (United States)

    Lalit, Harshad

    This study delves into computational modeling of soot and infrared radiation for turbulent reacting flows, detailed understanding of both of which is paramount in the design of cleaner engines and pollution control. In the first part of the study, the concept of Stochastic Time and Space Series Analysis (STASS) as a numerical tool to compute time dependent statistics of radiation intensity is introduced for a turbulent premixed flame. In the absence of high fidelity codes for large eddy simulation or direct numerical simulation of turbulent flames, the utility of STASS for radiation imaging of reacting flows to understand the flame structure is assessed by generating images of infrared radiation in spectral bands dominated by radiation from gas phase carbon dioxide and water vapor using an assumed PDF method. The study elucidates the need for time dependent computation of radiation intensity for validation with experiments and the need for accounting for turbulence radiation interactions for correctly predicting radiation intensity and consequently the flame temperature and NOx in a reacting fluid flow. Comparison of single point statistics of infrared radiation intensity with measurements show that STASS can not only predict the flame structure but also estimate the dynamics of thermochemical scalars in the flame with reasonable accuracy. While a time series is used to generate realizations of thermochemical scalars in the first part of the study, in the second part, instantaneous realizations of resolved scale temperature, CO2 and H2O mole fractions and soot volume fractions are extracted from a large eddy simulation (LES) to carry out quantitative imaging of radiation intensity (QIRI) for a turbulent soot generating ethylene diffusion flame. A primary motivation of the study is to establish QIRI as a computational tool for validation of soot models, especially in the absence of conventional flow field and measured scalar data for sooting flames. Realizations of

  4. On the improvement of two-dimensional curvature computation and its application to turbulent premixed flame correlations

    International Nuclear Information System (INIS)

    Chrystie, R S M; Burns, I S; Hult, J; Kaminski, C F

    2008-01-01

    Measurement of curvature of the flamefront of premixed turbulent flames is important for the validation of numerical models for combustion. In this work, curvature is measured from contours that outline the flamefront, which are generated from laser-induced fluorescence images. The contours are inherently digitized, resulting in pixelation effects that lead to difficulties in computing curvature of the flamefront accurately. A common approach is to fit functions locally to short sections along the flame contour, and this approach is also followed in this work; the method helps smoothen the pixelation before curvature is measured. However, the length and degree of the polynomial, and hence the amount of smoothing, must be correctly set in order to maximize the precision and accuracy of the curvature measurements. Other researchers have applied polynomials of different orders and over different segment lengths to circles of known curvature as a test to determine the appropriate choice of polynomial; it is shown here that this method results in a sub-optimal choice of polynomial function. Here, we determine more suitable polynomial functions through use of a circle whose radius is sinusoidally modulated. We show that this leads to a more consistent and reliable choice for the local polynomial functions fitted to experimental data. A polynomial function thus determined is then applied to flame contour data to measure curvature of experimentally acquired flame contours. The results show that there is an enhancement in local flame speed at sections of the flamefront with a non-zero curvature, and this agrees with numerical models

  5. Investigation of the effects of quarl and initial conditions on swirling non-premixed methane flames: Flow field, temperature, and species distributions

    KAUST Repository

    Elbaz, Ayman M.

    2015-12-19

    Detailed measurements are presented of the turbulent flow field, gas species concentrations and temperature field in a non-premixed methane swirl flame. Attention is given to the effect of the quarl geometry on the flame structure and emission characteristics due to its importance in gas turbine and industrial burner applications. Two different quarls were fitted to the burner exit, one a straight quarl and the other a diverging quarl of 15° half cone angle. Stereoscopic Particle Image Velocimetry (SPIV) was applied to obtain the three components of the instantaneous velocity on a vertical plane immediately downstream of the quarl exit. Temperature and gaseous species measurements were made both inside and downstream of the quarls, using a fine wire thermocouple and sampling probe, respectively. This work provides experimental verification by complementary techniques. The results showed that although the main flame structures were governed by the swirl motion imparted to the air stream, the quarl geometry, fuel loading and air loading also had a significant effect on the flow pattern, turbulence intensity, mixture formation, temperature distribution, emissions and flame stabilization. Particularly, in the case of the straight quarl flame, the flow pattern leads to strong, rapid mixing and reduces the residence time for NO formation within the internal recirculation zone (IRZ). However, for the diverging quarl flames, the recirculation zone is shifted radially outward, and the turbulent interaction between the central fuel jet and the internal recirculation zone IRZ induces another small vortex between these two flow features. Less mixing near the diverging quarl exit is observed, with a higher concentration of NO and CO in the post-combustion zone. The instantaneous flow field for both flames showed the existence of small scale vortical structure near the shear layers which were not apparent in the time averaged flow field. These structures, along with high levels

  6. NOx emissions from high swirl turbulent spray flames with highly oxygenated fuels

    KAUST Repository

    Bohon, Myles

    2013-01-01

    Combustion of fuels with fuel bound oxygen is of interest from both a practical and a fundamental viewpoint. While a great deal of work has been done studying the effect of oxygenated additives in diesel and gasoline engines, much less has been done examining combustion characteristics of fuels with extremely high mass fractions of fuel bound oxygen. This work presents an initial investigation into the very low NOx emissions resulting from the combustion of a model, high oxygen mass fraction fuel. Glycerol was chosen as a model fuel with a fuel bound oxygen mass fraction of 52%, and was compared with emissions measured from diesel combustion at similar conditions in a high swirl turbulent spray flame. This work has shown that high fuel bound oxygen mass fractions allow for combustion at low global equivalence ratios with comparable exhaust gas temperatures due to the significantly lower concentrations of diluting nitrogen. Despite similar exhaust gas temperatures, NOx emissions from glycerol combustion were up to an order of magnitude lower than those measured using diesel fuel. This is shown to be a result not of specific burner geometry, but rather is influenced by the presence of higher oxygen and lower nitrogen concentrations at the flame front inhibiting NOx production. © 2012 The Combustion Institute.

  7. Time-averaged probability density functions of soot nanoparticles along the centerline of a piloted turbulent diffusion flame using a scanning mobility particle sizer

    KAUST Repository

    Chowdhury, Snehaunshu; Boyette, Wesley; Roberts, William L.

    2017-01-01

    In this study, we demonstrate the use of a scanning mobility particle sizer (SMPS) as an effective tool to measure the probability density functions (PDFs) of soot nanoparticles in turbulent flames. Time-averaged soot PDFs necessary for validating

  8. Flame oscillations in tubes with nonslip at the walls

    Energy Technology Data Exchange (ETDEWEB)

    Akkerman, V' yacheslav; Bychkov, Vitaly; Petchenko, Arkady [Institute of Physics, Umeaa University, SE-901 87 Umeaa (Sweden); Eriksson, Lars-Erik [Department of Applied Mechanics, Chalmers University of Technology, 412 96 Goeteborg (Sweden)

    2006-06-15

    A laminar premixed flame front propagating in a two-dimensional tube is considered with nonslip at the walls and with both ends open. The problem of flame propagation is solved using direct numerical simulations of the complete set of hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. As a result, it is shown that flame interaction with the walls leads to the oscillating regime of burning. The oscillations involve variations of the curved flame shape and the velocity of flame propagation. The oscillation parameters depend on the characteristic tube width, which controls the Reynolds number of the flow. In narrow tubes the oscillations are rather weak, while in wider tubes they become stronger with well-pronounced nonlinear effects. The period of oscillations increases for wider tubes, while the average flame length scaled by the tube diameter decreases only slightly with increasing tube width. The average flame length calculated in the present work is in agreement with that obtained in the experiments. Numerical results reduce the gap between the theory of turbulent flames and the experiments on turbulent combustion in tubes. (author)

  9. The Coherent Flame Model for Turbulent Chemical Reactions

    Science.gov (United States)

    1977-01-01

    numerical integration of the resulting differential equations. The model predicts the flame length and superficial comparison with experiments suggest a...value for the single universal constant. The theory correctly predicts the change of flame length with changes in stoich- iometric ratio for the...indicate the X will be some where between 0.1 and 0.5. Figure 13 is presented to show the effect of equivalence ratio, , on the flame length when the

  10. Cellular structure of lean hydrogen flames in microgravity

    Science.gov (United States)

    Patnaik, G.; Kailasanath, K.

    1990-01-01

    Detailed, time-dependent, two-dimensional numerical simulations of premixed laminar flames have been used to study the initiation and subsequent development of cellular structures in lean hydrogen-air flames. The model includes detailed hydrogen-oxygen combustion with 24 elementary reactions of eight reactive species and a nitrogen diluent, molecular diffusion of all species, thermal conduction, viscosity, and convection. This model has been used to study the nonlinear evolution of cellular flame structure and shows that cell splitting, as observed in experiments, can be predicted numerically for sufficiently reactive mixtures. The structures that evolved also resembled the cellular structures observed in experiments. The present study shows that the 'cell-split limit' postulated from experimental observations is an intrinsic property of the mixture and that external factors such as heat losses are not necessary to cause this limit.

  11. Scalar energy fluctuations in Large-Eddy Simulation of turbulent flames: Statistical budgets and mesh quality criterion

    Energy Technology Data Exchange (ETDEWEB)

    Vervisch, Luc; Domingo, Pascale; Lodato, Guido [CORIA - CNRS and INSA de Rouen, Technopole du Madrillet, BP 8, 76801 Saint-Etienne-du-Rouvray (France); Veynante, Denis [EM2C - CNRS and Ecole Centrale Paris, Grande Voie des Vignes, 92295 Chatenay-Malabry (France)

    2010-04-15

    Large-Eddy Simulation (LES) provides space-filtered quantities to compare with measurements, which usually have been obtained using a different filtering operation; hence, numerical and experimental results can be examined side-by-side in a statistical sense only. Instantaneous, space-filtered and statistically time-averaged signals feature different characteristic length-scales, which can be combined in dimensionless ratios. From two canonical manufactured turbulent solutions, a turbulent flame and a passive scalar turbulent mixing layer, the critical values of these ratios under which measured and computed variances (resolved plus sub-grid scale) can be compared without resorting to additional residual terms are first determined. It is shown that actual Direct Numerical Simulation can hardly accommodate a sufficiently large range of length-scales to perform statistical studies of LES filtered reactive scalar-fields energy budget based on sub-grid scale variances; an estimation of the minimum Reynolds number allowing for such DNS studies is given. From these developments, a reliability mesh criterion emerges for scalar LES and scaling for scalar sub-grid scale energy is discussed. (author)

  12. Numerical Investigation of Soot Formation in Non-premixed Flames

    KAUST Repository

    Abdelgadir, Ahmed Gamaleldin

    2017-05-01

    Soot is a carbon particulate formed as a result of the combustion of fossil fuels. Due to the health hazard posed by the carbon particulate, government agencies have applied strict regulations to control soot emissions from road vehicles, airplanes, and industrial plants. Thus, understanding soot formation and evolution is critical. Practical combustion devices operate at high pressure and in the turbulent regime. Elevated pressures and turbulence on soot formation significantly and fundamental understanding of these complex interactions is still poor. In this study, the effects of pressure and turbulence on soot formation and growth are investigated numerically. As the first step, the evolution of the particle size distribution function (PSDF) and soot particles morphology are investigated in turbulent non-premixed flames. A Direct Simulation Monte Carlo (DSMC) code is developed and used. The stochastic reactor describes the evolution of soot in fluid parcels following Lagrangian trajectories in a turbulent flow field. The trajectories are sampled from a Direct Numerical Simulation (DNS) of an n-heptane turbulent non-premixed flame. Although individual trajectories display strong bimodality as in laminar flames, the ensemble-average PSDF possesses only one mode and a broad tail, which implies significant polydispersity induced by turbulence. Secondly, the effect of the flow and mixing fields on soot formation at atmospheric and elevated pressures is investigated in coflow laminar diffusion flames. The experimental observation and the numerical prediction of the spatial distribution are in good agreement. Based on the common scaling methodology of the flames (keeping the Reynolds number constant), the scalar dissipation rate decreases as pressure increases, promoting the formation of PAH species and soot. The decrease of the scalar dissipation rate significantly contributes to soot formation occurring closer to the nozzle and outward on the flames wings as pressure

  13. Log-Normality and Multifractal Analysis of Flame Surface Statistics

    Science.gov (United States)

    Saha, Abhishek; Chaudhuri, Swetaprovo; Law, Chung K.

    2013-11-01

    The turbulent flame surface is typically highly wrinkled and folded at a multitude of scales controlled by various flame properties. It is useful if the information contained in this complex geometry can be projected onto a simpler regular geometry for the use of spectral, wavelet or multifractal analyses. Here we investigate local flame surface statistics of turbulent flame expanding under constant pressure. First the statistics of local length ratio is experimentally obtained from high-speed Mie scattering images. For spherically expanding flame, length ratio on the measurement plane, at predefined equiangular sectors is defined as the ratio of the actual flame length to the length of a circular-arc of radius equal to the average radius of the flame. Assuming isotropic distribution of such flame segments we convolute suitable forms of the length-ratio probability distribution functions (pdfs) to arrive at corresponding area-ratio pdfs. Both the pdfs are found to be near log-normally distributed and shows self-similar behavior with increasing radius. Near log-normality and rather intermittent behavior of the flame-length ratio suggests similarity with dissipation rate quantities which stimulates multifractal analysis. Currently at Indian Institute of Science, India.

  14. Direct numerical simulation of turbulent reacting flows

    Energy Technology Data Exchange (ETDEWEB)

    Chen, J.H. [Sandia National Laboratories, Livermore, CA (United States)

    1993-12-01

    The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.

  15. High-Spatial-Resolution OH PLIF Visualization in a Cavity-Stabilized Ethylene-Air Turbulent Flame

    Science.gov (United States)

    Geipel, Clayton M.; Rockwell, Robert D.; Chelliah, Harsha K.; Cutler, Andrew D.; Spelker, Christopher A.; Hashem, Zeid; Danehy, Paul M.

    2017-01-01

    High-spatial-resolution OH planar laser-induced fluorescence was measured for a premixed ethylene-air turbulent flame in an electrically-heated Mach 2 continuous-flow facility (University of Virginia Supersonic Combustion Facility, Configuration E.) The facility comprised a Mach 2 nozzle, an isolator with flush-wall fuel injectors, a combustor with optical access, and an extender. The flame was anchored at a cavity flameholder with a backward-facing step of height 9 mm. The temperature-insensitive Q1(8) transition of OH was excited using laser light of wavelength 283.55 nm. A spatial filter was used to create a laser sheet approximately 25 microns thick based on full-width at half maximum (FWHM). Extension tubes increased the magnification of an intensified camera system, achieving in-plane resolution of 40 microns based on a 50% modulation transfer function (MTF). The facility was tested with total temperature 1200 K, total pressure 300 kPa, local fuel/air equivalence ratios of approximately 0.4, and local Mach number of approximately 0.73 in the combustor. A test case with reduced total temperature and another with reduced equivalence ratio were also tested. PLIF images were acquired along a streamwise plane bisecting the cavity flameholder, from the backward facing step to 120 mm downstream of the step. The smallest observed features in the flow had width of approximately 110 microns. Flame surface density was calculated for OH PLIF images.

  16. Effect of turbulence on deflagration to detonation transition

    Energy Technology Data Exchange (ETDEWEB)

    Radford, D D; Chan, C K [Atomic Energy of Canada Ltd., Pinawa, MB (Canada). Whiteshell Labs.; Azad, R S [Manitoba Univ., Winnipeg, MB (Canada). Dept. of Mechanical Engineering

    1996-12-31

    The interaction of a turbulent jet and an expanding flame kernel was examined using spark-schlieren photography and piezoelectric pressure transducers. Experiments were performed in a 9 by 9 cm, 4-m-long shock channel. Results show that an expanding flame kernel can be locally, or partially, quenched by flame stretching. The mixing of the hot combustion products, containing reactive species, with the unburnt gas in the turbulent flame-jet, created pockets of sensitized mixture. The subsequent re-ignition of the sensitized mixture could result in a local explosion. In a number of experiments the blast waves produced in the local explosion developed into detonation waves. A local explosion occurred only if there was partial quenching of the flame kernel. Partial quenching occurs when the Karlovitz-Kovaszney factor approaches unity and, therefore, it is possible to establish a set of conditions in terms of turbulent parameters for the transition to detonation. (author). 16 refs., 13 figs.

  17. Effect of turbulence on deflagration to detonation transition

    International Nuclear Information System (INIS)

    Radford, D.D.; Chan, C.K.; Azad, R.S.

    1995-01-01

    The interaction of a turbulent jet and an expanding flame kernel was examined using spark-schlieren photography and piezoelectric pressure transducers. Experiments were performed in a 9 by 9 cm, 4-m-long shock channel. Results show that an expanding flame kernel can be locally, or partially, quenched by flame stretching. The mixing of the hot combustion products, containing reactive species, with the unburnt gas in the turbulent flame-jet, created pockets of sensitized mixture. The subsequent re-ignition of the sensitized mixture could result in a local explosion. In a number of experiments the blast waves produced in the local explosion developed into detonation waves. A local explosion occurred only if there was partial quenching of the flame kernel. Partial quenching occurs when the Karlovitz-Kovaszney factor approaches unity and, therefore, it is possible to establish a set of conditions in terms of turbulent parameters for the transition to detonation. (author). 16 refs., 13 figs

  18. A computational study of soot formation in opposed-flow diffusion flame interacting with vortices

    KAUST Repository

    Selvaraj, Prabhu

    2017-01-05

    The flame-vortex interaction enables the study of basic phenomena that control the coupling between combustion and turbulence. Employing a gas phase reaction mechanism considering polycyclic aromatic hydrocarbons (PAH), a two dimensional counterflow ethylene-air flame is simulated. A reduced mechanism with PAH pathways that includes until coronene and method of moments with interpolative closure (MOMIC) has been employed to calculate the soot characteristics. Interaction of sooting flame with a prescribed decaying random velocity field is being investigated. Counterflow nonpremixed flames at low strain rate sooting conditions are considered. Effects of vortices are studied on the flame structures and its sensitivity on the soot formation characteristics. As the vortex rolls up the flame, integrated soot volume fraction is found to be larger for the air-side vortex. A detailed analysis on the flame structure and its influence on the formation of soot were carried out. The results indicate that the larger PAH species contributes to the soot formation in the airside perturbation regimes, whereas the soot formation is dominated by the soot transport in fuel-side perturbation.

  19. Analysis of jet flames and unignited jets from unintended releases of hydrogen

    Energy Technology Data Exchange (ETDEWEB)

    Houf, W.G.; Evans, G.H.; Schefer, R.W. [Sandia National Laboratories, Livermore, CA 94551-0969 (United States)

    2009-07-15

    A combined experimental and modeling program is being carried out at Sandia National Laboratories to characterize and predict the behavior of unintended hydrogen releases. In the case where the hydrogen leak remains unignited, knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited, a classic turbulent jet flame forms. Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. Depending on the effective diameter of the leak and the tank source pressure, free jet flames can be extensive in length and pose significant radiation and impingement hazard, resulting in consequence distances that are unacceptably large. One possible mitigation strategy to potentially reduce the exposure to jet flames is to incorporate barriers around hydrogen storage equipment. The reasoning is that walls will reduce the extent of unacceptable consequences due to jet releases resulting from accidents involving high-pressure equipment. While reducing the jet extent, the walls may introduce other hazards if not configured properly. The goal of this work is to provide guidance on configuration and placement of these walls to minimize overall hazards using a quantitative risk assessment approach. The program includes detailed CFD calculations of jet flames and unignited jets to predict how hydrogen leaks and jet flames interact with barriers, complemented by an experimental validation program that considers the interaction of jet flames and unignited jets with barriers. As a first step in this work on barrier release interaction the Sandia CFD model has been validated by computing the concentration decay of unignited turbulent free jets and comparing the results with the classic concentration decay laws for turbulent free jets taken from experimental data. Computations for turbulent hydrogen

  20. Modelling of turbulence and combustion for simulation of gas explosions in complex geometries

    Energy Technology Data Exchange (ETDEWEB)

    Arntzen, Bjoern Johan

    1998-12-31

    This thesis analyses and presents new models for turbulent reactive flows for CFD (Computational Fluid Dynamics) simulation of gas explosions in complex geometries like offshore modules. The course of a gas explosion in a complex geometry is largely determined by the development of turbulence and the accompanying increased combustion rate. To be able to model the process it is necessary to use a CFD code as a starting point, provided with a suitable turbulence and combustion model. The modelling and calculations are done in a three-dimensional finite volume CFD code, where complex geometries are represented by a porosity concept, which gives porosity on the grid cell faces, depending on what is inside the cell. The turbulent flow field is modelled with a k-{epsilon} turbulence model. Subgrid models are used for production of turbulence from geometry not fully resolved on the grid. Results from laser doppler anemometry measurements around obstructions in steady and transient flows have been analysed and the turbulence models have been improved to handle transient, subgrid and reactive flows. The combustion is modelled with a burning velocity model and a flame model which incorporates the burning velocity into the code. Two different flame models have been developed: SIF (Simple Interface Flame model), which treats the flame as an interface between reactants and products, and the {beta}-model where the reaction zone is resolved with about three grid cells. The flame normally starts with a quasi laminar burning velocity, due to flame instabilities, modelled as a function of flame radius and laminar burning velocity. As the flow field becomes turbulent, the flame uses a turbulent burning velocity model based on experimental data and dependent on turbulence parameters and laminar burning velocity. The laminar burning velocity is modelled as a function of gas mixture, equivalence ratio, pressure and temperature in reactant. Simulations agree well with experiments. 139

  1. Flame stabilization and mixing characteristics in a Stagnation Point Reverse Flow combustor

    Science.gov (United States)

    Bobba, Mohan K.

    A novel combustor design, referred to as the Stagnation Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures and with low NOx emissions even when the fuel and air are not premixed before entering the combustor. The primary objective of this work is to elucidate the underlying physics behind the excellent stability and emissions performance of the SPRF combustor. The approach is to experimentally characterize velocities, species mixing, heat release and flame structure in an atmospheric pressure SPRF combustor with the help of various optical diagnostic techniques: OH PLIF, chemiluminescence imaging, PIV and Spontaneous Raman Scattering. Results indicate that the combustor is primarily stabilized in a region downstream of the injector that is characterized by low average velocities and high turbulence levels; this is also the region where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product entrainment levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on chemical timescales and the flame structure is illustrated with simple reactor models. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor, and it tends to become more flamelet like with increasing distance from the injector. Fuel-air mixing measurements in case of non-premixed operation indicate that the fuel is shielded from hot products until it is fully mixed with air, providing nearly premixed performance without the safety issues associated with premixing. The reduction in NOx emissions in the SPRF

  2. Turbulent burning rates of methane and methane-hydrogen mixtures

    Energy Technology Data Exchange (ETDEWEB)

    Fairweather, M. [School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom); Ormsby, M.P.; Sheppard, C.G.W. [School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom); Woolley, R. [Department of Mechanical Engineering, University of Sheffield, Sheffield S1 3JD (United Kingdom)

    2009-04-15

    Methane and methane-hydrogen (10%, 20% and 50% hydrogen by volume) mixtures have been ignited in a fan stirred bomb in turbulence and filmed using high speed cine schlieren imaging. Measurements were performed at 0.1 MPa (absolute) and 360 K. A turbulent burning velocity was determined for a range of turbulence velocities and equivalence ratios. Experimental laminar burning velocities and Markstein numbers were also derived. For all fuels the turbulent burning velocity increased with turbulence velocity. The addition of hydrogen generally resulted in increased turbulent and laminar burning velocity and decreased Markstein number. Those flames that were less sensitive to stretch (lower Markstein number) burned faster under turbulent conditions, especially as the turbulence levels were increased, compared to stretch-sensitive (high Markstein number) flames. (author)

  3. Monte-Carlo computation of turbulent premixed methane/air ignition

    Science.gov (United States)

    Carmen, Christina Lieselotte

    The present work describes the results obtained by a time dependent numerical technique that simulates the early flame development of a spark-ignited premixed, lean, gaseous methane/air mixture with the unsteady spherical flame propagating in homogeneous and isotropic turbulence. The algorithm described is based upon a sub-model developed by an international automobile research and manufacturing corporation in order to analyze turbulence conditions within internal combustion engines. Several developments and modifications to the original algorithm have been implemented including a revised chemical reaction scheme and the evaluation and calculation of various turbulent flame properties. Solution of the complete set of Navier-Stokes governing equations for a turbulent reactive flow is avoided by reducing the equations to a single transport equation. The transport equation is derived from the Navier-Stokes equations for a joint probability density function, thus requiring no closure assumptions for the Reynolds stresses. A Monte-Carlo method is also utilized to simulate phenomena represented by the probability density function transport equation by use of the method of fractional steps. Gaussian distributions of fluctuating velocity and fuel concentration are prescribed. Attention is focused on the evaluation of the three primary parameters that influence the initial flame kernel growth-the ignition system characteristics, the mixture composition, and the nature of the flow field. Efforts are concentrated on the effects of moderate to intense turbulence on flames within the distributed reaction zone. Results are presented for lean conditions with the fuel equivalence ratio varying from 0.6 to 0.9. The present computational results, including flame regime analysis and the calculation of various flame speeds, provide excellent agreement with results obtained by other experimental and numerical researchers.

  4. The impact of hydrogen enrichment and bluff-body lip thickness on characteristics of blended propane/hydrogen bluff-body stabilized turbulent diffusion flames

    International Nuclear Information System (INIS)

    Kashir, Babak; Tabejamaat, Sadegh; Jalalatian, Nafiseh

    2015-01-01

    Highlights: • Characteristics of C 3 H 8 –H 2 bluff-body stabilized flames are investigated. • Decreasing the bluff-body lip thickness led into enhanced flame length. • CO mass fraction is increased with reducing hydrogen content in the fuel stream. • Augmenting hydrogen content increased the maximum temperature. • Jet-like zone in propane–hydrogen bluff-body stabilized flames is very unstable. - Abstract: At the beginning of this study, the well-known turbulent bluff-body stabilized diffusion flame of HM1 is simulated by a coupled flamelet/radiation approach. The HM1 flame comprises a CH 4 :H 2 [50:50 Vol.] jet flame at a Reynolds number of 15,800. The results showed reasonable agreement for the flow field and species. Afterwards, the abovementioned approach is employed to investigate the effects of hydrogen addition on bluff-body stabilized flames of propane–hydrogen. Adding hydrogen to the blended fuel of propane/hydrogen shifts the recirculation zone outwards the bluff-body and thus culminates in increased flame length. Besides this, the flame length is predicted to be enhanced with decreasing the lip thickness of the bluff-body configuration. The CO emission level is found to be decreased with hydrogen addition in near-burner and far field regions which might be attributed to the decrease of inflow carbon atoms. The local radiative heat power reveals higher values for fuel blends with decreased contents of hydrogen at the recirculation and jet-like zones. This might be attributed to the increased local heat release rate due to breaking further carbon bonds

  5. Cyber-Based Turbulent Combustion Simulation

    Science.gov (United States)

    2012-02-28

    in flow-field structures between the laminar and turbulent counter-flowing fuel injection is clearly illustrated in figure 1. As a consequence , it...flame thickness by comparing with benchmark of AFRL/RZ ( UNICORN ) suppressing the oscillatory numerical behavior. These improvements in numerical...fraction with the benchmark results of AFRL/RZ. This validating base is generated by the UNICORN program on the finest mesh available and the local

  6. Investigation of non-premixed flame combustion characters in GO2/GH2 shear coaxial injectors using non-intrusive optical diagnostics

    Science.gov (United States)

    Dai, Jian; Yu, NanJia; Cai, GuoBiao

    2015-12-01

    Single-element combustor experiments are conducted for three shear coaxial geometry configuration injectors by using gaseous oxygen and gaseous hydrogen (GO2/GH2) as propellants. During the combustion process, several spatially and timeresolved non-intrusive optical techniques, such as OH planar laser induced fluorescence (PLIF), high speed imaging, and infrared imaging, are simultaneously employed to observe the OH radical concentration distribution, flame fluctuations, and temperature fields. The results demonstrate that the turbulent flow phenomenon of non-premixed flame exhibits a remarkable periodicity, and the mixing ratio becomes a crucial factor to influence the combustion flame length. The high speed and infrared images have a consistent temperature field trend. As for the OH-PLIF images, an intuitionistic local flame structure is revealed by single-shot instantaneous images. Furthermore, the means and standard deviations of OH radical intensity are acquired to provide statistical information regarding the flame, which may be helpful for validation of numerical simulations in future. Parameters of structure configurations, such as impinging angle and oxygen post thickness, play an important role in the reaction zone distribution. Based on a successful flame contour extraction method assembled with non-linear anisotropic diffusive filtering and variational level-set, it is possible to implement a fractal analysis to describe the fractal characteristics of the non-premixed flame contour. As a result, the flame front cannot be regarded as a fractal object. However, this turbulent process presents a self-similarity characteristic.

  7. The PDF method for turbulent combustion

    Science.gov (United States)

    Pope, S. B.

    1991-01-01

    Probability Density Function (PDF) methods provide a means of calculating the properties of turbulent reacting flows. They have been successfully applied to many turbulent flames, including some with finite rate kinetic effects. Here the methods are reviewed with an emphasis on computational issues and their application to turbulent combustion.

  8. The dilution effect on the extinction of wall diffusion flame

    Directory of Open Access Journals (Sweden)

    Ghiti Nadjib

    2014-12-01

    Full Text Available The dynamic process of the interaction between a turbulent jet diffusion methane flame and a lateral wall was experimentally studied. The evolution of the flame temperature field with the Nitrogen dilution of the methane jet flame was examined. The interaction between the diffusion flame and the lateral wall was investigated for different distance between the wall and the central axes of the jet flame. The dilution is found to play the central role in the flame extinction process. The flame response as the lateral wall approaches from infinity and the increasing of the dilution rate make the flame extinction more rapid than the flame without dilution, when the nitrogen dilution rate increase the flame temperature decrease.

  9. Advancing predictive models for particulate formation in turbulent flames via massively parallel direct numerical simulations

    KAUST Repository

    Bisetti, Fabrizio

    2014-07-14

    Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multi-scale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physico-chemical mechanisms and to identify modelling needs. © 2014 The Author(s) Published by the Royal Society.

  10. Experimental characterization of methane inverse diffusion flame

    KAUST Repository

    Elbaz, Ayman M.

    2014-06-26

    This article presents 10-kHz images of OH-PLIF simultaneously with 2-D PIV measurements in an inverse methane diffusion flame. Under a constant fuel flow rate, the central air jet Re was varied, leading to air to fuel velocity ratio, Vr, to vary from 8.3 to 66.5. Starting from Vr = 20.7, the flame is commonly characterized by three distinct zones. The length of the lower fuel entrainment region is inversely proportional to Vr. The flames investigated resemble a string shear layer confining this zone, and converging into the second distinct region, the flame neck zone. The third region is the rest of the flame, which spreads in a jet-like manner. The inverse diffusion flames exhibit varying degrees of partial premixing, depending upon on the velocity ratio Vr, and this region of partial premixing evolves into a well-mixed reaction zone along the flame centerline. The OH distribution correlated with the changes in the mean characteristics of the flow through reduction in the local Reynolds number due to heat release. The existence of a flame suppresses or laminarizes the turbulence at early axial locations and promotes fluctuations at the flame tip for flames with Vr < 49.8. In addition, the flame jet width can be correlated to the OH distribution. In upstream regions of the flames, the breaks in OH are counterbalanced by flame closures and are governed by edge flame propagation. These local extinctions were found to occur at locations where large flow structures were impinging on the flame and are associated with a locally higher strain rate or correlated to the local high strain rates at the flame hole edges without this flow impinging. Another contributor to re-ignition was found to be growing flame kernels. As the flames approach global blow-off, these kernels become the main mechanism for re-ignition further downstream of the flames. At low Vr, laminarization within the early regions of the flame provides an effective shield, preventing the jet flow from

  11. Temperature response of an acoustically forced turbulent lean premixed flame: A quantitative experimental determination

    KAUST Repository

    Chrystie, Robin

    2013-01-02

    Temperature measurements have been taken on an acoustically forced lean premixed turbulent bluff-body stabilized flame. The burner used in this study is a test-bed to investigate thermoacoustic instability in gas-turbine engines at the University of Cambridge. Numerous experiments have been performed on the burner, one of which used two-line OH planar laser induced fluorescence to measure temperature. Here, we employ vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen as an alternative to measure temperature, circumventing the limitations of the former method. The use of nitrogen CARS avoids the problem of probing regions of the flame with low OH concentrations that resulted in erroneous temperature. Such an application of CARS showed that the results from previous efforts were systematically biased up to 47% close to the bluff-body. We also critically review the limitations of CARS used in our experiments, pertaining to spatial resolution and associated biasing further downstream from the bluff-body. Using the more accurate results from this work, more up-to-date computational fluid dynamical (CFD) models of the burner can be validated, with the aim of improved understanding and prediction of thermoacoustic instability in gas turbines. © 2013 Copyright Taylor and Francis Group, LLC.

  12. Temperature response of an acoustically forced turbulent lean premixed flame: A quantitative experimental determination

    KAUST Repository

    Chrystie, Robin; Burns, Iain Stewart; Kaminski, Clemens Friedrich

    2013-01-01

    Temperature measurements have been taken on an acoustically forced lean premixed turbulent bluff-body stabilized flame. The burner used in this study is a test-bed to investigate thermoacoustic instability in gas-turbine engines at the University of Cambridge. Numerous experiments have been performed on the burner, one of which used two-line OH planar laser induced fluorescence to measure temperature. Here, we employ vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen as an alternative to measure temperature, circumventing the limitations of the former method. The use of nitrogen CARS avoids the problem of probing regions of the flame with low OH concentrations that resulted in erroneous temperature. Such an application of CARS showed that the results from previous efforts were systematically biased up to 47% close to the bluff-body. We also critically review the limitations of CARS used in our experiments, pertaining to spatial resolution and associated biasing further downstream from the bluff-body. Using the more accurate results from this work, more up-to-date computational fluid dynamical (CFD) models of the burner can be validated, with the aim of improved understanding and prediction of thermoacoustic instability in gas turbines. © 2013 Copyright Taylor and Francis Group, LLC.

  13. A Burke-Schumann Analysis of Dual-Flame Structure Supported by a Burning Droplet

    Science.gov (United States)

    Nayagam, V.; Dietrich, D.; Williams, F. A.

    2016-01-01

    Droplet combustion experiments carried out onboard the International Space Station (ISS), using pure fuels and fuel mixtures, have shown that quasi-steady burning can be sustained by a non-traditional flame configuration, namely a "cool flame" burning in the "partial-burning" regime where both fuel and oxygen leak through the low-temperature controlled flame-sheet. Recent experiments involving large, bi-component fuel (n-decane and hexanol, 50/50 by volume) droplets at elevated pressures show that the visible, hot flame becomes extremely weak while the burning rate remains relatively high, suggesting the possibility of simultaneous presence of "cool" and "hot" flames of roughly equal importance. The radiant output from these bi-component droplets is relatively high and cannot be accounted for only by the presence of a visible hot-flame. In this analysis we explore the theoretical possibility of a dual-flame structure, where one flame lies close to the droplet surface called the "cool-flame," and other farther away from the droplet surface, termed the "hot-flame." A Burke-Schumann analysis of this dual-structure seems to indicate such flame structures are possible over a narrow range of initial conditions. Theoretical results can be compared against available experimental data for pure and bi-component fuel droplet combustion to test how realistic the model may be.

  14. Validation of a mixture-averaged thermal diffusion model for premixed lean hydrogen flames

    Science.gov (United States)

    Schlup, Jason; Blanquart, Guillaume

    2018-03-01

    The mixture-averaged thermal diffusion model originally proposed by Chapman and Cowling is validated using multiple flame configurations. Simulations using detailed hydrogen chemistry are done on one-, two-, and three-dimensional flames. The analysis spans flat and stretched, steady and unsteady, and laminar and turbulent flames. Quantitative and qualitative results using the thermal diffusion model compare very well with the more complex multicomponent diffusion model. Comparisons are made using flame speeds, surface areas, species profiles, and chemical source terms. Once validated, this model is applied to three-dimensional laminar and turbulent flames. For these cases, thermal diffusion causes an increase in the propagation speed of the flames as well as increased product chemical source terms in regions of high positive curvature. The results illustrate the necessity for including thermal diffusion, and the accuracy and computational efficiency of the mixture-averaged thermal diffusion model.

  15. Turbulent piloted partially-premixed flames with varying levels of O2/N2: stability limits and PDF calculations

    Science.gov (United States)

    Juddoo, Mrinal; Masri, Assaad R.; Pope, Stephen B.

    2011-12-01

    This paper reports measured stability limits and PDF calculations of piloted, turbulent flames of compressed natural gas (CNG) partially-premixed with either pure oxygen, or with varying levels of O2/N2. Stability limits are presented for flames of CNG fuel premixed with up to 20% oxygen as well as CNG-O2-N2 fuel where the O2 content is varied from 8 to 22% by volume. Calculations are presented for (i) Sydney flame B [Masri et al. 1988] which uses pure CNG as well as flames B15 to B25 where the CNG is partially-premixed with 15-25% oxygen by volume, respectively and (ii) Sandia methane-air (1:3 by volume) flame E [Barlow et al. 2005] as well as new flames E15 and E25 that are partially-premixed with 'reconstituted air' where the O2 content in nitrogen is 15 and 25% by volume, respectively. The calculations solve a transported PDF of composition using a particle-based Monte Carlo method and employ the EMST mixing model as well as detailed chemical kinetics. The addition of oxygen to the fuel increases stability, shortens the flames, broadens the reaction zone, and shifts the stoichiometric mixture fraction towards the inner side of the jet. It is found that for pure CNG flames where the reaction zone is narrow (∼0.1 in mixture fraction space), the PDF calculations fail to reproduce the correct level of local extinction on approach to blow-off. A broadening in the reaction zone up to about 0.25 in mixture fraction space is needed for the PDF/EMST approach to be able to capture these finite-rate chemistry effects. It is also found that for the same level of partial premixing, increasing the O2/N2 ratio increases the maximum levels of CO and NO but shifts the peak to richer mixture fractions. Over the range of oxygenation investigated here, stability limits have shown to improve almost linearly with increasing oxygen levels in the fuel and with increasing the contribution of release rate from the pilot.

  16. Multiscale coherent structures in tokamak plasma turbulence

    International Nuclear Information System (INIS)

    Xu, G. S.; Wan, B. N.; Zhang, W.; Yang, Q. W.; Wang, L.; Wen, Y. Z.

    2006-01-01

    A 12-tip poloidal probe array is used on the HT-7 superconducting tokamak [Li, Wan, and Mao, Plasma Phys. Controlled Fusion 42, 135 (2000)] to measure plasma turbulence in the edge region. Some statistical analysis techniques are used to characterize the turbulence structures. It is found that the plasma turbulence is composed of multiscale coherent structures, i.e., turbulent eddies and there is self-similarity in a relative short scale range. The presence of the self-similarity is found due to the structural similarity of these eddies between different scales. These turbulent eddies constitute the basic convection cells, so the self-similar range is just the dominant scale range relevant to transport. The experimental results also indicate that the plasma turbulence is dominated by low-frequency and long-wavelength fluctuation components and its dispersion relation shows typical electron-drift-wave characteristics. Some large-scale coherent structures intermittently burst out and exhibit a very long poloidal extent, even longer than 6 cm. It is found that these large-scale coherent structures are mainly contributed by the low-frequency and long-wavelength fluctuating components and their presence is responsible for the observations of long-range correlations, i.e., the correlation in the scale range much longer than the turbulence decorrelation scale. These experimental observations suggest that the coexistence of multiscale coherent structures results in the self-similar turbulent state

  17. Mixing Characteristics of Strongly-Forced Jet Flames in Crossflow

    Science.gov (United States)

    Marr, Kevin; Clemens, Noel; Ezekoye, Ofodike

    2008-11-01

    The effects of high frequency, large-amplitude forcing on the characteristics of a non-premixed jet flame in crossflow (JFICF) at mean Reynolds numbers of 3,200 and 4,850 are studied experimentally. Harmonic forcing of the jet fuel results in a drastic decrease in flame length and complete suppression of soot luminosity. Visualization by planar laser Mie scattering shows that forced JFICF, similar to forced free or coflow jet flames, are characterized by ejection of high-momentum, deeply penetrating vortical structures. These structures rapidly breakdown and promote intense turbulent mixing in the near region of the jet. The rapid mixing resembles a ``one-step'' process going from a fuel rich state far in the nozzle to a well-mixed, but significantly diluted, state just a few diameters from the jet exit plane. Exhaust gas emissions measurements indicate a decrease in NOx, but increases in CO and unburned hydrocarbons with increasing forcing amplitude. Acetone PLIF measurements are used to investigate the effect of partial-premixing on these emissions findings.

  18. Large eddy simulation of turbulent diffusion flame with hybrid fuel of CH4/H2 in various background conditions

    Science.gov (United States)

    Hong, Sungmin; Lee, Wook; Song, Han Ho; Kang, Seongwon

    2014-11-01

    A turbulent diffusion flame with hybrid fuel of methane and hydrogen is analyzed to investigate the effects of operating conditions on flame shape, rate of fuel consumption and pollutant formation. Various combinations of operating parameter, i.e. hydrogen concentration, background pressure and temperature, are examined in relatively high pressure and temperature conditions that can be found at the end of compression stroke in an internal combustion engine. A flamelet-progress variable approach (FPVA) and a dynamic subgrid scale (SGS) model are used for large eddy simulation (LES). A comparison with previous experiments and simulations in the standard condition shows a good agreement in the statistics of flow fields and chemical compositions, as well as in the resultant trends by similar parametric studies. As a result, the effects of added hydrogen are found to be consistent for most of the chemical species in the range of background pressure and temperature conditions. However, the flow fields of some species such as OH, NO, CO at a higher pressure and temperature state show a behavior different from the standard condition. Finally, hydrogen addition is shown to improve flame stability which is measured by the pressure fluctuations in all the tested conditions.

  19. NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

    Energy Technology Data Exchange (ETDEWEB)

    Moon, Hee Jang [Korea Aerospace University, Goyang (Korea, Republic of); Park, Yang Ho; Yoon, Young Bin [Seoul National University, Seoul (Korea, Republic of)

    2009-06-15

    The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO{sub 2}/H{sub 2}O{sub 2} reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect

  20. NOx emission characteristics in turbulent hydrogen jet flames with coaxial air

    International Nuclear Information System (INIS)

    Moon, Hee Jang; Park, Yang Ho; Yoon, Young Bin

    2009-01-01

    The characteristics of NOx emissions in pure hydrogen nonpremixed jet flames with coaxial air are analyzed numerically for a wide range of coaxial air conditions. Among the models tested in simple nonpremixed jet flame, the one-half power scaling law could be reproduced only by the Model C using the HO 2 /H 2 O 2 reaction, implying the importance of chemical nonequilibrium effect. The flame length is reduced significantly by augmenting coaxial air, and could be represented as a function of the ratio of coaxial air to fuel velocity. Predicted EINOx scaling showed a good concordance with experimental data, and the overall one-half power scaling was observed in coaxial flames with Model C when flame residence time was defined with flame volume instead of a cubic of the flame length. Different level of oxygen mass fraction at the stoichiometric surface was observed as coaxial air was increased. These different levels imply that the coaxial air strengthens the nonequilibrium effect

  1. Blow-out of nonpremixed turbulent jet flames at sub-atmospheric pressures

    KAUST Repository

    Wang, Qiang; Hu, Longhua; Chung, Suk-Ho

    2016-01-01

    Blow-out limits of nonpremixed turbulent jet flames in quiescent air at sub-atmospheric pressures (50–100 kPa) were studied experimentally using propane fuel with nozzle diameters ranging 0.8–4 mm. Results showed that the fuel jet velocity at blow-out limit increased with increasing ambient pressure and nozzle diameter. A Damköhler (Da) number based model was adopted, defined as the ratio of characteristic mixing time and characteristic reaction time, to include the effect of pressure considering the variations in laminar burning velocity and thermal diffusivity with pressure. The critical lift-off height at blow-out, representing a characteristic length scale for mixing, had a linear relationship with the theoretically predicted stoichiometric location along the jet axis, which had a weak dependence on ambient pressure. The characteristic mixing time (critical lift-off height divided by jet velocity) adjusted to the characteristic reaction time such that the critical Damköhler at blow-out conditions maintained a constant value when varying the ambient pressure.

  2. Blow-out of nonpremixed turbulent jet flames at sub-atmospheric pressures

    KAUST Repository

    Wang, Qiang

    2016-12-09

    Blow-out limits of nonpremixed turbulent jet flames in quiescent air at sub-atmospheric pressures (50–100 kPa) were studied experimentally using propane fuel with nozzle diameters ranging 0.8–4 mm. Results showed that the fuel jet velocity at blow-out limit increased with increasing ambient pressure and nozzle diameter. A Damköhler (Da) number based model was adopted, defined as the ratio of characteristic mixing time and characteristic reaction time, to include the effect of pressure considering the variations in laminar burning velocity and thermal diffusivity with pressure. The critical lift-off height at blow-out, representing a characteristic length scale for mixing, had a linear relationship with the theoretically predicted stoichiometric location along the jet axis, which had a weak dependence on ambient pressure. The characteristic mixing time (critical lift-off height divided by jet velocity) adjusted to the characteristic reaction time such that the critical Damköhler at blow-out conditions maintained a constant value when varying the ambient pressure.

  3. Premixed Flames Under Microgravity and Normal Gravity Conditions

    Science.gov (United States)

    Krikunova, Anastasia I.; Son, Eduard E.

    2018-03-01

    Premixed conical CH4-air flames were studied experimentally and numerically under normal straight, reversed gravity conditions and microgravity. Low-gravity experiments were performed in Drop tower. Classical Bunsen-type burner was used to find out features of gravity influence on the combustion processes. Mixture equivalence ratio was varied from 0.8 to 1.3. Wide range of flow velocity allows to study both laminar and weakly turbulized flames. High-speed flame chemoluminescence video-recording was used as diagnostic. The investigations were performed at atmospheric pressure. As results normalized flame height, laminar flame speed were measured, also features of flame instabilities were shown. Low- and high-frequency flame-instabilities (oscillations) have a various nature as velocity fluctuations, preferential diffusion instability, hydrodynamic and Rayleigh-Taylor ones etc., that was explored and demonstrated.

  4. Structure and modeling of turbulence

    International Nuclear Information System (INIS)

    Novikov, E.A.

    1995-01-01

    The open-quotes vortex stringsclose quotes scale l s ∼ LRe -3/10 (L-external scale, Re - Reynolds number) is suggested as a grid scale for the large-eddy simulation. Various aspects of the structure of turbulence and subgrid modeling are described in terms of conditional averaging, Markov processes with dependent increments and infinitely divisible distributions. The major request from the energy, naval, aerospace and environmental engineering communities to the theory of turbulence is to reduce the enormous number of degrees of freedom in turbulent flows to a level manageable by computer simulations. The vast majority of these degrees of freedom is in the small-scale motion. The study of the structure of turbulence provides a basis for subgrid-scale (SGS) models, which are necessary for the large-eddy simulations (LES)

  5. Experimental Investigation of Turbulence-Chemistry Interaction in High-Reynolds-Number Turbulent Partially Premixed Flames

    Science.gov (United States)

    2016-06-23

    AFRL-AFOSR-VA-TR-2016-0277 Experimental Investigation of Turbulence-Chemistry Interaction in High- Reynolds -Number Turbulent Partially Premixed...4. TITLE AND SUBTITLE [U] Experimental investigation of turbulence-chemistry interaction in high- Reynolds -number 5a. CONTRACT NUMBER turbulent...for public release Final Report: Experimental investigation of turbulence-chemistry interaction in high- Reynolds -number turbulent partially premixed

  6. Experimental investigation of the effects of heat release on mixing processes and flow structure in a high-speed subsonic turbulent H{sub 2} jet

    Energy Technology Data Exchange (ETDEWEB)

    Theron, M.; Bellenoue, M. [Laboratoire de Combustion et de Detonique, CNRS UPR 9028, Poitiers (France)

    2006-06-15

    In this paper, we explore the effects of heat release on mixing and flow structure in a high-speed subsonic turbulent H{sub 2} jet in an air coflow. Heat release effects are determined from the comparison of nonreacting and reacting jet behavior, boundary conditions being identical in both cases. Experiments are performed in a wind tunnel specifically designed for this purpose. Planar laser induced fluorescence on OH radicals and on acetone (seeded in the hydrogen jet) are used to characterize the cartography of scalars, and laser Doppler velocimetry is used to characterize velocity profiles in the far field of the H{sub 2} jet. Results show significant effects of heat release on mixing and flow structure, indicating an overall reduction of mixing and entrainment in the reacting jet compared to the nonreacting jet. First, a change is observed in the orientation of coherent structures originating from Kelvin-Helmholtz type instabilities, and responsible for air entrainment within the jet, which appear 'flatter' in the jet flame. Then, the flame length is increased over what would be predicted from the intersection of the mean stoichiometric contour with the centerline of the nonreacting jet. And finally, the longitudinal average velocity decrease along the jet axis is quicker in the nonreacting jet, and nondimensional transverse velocity fluctuations are about half as high in the reacting jet as in the nonreacting jet, indicating a reduction of the turbulence intensity of the flow in this direction in the jet flame. (author)

  7. Experimental and numerical study of temperature fields and flows in flame during the diffusion combustion of certain liquid fuels

    Science.gov (United States)

    Loboda, E. L.; Matvienko, O. V.; Agafontsev, M. V.; Reyno, V. V.

    2017-11-01

    The paper represents experimental studying the pulsations of temperature fields and the structure of a flow in the flame formed during the combustion of certain fuels. Also, the paper provides the mathematical modeling of a flow in the flame formed during the combustion of diesel fuels, as well as the comparison with experimental data and the estimation of the scale for turbulent vortices in flame. The experimental results are in satisfactory agreement with numerical modeling, which confirms the hypothesis of similarity for the pulsations of hydrodynamic and thermodynamic parameters.

  8. Experimental quantification of transient stretch effects from vortices interacting with premixed flames

    Science.gov (United States)

    Danby, Sean James

    The understanding of complex premixed combustion reactions is paramount to the development of new concepts and devices used to increase the overall usefulness and capabilities of current technology. The complex interactions which occur within any modern practical combustion device were studied by isolating a single turbulent scale of the turbulence-chemistry interaction. Methane-air flame equivalence ratios (φ = 0.64, 0.90, and 1.13) were chosen to observe the mild affects of thermo-diffusive stability on the methane-air flame. Nitrogen was used as a diluent to retard the flame speeds of the φ = 0.90, and 1.13 mixtures so that the undisturbed outwardly propagating spherical flame kernel propagation rates, drf/dt, were approximately equal. Five primary propane equivalence ratios were utilized for investigation: φ = 0.69, 0.87, 1.08, 1.32, and 1.49. The choice of equivalence ratio was strategically made so that the φ = 0.69/1.49 and φ = 0.87/1.32 mixtures have the same undiluted flame propagation rate, drf/dt. Therefore, in the undiluted case, there are three flame speeds (in laboratory coordinates, not to be confused with burning velocity) represented by these mixtures. Three vortices were selected to be used in this investigation. The vortex rotational velocities were measured to be 77 cm/s, 266 cm/s and 398 cm/s for the "weak", "medium" and "strong" vortices, respectively. Ignition of the flame occurred in two ways: (1) spark-ignition or (2) laser ignition using an Nd:YAG laser at its second harmonic (lambda = 532 nm) in order to quantify the effect of electrode interference. Accompanying high-speed chemiluminescence imaging measurements, instantaneous pressure measurements were obtained to give a more detailed understanding of the effect of vortex strength on the overall flame speed and heat release rate over an extended time scale and to explore the use of a simple measurement to describe turbulent mixing. Further local flame-vortex interface analysis was

  9. Modelling of local extinction and reignition of the flame

    Energy Technology Data Exchange (ETDEWEB)

    Brink, A.; Kilpinen, P.; Hupa, M. [Aabo Akademi, Turku (Finland); Kjaeldman, L. [VTT Energy, Espoo (Finland); Jaeaeskelaeinen, K. [Imatran Voima Oy, Helsinki (Finland)

    1996-12-31

    The influence of the relations between the chemical time scale and the turbulent time scale on local extinction in turbulent flames has been studied. The results from the numerical investigation of a non-swirling flame in a sudden-expansion combustor was compared with measurements and computations reported in the literature. The turbulence-chemistry interaction was modelled using the Eddy-Dissipation Concept (EDC). In the study, different turbulent time scales were used; the Kolmogorov related time scale proposed in the EDC model and two turbulent time scales related to k/{epsilon}. The chemical time scale has been obtained from a model based on calculations with a comprehensive chemical reaction scheme. The results indicate that the Kolmogorov related time scale of the EDC model is too short to be used as an extinction criterium. The two k/{epsilon} related time scales both resulted in a closer agreement between the numerically obtained and the measured results. The result indicates that the time scale used in the EDC model should be further investigated before confident results from modelling of flows with extinction effects can be obtained. (author)

  10. Modelling of local extinction and reignition of the flame

    Energy Technology Data Exchange (ETDEWEB)

    Brink, A; Kilpinen, P; Hupa, M [Aabo Akademi, Turku (Finland); Kjaeldman, L [VTT Energy, Espoo (Finland); Jaeaeskelaeinen, K [Imatran Voima Oy, Helsinki (Finland)

    1997-12-31

    The influence of the relations between the chemical time scale and the turbulent time scale on local extinction in turbulent flames has been studied. The results from the numerical investigation of a non-swirling flame in a sudden-expansion combustor was compared with measurements and computations reported in the literature. The turbulence-chemistry interaction was modelled using the Eddy-Dissipation Concept (EDC). In the study, different turbulent time scales were used; the Kolmogorov related time scale proposed in the EDC model and two turbulent time scales related to k/{epsilon}. The chemical time scale has been obtained from a model based on calculations with a comprehensive chemical reaction scheme. The results indicate that the Kolmogorov related time scale of the EDC model is too short to be used as an extinction criterium. The two k/{epsilon} related time scales both resulted in a closer agreement between the numerically obtained and the measured results. The result indicates that the time scale used in the EDC model should be further investigated before confident results from modelling of flows with extinction effects can be obtained. (author)

  11. Study of the performance of three micromixing models in transported scalar PDF simulations of a piloted jet diffusion flame ('Delft Flame III')

    Energy Technology Data Exchange (ETDEWEB)

    Merci, Bart [Department of Flow, Heat and Combustion Mechanics, Ghent University-UGent, B-9000 Ghent (Belgium); Roekaerts, Dirk [Department of Multi-Scale Physics, Delft University of Technology, Delft (Netherlands); Naud, Bertrand [CIEMAT, Madrid (Spain)

    2006-02-01

    Numerical simulation results are presented for a turbulent nonpremixed flame with local extinction and reignition. The transported scalar PDF approach is applied to the turbulence-chemistry interaction. The turbulent flow field is obtained with a nonlinear two-equation turbulence model. A C{sub 1} skeletal scheme is used as the chemistry model. The performance of three micromixing models is compared: the interaction by exchange with the mean model (IEM), the modified Curl's coalescence/dispersion model (CD) and the Euclidean minimum spanning tree model (EMST). With the IEM model, global extinction occurs. With the standard value of model constant C{sub f}=2, the CD model yields a lifted flame, unlike the experiments, while with the EMST model the correct flame shape is obtained. However, the conditional variances of the thermochemical quantities are underestimated with the EMST model, due to a lack of local extinction in the simulations. With the CD model, the flame becomes attached when either the value of C{sub f} is increased to 3 or the pilot flame thermal power is increased by a factor of 1.5. With increased value of C{sub f} better results for mixture fraction variance are obtained with both the CD and the EMST model. Lowering the value of C{sub f} leads to better predictions for mean temperature with EMST, but at the cost of stronger overprediction of mixture fraction variance. These trends are explained as a consequence of variance production by macroscopic inhomogeneity and the specific properties of the micromixing models. Local time stepping is applied so that convergence is obtained more quickly. Iteration averaging reduces statistical error so that the limited number of 50 particles per cell is sufficient to obtain accurate results. (author)

  12. Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct

    Energy Technology Data Exchange (ETDEWEB)

    Xiao, Huahua; Wang, Qingsong; He, Xuechao; Sun, Jinhua; Yao, Liyin [State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026 (China)

    2010-02-15

    Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion. (author)

  13. Radiative heat transfer in turbulent combustion systems theory and applications

    CERN Document Server

    Modest, Michael F

    2016-01-01

    This introduction reviews why combustion and radiation are important, as well as the technical challenges posed by radiation. Emphasis is on interactions among turbulence, chemistry and radiation (turbulence-chemistry-radiation interactions – TCRI) in Reynolds-averaged and large-eddy simulations. Subsequent chapters cover: chemically reacting turbulent flows; radiation properties, Reynolds transport equation (RTE) solution methods, and TCRI; radiation effects in laminar flames; TCRI in turbulent flames; and high-pressure combustion systems. This Brief presents integrated approach that includes radiation at the outset, rather than as an afterthought. It stands as the most recent developments in physical modeling, numerical algorithms, and applications collected in one monograph.

  14. Some aspects of numerical analysis of turbulent gaseous and spray combustion

    International Nuclear Information System (INIS)

    Takagi, T.

    1991-01-01

    In this paper numerical calculations and analysis on turbulent non-premixed gaseous and spray combustion are reviewed. Attentions were paid to the turbulent flow and combustion modeling applicable to predicting the flow, mixing and combustion of gaseous fuels and sprays. Some of the computed results of turbulent gaseous non-premixed (diffusion) flames with and without swirl and transient spray combustion were compared with experimental ones to understand the processes in the flame and to assure how the computations predict the experiments

  15. An Optical Study of Processes in Hydrogen Flame in a Tube

    Science.gov (United States)

    2002-07-01

    growth of the hydrogen- flame length with the hydrogen flow rate was observed, whereas for a turbulent hydrogen jet (Reynolds number Re > 104 [5]), the... flame length remained almost constant and varied only weakly with the flow rate of hydrogen. For a subsonic jet flow, flame images display an...There are some data in the literature which show how the diffusive- flame length varies with the rate of hydrogen flow [4, 7]. The length of a

  16. Two- and Three-Dimensional Measurements in Flames

    National Research Council Canada - National Science Library

    Long, Marshall

    1998-01-01

    Three-scalar measurements were made in a turbulent nonpremixed flame (Re=15000) to increase confidence in the two-scalar technique based on simultaneous imaging of Rayleigh scattering and fuel Raman scattering...

  17. Gravity Effects Observed In Partially Premixed Flames

    Science.gov (United States)

    Puri, Ishwar K.; Aggarwal, Suresh K.; Lock, Andrew J.; Gauguly, Ranjan; Hegde, Uday

    2003-01-01

    Partially premixed flames (PPFs) contain a rich premixed fuel air mixture in a pocket or stream, and, for complete combustion to occur, they require the transport of oxidizer from an appropriately oxidizer-rich (or fuel-lean) mixture that is present in another pocket or stream. Partial oxidation reactions occur in fuel-rich portions of the mixture and any remaining unburned fuel and/or intermediate species are consumed in the oxidizer-rich portions. Partial premixing, therefore, represents that condition when the equivalence ratio (phi) in one portion of the flowfield is greater than unity, and in another section its value is less than unity. In general, for combustion to occur efficiently, the global equivalence ratio is in the range fuel-lean to stoichiometric. These flames can be established by design by placing a fuel-rich mixture in contact with a fuel-lean mixture, but they also occur otherwise in many practical systems, which include nonpremixed lifted flames, turbulent nonpremixed combustion, spray flames, and unwanted fires. Other practical applications of PPFs are reported elsewhere. Although extensive experimental studies have been conducted on premixed and nonpremixed flames under microgravity, there is a absence of previous experimental work on burner stabilized PPFs in this regard. Previous numerical studies by our group employing a detailed numerical model showed gravity effects to be significant on the PPF structure. We report on the results of microgravity experiments conducted on two-dimensional (established on a Wolfhard-Parker slot burner) and axisymmetric flames (on a coannular burner) that were investigated in a self-contained multipurpose rig. Thermocouple and radiometer data were also used to characterize the thermal transport in the flame.

  18. Density Effects on Post-shock Turbulence Structure

    Science.gov (United States)

    Tian, Yifeng; Jaberi, Farhad; Livescu, Daniel; Li, Zhaorui; Michigan State University Collaboration; Los Alamos National Laboratory Collaboration; Texas A&M University-Corpus Christi Collaboration

    2017-11-01

    The effects of density variations due to mixture composition on post-shock turbulence structure are studied using turbulence-resolving shock-capturing simulations. This work extends the canonical Shock-Turbulence Interaction (STI) problem to involve significant variable density effects. The numerical method has been verified using a series of grid and LIA convergence tests, and is used to generate accurate post-shock turbulence data for a detailed flow study. Density effects on post-shock turbulent statistics are shown to be significant, leading to an increased amplification of turbulent kinetic energy (TKE). Eulerian and Lagrangian analyses show that the increase in the post-shock correlation between rotation and strain is weakened in the case with significant density variations (referred to as the ``multi-fluid'' case). Similar to previous single-fluid results and LIA predictions, the shock wave significantly changes the topology of the turbulent structures, exhibiting a symmetrization of the joint PDF of second and third invariant of the deviatoric part of velocity gradient tensor. In the multi-fluid case, this trend is more significant and mainly manifested in the heavy fluid regions. Lagrangian data are also used to study the evolution of turbulence structure away from the shock wave and assess the accuracy of Lagrangian dynamical models.

  19. Experimental Study of Hydrogen Addition Effects on a Swirl-Stabilized Methane-Air Flame

    Directory of Open Access Journals (Sweden)

    Mao Li

    2017-11-01

    Full Text Available The effects of H2 addition on a premixed methane-air flame was studied experimentally with a swirl-stabilized gas turbine model combustor. Experiments with 0%, 25%, and 50% H2 molar fraction in the fuel mixture were conducted under atmospheric pressure. The primary objectives are to study the impacts of H2 addition on flame lean blowout (LBO limits, flame shapes and anchored locations, flow field characteristics, precessing vortex core (PVC instability, as well as the CO emission performance. The flame LBO limits were identified by gradually reducing the equivalence ratio until the condition where the flame physically disappeared. The time-averaged CH chemiluminescence was used to reveal the characteristics of flame stabilization, e.g., flame structure and stabilized locations. In addition, the inverse Abel transform was applied to the time-averaged CH results so that the distribution of CH signal on the symmetric plane of the flame was obtained. The particle image velocimetry (PIV was used to detect the characteristics of the flow field with a frequency of 2 kHz. The snapshot method of POD (proper orthogonal decomposition and fast Fourier transform (FFT were adopted to capture the most prominent coherent structures in the turbulent flow field. CO emission was monitored with an exhaust probe that was installed close to the combustor exit. The experimental results indicated that the H2 addition extended the flame LBO limits and the operation range of low CO emission. The influence of H2 addition on the flame shape, location, and flow field was observed. With the assistance of POD and FFT, the combustion suppression impacts on PVC was found.

  20. Comparison of PDF and Moment Closure Methods in the Modeling of Turbulent Reacting Flows

    Science.gov (United States)

    Norris, Andrew T.; Hsu, Andrew T.

    1994-01-01

    In modeling turbulent reactive flows, Probability Density Function (PDF) methods have an advantage over the more traditional moment closure schemes in that the PDF formulation treats the chemical reaction source terms exactly, while moment closure methods are required to model the mean reaction rate. The common model used is the laminar chemistry approximation, where the effects of turbulence on the reaction are assumed negligible. For flows with low turbulence levels and fast chemistry, the difference between the two methods can be expected to be small. However for flows with finite rate chemistry and high turbulence levels, significant errors can be expected in the moment closure method. In this paper, the ability of the PDF method and the moment closure scheme to accurately model a turbulent reacting flow is tested. To accomplish this, both schemes were used to model a CO/H2/N2- air piloted diffusion flame near extinction. Identical thermochemistry, turbulence models, initial conditions and boundary conditions are employed to ensure a consistent comparison can be made. The results of the two methods are compared to experimental data as well as to each other. The comparison reveals that the PDF method provides good agreement with the experimental data, while the moment closure scheme incorrectly shows a broad, laminar-like flame structure.

  1. Development and application of a computer model for large-scale flame acceleration experiments

    International Nuclear Information System (INIS)

    Marx, K.D.

    1987-07-01

    A new computational model for large-scale premixed flames is developed and applied to the simulation of flame acceleration experiments. The primary objective is to circumvent the necessity for resolving turbulent flame fronts; this is imperative because of the relatively coarse computational grids which must be used in engineering calculations. The essence of the model is to artificially thicken the flame by increasing the appropriate diffusivities and decreasing the combustion rate, but to do this in such a way that the burn velocity varies with pressure, temperature, and turbulence intensity according to prespecified phenomenological characteristics. The model is particularly aimed at implementation in computer codes which simulate compressible flows. To this end, it is applied to the two-dimensional simulation of hydrogen-air flame acceleration experiments in which the flame speeds and gas flow velocities attain or exceed the speed of sound in the gas. It is shown that many of the features of the flame trajectories and pressure histories in the experiments are simulated quite well by the model. Using the comparison of experimental and computational results as a guide, some insight is developed into the processes which occur in such experiments. 34 refs., 25 figs., 4 tabs

  2. Simulation of lean premixed turbulent combustion

    International Nuclear Information System (INIS)

    Bell, J; Day, M; Almgren, A; Lijewski, M; Rendleman, C; Cheng, R; Shepherd, I

    2006-01-01

    There is considerable technological interest in developing new fuel-flexible combustion systems that can burn fuels such as hydrogen or syngas. Lean premixed systems have the potential to burn these types of fuels with high efficiency and low NOx emissions due to reduced burnt gas temperatures. Although traditional Scientific approaches based on theory and laboratory experiment have played essential roles in developing our current understanding of premixed combustion, they are unable to meet the challenges of designing fuel-flexible lean premixed combustion devices. Computation, with its ability to deal with complexity and its unlimited access to data, has the potential for addressing these challenges. Realizing this potential requires the ability to perform high fidelity simulations of turbulent lean premixed flames under realistic conditions. In this paper, we examine the specialized mathematical structure of these combustion problems and discuss simulation approaches that exploit this structure. Using these ideas we can dramatically reduce computational cost, making it possible to perform high-fidelity simulations of realistic flames. We illustrate this methodology by considering ultra-lean hydrogen flames and discuss how this type of simulation is changing the way researchers study combustion

  3. Characterisation of the Interaction between Toroidal Vortex Structures and Flame Front Propagation

    International Nuclear Information System (INIS)

    Long, E J; Hargrave, G K; Jarvis, S; Justham, T; Halliwell, N

    2006-01-01

    Experimental laser diagnostic data is presented for flame characterisation during interactions with toroidal vortices generated in the wake of an annular obstacle. A novel twin section combustion chamber has been utilised to allow the controlled formation of stable eddy structures into which a flame front can propagate. High speed laser sheet visualisation was employed to record the flow field and flame front temporal development and high-speed digital particle image velocimetry was used to quantify the velocity field of the unburnt mixture ahead of the flame front. Results provide characterisation of the toroidal vortex/flame front interaction for a range of vortex scales of and recirculation strengths

  4. Characterisation of the Interaction between Toroidal Vortex Structures and Flame Front Propagation

    Energy Technology Data Exchange (ETDEWEB)

    Long, E J; Hargrave, G K; Jarvis, S; Justham, T; Halliwell, N [Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU (United Kingdom)

    2006-07-15

    Experimental laser diagnostic data is presented for flame characterisation during interactions with toroidal vortices generated in the wake of an annular obstacle. A novel twin section combustion chamber has been utilised to allow the controlled formation of stable eddy structures into which a flame front can propagate. High speed laser sheet visualisation was employed to record the flow field and flame front temporal development and high-speed digital particle image velocimetry was used to quantify the velocity field of the unburnt mixture ahead of the flame front. Results provide characterisation of the toroidal vortex/flame front interaction for a range of vortex scales of and recirculation strengths.

  5. Characterisation of the Interaction between Toroidal Vortex Structures and Flame Front Propagation

    Science.gov (United States)

    Long, E. J.; Hargrave, G. K.; Jarvis, S.; Justham, T.; Halliwell, N.

    2006-07-01

    Experimental laser diagnostic data is presented for flame characterisation during interactions with toroidal vortices generated in the wake of an annular obstacle. A novel twin section combustion chamber has been utilised to allow the controlled formation of stable eddy structures into which a flame front can propagate. High speed laser sheet visualisation was employed to record the flow field and flame front temporal development and high-speed digital particle image velocimetry was used to quantify the velocity field of the unburnt mixture ahead of the flame front. Results provide characterisation of the toroidal vortex/flame front interaction for a range of vortex scales of and recirculation strengths.

  6. Analysis of the flamelet concept in the numerical simulation of laminar partially premixed flames

    Energy Technology Data Exchange (ETDEWEB)

    Consul, R.; Oliva, A.; Perez-Segarra, C.D.; Carbonell, D. [Centre Tecnologic de Transferencia de Calor (CTTC), Universitat Politecnica de Catalunya (UPC), Colom 11, E-08222, Terrassa, Barcelona (Spain); de Goey, L.P.H. [Eindhoven University of Technology, Department of Mechanical Engineering, P.O. Box 513, 5600 MB Eindhoven (Netherlands)

    2008-04-15

    The aim of this work is to analyze the application of flamelet models based on the mixture fraction variable and its dissipation rate to the numerical simulation of partially premixed flames. Although the main application of these models is the computation of turbulent flames, this work focuses on the performance of flamelet concept in laminar flame simulations removing, in this way, turbulence closure interactions. A well-known coflow methane/air laminar flame is selected. Five levels of premixing are taken into account from an equivalence ratio {phi}={infinity} (nonpremixed) to {phi}=2.464. Results obtained using the flamelet approaches are compared to data obtained from the detailed solution of the complete transport equations using primitive variables. Numerical simulations of a counterflow flame are also presented to support the discussion of the results. Special emphasis is given to the analysis of the scalar dissipation rate modeling. (author)

  7. Investigation on Flame Characteristics and Burner Operability Issues of Oxy-Fuel Combustion

    Energy Technology Data Exchange (ETDEWEB)

    Choudhuri, Ahsan [Univ. Of Texas, El Paso, TX (United States)

    2013-09-30

    Oxy-fuel combustion has been used previously in a wide range of industrial applications. Oxy- combustion is carried out by burning a hydrocarbon fuel with oxygen instead of air. Flames burning in this configuration achieve higher flame temperatures which present opportunities for significant efficiency improvements and direct capture of CO2 from the exhaust stream. In an effort to better understand and characterize the fundamental flame characteristics of oxy-fuel combustion this research presents the experimental measurements of flame stability of various oxyfuel flames. Effects of H2 concentration, fuel composition, exhaust gas recirculation ratio, firing inputs, and burner diameters on the flame stability of these fuels are discussed. Effects of exhaust gas recirculation i.e. CO2 and H2O (steam) acting as diluents on burner operability are also presented. The roles of firing input on flame stability are then analyzed. For this study it was observed that many oxy-flames did not stabilize without exhaust gas recirculation due to their higher burning velocities. In addition, the stability regime of all compositions was observed to decrease as the burner diameter increased. A flashback model is also presented, using the critical velocity gradient gF) values for CH4-O2-CO2 flames. The second part of the study focuses on the experimental measurements of the flow field characteristics of premixed CH4/21%O2/79%N2 and CH4/38%O2/72%CO2 mixtures at constant firing input of 7.5 kW, constant, equivalence ratio of 0.8, constant swirl number of 0.92 and constant Reynolds Numbers. These measurements were taken in a swirl stabilized combustor at atmospheric pressure. The flow field visualization using Particle Imaging Velocimetry (PIV) technique is implemented to make a better understanding of the turbulence characteristics of

  8. Control of confined nonpremixed flames using a microjet

    International Nuclear Information System (INIS)

    Sinha, Ashok; Ganguly, Ranjan; Puri, Ishwar K.

    2005-01-01

    Industrial burners, such as those used in materials processing furnaces, require precise control over the flame length, width, overall shape and other physical flame attributes. The mechanism used to control the flame topology should be relatively simple, safe, and devoid of an emissions penalty. We have explored the feasibility of hydrodynamic control of confined nonpremixed flames by injecting air through a high-momentum microjet. An innovative strategy for the control of flame shape and luminosity is demonstrated based on a high-momentum coaxial microjet injected along the center of a confined nonpremixed flame burning in a coflowing oxidizer stream. The introduction of the microjet shortens a nonpremixed flame and reduces the amplitude of the buoyancy-induced flickering. For a microjet-assisted flame, the flame length is more sensitive to the fuel flowrate than for laminar or turbulent nonpremixed flames. This provides greater flexibility for the dynamic control of their flame lengths. Measurements of NO x and CO emissions show that the method is robust. Effective flame control without an emissions penalty is possible over a large range of microjet velocities that significantly alter the flame shape. Since the influence of the microjet is primarily of a hydrodynamic nature, inert microjet fluids like recirculated exhaust gas can also be used in practical devices

  9. Control of confined nonpremixed flames using a microjet

    Energy Technology Data Exchange (ETDEWEB)

    Sinha, A.; Puri, I.K. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Engineering Science and Mechanics; Ganguly, R. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Engineering Science and Mechanics; Jadavpur Univ., Calcutta (India). Dept. of Power Engineering

    2005-06-01

    Industrial burners, such as those used in materials processing furnaces, require precise control over the flame length, width, overall shape and other physical flame attributes. The mechanism used to control the flame topology should be relatively simple, safe, and devoid of an emissions penalty. We have explored the feasibility of hydrodynamic control of confined nonpremixed flames by injecting air through a high-momentum microjet. An innovative strategy for the control of flame shape and luminosity is demonstrated based on a high-momentum coaxial microjet injected along the center of a confined nonpremixed flame burning in a coflowing oxidizer stream. The introduction of the microjet shortens a nonpremixed flame and reduces the amplitude of the buoyancy-induced flickering. For a microjet-assisted flame, the flame length is more sensitive to the fuel flowrate than for laminar or turbulent nonpremixed flames. This provides greater flexibility for the dynamic control of their flame lengths. Measurements of NO{sub x} and CO emissions show that the method is robust. Effective flame control without an emissions penalty is possible over a large range of microjet velocities that significantly alter the flame shape. Since the influence of the microjet is primarily of a hydrodynamic nature, inert microjet fluids like recirculated exhaust gas can also be used in practical devices. (Author)

  10. Flame acceleration of hydrogen - air - diluent mixtures at middle scale using ENACCEF: experiments and modelling

    International Nuclear Information System (INIS)

    Fabrice Malet; Nathalie Lamoureux; Nabiha Djebaili-Chaumeix; Claude-Etienne Paillard; Pierre Pailhories; Jean-Pierre L'heriteau; Bernard Chaumont; Ahmed Bentaib

    2005-01-01

    Full text of publication follows: In the case of hypothetic severe accident on light water nuclear reactor, hydrogen would be produced during reactor core degradation and released to the reactor building which could subsequently raise a combustion hazard. A local ignition of the combustible mixture would give birth initially to a slow flame which can be accelerated due to turbulence. Depending on the geometry and the premixed combustible mixture composition, the flame can accelerate and for some conditions transit to detonation or be quenched after a certain distance. The flame acceleration is responsible for the generation of high pressure loads that could damage the reactor's building. Moreover, geometrical configuration is a major factor leading to flame acceleration. Thus, recording experimental data notably on mid-size installations is required for the numeric simulations validation before modelling realistic scales. The ENACCEF vertical facility is a 6 meters high acceleration tube aimed at representing steam generator room leading to containment dome. This setup can be equipped with obstacles of different blockage ratios and shapes in order to obtain an acceleration of the flame. Depending on the geometrical characteristics of these obstacles, different regimes of the flame propagation can be achieved. The mixture composition's influence on flame velocity and acceleration has been investigated. Using a steam physical-like diluent (40% He - 60% CO 2 ), influence of dilution on flame speed and acceleration has been investigated. The flame front has also been recorded with ultra fast ombroscopy visualization, both in the tube and in dome's the entering. The flame propagation is computed using the TONUS code. Based on Euler's equation solving code using structured finite volumes, it includes the CREBCOM flames modelling and simulates the hydrogen/air turbulent flame propagation, taking into account 3D complex geometry and reactants concentration gradients. Since

  11. Chemical Reactions in Turbulent Mixing Flows. Revision.

    Science.gov (United States)

    1983-08-02

    jet diameter F2 fluorine H2 hydrogen HF hydrogen fluoride I(y) instantaneous fluorescence intensity distribution L-s flame length measured from...virtual origin -.4 of turbulent region (L-s). flame length at high Reynolds number LIF laser induced fluorescence N2 nitrogen PI product thickness (defined...mixing is attained as a function of the equivallence ratio. For small values of the equivalence ratio f, the flame length - defined here as the

  12. Effect of flame-tube head structure on combustion chamber performance

    Science.gov (United States)

    Gu, Minqqi

    1986-01-01

    The experimental combustion performance of a premixed, pilot-type flame tube with various head structures is discussed. The test study covers an extensive area: efficiency of the combustion chamber, quality of the outlet temperature field, limit of the fuel-lean blowout, ignition performance at ground starting, and carbon deposition. As a result of these tests, a nozzle was found which fits the premixed pilot flame tube well. The use of this nozzle optimized the performance of the combustion chamber. The tested models had premixed pilot chambers with two types of air-film-cooling structures, six types of venturi-tube structures, and secondary fuel nozzles with two small spray-cone angles.

  13. Ignition of turbulent swirling n-heptane spray flames using single and multiple sparks

    Energy Technology Data Exchange (ETDEWEB)

    Marchionea, T.; Ahmeda, S.F.; Mastorakos, E. [Department of Engineering, University of Cambridge (United Kingdom)

    2009-01-15

    This paper examines ignition processes of an n-heptane spray in a flow typical of a liquid-fuelled burner. The spray is created by a hollow-cone pressure atomiser placed in the centre of a bluff body, around which swirling air induces a strong recirculation zone. Ignition was achieved by single small sparks of short duration (2 mm; 0.5 ms), located at various places inside the flow so as to identify the most ignitable regions, or larger sparks of longer duration (5 mm; 8 ms) repeated at 100 Hz, located close to the combustion chamber enclosure so as to mimic the placement and characteristics of a gas turbine combustor surface igniter. The air and droplet velocities, the droplet diameter, and the total (i.e. liquid plus vapour) equivalence ratio were measured in inert flow by phase Doppler anemometry and sampling respectively. Fast camera imaging suggested that successful ignition events were associated with flamelets that propagated back towards the spray nozzle. Measurements of ignition probability with the single spark showed that localised ignition inside the spray is more likely to result in successful flame establishment when the spark is located in a region of negative velocity, relatively small droplet Sauter mean diameter, and mean equivalence ratio within the flammability limits. Ignition with the single spark was not possible at the location where the multiple spark experiments were performed. For those, the multiple spark sequence lasted approximately 1 to 5 s. It was found that a long spark sequence increases the ignition efficiency, which reached a maximum of 100% at the axial distance where the recirculation zone had maximum width. Ignition was not feasible with the spark downstream of about two burner diameters. Visualisation showed that small flame kernels emanate very often from the spark, which can be stretched as far as 20 mm from the electrodes by the turbulent velocity fluctuations. These kernels survive very little time. Successful overall

  14. Computational Analysis of Spray Jet Flames

    Science.gov (United States)

    Jain, Utsav

    There is a boost in the utilization of renewable sources of energy but because of high energy density applications, combustion will never be obsolete. Spray combustion is a type of multiphase combustion which has tremendous engineering applications in different fields, varying from energy conversion devices to rocket propulsion system. Developing accurate computational models for turbulent spray combustion is vital for improving the design of combustors and making them energy efficient. Flamelet models have been extensively used for gas phase combustion because of their relatively low computational cost to model the turbulence-chemistry interaction using a low dimensional manifold approach. This framework is designed for gas phase non-premixed combustion and its implementation is not very straight forward for multiphase and multi-regime combustion such as spray combustion. This is because of the use of a conserved scalar and various flamelet related assumptions. Mixture fraction has been popularly employed as a conserved scalar and hence used to parameterize the characteristics of gaseous flamelets. However, for spray combustion, the mixture fraction is not monotonic and does not give a unique mapping in order to parameterize the structure of spray flames. In order to develop a flamelet type model for spray flames, a new variable called the mixing variable is introduced which acts as an ideal conserved scalar and takes into account the convection and evaporation of fuel droplets. In addition to the conserved scalar, it has been observed that though gaseous flamelets can be characterized by the conserved scalar and its dissipation, this might not be true for spray flamelets. Droplet dynamics has a significant influence on the spray flamelet and because of effects such as flame penetration of droplets and oscillation of droplets across the stagnation plane, it becomes important to accommodate their influence in the flamelet formulation. In order to recognize the

  15. Direct Numerical Simulation of Turbulent Multi-Stage Autoignition Relevant to Engine Conditions

    Science.gov (United States)

    Chen, Jacqueline

    2017-11-01

    Due to the unrivaled energy density of liquid hydrocarbon fuels combustion will continue to provide over 80% of the world's energy for at least the next fifty years. Hence, combustion needs to be understood and controlled to optimize combustion systems for efficiency to prevent further climate change, to reduce emissions and to ensure U.S. energy security. In this talk I will discuss recent progress in direct numerical simulations of turbulent combustion focused on providing fundamental insights into key `turbulence-chemistry' interactions that underpin the development of next generation fuel efficient, fuel flexible engines for transportation and power generation. Petascale direct numerical simulation (DNS) of multi-stage mixed-mode turbulent combustion in canonical configurations have elucidated key physics that govern autoignition and flame stabilization in engines and provide benchmark data for combustion model development under the conditions of advanced engines which operate near combustion limits to maximize efficiency and minimize emissions. Mixed-mode combustion refers to premixed or partially-premixed flames propagating into stratified autoignitive mixtures. Multi-stage ignition refers to hydrocarbon fuels with negative temperature coefficient behavior that undergo sequential low- and high-temperature autoignition. Key issues that will be discussed include: 1) the role of mixing in shear driven turbulence on the dynamics of multi-stage autoignition and cool flame propagation in diesel environments, 2) the role of thermal and composition stratification on the evolution of the balance of mixed combustion modes - flame propagation versus spontaneous ignition - which determines the overall combustion rate in autoignition processes, and 3) the role of cool flames on lifted flame stabilization. Finally prospects for DNS of turbulent combustion at the exascale will be discussed in the context of anticipated heterogeneous machine architectures. sponsored by DOE

  16. Effects of non-unity Lewis number of gas-phase species in turbulent nonpremixed sooting flames

    KAUST Repository

    Attili, Antonio

    2016-02-13

    Turbulence statistics from two three-dimensional direct numerical simulations of planar n-heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20-30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors\\' concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers. © 2016 The Combustion Institute.

  17. Effects of non-unity Lewis number of gas-phase species in turbulent nonpremixed sooting flames

    KAUST Repository

    Attili, Antonio; Bisetti, Fabrizio; Mueller, Michael E.; Pitsch, Heinz

    2016-01-01

    Turbulence statistics from two three-dimensional direct numerical simulations of planar n-heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20-30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors' concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers. © 2016 The Combustion Institute.

  18. Conditional Moment Closure Modelling of a Lifted H2/N2 Turbulent Jet Flame Using the Presumed Mapping Function Approach

    Directory of Open Access Journals (Sweden)

    Ahmad El Sayed

    2015-01-01

    Full Text Available A lifted hydrogen/nitrogen turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure (CMC supplemented by the presumed mapping function (PMF approach for the modelling of conditional mixing and velocity statistics. Using a prescribed reference field, the PMF approach yields a presumed probability density function (PDF for the mixture fraction, which is then used in closing the conditional scalar dissipation rate (CSDR and conditional velocity in a fully consistent manner. These closures are applied to a lifted flame and the findings are compared to previous results obtained using β-PDF-based closures over a range of coflow temperatures (Tc. The PMF results are in line with those of the β-PDF and compare well to measurements. The transport budgets in mixture fraction and physical spaces and the radical history ahead of the stabilisation height indicate that the stabilisation mechanism is susceptible to Tc. As in the previous β-PDF calculations, autoignition around the “most reactive” mixture fraction remains the controlling mechanism for sufficiently high Tc. Departure from the β-PDF predictions is observed when Tc is decreased as PMF predicts stabilisation by means of premixed flame propagation. This conclusion is based on the observation that lean mixtures are heated by downstream burning mixtures in a preheat zone developing ahead of the stabilization height. The spurious sources, which stem from inconsistent CSDR modelling, are further investigated. The findings reveal that their effect is small but nonnegligible, most notably within the flame zone.

  19. Coherent Structures and Intermittency in Plasma Turbulence

    International Nuclear Information System (INIS)

    Das, Amita; Kaw, Predhiman; Sen, Abhijit

    2008-01-01

    The paper discusses some fundamental issues related to the phenomenon of intermittency in plasma turbulence with particular reference to experimental observations in fusion devices. Intermittency is typically associated with the presence of coherent structures in turbulence. Since coherent structures can play an important role in governing the transport properties of a system they have received a great deal of attention in fusion research. We review some of the experimental measurements and numerical simulation studies on the presence and formation of coherent structures in plasmas and discuss their relevance to intermittency. Intermittency, as widely discussed in the context of neutral fluid turbulence, implies multiscaling behaviour in contrast to self-similar scaling patterns observed in self organized criticality (SOC) phenomenon. The experimental evidence from plasma turbulence measurements reveal a mixed picture--while some observations support the SOC model description others indicate the presence of multiscaling behaviour. We discuss these results in the light of our present understanding of plasma turbulence and in terms of certain unique aspects of intermittency as revealed by fluid models of plasmas.

  20. Heat release effects on mixing scales of non-premixed turbulent wall-jets: A direct numerical simulation study

    International Nuclear Information System (INIS)

    Pouransari, Zeinab; Vervisch, Luc; Johansson, Arne V.

    2013-01-01

    Highlights: ► A non-premixed turbulent flame close to a solid surface is studied using DNS. ► Heat release effects delay transition and enlarge fluctuation of density and pressure. ► The fine-scale structures damped and surface wrinkling diminished due to heat-release. ► Using semilocal scaling improves the collapse of turbulence statistic in inner region. ► There are regions of the flame where considerable (up to 10%) premixed burning occurs. -- Abstract: The present study concerns the role of heat release effects on characteristics mixing scales of turbulence in reacting wall-jet flows. Direct numerical simulations of exothermic reacting turbulent wall-jets are performed and compared to the isothermal reacting case. An evaluation of the heat-release effects on the structure of turbulence is given by examining the mixture fraction surface characteristics, diagnosing vortices and exploring the dissipation rate of the fuel and passive scalar concentrations, and moreover by illustration of probability density functions of reacting species and scatter plots of the local temperature against the mixture fraction. Primarily, heat release effects delay the transition, enlarge the fluctuation intensities of density and pressure and also enhance the fluctuation level of the species concentrations. However, it has a damping effect on all velocity fluctuation intensities and the Reynolds shear stress. A key result is that the fine-scale structures of turbulence are damped, the surface wrinkling is diminished and the vortices become larger due to heat-release effects. Taking into account the varying density by using semi-local scaling improves the collapse of the turbulence statistics in the inner region, but does not eliminate heat release induced differences in the outer region. Examining the two-dimensional premultiplied spanwise spectra of the streamwise velocity fluctuations indicates a shifting in the positions of the outer peaks, associated with large

  1. Correspondence Between “Stable” Flame Macrostructure and Thermo-acoustic Instability in Premixed Swirl-Stabilized Turbulent Combustion

    KAUST Repository

    Taamallah, Soufien; LaBry, Zachary A.; Shanbhogue, Santosh J.; Habib, Mohamed A. M.; Ghoniem, Ahmed F.

    2014-01-01

    Copyright © 2015 by ASME. In this paper, we conduct an experimental investigation to study the link between the flame macroscale structure - or flame brush spatial distribution - and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. We operate the combustor with premixed methane-air in the range of equivalence ratio (Φ) from the lean blowout limit to Φ = 0. 75. First, we observe the different dynamic modes in this lean range as Φ is raised. We also document the effect of Φ on the flame macrostructure. Next, we examine the correspondence between dynamic mode transitions and changes in flame macrostructure. To do so, we modify the combustor length - by downstream truncation - without changing the underlying flow upstream. Thus, the resonant frequencies of the geometry are altered allowing for decoupling the heat release rate fluctuations and the acoustic feedback. Mean flame configurations in the modified combustor and for the same range of equivalence ratio are examined, following the same experimental protocol. It is found that not only the same sequence of flame macrostructures is observed in both combustors but also that the transitions occur at a similar set of equivalence ratio. In particular, the appearance of the flame in the outside recirculation zone (ORZ) in the long combustor - which occurs simultaneously with the onset of instability at the fundamental frequency - happens at similar Φ when compared to the short combustor, but without being in latter case accompanied by a transition to thermo-acoustic instability. Then, we interrogate the flow field by analyzing the streamlines, mean, and rms velocities for the nonreacting flow and the different flame types. Finally, we focus on the transition of the flame to the ORZ in the acoustically decoupled case. Our analysis of this transition shows that it occurs gradually with an intermittent appearance of a flame in the ORZ and an increasing probability with Φ. The spectral

  2. Correspondence Between “Stable” Flame Macrostructure and Thermo-acoustic Instability in Premixed Swirl-Stabilized Turbulent Combustion

    KAUST Repository

    Taamallah, Soufien

    2014-12-23

    Copyright © 2015 by ASME. In this paper, we conduct an experimental investigation to study the link between the flame macroscale structure - or flame brush spatial distribution - and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. We operate the combustor with premixed methane-air in the range of equivalence ratio (Φ) from the lean blowout limit to Φ = 0. 75. First, we observe the different dynamic modes in this lean range as Φ is raised. We also document the effect of Φ on the flame macrostructure. Next, we examine the correspondence between dynamic mode transitions and changes in flame macrostructure. To do so, we modify the combustor length - by downstream truncation - without changing the underlying flow upstream. Thus, the resonant frequencies of the geometry are altered allowing for decoupling the heat release rate fluctuations and the acoustic feedback. Mean flame configurations in the modified combustor and for the same range of equivalence ratio are examined, following the same experimental protocol. It is found that not only the same sequence of flame macrostructures is observed in both combustors but also that the transitions occur at a similar set of equivalence ratio. In particular, the appearance of the flame in the outside recirculation zone (ORZ) in the long combustor - which occurs simultaneously with the onset of instability at the fundamental frequency - happens at similar Φ when compared to the short combustor, but without being in latter case accompanied by a transition to thermo-acoustic instability. Then, we interrogate the flow field by analyzing the streamlines, mean, and rms velocities for the nonreacting flow and the different flame types. Finally, we focus on the transition of the flame to the ORZ in the acoustically decoupled case. Our analysis of this transition shows that it occurs gradually with an intermittent appearance of a flame in the ORZ and an increasing probability with Φ. The spectral

  3. Interaction between combustion and turbulence in modelling of emissions; Palamisen ja turbulenssin vuorovaikutus paeaestoejen mallinnuksessa

    Energy Technology Data Exchange (ETDEWEB)

    Oksanen, A.; Maeki-Mantila, E. [Tampere Univ. of Technology (Finland). Inst. of Energy and Process Technology

    1997-10-01

    The aim of the project has been to model and simulate gas phase combustion taking into account the interaction between the chemical reactions and turbulence, respectively. Especially the modelling of nitric oxide and carbon monoxide were included in the computations which were applied into two laboratory-scale test cases namely into the about 300 kW natural gas burner by International Flame Research Foundation and into the smaller natural gas jet flame by delft University of Technology. Both test cases were calculated in two dimensional axially symmetric chambers with the swirl numbers equal to 0.56 and zero in the IFRF and Delft flames, respectively. In this study it was necessary to take into account as well as possible the effect of turbulence on the chemical reactions. Therefore, the Eddy Dissipation Concept Model (EDC) together with the local extinction was chosen to describe both the combustion reactions of methane and carbon monoxide and the formation and reduction of nitric oxide, too. In this study two different turbulent time scales were used namely the Kolmogorov time scale in the fine structure conditions without and with the factor taking more into account the fine structure conditions, respectively. It can be noticed the computational results are more similar with the experimental data when the factor was used. The prediction of chemical time scale was based on the principle by Gran et Melaaen and Magnussen. (orig.)

  4. Coherent structures in two-dimensional plasma turbulence

    DEFF Research Database (Denmark)

    Huld, T.; Nielsen, A.H.; Pécseli, H.L.

    1991-01-01

    -band turbulent fluctuations is demonstrated by a conditional sampling technique. Depending on plasma parameters, the dominant structures can appear as monopole or multipole vortices, dipole vortices in particular. The importance of large structures for the turbulent plasma diffusion is discussed. A statistical...... analysis of the randomly varying plasma flux is presented....

  5. Application of foam-extend on turbulent fluid-structure interaction

    Science.gov (United States)

    Rege, K.; Hjertager, B. H.

    2017-12-01

    Turbulent flow around flexible structures is likely to induce structural vibrations which may eventually lead to fatigue failure. In order to assess the fatigue life of these structures, it is necessary to take the action of the flow on the structure into account, but also the influence of the vibrating structure on the fluid flow. This is achieved by performing fluid-structure interaction (FSI) simulations. In this work, we have investigated the capability of a FSI toolkit for the finite volume computational fluid dynamics software foam-extend to simulate turbulence-induced vibrations of a flexible structure. A large-eddy simulation (LES) turbulence model has been implemented to a basic FSI problem of a flexible wall which is placed in a confined, turbulent flow. This problem was simulated for 2.32 seconds. This short simulation required over 200 computation hours, using 20 processor cores. Thereby, it has been shown that the simulation of FSI with LES is possible, but also computationally demanding. In order to make turbulent FSI simulations with foam-extend more applicable, more sophisticated turbulence models and/or faster FSI iteration schemes should be applied.

  6. Aerodynamic characteristics and thermal structure of nonpremixed reacting swirling wakes at low Reynolds numbers

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Rong F. [Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei (China); Yen, Shun C. [Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung (China)

    2008-12-15

    The aerodynamic characteristics and thermal structure of uncontrolled and controlled swirling double-concentric jet flames at low Reynolds numbers are experimentally studied. The swirl and Reynolds numbers are lower than 0.6 and 2000, respectively. The flow characteristics are diagnosed by the laser-light-sheet-assisted Mie scattering flow visualization method and particle image velocimetry (PIV). The thermal structure is measured by a fine-wire thermocouple. The flame shapes, combined images of flame and flow, velocity vector maps, streamline patterns, velocity and turbulence distributions, flame lengths, and temperature distributions are discussed. The flow patterns of the no-control case exhibit an open-top, single-ring vortex sitting on the blockage disc with a jetlike swirling flow evolving from the central disc face toward the downstream area. The rotation direction and size of the near-disc vortex, as well as the flow properties, change in different ranges of annulus swirl number and therefore induce three characteristic flame modes: weak swirling flame, lifted flame, and turbulent reattached flame. Because the near-disc vortex is open-top, the radial dispersion of the fuel-jet fluids is not significantly enhanced by the annulus swirling flow. The flows of the reacting swirling double-concentric jets at such low swirl and Reynolds numbers therefore present characteristics of diffusion jet flames. In the controlled case, the axial momentum of the central fuel jet is deflected radially by a control disc placed above the blockage disc. This arrangement can induce a large near-disc recirculation bubble and high turbulence intensities. The enhanced mixing hence tremendously shortens the flame length and enlarges the flame width. (author)

  7. Symposium on Turbulent Shear Flows, 6th, Universite de Toulouse III, France, Sept. 7-9, 1987, Proceedings

    International Nuclear Information System (INIS)

    Anon.

    1987-01-01

    This symposium includes topics on wall flows, unsteady flows, scalar and buoyant transport, instrumentation and techniques, combustion, aerodynamic flows, free flows, geophysical flows, complex flows, separated flows, coherent structures, closures, numerical simulation, and two-phase flows. Papers are presented on the effect of favorable pressure gradients on turbulent boundary layers, the models of hydrodynamic resonances in separated shear flows, the transport of passive scalars in a turbulent channel flow, a pulsed hot-wire probe for near-wall measurements, and vortex dynamics in diffusion flames. Consideration is also given to time-dependent structure in wing-body junction flows, bifurcating air jets at high subsonic speeds, the wake of an axisymmetric body with or without tail separation, coherent structures in quasi-geostrophic jets, and separated flow predictions using a new turbulence model. Additional papers are on stochastic estimation of organized structures in turbulent channel flow, a comparative study of eleven models of turbulence, and a numerical study of a stably stratified mixing layer

  8. Structure of Unsteady Partially Premixed Flames and the Existence of State Relationships

    Directory of Open Access Journals (Sweden)

    Suresh K. Aggarwal

    2009-09-01

    Full Text Available In this study, we examine the structure and existence of state relationships in unsteady partially premixed flames (PPFs subjected to buoyancy-induced and external perturbations. A detailed numerical model is employed to simulate the steady and unsteady two-dimensional PPFs established using a slot burner under normal and zero-gravity conditions. The coflow velocity is parametrically varied. The methane-air chemistry is modeled using a fairly detailed mechanism that contains 81 elementary reactions and 24 species. Validation of the computational model is provided through comparisons of predictions with nonintrusive measurements. The combustion proceeds in two reaction zones, one a rich premixed zone and the other a nonpremixed zone. These reaction zones are spatially separated, but involve strong interactions between them due to thermochemistry and scalar transport. The fuel is mostly consumed in the premixed zone to produce CO and H2, which are transported to and consumed in the nonpremixed zone. The nonpremixed zone in turn provides heat and H-atoms to the premixed zone. For the range of conditions investigated, the zero-g partially premixed flames exhibit a stable behavior and a remarkably strong resistance to perturbations. In contrast, the corresponding normal-gravity flames exhibit oscillatory behavior at low coflow velocities but a stable behavior at high coflow velocities, and the behavior can be explained in terms of a global and convective instabilities. The effects of coflow and gravity on the flames are characterized through a parameter VR, defined as the ratio of coflow velocity to jet velocity. For VR ≤ 1 (low coflow velocity regime, the structures of both 0- and 1-g flames are strongly sensitive to changes in VR, while they are only mildly affected by coflow in the high coflow velocity regime (VR > 1. In addition, the spatio-temporal characteristics of the 0- and 1-g flames are markedly different in the first regime, but are

  9. Design of a High Intensity Turbulent Combustion System

    Science.gov (United States)

    2015-05-01

    mixing of the reactants in chemical reactors, boilers , furnaces and mixing of fuel and air in engines take place in turbulent flow. One of the most...determining flame speed. When a tube containing combustible mixture, the flame speed is the propagation speed of the flame front towards the unburned...stress criteria is effective when the material is ductile in nature. This stress theory is developed from the 44 ’distortion energy failure theory

  10. Conical quarl swirl stabilized non-premixed flames: flame and flow field interaction

    KAUST Repository

    Elbaz, Ayman M.; Roberts, William L.

    2017-01-01

    The flame-flow field interaction is studied in non-premixed methane swirl flames stabilized in quartz quarl via simultaneous measurements of the flow field using a stereo PIV and OH-PLIF at 5 KHz repetition rate. Under the same swirl intensity, two flames with different fuel jet velocity were investigated. The time-averaged flow field shows a unique flow pattern at the quarl exit, where two recirculation vortices are formed; a strong recirculation zone formed far from the quarl exit and a larger recirculation zone extending inside the quarl. However, the instantaneous images show that, the flow pattern near the quarl exit plays a vital role in the spatial location and structure of the reaction zone. In the low fuel jet velocity flame, a pair of vortical structures, located precisely at the corners of the quarl exit, cause the flame to roll up into the central region of low speed flow, where the flame sheet then tracks the axial velocity fluctuations. The vorticity field reveals a vortical structure surrounding the reaction zones, which reside on a layer of low compressive strain adjacent to that vortical structure. In the high fuel jet velocity flame, initially a laminar flame sheet resides at the inner shear layer of the main jet, along the interface between incoming fresh gas and high temperature recirculating gas. Further downstream, vortex breakdown alters the flame sheet path toward the central flame region. The lower reaction zones show good correlation to the regions of maximum vorticity and track the regions of low compressive strain associated with the inner shear layer of the jet flow. In both flames the reactions zones conform the passage of the large structure while remaining inside the low speed regions or at the inner shear layer.

  11. Conical quarl swirl stabilized non-premixed flames: flame and flow field interaction

    KAUST Repository

    Elbaz, Ayman M.

    2017-09-19

    The flame-flow field interaction is studied in non-premixed methane swirl flames stabilized in quartz quarl via simultaneous measurements of the flow field using a stereo PIV and OH-PLIF at 5 KHz repetition rate. Under the same swirl intensity, two flames with different fuel jet velocity were investigated. The time-averaged flow field shows a unique flow pattern at the quarl exit, where two recirculation vortices are formed; a strong recirculation zone formed far from the quarl exit and a larger recirculation zone extending inside the quarl. However, the instantaneous images show that, the flow pattern near the quarl exit plays a vital role in the spatial location and structure of the reaction zone. In the low fuel jet velocity flame, a pair of vortical structures, located precisely at the corners of the quarl exit, cause the flame to roll up into the central region of low speed flow, where the flame sheet then tracks the axial velocity fluctuations. The vorticity field reveals a vortical structure surrounding the reaction zones, which reside on a layer of low compressive strain adjacent to that vortical structure. In the high fuel jet velocity flame, initially a laminar flame sheet resides at the inner shear layer of the main jet, along the interface between incoming fresh gas and high temperature recirculating gas. Further downstream, vortex breakdown alters the flame sheet path toward the central flame region. The lower reaction zones show good correlation to the regions of maximum vorticity and track the regions of low compressive strain associated with the inner shear layer of the jet flow. In both flames the reactions zones conform the passage of the large structure while remaining inside the low speed regions or at the inner shear layer.

  12. Dissipative structures in magnetorotational turbulence

    Science.gov (United States)

    Ross, Johnathan; Latter, Henrik N.

    2018-03-01

    Via the process of accretion, magnetorotational turbulence removes energy from a disk's orbital motion and transforms it into heat. Turbulent heating is far from uniform and is usually concentrated in small regions of intense dissipation, characterised by abrupt magnetic reconnection and higher temperatures. These regions are of interest because they might generate non-thermal emission, in the form of flares and energetic particles, or thermally process solids in protoplanetary disks. Moreover, the nature of the dissipation bears on the fundamental dynamics of the magnetorotational instability (MRI) itself: local simulations indicate that the large-scale properties of the turbulence (e.g. saturation levels, the stress-pressure relationship) depend on the short dissipative scales. In this paper we undertake a numerical study of how the MRI dissipates and the small-scale dissipative structures it employs to do so. We use the Godunov code RAMSES and unstratified compressible shearing boxes. Our simulations reveal that dissipation is concentrated in ribbons of strong magnetic reconnection that are significantly elongated in azimuth, up to a scale height. Dissipative structures are hence meso-scale objects, and potentially provide a route by which large scales and small scales interact. We go on to show how these ribbons evolve over time — forming, merging, breaking apart, and disappearing. Finally, we reveal important couplings between the large-scale density waves generated by the MRI and the small-scale structures, which may illuminate the stress-pressure relationship in MRI turbulence.

  13. A Numerical Study on Premixed Bluff Body Flame of Different Bluff Apex Angle

    Directory of Open Access Journals (Sweden)

    Gelan Yang

    2013-01-01

    Full Text Available In order to investigate effects of apex angle (α on chemically reacting turbulent flow and thermal fields in a channel with a bluff body V-gutter flame holder, a numerical study has been carried out in this paper. With a basic geometry used in a previous experimental study, the apex angle was varied from 45° to 150°. Eddy dissipation concept (EDC combustion model was used for air and propane premixed flame. LES-Smagorinsky model was selected for turbulence. The gird-dependent learning and numerical model verification were done. Both nonreactive and reactive conditions were analyzed and compared. The results show that as α increases, recirculation zone becomes bigger, and Strouhal number increases a little in nonreactive cases while decreases a little in reactive cases, and the increase of α makes the flame shape wider, which will increase the chamber volume heat release ratio and enhance the flame stability.

  14. Augmenting the Structures in a Swirling Flame via Diffusive Injection

    Directory of Open Access Journals (Sweden)

    Jonathan Lewis

    2014-01-01

    Full Text Available Small scale experimentation using particle image velocimetry investigated the effect of the diffusive injection of methane, air, and carbon dioxide on the coherent structures in a swirling flame. The interaction between the high momentum flow region (HMFR and central recirculation zone (CRZ of the flame is a potential cause of combustion induced vortex breakdown (CIVB and occurs when the HMFR squeezes the CRZ, resulting in upstream propagation. The diffusive introduction of methane or carbon dioxide through a central injector increased the size and velocity of the CRZ relative to the HMFR whilst maintaining flame stability, reducing the likelihood of CIVB occurring. The diffusive injection of air had an opposing effect, reducing the size and velocity of the CRZ prior to eradicating it completely. This would also prevent combustion induced vortex breakdown CIVB occurring as a CRZ is fundamental to the process; however, without recirculation it would create an inherently unstable flame.

  15. Improved Modeling of Finite-Rate Turbulent Combustion Processes in Research Combustors

    Science.gov (United States)

    VanOverbeke, Thomas J.

    1998-01-01

    The objective of this thesis is to further develop and test a stochastic model of turbulent combustion in recirculating flows. There is a requirement to increase the accuracy of multi-dimensional combustion predictions. As turbulence affects reaction rates, this interaction must be more accurately evaluated. In this work a more physically correct way of handling the interaction of turbulence on combustion is further developed and tested. As turbulence involves randomness, stochastic modeling is used. Averaged values such as temperature and species concentration are found by integrating the probability density function (pdf) over the range of the scalar. The model in this work does not assume the pdf type, but solves for the evolution of the pdf using the Monte Carlo solution technique. The model is further developed by including a more robust reaction solver, by using accurate thermodynamics and by more accurate transport elements. The stochastic method is used with Semi-Implicit Method for Pressure-Linked Equations. The SIMPLE method is used to solve for velocity, pressure, turbulent kinetic energy and dissipation. The pdf solver solves for temperature and species concentration. Thus, the method is partially familiar to combustor engineers. The method is compared to benchmark experimental data and baseline calculations. The baseline method was tested on isothermal flows, evaporating sprays and combusting sprays. Pdf and baseline predictions were performed for three diffusion flames and one premixed flame. The pdf method predicted lower combustion rates than the baseline method in agreement with the data, except for the premixed flame. The baseline and stochastic predictions bounded the experimental data for the premixed flame. The use of a continuous mixing model or relax to mean mixing model had little effect on the prediction of average temperature. Two grids were used in a hydrogen diffusion flame simulation. Grid density did not effect the predictions except

  16. Flame Acceleration and Transition to Detonation in High-Speed Turbulent Combustion

    Science.gov (United States)

    2016-12-21

    ficult to overestimate, as it is the main process in all internal-combustion engines used for propulsion and energy generation. These include piston ...distorted tulip flame develops a double -cusped, concave flame front (6.91 and 7.34 ms) . By t his time, the pressure waves are amplified , and

  17. Dynamics of Practical Premixed Flames, Part I: Model Structure and Identification

    Directory of Open Access Journals (Sweden)

    A. Huber

    2009-06-01

    Full Text Available For the analysis of thermoacoustic instabilities it is most important to determine the dynamic flame response to acoustic disturbances. Premixed flames are often modelled as single-input single-output system, where the “output” (the overall rate of heat release responds to a single “input” variable (often the velocity at the exit of the burner nozzle. However, for practical premixed flames, where perturbations of pressure or velocity at the fuel injector will modulate the fuel equivalence ratio, the heat release rate will respond to fluctuations of equivalence ratio as well as nozzle mass flow rate. In this case, a multiple-input, single-output (MISO model structure for the flame is appropriate. Such a model structure is developed in the present paper. Staged fuel injection as well as fuel line impedances can be taken into account, the integration with low-order or finite-element based models for stability analysis is straightforward. In order to determine unit impulse and frequency response functions for such a model structure, an identification scheme based on unsteady CFD calculation with broadband excitation followed by correlation analysis is proposed and validated successfully. Identification of MISO model coefficients is a challenging task, especially in the presence of noise. Therefore criteria are introduced which allow to ascertain a posteriori how well the identified model represents the true system dynamics. Using these criteria, it is investigated how excitation signal type, time series length and signal-to-noise ratio influence the results of the identification process. Consequences for passive design strategies based on multi-stage fuel injection and experimental work on practical premixed flame dynamics are discussed.

  18. Highly stabilized partially premixed flames of propane in a concentric flow conical nozzle burner with coflow

    KAUST Repository

    Elbaz, Ayman M.

    2018-01-11

    Partially premixed turbulent flames with non-homogeneous jet of propane were generated in a concentric flow conical nozzle burner in order to investigate the effect of the coflow on the stability and flame structure. The flame stability is first mapped and then high-speed stereoscopic particle image velocimetry, SPIV, plus OH planar laser-induced fluorescence, OH-PLIF, measurements were conducted on a subset of four flames. The jet equivalence ratio Φ = 2, Jet exit Reynolds number Re = 10,000, and degree of premixing are kept constant for the selected flames, while the coflow velocity, Uc, is progressively changed from 0 to 15 m/s. The results showed that the flame is stable between two extinction limits of mixture inhomogeneity, and the optimum stability is obtained at certain degree of mixture inhomogeneity. Increasing Φ, increases the span between these two extinction limits, while these limits converge to a single point (corresponding to optimum mixture inhomogeneity) with increasing Re. Regardless the value of Φ, increasing the coflow velocity improves the flame stability. The correlation between recessed distance of the burner tubes and the fluctuation of the mixture fraction, Δξ, shows that at Δξ around 40% of the flammability limits leads to optimum flame stability. The time averaged SPIV results show that the coflow induces a big annular recirculation zone surrounds the jet flames. The size and the location of this zone is seen to be sensitive to Uc. However, the instantaneous images show the existence of a small vortical structure close to the shear layer, where the flame resides there in the case of no-coflow. These small vertical structures are seen playing a vital role in the flame structure, and increasing the flame corrugation close to the nozzle exit. Increasing the coflow velocity expands the central jet at the expense of the jet velocity, and drags the flame in the early flame regions towards the recirculation zone, where the flame tracks

  19. The physics of flames in Type Ia supernovae

    International Nuclear Information System (INIS)

    Zingale, M; Woosley, S E; Bell, J B; Day, M S; Rendleman, C A

    2005-01-01

    We extend a low Mach number hydrodynamics method developed for terrestrial combustion, to the study of thermonuclear flames in Type Ia supernovae. We discuss the differences between 2-D and 3-D Rayleigh-Taylor unstable flame simulations, and give detailed diagnostics on the turbulence, showing that the kinetic energy power spectrum obeys Bolgiano-Obukhov statistics in 2-D, but Kolmogorov statistics in 3-D. Preliminary results from 3-D reacting bubble calculations are shown, and their implications for ignition are discussed

  20. Hydrodynamic model of hydrogen-flame propagation in reactor vessels

    International Nuclear Information System (INIS)

    Baer, M.R.; Ratzel, A.C.

    1982-01-01

    A hydrodynamic model for hydrogen flame propagation in reactor geometries is presented. This model is consistent with the theory of slow combustion in which the gasdynamic field equations are treated in the limit of small Mach numbers. To the lowest order, pressure is spatially uniform. The flame is treated as a density and entropy discontinuity which propagates at prescribed burning velocities, corresponding to laminar or turbulent flames. Radiation cooling of the burned combustion gases and possible preheating of the unburned gases during propagation of the flame is included using a molecular gas-band thermal radiation model. Application of this model has been developed for 1-D variable area flame propagation. Multidimensional effects induced by hydrodynamics and buoyancy are introduced as a correction to the burn velocity (which reflects a modification of planar flame surface to a distorted surface) using experimentally measured pressure-rise time data for hydrogen/air deflagrations in cylindrical vessels

  1. Imaging hydrogen flames by two-photon, laser-induced fluorescence

    Science.gov (United States)

    Miles, R.; Lempert, W.; Kumar, V.; Diskin, G.

    1991-01-01

    A nonintrusive multicomponent imaging system is developed which can image hydrogen, hot oxygen, and air simultaneously. An Ar-F excimer laser is injection-locked to cover the Q1 two-photon transition in molecular hydrogen which allows the observation of both hot oxygen and cold hydrogen. Rayleigh scattering from the water molecules occurs at the same frequency as the illuminating laser allowing analysis of the air density. Images of ignited and nonignited hydrogen jets are recorded with a high-sensitivity gated video camera. The images permit the analysis of turbulent hydrogen-core jet, the combustion zone, and the surrounding air, and two-dimensional spatial correlations can be made to study the turbulent structure and couplings between different regions of the flow field. The method is of interest to the study of practical combustion systems which employ hydrogen-air diffusion flames.

  2. Anomalous roughness of turbulent interfaces with system size dependent local roughness exponent

    International Nuclear Information System (INIS)

    Balankin, Alexander S.; Matamoros, Daniel Morales

    2005-01-01

    In a system far from equilibrium the system size can play the role of control parameter that governs the spatiotemporal dynamics of the system. Accordingly, the kinetic roughness of interfaces in systems far from equilibrium may depend on the system size. To get an insight into this problem, we performed a detailed study of rough interfaces formed in paper combustion experiments. Using paper sheets of different width λ, we found that the turbulent flame fronts display anomalous multi-scaling characterized by non-universal global roughness exponent α and by the system size dependent spectrum of local roughness exponents, ζ q (λ)=ζ 1 (1)q -ω λ φ q =0.93q -0.15 . The structure factor of turbulent flame fronts also exhibits unconventional scaling dependence on λ. These results are expected to apply to a broad range of far from equilibrium systems when the kinetic energy fluctuations exceed a certain critical value.

  3. Flame structure of methane inverse diffusion flame

    KAUST Repository

    Elbaz, Ayman M.; Roberts, William L.

    2014-01-01

    This paper presents high speed images of OH-PLIF at 10. kHz simultaneously with 2D PIV (particle image velocimetry) measurements collected along the entire length of an inverse diffusion flame with circumferentially arranged methane fuel jets. For a

  4. Terascale High-Fidelity Simulations of Turbulent Combustion with Detailed Chemistry

    Energy Technology Data Exchange (ETDEWEB)

    Hong G. Im; Arnaud Trouve; Christopher J. Rutland; Jacqueline H. Chen

    2009-02-02

    The TSTC project is a multi-university collaborative effort to develop a high-fidelity turbulent reacting flow simulation capability utilizing terascale, massively parallel computer technology. The main paradigm of our approach is direct numerical simulation (DNS) featuring highest temporal and spatial accuracy, allowing quantitative observations of the fine-scale physics found in turbulent reacting flows as well as providing a useful tool for development of sub-models needed in device-level simulations. The code named S3D, developed and shared with Chen and coworkers at Sandia National Laboratories, has been enhanced with new numerical algorithms and physical models to provide predictive capabilities for spray dynamics, combustion, and pollutant formation processes in turbulent combustion. Major accomplishments include improved characteristic boundary conditions, fundamental studies of auto-ignition in turbulent stratified reactant mixtures, flame-wall interaction, and turbulent flame extinction by water spray. The overarching scientific issue in our recent investigations is to characterize criticality phenomena (ignition/extinction) in turbulent combustion, thereby developing unified criteria to identify ignition and extinction conditions. The computational development under TSTC has enabled the recent large-scale 3D turbulent combustion simulations conducted at Sandia National Laboratories.

  5. Terascale High-Fidelity Simulations of Turbulent Combustion with Detailed Chemistry

    Energy Technology Data Exchange (ETDEWEB)

    Im, Hong G [University of Michigan; Trouve, Arnaud [University of Maryland; Rutland, Christopher J [University of Wisconsin; Chen, Jacqueline H [Sandia National Laboratories

    2012-08-13

    The TSTC project is a multi-university collaborative effort to develop a high-fidelity turbulent reacting flow simulation capability utilizing terascale, massively parallel computer technology. The main paradigm of our approach is direct numerical simulation (DNS) featuring highest temporal and spatial accuracy, allowing quantitative observations of the fine-scale physics found in turbulent reacting flows as well as providing a useful tool for development of sub-models needed in device-level simulations. The code named S3D, developed and shared with Chen and coworkers at Sandia National Laboratories, has been enhanced with new numerical algorithms and physical models to provide predictive capabilities for spray dynamics, combustion, and pollutant formation processes in turbulent combustion. Major accomplishments include improved characteristic boundary conditions, fundamental studies of auto-ignition in turbulent stratified reactant mixtures, flame-wall interaction, and turbulent flame extinction by water spray. The overarching scientific issue in our recent investigations is to characterize criticality phenomena (ignition/extinction) in turbulent combustion, thereby developing unified criteria to identify ignition and extinction conditions. The computational development under TSTC has enabled the recent large-scale 3D turbulent combustion simulations conducted at Sandia National Laboratories.

  6. Measurements and correlations of turbulent burning velocities over wide ranges of fuels and elevated pressures

    KAUST Repository

    Bradley, Derek; Lawes, Malcolm; Liu, Kexin; Mansour, Morkous S.

    2013-01-01

    The implosion technique has been used to extend measurements of turbulent burning velocities over greater ranges of fuels and pressures. Measurements have been made up to 3.5 MPa and at strain rate Markstein numbers as low as 23. The implosion technique, with spark ignition at two opposite wall positions within a fan-stirred spherical bomb is capable of measuring turbulent burning velocities, at higher pressures than is possible with central ignition. Pressure records and schlieren high speed photography define the rate of burning and the smoothed area of the flame front. The first aim of the study was to extend the previous measurements with ethanol and propane-air, with further measurements over wider ranges of fuels and equivalence ratios with mixtures of hydrogen, methane, 10% hydrogen-90% methane, toluene, and i-octane, with air. The second aim was to study further the low turbulence regime in which turbulent burning co-exists with laminar flame instabilities. Correlations are presented of turbulent burning velocity normalised by the effective rms turbulent velocity acting on the flame front, ut=u0k , with the Karlovitz stretch factor, K, for different strain rate Markstein numbers, a decrease in which increases ut=u0k . Experimental correlations are presented for the present measurements, combined with previous ones. Different burning regimes are also identified, extending from that of mixed turbulence/laminar instability at low values of K to that at high values of K, in which ut=u0k is gradually reduced due to increasing localised flame extinctions. © 2012 The Combustion Institute.

  7. Study of scattering from turbulence structure generated by propeller with FLUENT

    Science.gov (United States)

    Luo, Gen

    2017-07-01

    In this article, the turbulence structure generated by a propeller is simulated with the computational fluid dynamics (CFD) software FLUENT. With the method of moments, the backscattering radar cross sections (RCS) of the turbulence structure are calculated. The scattering results can reflect the turbulent intensity of the wave profiles. For the wake turbulence with low rotating speed, the scattering intensity of HH polarization is much smaller than VV polarization at large incident angles. When the turbulence becomes stronger with high rotating speed, the scattering intensity of HH polarization also becomes stronger at large incident angles, which is almost the same with VV polarization. And also, the bistatic scattering of the turbulence structure has the similar situation. These scattering results indicate that the turbulence structure can also give rise to an anomaly compared with traditional sea surface. The study of electromagnetic (EM) scattering from turbulence structure generated by the propeller can help in better understanding of the scattering from different kinds of waves and provide more bases to explain the anomalies of EM scattering from sea surfaces.

  8. Acoustic radiation from weakly wrinkled premixed flames

    Energy Technology Data Exchange (ETDEWEB)

    Lieuwen, Tim; Mohan, Sripathi; Rajaram, Rajesh; Preetham, [School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0150 (United States)

    2006-01-01

    This paper describes a theoretical analysis of acoustic radiation from weakly wrinkled (i.e., u'/S{sub L}<1) premixed flames. Specifically, it determines the transfer function relating the spectrum of the acoustic pressure oscillations, P'({omega}), to that of the turbulent velocity fluctuations in the approach flow, U'({omega}). In the weakly wrinkled limit, this transfer function is local in frequency space; i.e., velocity fluctuations at a frequency {omega} distort the flame and generate sound at the same frequency. This transfer function primarily depends upon the flame Strouhal number St (based on mean flow velocity and flame length) and the correlation length, {lambda}, of the flow fluctuations. For cases where the ratio of the correlation length and duct radius {lambda}/a>>1, the acoustic pressure and turbulent velocity power spectra are related by P'({omega})-{omega}{sup 2}U'({omega}) and P'({omega})-U'({omega}) for St<<1 and St>>1, respectively. For cases where {lambda}/a<<1, the transfer functions take the form P'({omega})-{omega}{sup 2}({lambda}/a){sup 2}U'({omega}) and P'({omega})-{omega}{sup 2}({lambda}/a){sup 2}({psi}-{delta}ln({lambda}/a))U'({omega}) for St<<1 and St>>1, respectively, where (PS) and {delta} are constants. The latter result demonstrates that this transfer function does not exhibit a simple power law relationship in the high frequency region of the spectra. The simultaneous dependence of this pressure-velocity transfer function upon the Strouhal number and correlation length suggests a mechanism for the experimentally observed maximum in acoustic spectra and provides some insight into the controversy in the literature over how this peak should scale with the flame Strouhal number.

  9. On some structure-turbulence interaction problems

    Science.gov (United States)

    Maekawa, S.; Lin, Y. K.

    1976-01-01

    The interactions between a turbulent flow structure; responding to its excitation were studied. The turbulence was typical of those associated with a boundary layer, having a cross-spectral density indicative of convection and statistical decay. A number of structural models were considered. Among the one-dimensional models were an unsupported infinite beam and a periodically supported infinite beam. The fuselage construction of an aircraft was then considered. For the two-dimensional case a simple membrane was used to illustrate the type of formulation applicable to most two-dimensional structures. Both the one-dimensional and two-dimensional structures studied were backed by a cavity filled with an initially quiescent fluid to simulate the acoustic environment when the structure forms one side of a cabin of a sea vessel or aircraft.

  10. The structure and statistics of interstellar turbulence

    International Nuclear Information System (INIS)

    Kritsuk, A G; Norman, M L; Ustyugov, S D

    2017-01-01

    We explore the structure and statistics of multiphase, magnetized ISM turbulence in the local Milky Way by means of driven periodic box numerical MHD simulations. Using the higher order-accurate piecewise-parabolic method on a local stencil (PPML), we carry out a small parameter survey varying the mean magnetic field strength and density while fixing the rms velocity to observed values. We quantify numerous characteristics of the transient and steady-state turbulence, including its thermodynamics and phase structure, kinetic and magnetic energy power spectra, structure functions, and distribution functions of density, column density, pressure, and magnetic field strength. The simulations reproduce many observables of the local ISM, including molecular clouds, such as the ratio of turbulent to mean magnetic field at 100 pc scale, the mass and volume fractions of thermally stable Hi, the lognormal distribution of column densities, the mass-weighted distribution of thermal pressure, and the linewidth-size relationship for molecular clouds. Our models predict the shape of magnetic field probability density functions (PDFs), which are strongly non-Gaussian, and the relative alignment of magnetic field and density structures. Finally, our models show how the observed low rates of star formation per free-fall time are controlled by the multiphase thermodynamics and large-scale turbulence. (paper)

  11. Coherent structures in wall-bounded turbulence

    Science.gov (United States)

    Jiménez, Javier

    2018-05-01

    This article discusses the description of wall-bounded turbulence as a deterministic high-dimensional dynamical system of interacting coherent structures, defined as eddies with enough internal dynamics to behave relatively autonomously from any remaining incoherent part of the flow. The guiding principle is that randomness is not a property, but a methodological choice of what to ignore in the flow, and that a complete understanding of turbulence, including the possibility of control, requires that it be kept to a minimum. After briefly reviewing the underlying low-order statistics of flows at moderate Reynolds numbers, the article examines what two-point statistics imply for the decomposition of the flow into individual eddies. Intense eddies are examined next, including their temporal evolution, and shown to satisfy many of the properties required for coherence. In particular, it is shown that coherent structures larger than the Corrsin scale are a natural consequence of the shear. In wall-bounded turbulence, they can be classified into coherent dispersive waves and transient bursts. The former are found in the viscous layer near the wall and as very-large structures spanning the boundary layer thickness. Although they are shear-driven, these waves have enough internal structure to maintain a uniform advection velocity. Conversely, bursts exist at all scales, are characteristic of the logarithmic layer, and interact almost linearly with the shear. While the waves require a wall to determine their length scale, the bursts are essentially independent from it. The article concludes with a brief review of our present theoretical understanding of turbulent structures, and with a list of open problems and future perspectives.

  12. Analysis of lift-off height and structure of n-heptane tribrachial flames in laminar jet configuration

    KAUST Repository

    Luca, Stefano

    2015-03-30

    A set of lifted tribrachial n-heptane flames in a laminar jet configuration are simulated. The simulations are performed using finite rate chemistry and detailed transport, and aim at investigating the propagation of tribrachial flames. Varying the inlet velocity of the fuel, different stabilization heights are obtained, and the dependence of the stabilization height in the inlet velocity is compared with experimental data. A detailed analysis of the flame geometry is performed by comparingthe flame structure to that of unstretched premixed flames. Issues related to differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front are discussed.

  13. Analysis of lift-off height and structure of n-heptane tribrachial flames in laminar jet configuration

    KAUST Repository

    Luca, Stefano; Bisetti, Fabrizio

    2015-01-01

    A set of lifted tribrachial n-heptane flames in a laminar jet configuration are simulated. The simulations are performed using finite rate chemistry and detailed transport, and aim at investigating the propagation of tribrachial flames. Varying the inlet velocity of the fuel, different stabilization heights are obtained, and the dependence of the stabilization height in the inlet velocity is compared with experimental data. A detailed analysis of the flame geometry is performed by comparingthe flame structure to that of unstretched premixed flames. Issues related to differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front are discussed.

  14. Experimental Studies of Turbulent Intensity around a Tidal Turbine Support Structure

    Directory of Open Access Journals (Sweden)

    Stuart Walker

    2017-04-01

    Full Text Available Tidal stream energy is a low-carbon energy source. Tidal stream turbines operate in a turbulent environment, and the effect of the structure between the turbine and seabed on this environment is not fully understood. An experimental study using 1:72 scale models based on a commercial turbine design was carried out to study the support structure influence on turbulent intensity around the turbine blades. The study was conducted using the wave-current tank at the Laboratory of Maritime Engineering (LABIMA, University of Florence. A realistic flow environment (ambient turbulent intensity = 11% was established. Turbulent intensity was measured upstream and downstream of a turbine mounted on two different support structures (one resembling a commercial design, the other the same with an additional vertical element, in order to quantify any variation in turbulence and performance between the support structures. Turbine drive power was used to calculate power generation. Acoustic Doppler velocimetry (ADV was used to record and calculate upstream and downstream turbulent intensity. In otherwise identical conditions, performance variation of only 4% was observed between two support structures. Turbulent intensity at 1, 3 and 5 blade diameters, both upstream and downstream, showed variation up to 21% between the two cases. The additional turbulent structures generated by the additional element of the second support structure appears to cause this effect, and the upstream propagation of turbulent intensity is believed to be permitted by surface waves. This result is significant for the prediction of turbine array performance.

  15. LES of the interaction between a premixed flame and complex turbulent swirling flow

    International Nuclear Information System (INIS)

    Iudiciani, P; Duwig, C; Szasz, R Z; Fuchs, L; Gutmark, E

    2011-01-01

    In this paper the Triple Annular Research Swirler, a fuel injector characterized by complex design with three concentric air passages, has been studied numerically. A swirl-stabilized lean premixed flame has been simulated by means of Large Eddy Simulation. The computations characterize successfully the dynamics of the flame and their interactions with the complex swirling flow. The flame is stabilized upstream the fuel injector exit, and the dynamics are led by a Precessing Vortex Core which seems to originate in the inner air passage. The results obtained by Proper Orthogonal Decomposition analysis are in agreement with previous findings in the context of swirling flows/flames.

  16. Investigations into the Impact of the Equivalence Ratio on Turbulent Premixed Combustion Using Particle Image Velocimetry and Large Eddy Simulation Techniques: “V” and “M” Flame Configurations in a Swirl Combustor

    KAUST Repository

    Kewlani, Gaurav

    2016-03-24

    Turbulent premixed combustion is studied using experiments and numerical simulations in an acoustically uncoupled cylindrical sudden-expansion swirl combustor, and the impact of the equivalence ratio on the flame–flow characteristics is analyzed. In order to numerically capture the inherent unsteadiness exhibited in the flow, the large eddy simulation (LES) technique based on the artificial flame thickening combustion model is employed. The experimental data are obtained using particle image velocimetry. It is observed that changes in heat loading, in the presence of wall confinement, significantly influence the flow field in the wake region, the stabilization location of the flame, and the flame intensity. Specifically, increasing the equivalence ratio drastically reduces the average inner recirculation zone size and causes transition of the flame macrostructure from the “V” configuration to the “M” configuration. In other words, while the flame stabilizes along the inner shear layer for the V flame, a persistent diffuse reaction zone is also manifested along the outer shear layer for the M flame. The average chemiluminescence intensity increases in the case of the M flame macrostructure, while the axial span of the reaction zone within the combustion chamber decreases. The predictions of the numerical approach resemble the experimental observations, suggesting that the LES framework can be an effective tool for examining the effect of heat loading on flame–flow interactions and the mechanism of transition of the flame macrostructure with a corresponding change in the equivalence ratio.

  17. Distillation and Visualization of Spatiotemporal Structures in Turbulent Flow Fields

    International Nuclear Information System (INIS)

    Hege, Hans-Christian; Hotz, Ingrid; Kasten, Jens

    2011-01-01

    Although turbulence suggests randomness and disorder, organized motions that cause spatiotemporal 'coherent structures' are of particular interest. Revealing such structures in numerically given turbulent or semi-turbulent flows is of interest both for practically working engineers and theoretically oriented physicists. However, as long as there is no common agreement about the mathematical definition of coherent structures, extracting such structures is a vaguely defined task. Instead of searching for a general definition, the data visualization community takes a pragmatic approach and provides various tool chains implemented in flexible software frameworks that allow the user to extract distinct flow field structures. Thus physicists or engineers can select those flow structures which might advance their insight best. We present different approaches to distill important features from turbulent flows and discuss the necessary steps to be taken on the example of Lagrangian coherent structures.

  18. Application of a Novel Laser-Doppler Velocimeter for Turbulence: Structural Measurements in Turbulent Boundary Layers

    National Research Council Canada - National Science Library

    Lowe, Kevin T; Simpson, Roger L

    2006-01-01

    An advanced laser-Doppler velocimeter (LDV), deemed the 'comprehensive LDV', is designed to acquire fully-resolved turbulence structural measurements in high Reynolds number two- and three-dimensional turbulent boundary layers...

  19. Eulerian–Lagrangian RANS Model Simulations of the NIST Turbulent Methanol Spray Flame

    NARCIS (Netherlands)

    Zhu, Shanglong; Roekaerts, Dirk; Pozarlik, Artur Krzysztof; van der Meer, Theodorus H.

    2015-01-01

    A methanol spray flame in a combustion chamber of the NIST was simulated using an Eulerian–Lagrangian RANS model. Experimental data and previous numerical investigations by other researchers on this flame were analyzed to develop methods for more comprehensive model validation. The inlet boundary

  20. Turbulence and turbulence-generated structural loading in wind turbine clusters

    Energy Technology Data Exchange (ETDEWEB)

    Frandsen, Sten

    2007-01-15

    Turbulence, in terms of standard deviation of wind speed fluctuations, and other flow characteristics are different in the interior of wind farms relative to the free flow and action must be taken to ensure sufficient structural sustainability of the wind turbines exposed to 'wind farm flow'. The standard deviation of wind speed fluctuations is a known key parameter for both extreme- and fatigue loading, and it is argued and found to be justified that a model for change in turbulence intensity alone may account for increased fatigue loading in wind farms. Changes in scale of turbulence and horizontal flow-shear also influence the dynamic response and thus fatigue loading. However, these parameters are typically negatively or positively correlated with the standard deviation of wind speed fluctuations, which therefore can, if need be, represent these other variables. Thus, models for spatially averaged turbulence intensity inside the wind farm and direct-wake turbulence intensity are being devised and a method to combine the different load situations is proposed. The combination of the load cases implies a weighting method involving the slope of the considered material's Woehler curve. In the context, this is novel and necessary to avoid excessive safety for fatigue estimation of the structure's steel components, and non-conservatism for fibreglass components. The proposed model offers significant reductions in computational efforts in the design process. The status for the implementation of the model is that it became part of the Danish standard for wind turbine design DS 472 (2001) in August 2001 and it is part of the corresponding international standard, IEC61400-1 (2005). Also, extreme loading under normal operation for wake conditions and the efficiency of very large wind farms are discussed. (au)

  1. DNS and LES/FMDF of turbulent jet ignition and combustion

    Science.gov (United States)

    Validi, Abdoulahad; Jaberi, Farhad

    2014-11-01

    The ignition and combustion of lean fuel-air mixtures by a turbulent jet flow of hot combustion products injected into various geometries are studied by high fidelity numerical models. Turbulent jet ignition (TJI) is an efficient method for starting and controlling the combustion in complex propulsion systems and engines. The TJI and combustion of hydrogen and propane in various flow configurations are simulated with the direct numerical simulation (DNS) and the hybrid large eddy simulation/filtered mass density function (LES/FMDF) models. In the LES/FMDF model, the filtered form of the compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity and the FMDF transport equation is solved with a Lagrangian stochastic method to obtain the scalar field. The DNS and LES/FMDF data are used to study the physics of TJI and combustion for different turbulent jet igniter and gas mixture conditions. The results show the very complex and different behavior of the turbulence and the flame structure at different jet equivalence ratios.

  2. Type III radio bursts in a flaming structure

    International Nuclear Information System (INIS)

    Karlicky, M.; Tlamicha, A.

    1977-01-01

    An interpretation is presented of the burst of 3.7.1974. The slowly drifting, fine structure in this type III burst is evidence of the existence of very fast, spatially extensive processes in the corona. The concept is presented of a rapidly varying, magnetohydrodynamically unstable, flaming structure of the magnetic field and, using this model, the intensities were computed of the magnetic field at certain altitudes and at two moments differing by 1.4 s. (author)

  3. Flame-Vortex Interactions Imaged in Microgravity - To Assess the Theory Flame Stretch

    Science.gov (United States)

    Driscoll, James F.

    2001-01-01

    The goals of this research are to: 1) Assess the Theory of Flame Stretch by operating a unique flame-vortex experiment under microgravity conditions in the NASA Glenn 2.2 Second Drop Tower (drops to identify operating conditions have been completed); 2) Obtain high speed shadowgraph images (500-1000 frames/s) using the drop rig (images were obtained at one-g, and the NASA Kodak RO camera is being mounted on the drop rig); 3) Obtain shadowgraph and PIV images at 1-g while varying the effects of buoyancy by controlling the Froude number (completed); 4) Numerically model the inwardly-propagating spherical flame that is observed in the experiment using full chemistry and the RUN 1DL code (completed); 5) Send images of the flame shape to Dr. G. Patniak at NRL who is numerically simulating the entire flame-vortex interaction of the present experiment (data transfer completed); and 6) Assess the feasibility of obtaining PIV velocity field images in the drop rig, which would be useful (but not required) for our assessment of the Theory of Flame Stretch (PIV images were obtained at one-g using same low laser power that is available from fiber optic cable in drop tower). The motivation for the work is to obtain novel measurement needed to develop a physically accurate model of turbulent combustion that can help in the control of engine pollutants. The unique experiment allows, for the first time, the detailed study of a negatively-curved (negatively stretched) flame, which is one of the five fundamental types of premixed flames. While there have been studies of flat flames, positively-curved (outwardly-propagating) cases and positively-strained (counterflow) cases, this is the first detailed study of a negatively-curved (inwardly-propagating) flame. The first set of drops in the 2.2 Second Drop Tower showed that microgravity provides more favorable conditions for achieving inwardly-propagating flames (IPFs) than 1-g. A vortex interacts with a flame and creates a spherical

  4. Premixed and non-premixed generated manifolds in large-eddy simulation of Sandia flame D and F

    NARCIS (Netherlands)

    Vreman, A.W.; Albrecht, B.A.; Oijen, van J.A.; Goey, de L.P.H.; Bastiaans, R.J.M.

    2008-01-01

    Premixed and nonpremixed flamelet-generated manifolds have been constructed and applied to large-eddy simulation of the piloted partially premixed turbulent flames Sandia Flame D and F. In both manifolds the chemistry is parameterized as a function of the mixture fraction and a progress variable.

  5. Analytic prediction of unconfined boundary layer flashback limits in premixed hydrogen-air flames

    Science.gov (United States)

    Hoferichter, Vera; Hirsch, Christoph; Sattelmayer, Thomas

    2017-05-01

    Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature.

  6. Characteristics of premixed flames stabilized in an axisymmetric curved-wall jet burner with tip modification

    KAUST Repository

    Kim, Daejoong

    2009-11-10

    The stabilization characteristics of premixed flames in an axisymmetric curved-wall jet burner have been experimentally investigated. This burner utilized the Coanda effect on top of a burner tip. The initially spherical burner tip was modified to a flat tip and a concave tip in order to improve flame stabilization by providing enough space for flow recirculation above the burner tip region. The flow characteristics have been visualized using a schlieren technique. Small-scale turbulence structure has been observed mainly in the interaction jet region (located downstream of the recirculation region) for large jet velocity (Reynolds number >11,500). An appreciable amount of air entrainment was exhibited from the half-angle of the jet spread, approximately 20. The averaged planar laser-induced fluorescence images of the flames for this large velocity demonstrated that the strong signal of OH radicals, representing reaction zones, existed in the recirculation zone, while it was weak in the interaction jet region due to intermittency and local extinction by the generation of small scale turbulence. The OH radical signals strengthened again in the merged jet region (downstream of the interaction jet region). In extreme cases of Reynolds number over 19,000, a unique flame exhibiting OH radicals only in the recirculation zone was observed for the concave tip. The flame stabilization has been mapped by varying jet velocity and equivalence ratio, and the result showed that the stabilization characteristics were improved appreciably from the initial spherical tip design, especially for rich mixtures. The flow fields measured by a laser Doppler velocimetry confirmed the existence of recirculation zone and the expansion of the recirculation zones for the modified tips. The temperature profile measured by a coherent anti-Stokes Raman spectroscopy exhibited an intermittent nature, especially near the recirculation zone.

  7. Turbulent burning characteristics of FACE-C gasoline and TPRF blend associated with the same RON at elevated pressures

    KAUST Repository

    Mannaa, Ossama

    2018-04-06

    Fuels for Advanced Combustion Engine (FACE)-C gasoline/air and toluene primary reference fuel (TPRF) (51.6 vol% iso-octane, 21.5 vol% n-heptane and 26.9 vol% toluene)/air mixtures corresponding to the same Research Octane numbers (RON) of 85 were characterized in terms of determining their burning rates in a fan stirred turbulent vessel and filmed using a high-speed dual Schlieren imaging technique. Also, a Mie scattering planar laser tomography was employed to characterize the variations of flame morphology induced by the simultaneous existences of different turbulent length scales and the susceptibility to develop cellular structures at elevated pressures (through the Darrieus-Landau instability). Measurements were performed in a well-controlled environment of initial pressures 0.1, 0.5 and 1.0 MPa at a fixed initial temperature of 358 K at a range of measured turbulence intensities from 0.5 to 2.0 m/s. The enhancement of turbulent burning velocity ST as a function of turbulence intensity was evaluated. The absence of bending regime was accounted for based on the size of the vessel and limited range of turbulent intensities investigated in the present work. All the present data were empirically correlated by power-law correlation derived for a different flame-type configuration to test its sensitivity to the geometry and type of the burner investigated.

  8. CFD analysis of a scramjet combustor with cavity based flame holders

    Science.gov (United States)

    Kummitha, Obula Reddy; Pandey, Krishna Murari; Gupta, Rajat

    2018-03-01

    Numerical analysis of a scramjet combustor with different cavity flame holders has been carried out using ANSYS 16 - FLUENT tool. In this research article the internal fluid flow behaviour of the scramjet combustor with different cavity based flame holders have been discussed in detail. Two dimensional Reynolds-Averaged Navier-Stokes governing(RANS) equations and shear stress turbulence (SST) k - ω model along with finite rate/eddy dissipation chemistry turbulence have been considered for modelling chemical reacting flows. Due to the advantage of less computational time, global one step reaction mechanism has been used for combustion modelling of hydrogen and air. The performance of the scramjet combustor with two different cavities namely spherical and step cavity has been compared with the standard DLR scramjet. From the comparison of numerical results, it is found that the development of recirculation regions and additional shock waves from the edge of cavity flame holder is increased. And also it is observed that with the cavity flame holder the residence time of air in the scramjet combustor is also increased and achieved stabilized combustion. From this research analysis, it has been found that the mixing and combustion efficiency of scramjet combustor with step cavity design is optimum as compared to other models.

  9. An experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon

    Science.gov (United States)

    Tran, L.S.; Glaude, P.A.; Battin-Leclerc, F.

    2013-01-01

    The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of concentration profiles of methane, ethanol, O2, Ar, CO, CO2, H2O, H2, C2H6, C2H4, C2H2, C3H8, C3H6, p-C3H4, a-C3H4, CH2O, CH3HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained using a PtRh (6%)-PtRh (30%) type B thermocouple. The similarities and differences between the three flames were analyzed. The results show that, in these three flames, the concentration of the C2 intermediates is much larger than that of the C3 species. In general, mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame. PMID:24092946

  10. The spatio-temporal structure of electrostatic turbulence in the WEGA stellarator

    International Nuclear Information System (INIS)

    Marsen, Stefan

    2008-03-01

    The main object of this work is to provide a detailed characterisation of electrostatic turbulence in WEGA and to identify the underlying instability mechanism driving turbulence. The spatio-temporal structure of turbulence is studied using multiple Langmuir probes providing a sufficiently high spatial and temporal resolution. Turbulence in WEGA is dominated by drift wave dynamics. The phase shift between density and potential fluctuations is close to zero, fluctuations are mainly driven by the density gradient, and the phase velocity of turbulent structures points in the direction of the electron diamagnetic drift. The structure of turbulence is studied mainly in the plasma edge region inside the last closed flux surface. WEGA can be operated in two regimes differing in the magnetic field strength by almost one order of magnitude (57 mT and 500 mT, respectively). At 57 mT large structures with a poloidal extent comparable to the machine dimensions are observed, whereas at 500 mT turbulent structures are much smaller. The poloidal structure size scales nearly linearly with the inverse magnetic field strength. This scaling may be argued to be related to the drift wave dispersion scale, ρ s =√(m i k B T e )/(qB). However, the structure size remains unchanged when the ion mass is changed by using different discharge gases. Inside the last closed flux surface the poloidal E x B drift in WEGA is negligible. The three-dimensional structure is studied in detail using probes which are toroidally separated but aligned along connecting magnetic field lines. A small but finite parallel wavenumber is found. The ratio between the average parallel and perpendicular wavenumber is in the order of anti κ parallel / anti κ θ ∼ 10 -2 . The parallel phase velocity of turbulent structures is in-between the ion sound velocity and the Alfven velocity. In the parallel dynamics a fundamental difference between the two operational regimes at different magnetic field strength is

  11. Coherent vortical structures in two-dimensional plasma turbulence

    DEFF Research Database (Denmark)

    Pécseli, H.L.; Coutsias, E.A.; Huld, T.

    1992-01-01

    A laboratory experiment was carried out in order to study the nonlinear saturated stage of the cross-field electrostatic Kelvin-Helmholtz instability in a strongly magnetized plasma. The presence of large vortex-like structures in a background of wide-band turbulent fluctuations was demonstrated...... simulations. The importance of the large scale structures for the turbulent plasma transport across magnetic field lines was analyzed in detail....

  12. Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

    KAUST Repository

    Hong, Seunghyuck

    2015-01-01

    © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. We investigate the dependence of the recirculation zone (RZ) size and structure on the fuel composition using high-speed particle image velocimetry (PIV) and chemiluminescence measurements for C3H8/H2/air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low equivalence ratios, the time-averaged RZ is comprised of two counter rotating eddies: a primary eddy (PE) between the shear layer and the bottom wall; and a secondary eddy (SE) between the vertical step wall and the PE. The flame stabilizes downstream of the saddle point of the dividing streamline between the two eddies. As equivalence ratio is raised, the flame moves upstream, pushing the saddle point with it and reducing the size of the SE. Higher temperature of the products reduces the velocity gradient in the shear layer and thus the reattachment length. As equivalence ratio approaches a critical value, the saddle point reaches the step and the SE collapses while the flame starts to exhibit periodic flapping motions, suggesting a correlation between the RZ structure and flame anchoring. The overall trend in the flow field is the same as we add hydrogen to the fuel at a fixed equivalence ratio, demonstrating the impact of fuel composition on the flow field. We show that the reattachment lengths (LR), which are shown to encapsulate the mean RZ structure, measured over a range of fuel composition and equivalence ratio collapse if plotted against the strained consumption speed (Sc). Results indicate that for the flame to remain anchored, the RZ structure should satisfy lR,isothermal/L R,reacting · S c/U ∞ ∼ 0.1. If this criterion cannot be met, the flame blows off, flashes back or becomes thermoacoustically unstable, suggesting a Damköhler-like criterion for

  13. Turbulence and turbulence-generated structural loading in wind turbine clusters

    DEFF Research Database (Denmark)

    Frandsen, Sten Tronæs

    2007-01-01

    of the model is that it became part of the Danish standard for wind turbine design DS 472 (2001) in August 2001 and it is part of the corresponding international standard, IEC61400-1 (2005). Also, extreme loading under normal operation for wake conditions and the efficiency of very large wind farms......Turbulence - in terms of standard deviation of wind speed fluctuations - and other flow characteristics are different in the interior of wind farms relative to the free flow and action must be taken to ensure sufficient structural sustainability of the wind turbines exposed to “wind farm flow......”. The standard deviation of wind speed fluctuations is a known key parameter for both extreme- and fatigue loading, and it is argued and found to be justified that a model for change in turbulence intensity alone may account for increased fatigue loading in wind farms. Changes in scale of turbulence...

  14. The role of the intense vorticity structures in the turbulent structure of the jet edge

    Science.gov (United States)

    Reis, Ricardo J. N.; da Silva, Carlos B.; Pereira, José C. F.

    In free shear flows (jets, mixing layers and wakes) there is an highly contorted interface dividing the turbulent from the non-turbulent flow: the turbulent/non-turbulent (T/NT) interface. Across this interface important exchanges of mass, momentum and heat take place, in a process known as turbulent entrainment. Recently, the classical idea of the turbulent entrainment caused by engulfing [1] have been questioned, and it has been shown that the entrainment is mainly caused by small scale eddy motions (nibbling) [2, 3]). However, it is still argued that the entrainment rate is still largely governed by the large scale motions induced by the intense vorticity structures (IVS). The goal of the present work is to assess characterize the geometry and analyze the influence of these large scales structures in shaping the turbulent/nonturbulent interface.

  15. A parallel adaptive mesh refinement algorithm for predicting turbulent non-premixed combusting flows

    International Nuclear Information System (INIS)

    Gao, X.; Groth, C.P.T.

    2005-01-01

    A parallel adaptive mesh refinement (AMR) algorithm is proposed for predicting turbulent non-premixed combusting flows characteristic of gas turbine engine combustors. The Favre-averaged Navier-Stokes equations governing mixture and species transport for a reactive mixture of thermally perfect gases in two dimensions, the two transport equations of the κ-ψ turbulence model, and the time-averaged species transport equations, are all solved using a fully coupled finite-volume formulation. A flexible block-based hierarchical data structure is used to maintain the connectivity of the solution blocks in the multi-block mesh and facilitate automatic solution-directed mesh adaptation according to physics-based refinement criteria. This AMR approach allows for anisotropic mesh refinement and the block-based data structure readily permits efficient and scalable implementations of the algorithm on multi-processor architectures. Numerical results for turbulent non-premixed diffusion flames, including cold- and hot-flow predictions for a bluff body burner, are described and compared to available experimental data. The numerical results demonstrate the validity and potential of the parallel AMR approach for predicting complex non-premixed turbulent combusting flows. (author)

  16. Modelling thermal radiation and soot formation in buoyant diffusion flames

    International Nuclear Information System (INIS)

    Demarco Bull, R.A.

    2012-01-01

    The radiative heat transfer plays an important role in fire problems since it is the dominant mode of heat transfer between flames and surroundings. It controls the pyrolysis, and therefore the heat release rate, and the growth rate of the fire. In the present work a numerical study of buoyant diffusion flames is carried out, with the main objective of modelling the thermal radiative transfer and the soot formation/destruction processes. In a first step, different radiative property models were tested in benchmark configurations. It was found that the FSCK coupled with the Modest and Riazzi mixing scheme was the best compromise in terms of accuracy and computational requirements, and was a good candidate to be implemented in CFD codes dealing with fire problems. In a second step, a semi-empirical soot model, considering acetylene and benzene as precursor species for soot nucleation, was validated in laminar co flow diffusion flames over a wide range of hydrocarbons (C1-C3) and conditions. In addition, the optically-thin approximation was found to produce large discrepancies in the upper part of these small laminar flames. Reliable predictions of soot volume fractions require the use of an advanced radiation model. Then the FSCK and the semi-empirical soot model were applied to simulate laboratory-scale and intermediate-scale pool fires of methane and propane. Predicted flame structures as well as the radiant heat flux transferred to the surroundings were found to be in good agreement with the available experimental data. Finally, the interaction between radiation and turbulence was quantified. (author)

  17. Analysis of flame acceleration in open or vented obstructed pipes

    Science.gov (United States)

    Bychkov, Vitaly; Sadek, Jad; Akkerman, V'yacheslav

    2017-01-01

    While flame propagation through obstacles is often associated with turbulence and/or shocks, Bychkov et al. [V. Bychkov et al., Phys. Rev. Lett. 101, 164501 (2008), 10.1103/PhysRevLett.101.164501] have revealed a shockless, conceptually laminar mechanism of extremely fast flame acceleration in semiopen obstructed pipes (one end of a pipe is closed; a flame is ignited at the closed end and propagates towards the open one). The acceleration is devoted to a powerful jet flow produced by delayed combustion in the spaces between the obstacles, with turbulence playing only a supplementary role in this process. In the present work, this formulation is extended to pipes with both ends open in order to describe the recent experiments and modeling by Yanez et al. [J. Yanez et al., arXiv:1208.6453] as well as the simulations by Middha and Hansen [P. Middha and O. R. Hansen, Process Safety Prog. 27, 192 (2008) 10.1002/prs.10242]. It is demonstrated that flames accelerate strongly in open or vented obstructed pipes and the acceleration mechanism is similar to that in semiopen ones (shockless and laminar), although acceleration is weaker in open pipes. Starting with an inviscid approximation, we subsequently incorporate hydraulic resistance (viscous forces) into the analysis for the sake of comparing its role to that of a jet flow driving acceleration. It is shown that hydraulic resistance is actually not required to drive flame acceleration. In contrast, this is a supplementary effect, which moderates acceleration. On the other hand, viscous forces are nevertheless an important effect because they are responsible for the initial delay occurring before the flame acceleration onset, which is observed in the experiments and simulations. Accounting for this effect provides good agreement between the experiments, modeling, and the present theory.

  18. Simplified hydrodynamic model of hydrogen-flame propagation in reactor vessels

    International Nuclear Information System (INIS)

    Baer, M.R.; Ratzel, A.C.

    1983-01-01

    The model is consistent with the theory of slow combustion in which the gasdynamic field equations are treated in the limit of small Mach numbers. To the lowest order, pressure is spatially uniform. The flame is treated as a density and entropy discontinuity which propagates at prescribed burning velocities, corresponding to laminar or turbulent flames. Radiation cooling of the burned combustion gases and possible preheating of the unburned gases during propagation of the flame is included using a molecular gas-band thermal radiation model. Application of this model has been developed for 1-D variable-area flame propagation. Multidimensional effects induced by hydrodynamics and buoyancy are corrected for. This model of flame propagation reduces to differential equations which describes the temporal variations of vessel pressure, burned volume and gas entropy. The thermodynamic state of the burned gas immediately following the flame is determined using an isobaric Hugoniot relation. At other locations the burned-gas thermodynamic states are determined using a Lagrangian particle tracking method. Results of a computer code using the method are presented. 11 figures

  19. Iron Oxide Doped Alumina-Zirconia Nanoparticle Synthesis by Liquid Flame Spray from Metal Organic Precursors

    OpenAIRE

    Juha-Pekka Nikkanen; Helmi Keskinen; Mikko Aromaa; Mikael Järn; Tomi Kanerva; Erkki Levänen; Jyrki M. Mäkelä; Tapio Mäntylä

    2008-01-01

    The liquid flame spray (LFS) method was used to make iron oxide doped alumina-zirconia nanoparticles. Nanoparticles were generated using a turbulent, high-temperature (Tmax⁡∼3000 K) H2-O2 flame. The precursors were aluminium-isopropoxide, zirconium-n-propoxide, and ferrocene in xylene solution. The solution was atomized into micron-sized droplets by high velocity H2 flow and introduced into the flame where nanoparticles were formed. The particle morphology, size, phase, and chemical compositi...

  20. Numerical study of laminar nonpremixed methane flames in coflow jets: Autoignited lifted flames with tribrachial edges and MILD combustion at elevated temperatures

    KAUST Repository

    M. Al-Noman, Saeed

    2016-07-07

    Autoignition characteristics of laminar nonpremixed methane jet flames in high-temperature coflow air are studied numerically. Several flame configurations are investigated by varying the initial temperature and fuel mole fraction. At a relatively low initial temperature, a non-autoignited nozzle-attached flame is simulated at relatively low jet velocity. When the initial temperature is higher than that required for autoignition, two regimes are investigated: an autoignited lifted flame with tribrachial edge structure and an autoignited lifted flame with Mild combustion. The autoignited lifted flame with tribrachial edge exhibited three branches: lean and rich premixed flame wings and a trailing diffusion flame. Characteristics of kinetic structure for autoignited lifted flames are discussed based on the kinetic structures of homogeneous autoignition and flame propagation of stoichiometric mixture. Results showed that a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. The autoignited lifted flame with Mild combustion occurs when methane fuel is highly diluted with nitrogen. The kinetic structure analysis shows that the characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to nozzle-attached flame was investigated by increasing the fuel mole fraction. As the maximum flame temperature increases with decreasing liftoff height, the kinetic structure showed a transition behavior from autoignition to flame propagation of a lean premixed flame. © 2016 The Combustion Institute

  1. An experimental investigation on self-acceleration of cellular spherical flames

    DEFF Research Database (Denmark)

    Wu, Fujia; Jomaas, Grunde; Law, Chung K.

    2013-01-01

    , whether it could also be self-turbulizing. Extensive experiments at elevated pressures and thereby reduced laminar flame thicknesses and enhanced propensity to exhibit Darrieus-Landau instability were conducted for hydrogen/air mixtures over an extensive range of equivalence ratios. The results...

  2. Modeling local extinction in turbulent combustion using an embedding method

    Science.gov (United States)

    Knaus, Robert; Pantano, Carlos

    2012-11-01

    Local regions of extinction in diffusion flames, called ``flame holes,'' can reduce the efficiency of combustion and increase the production of certain pollutants. At sufficiently high speeds, a flame may also be lifted from the rim of the burner to a downstream location that may be stable. These two phenomena share a common underlying mechanism of propagation related to edge-flame dynamics where chemistry and fluid mechanics are equally important. We present a formulation that describes the formation, propagation, and growth of flames holes on the stoichiometric surface using edge flame dynamics. The boundary separating the flame from the quenched region is modeled using a progress variable defined on the moving stoichiometric surface that is embedded in the three-dimensional space using an extension algorithm. This Cartesian problem is solved using a high-order finite-volume WENO method extended to this nonconservative problem. This algorithm can track the dynamics of flame holes in a turbulent reacting-shear layer and model flame liftoff without requiring full chemistry calculations.

  3. The spatio-temporal structure of electrostatic turbulence in the WEGA stellarator

    Energy Technology Data Exchange (ETDEWEB)

    Marsen, Stefan

    2008-03-15

    The main object of this work is to provide a detailed characterisation of electrostatic turbulence in WEGA and to identify the underlying instability mechanism driving turbulence. The spatio-temporal structure of turbulence is studied using multiple Langmuir probes providing a sufficiently high spatial and temporal resolution. Turbulence in WEGA is dominated by drift wave dynamics. The phase shift between density and potential fluctuations is close to zero, fluctuations are mainly driven by the density gradient, and the phase velocity of turbulent structures points in the direction of the electron diamagnetic drift. The structure of turbulence is studied mainly in the plasma edge region inside the last closed flux surface. WEGA can be operated in two regimes differing in the magnetic field strength by almost one order of magnitude (57 mT and 500 mT, respectively). At 57 mT large structures with a poloidal extent comparable to the machine dimensions are observed, whereas at 500 mT turbulent structures are much smaller. The poloidal structure size scales nearly linearly with the inverse magnetic field strength. This scaling may be argued to be related to the drift wave dispersion scale, {rho}{sub s}={radical}(m{sub i}k{sub B}T{sub e})/(qB). However, the structure size remains unchanged when the ion mass is changed by using different discharge gases. Inside the last closed flux surface the poloidal E x B drift in WEGA is negligible. The three-dimensional structure is studied in detail using probes which are toroidally separated but aligned along connecting magnetic field lines. A small but finite parallel wavenumber is found. The ratio between the average parallel and perpendicular wavenumber is in the order of anti {kappa} {sub parallel} / anti {kappa}{sub {theta}} {approx} 10{sup -2}. The parallel phase velocity of turbulent structures is in-between the ion sound velocity and the Alfven velocity. In the parallel dynamics a fundamental difference between the two

  4. Flow topologies in different regimes of premixed turbulent combustion: A direct numerical simulation analysis

    KAUST Repository

    Wacks, Daniel H.

    2016-12-02

    The distributions of flow topologies within the flames representing the corrugated flamelets, thin reaction zones, and broken reaction zone regimes of premixed turbulent combustion are investigated using direct numerical simulation data of statistically planar turbulent H-2-air flames with an equivalence ratio phi = 0.7. It was found that the diminishing influence of dilatation rate with increasing Karlovitz number has significant influences on the statistical behaviors of the first, second, and third invariants (i.e., P, Q, and R) of the velocity gradient tensor. These differences are reflected in the distributions of the flow topologies within the flames considered in this analysis. This has important consequences for those topologies that make dominant contributions to the scalar-turbulence interaction and vortex-stretching terms in the scalar dissipation rate and enstrophy transport equations, respectively. Detailed physical explanations are provided for the observed regime dependences of the flow topologies and their implications on the scalar dissipation rate and enstrophy transport.

  5. Flow topologies in different regimes of premixed turbulent combustion: A direct numerical simulation analysis

    KAUST Repository

    Wacks, Daniel H.; Chakraborty, Nilanjan; Klein, Markus; Arias, Paul G.; Im, Hong G.

    2016-01-01

    The distributions of flow topologies within the flames representing the corrugated flamelets, thin reaction zones, and broken reaction zone regimes of premixed turbulent combustion are investigated using direct numerical simulation data of statistically planar turbulent H-2-air flames with an equivalence ratio phi = 0.7. It was found that the diminishing influence of dilatation rate with increasing Karlovitz number has significant influences on the statistical behaviors of the first, second, and third invariants (i.e., P, Q, and R) of the velocity gradient tensor. These differences are reflected in the distributions of the flow topologies within the flames considered in this analysis. This has important consequences for those topologies that make dominant contributions to the scalar-turbulence interaction and vortex-stretching terms in the scalar dissipation rate and enstrophy transport equations, respectively. Detailed physical explanations are provided for the observed regime dependences of the flow topologies and their implications on the scalar dissipation rate and enstrophy transport.

  6. Turbulent structure of stably stratified inhomogeneous flow

    Science.gov (United States)

    Iida, Oaki

    2018-04-01

    Effects of buoyancy force stabilizing disturbances are investigated on the inhomogeneous flow where disturbances are dispersed from the turbulent to non-turbulent field in the direction perpendicular to the gravity force. Attaching the fringe region, where disturbances are excited by the artificial body force, a Fourier spectral method is used for the inhomogeneous flow stirred at one side of the cuboid computational box. As a result, it is found that the turbulent kinetic energy is dispersed as layered structures elongated in the streamwise direction through the vibrating motion. A close look at the layered structures shows that they are flanked by colder fluids at the top and hotter fluids at the bottom, and hence vertically compressed and horizontally expanded by the buoyancy related to the countergradient heat flux, though they are punctuated by the vertical expansion of fluids at the forefront of the layered structures, which is related to the downgradient heat flux, indicating that the layered structures are gravity currents. However, the phase between temperature fluctuations and vertical velocity is shifted by π/2 rad, indicating that temperature fluctuations are generated by the propagation of internal gravity waves.

  7. Laser-Induced Breakdown Spectroscopy (LIBS) for the Measurement of Spatial Structures and Fuel Distribution in Flames.

    Science.gov (United States)

    Kotzagianni, Maria; Kakkava, Eirini; Couris, Stelios

    2016-04-01

    Laser-induced breakdown spectroscopy (LIBS) is used for the mapping of local structures (i.e., reactants and products zones) and for the determination of fuel distribution by means of the local equivalence ratio ϕ in laminar, premixed air-hydrocarbon flames. The determination of laser threshold energy to induce breakdown in the different zones of flames is employed for the identification and demarcation of the local structures of a premixed laminar flame, while complementary results about fuel concentration were obtained from measurements of the cyanogen (CN) band Β(2)Σ(+)--Χ(2)Σ(+), (Δυ = 0) at 388.3 nm and the ratio of the atomic lines of hydrogen (Hα) and oxygen (O(I)), Hα/O. The combination of these LIBS-based methods provides a relatively simple to use, rapid, and accurate tool for online and in situ combustion diagnostics, providing valuable information about the fuel distribution and the spatial variations of the local structures of a flame. © The Author(s) 2016.

  8. Large-scale structures in turbulent Couette flow

    Science.gov (United States)

    Kim, Jung Hoon; Lee, Jae Hwa

    2016-11-01

    Direct numerical simulation of fully developed turbulent Couette flow is performed with a large computational domain in the streamwise and spanwise directions (40 πh and 6 πh) to investigate streamwise-scale growth mechanism of the streamwise velocity fluctuating structures in the core region, where h is the channel half height. It is shown that long streamwise-scale structures (> 3 h) are highly energetic and they contribute to more than 80% of the turbulent kinetic energy and Reynolds shear stress, compared to previous studies in canonical Poiseuille flows. Instantaneous and statistical analysis show that negative-u' structures on the bottom wall in the Couette flow continuously grow in the streamwise direction due to mean shear, and they penetrate to the opposite moving wall. The geometric center of the log layer is observed in the centerline with a dominant outer peak in streamwise spectrum, and the maximum streamwise extent for structure is found in the centerline, similar to previous observation in turbulent Poiseuille flows at high Reynolds number. Further inspection of time-evolving instantaneous fields clearly exhibits that adjacent long structures combine to form a longer structure in the centerline. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057031).

  9. Direct numerical simulation of turbulent, chemically reacting flows

    Science.gov (United States)

    Doom, Jeffrey Joseph

    This dissertation: (i) develops a novel numerical method for DNS/LES of compressible, turbulent reacting flows, (ii) performs several validation simulations, (iii) studies auto-ignition of a hydrogen vortex ring in air and (iv) studies a hydrogen/air turbulent diffusion flame. The numerical method is spatially non-dissipative, implicit and applicable over a range of Mach numbers. The compressible Navier-Stokes equations are rescaled so that the zero Mach number equations are discretely recovered in the limit of zero Mach number. The dependent variables are co--located in space, and thermodynamic variables are staggered from velocity in time. The algorithm discretely conserves kinetic energy in the incompressible, inviscid, non--reacting limit. The chemical source terms are implicit in time to allow for stiff chemical mechanisms. The algorithm is readily applicable to complex chemical mechanisms. Good results are obtained for validation simulations. The algorithm is used to study auto-ignition in laminar vortex rings. A nine species, nineteen reaction mechanism for H2/air combustion proposed by Mueller et al. [37] is used. Diluted H 2 at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratio, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto--ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a 'most reactive' mixture fraction, zeta MR (Mastorakos et al. [32]). Subsequent evolution of the flame is not predicted by zetaMR; a most reactive temperature TMR is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke

  10. Investigating Coherent Structures in the Standard Turbulence Models using Proper Orthogonal Decomposition

    International Nuclear Information System (INIS)

    Eliassen, Lene; Andersen, Søren

    2016-01-01

    The wind turbine design standards recommend two different methods to generate turbulent wind for design load analysis, the Kaimal spectra combined with an exponential coherence function and the Mann turbulence model. The two turbulence models can give very different estimates of fatigue life, especially for offshore floating wind turbines. In this study the spatial distributions of the two turbulence models are investigated using Proper Orthogonal Decomposition, which is used to characterize large coherent structures. The main focus has been on the structures that contain the most energy, which are the lowest POD modes. The Mann turbulence model generates coherent structures that stretches in the horizontal direction for the longitudinal component, while the structures found in the Kaimal model are more random in their shape. These differences in the coherent structures at lower frequencies for the two turbulence models can be the reason for differences in fatigue life estimates for wind turbines. (paper)

  11. Turbulent combustion modelization via a tabulation method of detailed kinetic chemistry coupled to Probability Density Function. Application to aeronautical engines; Modelisation de la combustion turbulente via une methode tabulation de la cinetique chimique detaillee couplee a des fonctions densites de probabilite. Application aux foyers aeronautiques

    Energy Technology Data Exchange (ETDEWEB)

    Rullaud, M

    2004-06-01

    A new modelization of turbulent combustion is proposed with detailed chemistry and probability density functions (PDFs). The objective is to capture temperature and species concentrations, mainly the CO. The PCM-FTC model, Presumed Conditional Moment - Flame Tabulated Chemistry, is based on the tabulation of laminar premixed and diffusion flames to capture partial pre-mixing present in aeronautical engines. The presumed PDFs is introduced to predict averaged values. The tabulation method is based on the analysis of the chemical structure of laminar premixed and diffusion flames. Hypothesis are presented, tested and validated with Sandia experimental data jet flames. Then, the model is introduced in a turbulent flow simulation software. Three configurations are retained to quantify the level of prediction of this formulation: the D and F-Flames of Sandia and lifted jet flames of methane/air of Stanford. A good agreement is observed between experiments and simulations. The validity of this method is then demonstrated. (author)

  12. Interfacial structures in confined cap-turbulent and churn-turbulent flows

    International Nuclear Information System (INIS)

    Sun Xiaodong; Kim, Seungjin; Cheng Ling; Ishii, Mamoru; Beus, Stephen G.

    2004-01-01

    The objective of the present work is to study and model the interfacial structure development of air-water two-phase flow in a confined flow passage. Experiments of a total of 13 flow conditions in cap-turbulent and churn-turbulent flow regimes are carried out in a vertical air-water upward two-phase flow experimental loop with a test section of 200 mm in width and 10 mm in gap. Miniaturized four-sensor conductivity probes are used to measure local two-phase parameters at three different elevations for each flow condition. Bubble characteristics captured by the probes are categorized into two groups in view of the two-group interfacial area transport equation, i.e., spherical/distorted bubbles as Group 1 and cap/churn-turbulent bubbles as Group 2. The acquired local parameters are time-averaged void fraction, interfacial velocity, bubble number frequency, interfacial area concentration, and bubble Sauter mean diameter for each group of bubbles. Also, the line-averaged and area-averaged data are presented and discussed in detail. The comparisons of these parameters at different elevations demonstrate the development of interfacial structures along the flow direction due to bubble interactions and the hydrodynamic effects. Furthermore, these data can serve as one part of the experimental data for investigation of the interfacial area transport in a confined two-phase flow

  13. Evaluation of Presumed Probability-Density-Function Models in Non-Premixed Flames by using Large Eddy Simulation

    International Nuclear Information System (INIS)

    Cao Hong-Jun; Zhang Hui-Qiang; Lin Wen-Yi

    2012-01-01

    Four kinds of presumed probability-density-function (PDF) models for non-premixed turbulent combustion are evaluated in flames with various stoichiometric mixture fractions by using large eddy simulation (LES). The LES code is validated by the experimental data of a classical turbulent jet flame (Sandia flame D). The mean and rms temperatures obtained by the presumed PDF models are compared with the LES results. The β-function model achieves a good prediction for different flames. The predicted rms temperature by using the double-δ function model is very small and unphysical in the vicinity of the maximum mean temperature. The clip-Gaussian model and the multi-δ function model make a worse prediction of the extremely fuel-rich or fuel-lean side due to the clip at the boundary of the mixture fraction space. The results also show that the overall prediction performance of presumed PDF models is better at mediate stoichiometric mixture fractions than that at very small or very large ones. (fundamental areas of phenomenology(including applications))

  14. Gas Temperature and Radiative Heat Transfer in Oxy-fuel Flames

    DEFF Research Database (Denmark)

    Bäckström, Daniel; Johansson, Robert; Andersson, Klas

    This work presents measurements of the gas temperature, including fluctuations, and its influence on the radiative heat transfer in oxy-fuel flames. The measurements were carried out in the Chalmers 100 kW oxy-fuel test unit. The in-furnace gas temperature was measured by a suction pyrometer...... on the radiative heat transfer shows no effect of turbulence-radiation interaction. However, by comparing with temperature fluctuations in other flames it can be seen that the fluctuations measured here are relatively small. Further research is needed to clarify to which extent the applied methods can account...

  15. Chemical structures of an n-butanol counterflow flame

    Energy Technology Data Exchange (ETDEWEB)

    Sarathy, S.M.; Thomson, M.J. [Toronto Univ., ON (Canada). Dept. of Mechanical and Industrial Engineering

    2007-07-01

    N-butanol, also known as biobutanol, is an attractive alternative biofuel that can replace gasoline in transportation applications. Biobutanol can be produced via the fermentation of sugars, starches, and lignocellulose obtained from agricultural feedstocks. Although biobutanol offers several advantages over ethanol, its detailed combustion characteristics are not well known. In order to determine the effect of fuel structure on combustion products, this paper presented the results of a study that examined the emission and temperature profiles of an n-butanol counterflow flame. The paper presented the experimental data and discussed the potential reaction mechanisms that rationalized the observed species profiles. It was found that significant quantities of acetylene, acetaldehyde, ethane, and propene were measured in the n-butanol flame and that the reaction pathways leading to the formation of these compounds were yet to be identified. In addition, significant concentrations of formaldehyde and acetaldehyde were found. Results will be utilized to validate a detailed chemical kinetic model for n-butanol combustion. 13 refs., 3 figs.

  16. Turbulent flame speeds and NOx kinetics of HHC fuels with contaminants and high dilution levels

    Energy Technology Data Exchange (ETDEWEB)

    Petersen, Eric; Krejci, Michael; Mathieu, Olivier; Vissotski, Andrew; Ravi, Sankar; Plichta, Drew; Sikes, Travis; Levacque, Anthony; Aul, Christopher; Petersen, Eric

    2012-09-30

    This progress report documents the second year of the project, from October 1, 2011 through September 30, 2012. Characterization of the new turbulent flame speed vessel design was completed. Turbulence statistics of three impellers with different geometric features were measured using particle image velocimetry inside a Plexiglas model (~1:1 scale) of a cylindrical flame speed vessel (30.5 cm ID × 35.6 cm L). With four impellers arranged in a central-symmetric configuration, turbulence intensities between 1.2 and 1.7 m/s with negligible mean flow (0.1u´) were attained at the lowest fan speeds. Acceptable ranges for homogeneity and isotropy ratios of the velocity fields were set within a narrow bandwidth near unity (0.9-1.1). Homogeneity ratios were unaffected by changes to the impeller geometry, and the prototype with the higher number of blades caused the flow to become anisotropic. The integral length scale of the flow fields varied between 27 and 20 mm, which correlates well with those typically observed inside a gas turbine combustor. The mechanism to independently vary the intensity level and the integral length scale was established, where turbulence intensity level was dependent on the rotational speed of the fan, and the integral length scale decreased with increasing blade pitch angle. Ignition delay times of H₂/O₂ mixtures highly diluted with Ar and doped with various amounts of N₂O (100, 400, 1600, 3200 ppm) were measured in a shock tube behind reflected shock waves over a wide range of temperatures (940-1675 K). The pressure range investigated during this work (around 1.6, 13, and 30 atm) allows studying the effect of N₂O on hydrogen ignition at pressure conditions that have never been heretofore investigated. Ignition delay times were decreased when N₂O was added to the mixture only for the higher nitrous oxide concentrations, and some changes in the activation energy were also observed at 1.5 and 30 atm. When it occurred, the decrease in

  17. Vertical structure of turbulence in offshore flow during RASEX

    DEFF Research Database (Denmark)

    Mahrt, L.; Vickers, D.; Edson, J.

    2001-01-01

    and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly...... with height and downward transport of turbulence energy toward the surface. With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained...

  18. Spatial structure of ion-scale plasma turbulence

    Directory of Open Access Journals (Sweden)

    Yasuhito eNarita

    2014-03-01

    Full Text Available Spatial structure of small-scale plasma turbulence is studied under different conditions of plasma parameter beta directly in the three-dimensional wave vector domain. Two independent approaches are taken: observations of turbulent magnetic field fluctuations in the solar wind measured by four Cluster spacecraft, and direct numerical simulations of plasma turbulence using the hybrid code AIKEF, both resolving turbulence on the ion kinetic scales. The two methods provide independently evidence of wave vector anisotropy as a function of beta. Wave vector anisotropy is characterized primarily by an extension of the energy spectrum in the direction perpendicular to the large-scale magnetic field. The spectrum is strongly anisotropic at lower values of beta, and is more isotropic at higher values of beta. Cluster magnetic field data analysis also provides evidence of axial asymmetry of the spectrum in the directions around the large-scale field. Anisotropy is interpreted as filament formation as plasma evolves into turbulence. Axial asymmetry is interpreted as the effect of radial expansion of the solar wind from the corona.

  19. Enstrophy transport conditional on local flow topologies in different regimes of premixed turbulent combustion

    KAUST Repository

    Papapostolou, Vassilios

    2017-09-11

    Enstrophy is an intrinsic feature of turbulent flows, and its transport properties are essential for the understanding of premixed flame-turbulence interaction. The interrelation between the enstrophy transport and flow topologies, which can be assigned to eight categories based on the three invariants of the velocity-gradient tensor, has been analysed here. The enstrophy transport conditional on flow topologies in turbulent premixed flames has been analysed using a Direct Numerical Simulation database representing the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) combustion regimes. The flame in the CF regime exhibits considerable flame-generated enstrophy, and the dilatation rate and baroclinic torque contributions to the enstrophy transport act as leading order sink and source terms, respectively. Consequently, flow topologies associated with positive dilatation rate values, contribute significantly to the enstrophy transport in the CF regime. By contrast, enstrophy decreases from the unburned to the burned gas side for the cases representing the TRZ and BRZ regimes, with diminishing influences of dilatation rate and baroclinic torque. The enstrophy transport in the TRZ and BRZ regimes is governed by the vortex-stretching and viscous dissipation contributions, similar to non-reacting flows, and topologies existing for all values of dilatation rate remain significant contributors.

  20. The structure of vortex tube segments in fluid turbulence

    International Nuclear Information System (INIS)

    Wang Lipo

    2011-01-01

    Geometrical description of the flow fields is an important direction to understand the physics of turbulence. Recently several new analysis approaches addressing the entire field properties have been developed, such as dissipation element analysis for the scalar fields and streamtube segment analysis (J. Fluid Mech. 2010, 648: 183-203) for the velocity vector field. By decomposing into a fundamental structure, i.e. stream-tube segments, the velocity field can be understood from the statistics of these relative simple units. Similar idea can be adopted to analyze the vorticity field. The classic concept of vortex tube has been remaining as a topic of essential importance in many aspects. However, the vortex tube structure is not complete to represent the entire turbulent fields, because of its ambiguous definition and small volume portion. This work presents tentatively the vorticitytube segment structure to overcome the existing deficiency. Vorticitytube segments reveal an inherent topology of turbulence vorticity fields. Based on statistics conditioned on different vorticitytube segments, some problems can be newly understood, such as the enstrophy production. Results hereof may also serve for turbulence modeling.

  1. Tabulated Combustion Model Development For Non-Premixed Flames

    Science.gov (United States)

    Kundu, Prithwish

    Turbulent non-premixed flames play a very important role in the field of engineering ranging from power generation to propulsion. The coupling of fluid mechanics and complicated combustion chemistry of fuels pose a challenge for the numerical modeling of these type of problems. Combustion modeling in Computational Fluid Dynamics (CFD) is one of the most important tools used for predictive modeling of complex systems and to understand the basic fundamentals of combustion. Traditional combustion models solve a transport equation of each species with a source term. In order to resolve the complex chemistry accurately it is important to include a large number of species. However, the computational cost is generally proportional to the cube of number of species. The presence of a large number of species in a flame makes the use of CFD computationally expensive and beyond reach for some applications or inaccurate when solved with simplified chemistry. For highly turbulent flows, it also becomes important to incorporate the effects of turbulence chemistry interaction (TCI). The aim of this work is to develop high fidelity combustion models based on the flamelet concept and to significantly advance the existing capabilities. A thorough investigation of existing models (Finite-rate chemistry and Representative Interactive Flamelet (RIF)) and comparative study of combustion models was done initially on a constant volume combustion chamber with diesel fuel injection. The CFD modeling was validated with experimental results and was also successfully applied to a single cylinder diesel engine. The effect of number of flamelets on the RIF model and flamelet initialization strategies were studied. The RIF model with multiple flamelets is computationally expensive and a model was proposed on the frame work of RIF. The new model was based on tabulated chemistry and incorporated TCI effects. A multidimensional tabulated chemistry database generation code was developed based on the 1

  2. Combustion chemistry of alcohols: Experimental and modeled structure of a premixed 2-methylbutanol flame

    KAUST Repository

    Lucassen, Arnas; Park, Sungwoo; Hansen, Nils; Sarathy, Mani

    2014-01-01

    This paper presents a detailed investigation of 2-methylbutanol combustion chemistry in low-pressure premixed flames. This chemistry is of particular interest to study because this compound is potentially a lignocellulosic-based, next-generation biofuel. The detailed chemical structure of a stoichiometric low-pressure (25 Torr) flame was determined using flame-sampling molecular-beam mass spectrometry. A total of 55 species were identified and subsequently quantitative mole fraction profiles as function of distance from the burner surface were determined. In an independent effort, a detailed flame chemistry model for 2-methylbutanol was assembled based on recent knowledge gained from combustion chemistry studies for butanol isomers ([Sarathy et al. Combust. Flame 159 (6) (2012) 2028-2055]) and iso-pentanol (3-methylbutanol) [Sarathy et al. Combust. Flame 160 (12) (2013) 2712-2728]. Experimentally determined and modeled mole fraction profiles were compared to demonstrate the model's capabilities. Examples of individual mole fraction profiles are discussed together with the most significant fuel consumption pathways to highlight the combustion chemistry of 2-methylbutanol. Discrepancies between experimental and modeling results are used to suggest areas where improvement of the kinetic model would be needed. © 2014.

  3. Combustion chemistry of alcohols: Experimental and modeled structure of a premixed 2-methylbutanol flame

    KAUST Repository

    Lucassen, Arnas

    2014-06-14

    This paper presents a detailed investigation of 2-methylbutanol combustion chemistry in low-pressure premixed flames. This chemistry is of particular interest to study because this compound is potentially a lignocellulosic-based, next-generation biofuel. The detailed chemical structure of a stoichiometric low-pressure (25 Torr) flame was determined using flame-sampling molecular-beam mass spectrometry. A total of 55 species were identified and subsequently quantitative mole fraction profiles as function of distance from the burner surface were determined. In an independent effort, a detailed flame chemistry model for 2-methylbutanol was assembled based on recent knowledge gained from combustion chemistry studies for butanol isomers ([Sarathy et al. Combust. Flame 159 (6) (2012) 2028-2055]) and iso-pentanol (3-methylbutanol) [Sarathy et al. Combust. Flame 160 (12) (2013) 2712-2728]. Experimentally determined and modeled mole fraction profiles were compared to demonstrate the model\\'s capabilities. Examples of individual mole fraction profiles are discussed together with the most significant fuel consumption pathways to highlight the combustion chemistry of 2-methylbutanol. Discrepancies between experimental and modeling results are used to suggest areas where improvement of the kinetic model would be needed. © 2014.

  4. Ignition parameters and early flame kernel development of laser-ignited combustible gas mixtures

    International Nuclear Information System (INIS)

    Kopecek, H.; Wintner, E.; Ruedisser, D.; Iskra, K.; Neger, T.

    2002-01-01

    Full text: Laser induced breakdown of focused pulsed laser radiation, the subsequent plasma formation and thermalization offers a possibility of ignition of combustible gas mixtures free from electrode interferences, an arbitrary choice of the location within the medium and exact timing regardless of the degree of turbulence. The development and the decreasing costs of solid state laser technologies approach the pay-off for the higher complexity of such an ignition system due to several features unique to laser ignition. The feasability of laser ignition was demonstrated in an 1.5 MW(?) natural gas engine, and several investigations were performed to determine optimal ignition energies, focus shapes and laser wavelengths. The early flame kernel development was investigated by time resolved planar laser induced fluorescence of the OH-radical which occurs predominantly in the flame front. The flame front propagation showed typical features like toroidal initial flame development, flame front return and highly increased flame speed along the laser focus axis. (author)

  5. Application of a robust and efficient Lagrangian particle scheme to soot transport in turbulent flames

    KAUST Repository

    Attili, Antonio

    2013-09-01

    A Lagrangian particle scheme is applied to the solution of soot dynamics in turbulent nonpremixed flames. Soot particulate is described using a method of moments and the resulting set of continuum advection-reaction equations is solved using the Lagrangian particle scheme. The key property of the approach is the independence between advection, described by the movement of Lagrangian notional particles along pathlines, and internal aerosol processes, evolving on each notional particle via source terms. Consequently, the method overcomes the issues in Eulerian grid-based schemes for the advection of moments: errors in the advective fluxes pollute the moments compromising their realizability and the stiffness of source terms weakens the stability of the method. The proposed scheme exhibits superior properties with respect to conventional Eulerian schemes in terms of stability, accuracy, and grid convergence. Taking into account the quality of the solution, the Lagrangian approach can be computationally more economical than commonly used Eulerian schemes as it allows the resolution requirements dictated by the different physical phenomena to be independently optimized. Finally, the scheme posseses excellent scalability on massively parallel computers. © 2013 Elsevier Ltd.

  6. Large Eddy Simulations of Two-phase Turbulent Reactive Flows in IC Engines

    Science.gov (United States)

    Banaeizadeh, Araz; Schock, Harold; Jaberi, Farhad

    2008-11-01

    The two-phase filtered mass density function (FMDF) subgrid-scale (SGS) model is used for large-eddy simulation (LES) of turbulent spray combustion in internal combustion (IC) engines. The LES/FMDF is implemented via an efficient, hybrid numerical method. In this method, the filtered compressible Navier-Stokes equations in curvilinear coordinate systems are solved with a generalized, high-order, multi-block, compact differencing scheme. The spray and the FMDF are implemented with Lagrangian methods. The reliability and the consistency of the numerical methods are established for different IC engines and the complex interactions among mean and turbulent velocity fields, fuel droplets and combustion are shown to be well captured with the LES/FMDF. In both spark-ignition/direct-injection and diesel engines, the droplet size and velocity distributions are found to be modified by the unsteady, vortical motions generated by the incoming air during the intake stroke. In turn, the droplets are found to change the in-cylinder flow structure. In the spark-ignition engine, flame propagation is similar to the experiment. In the diesel engine, the maximum evaporated fuel concentration is near the cylinder wall where the flame starts, which is again consistent with the experiment.

  7. Inadequacy representation of flamelet-based RANS model for turbulent non-premixed flame

    Science.gov (United States)

    Lee, Myoungkyu; Oliver, Todd; Moser, Robert

    2017-11-01

    Stochastic representations for model inadequacy in RANS-based models of non-premixed jet flames are developed and explored. Flamelet-based RANS models are attractive for engineering applications relative to higher-fidelity methods because of their low computational costs. However, the various assumptions inherent in such models introduce errors that can significantly affect the accuracy of computed quantities of interest. In this work, we develop an approach to represent the model inadequacy of the flamelet-based RANS model. In particular, we pose a physics-based, stochastic PDE for the triple correlation of the mixture fraction. This additional uncertain state variable is then used to construct perturbations of the PDF for the instantaneous mixture fraction, which is used to obtain an uncertain perturbation of the flame temperature. A hydrogen-air non-premixed jet flame is used to demonstrate the representation of the inadequacy of the flamelet-based RANS model. This work was supported by DARPA-EQUiPS(Enabling Quantification of Uncertainty in Physical Systems) program.

  8. Regeneration of near-wall turbulence structures

    Science.gov (United States)

    Hamilton, James M.; Kim, John J.; Waleffe, Fabian A.

    1993-01-01

    An examination of the regeneration mechanisms of near-wall turbulence and an attempt to investigate the critical Reynolds number conjecture of Waleffe & Kim is presented. The basis is an extension of the 'minimal channel' approach of Jimenez and Moin which emphasizes the near-wall region and further reduces the complexity of the turbulent flow. Reduction of the flow Reynolds number to the minimum value which will allow turbulence to be sustained has the effect of reducing the ratio of the largest scales to the smallest scales or, equivalently, of causing the near-wall region to fill more of the area between the channel walls. In addition, since each wall may have an active near-wall region, half of the channel is always somewhat redundant. If a plane Couette flow is instead chosen as the base flow, this redundancy is eliminated: the mean shear of a plane Couette flow has a single sign, and at low Reynolds numbers, the two wall regions share a single set of structures. A minimal flow with these modifications possesses, by construction, the strongest constraints which allow sustained turbulence, producing a greatly simplified flow in which the regeneration process can be examined.

  9. Flame Structure of Vitiated Fuel-Rich Inverse Diffusion Flames in a Cross-Flow (Postprint)

    Science.gov (United States)

    2011-12-01

    downstream of the slot. The flame length increases as the blowing ratio increases as a result of the greater mass of air which reacts. Ignition of...attributed to the greater penetration of the jet into the cross-stream. It is noted that the flame lengths are similar for the different blowing ratios

  10. Turbulent intermittent structure in non-homogeneous non-local flows

    Science.gov (United States)

    Mahjoub, O. B.; Castilla, R.; Vindel, J. M.; Redondo, J. M.

    2010-05-01

    Data from SABLES98 experimental campaign have been used in order to study the influence of stability (from weak to strong stratification) on intermittency [1]. Standard instrumentation, 14 thermocouples and 3 sonic anemometers at three levels (5.8, 13.5 and 32 m) were available in September 1998 and calculations are done in order to evaluate structure functions and the scale to scale characteristics. Using BDF [2-4] as well as other models of cascades, the spectral equilibrium values were used to calculate fluxes of momentum and heat as well as non-homogeneous models and the turbulent mixing produced. The differences in structure and higher order moments between stable, convective and neutral turbulence were used to identify differences in turbulent intermittent mixing and velocity PDF's. The intermittency of atmospheric turbulence in strongly stable situations affected by buoyancy and internal waves are seen to modify the structure functions exponents and intermittency, depending on the modulus of the Richardson's number,Ri, as well as of the Monin-Obukhov and Ozmidov lengthscales. The topological aspects of the turbulence affected by stratification reduce the vertical length-scales to a maximum described by the Thorpe and the Ozmidov lenth-scales, but intermittency, Kurtosis and other higher order descriptors of the turbulence based on spectral wavelet analysis are also affected in a complex way [5,6]. The relationship between stratification, intermittency, µ(Ri) and the fractal dimension of the stable flows and between the dispersion, the fractal dimension are discussed. The data analyzed is from the campaign SABLES-98 at the north-west Iberian Peninsula plateau.(Cuxart et al. 2000). Conditional statistics of the relationship between µ(Ri) are confirmed as in (Vindel et al 2008)[4] and compared with laboratory experiments and with 2D-3D aspects of the turbulence cascade. The use of BDF [3] model comparing the corresponding relative scaling exponents which are

  11. The Multifractal Structure of Small-Scale Artificial Ionospheric Turbulence

    Directory of Open Access Journals (Sweden)

    Vybornov F. I.

    2013-03-01

    Full Text Available We present the results of investigation of a multifractal structure of the artificial ionospheric turbulence when the midlatitude ionosphere is affected by high-power radio waves. The experimental studies were performed on the basis of the SURA heating facility with the help of radio sounding of the disturbed region of ionospheric plasma by signals from the Earth’s orbital satellities. In the case of vertical radio sounding of the disturbed ionosphere region, the measured multipower and generalized multifractal spectra of turbulence coincide well with similar multifractal characteristics of the ionosperic turbulence under the natural conditions. In the case of oblique sounding of the disturbance region at small angles between the line of sight to the satellite and the direction of the Earth’s magnetic field, a nonuniform structure of the small-scale turbulence with a relatively narrow multipower spectrum and small variations in the generalized multifractal spectrum of the electron density was detected.

  12. Simultaneous multipoint measurements of density gradients and temperature in a flame. Progress report, July 1, 1982-January 31, 1983

    International Nuclear Information System (INIS)

    Chang, R.K.; Chu, B.T.; Long, M.B.

    1983-02-01

    An account is given of recent progress in the development of nonintrusive optical diagnostic techniques and the application of these techniques to turbulent combustion systems. The primary focus of the work over the past year has been in: (1) the use of a broadband rotational CARS technique for the measurement of temperature in flames, and (2) the use of spontaneous Raman scattering to simultaneously map out the fuel gas concentration in a turbulent diffusion flame at 2500 points in a plane intersecting the flow. A summary of new results in each of these areas is given

  13. STRUCTURES OF TURBULENT VORTICES AND THEIR INFLUENCE ON FLOW PROPERTIES

    Directory of Open Access Journals (Sweden)

    Alfonsas Rimkus

    2015-03-01

    Full Text Available In spite of the many investigations that have been conducted on turbulent flows, the generation and development of turbulent vortices has not been investigated sufficiently yet. This prevents to understand well the processes involved in the flow. That is unfavorable for the further investigations. The developing vortex structures are interacting, and this needs to be estimated. Physical summing of velocities, formed by all structures, can be unfavorable for investigations, therefore they must be separated; otherwise bias errors can occur. The difficulty for investigations is that the widely employed Particle Image Velocity (PIV method, when a detailed picture of velocity field picture is necessary, can provide photos covering only a short interval of flow, which can’t include the largest flow structures, i.e. macro whirlpools. Consequently, action of these structures could not be investigated. Therefore, in this study it is tried to obtain the necessary data about the flow structure by analyzing the instantaneous velocity measurements by 3D means, which lasts for several minutes, therefore the existence and interaction of these structures become visible in measurement data. The investigations conducted in this way have been already discussed in the article, published earlier. Mostly the generation and development of bottom vortices was analyzed. In this article, the analysis of these turbulent velocity measurements is continued and the additional data about the structure of turbulent vortices is obtained.

  14. A simplified hydrodynamic model of hydrogen flame propagation in reactor vessels

    International Nuclear Information System (INIS)

    Baer, M.; Ratzel, A.

    1983-01-01

    A hydrodynamic model for hydrogen flame propagation in reactor geometries is presented. This model is consistent with the theory of slow combustion in which the gasdynamic field equations are treated in the limit of small Mach numbers. To the lowest order, pressure is spatially uniform. The flame is treated as a density and entropy discontinuity which propagates at prescribed burning velocities, corresponding to laminar or turbulent flames. Radiation cooling of the burned combustion gases and possible preheating of the unburned gases during propagation of the flame is included using a molecular gas-band thermal radiation model. Application of this model has been developed for 1-D variable area flame propagation. Multidimensional effects induced by hydrodynamics and buoyancy are introduced as a correction to the burn velocity (which reflects a modification of planar flame surface to a distorted surface) using experimentally measured pressure-rise time data for hydrogen/air deflagrations in cylindrical vessels. This semianalytical model of flame propagation reduces to a set of ordinary differential equations which describes the temporal variations of vessel pressure, burned volume and gas entropy. The thermodynamic state of the burned gas immediately following the flame is determined using an isobaric Hugoniot relationship. At other locations the burned gas thermodynamic states are determined using a Lagrangian particle tracking method. Results of a computer code using the method are presented

  15. Large eddy simulation of turbulent premixed combustion flows over backward facing step

    Energy Technology Data Exchange (ETDEWEB)

    Park, Nam Seob [Yuhan University, Bucheon (Korea, Republic of); Ko, Sang Cheol [Jeju National University, Jeju (Korea, Republic of)

    2011-03-15

    Large eddy simulation (LES) of turbulent premixed combustion flows over backward facing step has been performed using a dynamic sub-grid G-equation flamelet model. A flamelet model for the premixed flame is combined with a dynamic sub-grid combustion model for the filtered propagation of flame speed. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model in a complex turbulent premixed combustion flow. For the purpose of validating the LES combustion model, the LES of isothermal and reacting shear layer formed at a backward facing step is carried out. The calculated results are compared with the experimental results, and a good agreement is obtained.

  16. Large eddy simulation of turbulent premixed combustion flows over backward facing step

    International Nuclear Information System (INIS)

    Park, Nam Seob; Ko, Sang Cheol

    2011-01-01

    Large eddy simulation (LES) of turbulent premixed combustion flows over backward facing step has been performed using a dynamic sub-grid G-equation flamelet model. A flamelet model for the premixed flame is combined with a dynamic sub-grid combustion model for the filtered propagation of flame speed. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model in a complex turbulent premixed combustion flow. For the purpose of validating the LES combustion model, the LES of isothermal and reacting shear layer formed at a backward facing step is carried out. The calculated results are compared with the experimental results, and a good agreement is obtained

  17. On mathematical modeling and numerical simulation of chemical kinetics in turbulent lean premixed combustion

    Energy Technology Data Exchange (ETDEWEB)

    Lilleberg, Bjorn

    2011-07-01

    This thesis investigates turbulent reacting lean premixed flows with detailed treatment of the chemistry. First, the fundamental equations which govern laminar and turbulent reacting flows are presented. A perfectly stirred reactor numerical code is developed to investigate the role of unmixedness and chemical kinetics in driving combustion instabilities. This includes both global single-step and detailed chemical kinetic mechanisms. The single-step mechanisms predict to some degree a similar behavior as the detailed mechanisms. However, it is shown that simple mechanisms can by themselves introduce instabilities. Magnussens Eddy Dissipation Concept (EDC) for turbulent combustion is implemented in the open source CFD toolbox OpenFOAM R for treatment of both fast and detailed chemistry. RANS turbulence models account for the turbulent compressible flow. A database of pre-calculated chemical time scales, which contains the influence of chemical kinetics, is coupled to EDC with fast chemistry to account for local extinction in both diffusion and premixed flames. Results are compared to fast and detailed chemistry calculations. The inclusion of the database shows significantly better results than the fast chemistry calculations while having a comparably small computational cost. Numerical simulations of four piloted lean premixed jet flames falling into the 'well stirred reactor/broken reaction zones' regime, with strong finite-rate chemistry effects, are performed. Measured and predicted scalars compare well for the two jets with the lowest velocities. The two jets with the highest velocities experience extinction and reignition, and the simulations are able to capture the decrease and increase of the OH mass fractions, but the peak values are higher than in the experiments. Also numerical simulations of a lean premixed lifted jet flame with high sensitivity to turbulence modeling and chemical kinetics are performed. Limitations of the applied turbulence and

  18. FLAME facility: The effect of obstacles and transverse venting on flame acceleration and transition on detonation for hydrogen-air mixtures at large scale

    International Nuclear Information System (INIS)

    Sherman, M.P.; Tieszen, S.R.; Benedick, W.B.

    1989-04-01

    This report describes research on flame acceleration and deflagration-to-detonation transition (DDT) for hydrogen-air mixtures carried out in the FLAME facility, and describes its relevance to nuclear reactor safety. Flame acceleration and DDT can generate high peak pressures that may cause failure of containment. FLAME is a large rectangular channel 30.5 m long, 2.44 m high, and 1.83 m wide. It is closed on the ignition end and open on the far end. The three test variables were hydrogen mole fraction (12--30%), degree of transverse venting (by moving steel top plates---0%, 13%, and 50%), and the absence or presence of certain obstacles in the channel (zero or 33% blockage ratio). The most important variable was the hydrogen mole fraction. The presence of the obstacles tested greatly increased the flame speeds, overpressures, and tendency for DDT compared to similar tests without obstacles. Different obstacle configurations could have greater or lesser effects on flame acceleration and DDT. Large degrees of transverse venting reduced the flame speeds, overpressures, and possibility of DDT. For small degrees of transverse venting (13% top venting), the flame speeds and overpressures were higher than for no transverse venting with reactive mixtures (>18% H 2 ), but they were lower with leaner mixtures. The effect of the turbulence generated by the flow out the vents on increasing flame speed can be larger than the effect of venting gas out of the channel and hence reducing the overpressure. With no obstacles and 50% top venting, the flame speeds and overpressures were low, and there was no DDT. For all other cases, DDT was observed above some threshold hydrogen concentration. DDT was obtained at 15% H 2 with obstacles and no transverse venting. 67 refs., 62 figs

  19. Gaussian free turbulence: structures and relaxation in plasma models

    International Nuclear Information System (INIS)

    Gruzinov, A.V.

    1993-01-01

    Free-turbulent relaxation in two-dimensional MHD, the degenerate Hasegawa-Mima equation and a two-dimensional microtearing model are studied. The Gibbs distributions of these three systems can be completely analyzed, due to the special structure of their invariants and due to the existence of ultraviolet catastrophe. The free-turbulent field is seen to be a sum of a certain coherent structure (statistical attractor) and Gaussian random noise. Two-dimensional current layers are shown to be statistical attractors in 2D MHD. (author)

  20. Experimental study of flame characteristics during the combustion of certain types of liquid hydrocarbon fuels

    Science.gov (United States)

    Loboda, E. L.; Anufriev, I. S.; Agafontsev, M. V.; Kop'yev, E. P.; Reyno, Vladimir

    2017-11-01

    The paper represents experimental studying the scales of turbulent vortices in diffusion flames by using the measurements of thermodynamic characteristics obtained by thermography and the aerodynamic characteristics obtained by the PIV method.

  1. Automatic, non-intrusive, flame detection in pipelines

    Energy Technology Data Exchange (ETDEWEB)

    Morgan, M.D.; Mehta, S.A.; Moore, R.G. [Calgary Univ., AB (Canada). Dept. of Chemical and Petroleum Engineering; Al-Himyary, T.J. [Al-Himyary Consulting Inc., Calgary, AB (Canada)

    2004-07-01

    Flames have been known to occur within small diameter pipes operating under conditions of high turbulent flow. Although there are several methods of flame detection, few offer remote, non-line-of-site detection. In particular, combustion cannot be detected in cases where flammable mixtures are carried in flare lines, storage tank vents, air drilling or improperly designed purging operations. Combustion noise is being examined as a means to address this problem. A study was conducted in which flames within a small diameter tube were automatically detected using high speed pressure measurements and a newly developed algorithm. Commercially available, high-pressure, dynamic-pressure transducers were used for the measurements. The results of an experimental study showed that combustion noise can be distinguished from other sources of noise by its inverse power law relationship with frequency. This paper presented a newly developed algorithm which provides early detection of flames when combined with high-speed pressure measurements. The algorithm can also separate combustion noise automatically from other sources of noise when combined with other filters. In this study, the noise generated by a fluttering check valve was attenuated using a stop band filter. This detection method was found to be very reliable under the conditions tests, as long as there was no flow restriction between the sensor and the flame. A flow restriction would have resulted in the detection of only the strongest flame noise. It was shown that acoustic flame detection can be applied successfully in flare stacks, industrial burners and turbine combustors. It can be 15 times more sensitive than optical or electrical methods in diagnosing combustion problems with lean burning combustors. It may also be the only method available in applications that require remote, non-line-of-sight detection. 11 refs., 3 tabs., 15 figs.

  2. Ion structure and sequence of ion formation in acetylene flames

    Energy Technology Data Exchange (ETDEWEB)

    Larionova, I.A.; Fialkov, B.S.; Kalinich, K.YA.; Fialkov, A.B.; Ospanov, B.S.

    1993-06-01

    Results of a study of the ion composition of acetylene-air flames burning at low pressures are reported. Data on ion formation are compared for flames of saturated hydrocarbons, oxygen-containing fuels, and acetylene. It is shown that the characteristics of ion formation in the flame front and directly ahead of it are similar to those observed in flames of other fuels. These characteristics, however, are different in the low-temperature region. 9 refs.

  3. Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

    KAUST Repository

    Hong, Seunghyuck; Shanbhogue, Santosh J.; Ghoniem, Ahmed F.

    2015-01-01

    ) and chemiluminescence measurements for C3H8/H2/air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low

  4. Large eddy simulations of coal jet flame ignition using the direct quadrature method of moments

    Science.gov (United States)

    Pedel, Julien

    The Direct Quadrature Method of Moments (DQMOM) was implemented in the Large Eddy Simulation (LES) tool ARCHES to model coal particles. LES coupled with DQMOM was first applied to nonreacting particle-laden turbulent jets. Simulation results were compared to experimental data and accurately modeled a wide range of particle behaviors, such as particle jet waviness, spreading, break up, particle clustering and segregation, in different configurations. Simulations also accurately predicted the mean axial velocity along the centerline for both the gas phase and the solid phase, thus demonstrating the validity of the approach to model particles in turbulent flows. LES was then applied to the prediction of pulverized coal flame ignition. The stability of an oxy-coal flame as a function of changing primary gas composition (CO2 and O2) was first investigated. Flame stability was measured using optical measurements of the flame standoff distance in a 40 kW pilot facility. Large Eddy Simulations (LES) of the facility provided valuable insight into the experimentally observed data and the importance of factors such as heterogeneous reactions, radiation or wall temperature. The effects of three parameters on the flame stand-off distance were studied and simulation predictions were compared to experimental data using the data collaboration method. An additional validation study of the ARCHES LES tool was then performed on an air-fired pulverized coal jet flame ignited by a preheated gas flow. The simulation results were compared qualitatively and quantitatively to experimental observations for different inlet stoichiometric ratios. LES simulations were able to capture the various combustion regimes observed during flame ignition and to accurately model the flame stand-off distance sensitivity to the stoichiometric ratio. Gas temperature and coal burnout predictions were also examined and showed good agreement with experimental data. Overall, this research shows that high

  5. 3rd International Workshop on Turbulent Spray Combustion

    CERN Document Server

    Gutheil, Eva

    2014-01-01

    This book reflects the results of the 2nd and 3rd International Workshops on Turbulent Spray Combustion. The focus is on progress in experiments and numerical simulations for two-phase flows, with emphasis on spray combustion. Knowledge of the dominant phenomena and their interactions allows development of predictive models and their use in combustor and gas turbine design. Experts and young researchers present the state-of-the-art results, report on the latest developments and exchange ideas in the areas of experiments, modelling and simulation of reactive multiphase flows. The first chapter reflects on flame structure, auto-ignition and atomization with reference to well-characterized burners, to be implemented by modellers with relative ease. The second chapter presents an overview of first simulation results on target test cases, developed at the occasion of the 1st International Workshop on Turbulent Spray Combustion. In the third chapter, evaporation rate modelling aspects are covered, while the fourth ...

  6. Flame Structure and Dynamics for an Array of Premixed Methane-Air Jets

    Science.gov (United States)

    Nigam, Siddharth P.; Lapointe, Caelan; Christopher, Jason D.; Wimer, Nicholas T.; Hayden, Torrey R. S.; Rieker, Gregory B.; Hamlington, Peter E.

    2017-11-01

    Premixed flames have been studied extensively, both experimentally and computationally, and their properties are reasonably well characterized for a range of conditions and configurations. However, the premixed combustion process is potentially much more difficult to predict when many such flames are arranged in a closely spaced array. These arrays must be better understood, in particular, for the design of industrial burners used in chemical and heat treatment processes. Here, the effects of geometric array parameters (e.g., angle and diameter of jet inlets, number of inlets and their respective orientation) and operating conditions (e.g., jet velocities, fuel-air ratio) on flame structure and dynamics are studied using large eddy simulations (LES). The simulations are performed in OpenFOAM using multi-step chemistry for a methane-air mixture, and temperature and chemical composition fields are characterized for a variety of configurations as functions of height above the array. Implications of these results for the design and operation of industrial burners are outlined.

  7. Effects of Turbulent Reynolds Number on the Displacement Speed Statistics in the Thin Reaction Zones Regime of Turbulent Premixed Combustion

    Directory of Open Access Journals (Sweden)

    Nilanjan Chakraborty

    2011-01-01

    nature of the correlations remains unaffected. The dependence of displacement speed on strain rate and curvature is found to weaken with increasing turbulent Reynolds number when either Damköhler or Karlovitz number is held constant, but the qualitative nature of the correlation remains unaltered. The implications of turbulent Reynolds number effects in the context of Flame Surface Density (FSD modelling have also been addressed, with emphasis on the influence of displacement speed on the curvature and propagation terms in the FSD balance equation.

  8. Autoignited laminar lifted flames of methane/hydrogen mixtures in heated coflow air

    KAUST Repository

    Choi, Byungchul

    2012-04-01

    Autoignited lifted flame behavior in laminar jets of methane/hydrogen mixture fuels has been investigated experimentally in heated coflow air. Three regimes of autoignited lifted flames were identified depending on initial temperature and hydrogen to methane ratio. At relatively high initial temperature, addition of a small amount of hydrogen to methane improved ignition appreciably such that the liftoff height decreased significantly. In this hydrogen-assisted autoignition regime, the liftoff height increased with jet velocity, and the characteristic flow time - defined as the ratio of liftoff height to jet velocity - correlated well with the square of the adiabatic ignition delay time. At lower temperature, the autoignited lifted flame demonstrated a unique feature in that the liftoff height decreased with increasing jet velocity. Such behavior has never been observed in lifted laminar and turbulent jet flames. A transition regime existed between these two regimes at intermediate temperature. © 2011 The Combustion Institute.

  9. Plasma turbulence. Structure formation, selection rule, dynamic response and dynamics transport

    International Nuclear Information System (INIS)

    Ito, Sanae I.

    2010-01-01

    The five-year project of Grant-in-Aid for Specially Promoted Research entitled general research on the structure formation and selection rule in plasma turbulence had brought many outcomes. Based on these outcomes, the Grant-in-Aid for Scientific Research (S) program entitled general research on dynamic response and dynamic transport in plasma turbulence has started. In the present paper, the state-of-the-art of the research activities on the structure formation, selection rule and dynamics in plasma turbulence are reviewed with reference to outcomes of these projects. (author)

  10. Measurements of a high-luminosity flame structure by a shuttered PIV system

    International Nuclear Information System (INIS)

    Li, Yueh-Heng; Wu, Chih-Yung; Chen, Bi-Chian; Chao, Yei-Chin

    2008-01-01

    It is difficult to measure the velocity distribution inside a high-luminosity flame by using the particle image velocimetry (PIV) system with a double-shutter mode CCD camera. The second raw image of the PIV image pair is usually contaminated by flame emission. The main cause of the problem is an excess exposure time which lets the flame emission overlap the particle image in the second frame. If the flame-contamination problem is not significant, for example in faint flames, digital image processing can improve the image to an acceptable level. Nevertheless, when the PIV technique is applied to high-luminosity flames, the second raw particle image would be contaminated by flame emission. In this paper, incorporating a mechanical shutter in the PIV system with a double-shutter CCD camera is proposed to improve PIV measurements in high-luminosity flames. Measurements in faint, high-luminosity as well as very bright flames were tested. The results show that the present setup can accurately resolve the flow velocity field inside the flame cone, through the flame and in the post flame zone for all the flame conditions analyzed. The velocity distributions and streamline patterns measured by the present equipment are reasonable and meaningful

  11. Stabilization and structure of n-heptane tribrachial flames in axisymmetric laminar jets

    KAUST Repository

    Bisetti, Fabrizio

    2015-01-01

    A set of tribrachial flames of n-heptane/air is simulated with finite rate chemistry and detailed transport in a realistic laminar jet configuration for which experimental data are available. The flames differ by the temperature of the unburnt mixture and stabilization height, which controls the mixture fraction gradient ahead of the flame front. The simulations reproduce the lift-off heights in the experiments, showing that the flame stabilizes further downstream as the unburnt temperature decreases. For the lowest unburnt temperature, resulting in a weak mixture fraction gradient at the tribrachial point, positive stretch along the rich premixed wing leads to an increase in the rate of chemical reaction in the whole flame. The tribrachial flame burning velocity exceeds that in the unstretched, one-dimensional flame. For the highest temperature, the flame stabilizes closest to the nozzle. Large flame tilt, large mixture fraction gradient, and small radius of curvature lead to a reduction in the heat release rate and the flame propagates slower than its one-dimensional counterpart. The observed behavior is explained with a detailed analysis of the flame geometry, differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front. © 2014 The Combustion Institute.

  12. Effect of Electric Field on Outwardly Propagating Spherical Flame

    KAUST Repository

    Mannaa, Ossama

    2012-06-01

    The thesis comprises effects of electric fields on a fundamental study of spheri­cal premixed flame propagation.Outwardly-propagating spherical laminar premixed flames have been investigated in a constant volume combustion vessel by applying au uni-directional electric potential.Direct photography and schlieren techniques have been adopted and captured images were analyzed through image processing. Unstretched laminar burning velocities under the influence of electric fields and their associated Markstein length scales have been determined from outwardly prop­agating spherical flame at a constant pressure. Methane and propane fuels have been tested to assess the effect of electric fields on the differential diffusion of the two fuels.The effects of varying equivalence ratios and applied voltages have been in­vestigated, while the frequency of AC was fixed at 1 KHz. Directional propagating characteristics were analyzed to identify the electric filed effect. The flame morphology varied appreciably under the influence of electric fields which in turn affected the burning rate of mixtures.The flame front was found to propagate much faster toward to the electrode at which the electric fields were supplied while the flame speeds in the other direction were minimally influenced. When the voltage was above 7 KV the combustion is markedly enhanced in the downward direction since intense turbulence is generated and as a result the mixing process or rather the heat and mass transfer within the flame front will be enhanced.The com­bustion pressure for the cases with electric fields increased rapidly during the initial stage of combustion and was relatively higher since the flame front was lengthened in the downward direction.

  13. PIV Measurement of Transient 3-D (Liquid and Gas Phases) Flow Structures Created by a Spreading Flame over 1-Propanol

    Science.gov (United States)

    Hassan, M. I.; Kuwana, K.; Saito, K.

    2001-01-01

    In the past, we measured three-D flow structure in the liquid and gas phases that were created by a spreading flame over liquid fuels. In that effort, we employed several different techniques including our original laser sheet particle tracking (LSPT) technique, which is capable of measuring transient 2-D flow structures. Recently we obtained a state-of-the-art integrated particle image velocimetry (IPIV), whose function is similar to LSPT, but it has an integrated data recording and processing system. To evaluate the accuracy of our IPIV system, we conducted a series of flame spread tests using the same experimental apparatus that we used in our previous flame spread studies and obtained a series of 2-D flow profiles corresponding to our previous LSPT measurements. We confirmed that both LSPT and IPIV techniques produced similar data, but IPIV data contains more detailed flow structures than LSPT data. Here we present some of newly obtained IPIV flow structure data, and discuss the role of gravity in the flame-induced flow structures. Note that the application of IPIV to our flame spread problems is not straightforward, and it required several preliminary tests for its accuracy including this IPIV comparison to LSPT.

  14. Wall-attached structures of streamwise velocity fluctuations in turbulent boundary layer

    Science.gov (United States)

    Hwang, Jinyul; Sung, Hyung Jin

    2017-11-01

    The wall-attached structures of streamwise velocity fluctuations (u) are explored using direct numerical simulation data of turbulent boundary layer at Reτ = 1000 . We identify the structures of u, which are extended close to the wall. Their height (ly) ranges from the near-wall region to the edge of turbulent boundary layer. They are geometrically self-similar in a sense that the length and width of the structures are proportional to the distance from the wall. The population density of the attached structures shows that the tall attached structures (290 wall. The wall-attached structures of u identified in the present work are a proper candidate for Townsend's attached eddy hypothesis and these structures exist in the low Reynolds number turbulent boundary layer. This work was supported by the Creative Research Initiatives (No. 2017-013369) program of the National Research Foundation of Korea (MSIP) and supported by the Supercomputing Center (KISTI).

  15. Thermal fluid characteristics in diffusion flame formed by coaxial flow configuration

    Energy Technology Data Exchange (ETDEWEB)

    Torii, S. [Kumamoto Univ., Kumamoto (Japan). Dept. of Mechanical Engineering and Materials Science

    2005-07-01

    A numerical and experimental study was performed on the thermal transport phenomena of turbulent jet diffusion flames formed by coaxial flow configuration. Consideration was given to the effect of the flow rates of air and fuel on the flame morphology. It was noted that as the air flow rate increases, the augmentation of flow shear effect exerted on the shear layer form between the flame jet and the air flow induced the fuel-to-air mixture. Thermal diffusion was amplified with an increase in the Reynolds number. As the velocity ratio was increased, the streamwise velocity gradient along the radial axis was intensified, resulting in an amplification of thermal diffusion. Details of the experimental apparatus and method were provided, along with governing equations and numerical methods. It was concluded that the suppression of the flame length and an extension of flame blowoff limit caused an annular jet diffusion flame. An increase in the velocity ratio of air to fuel showed the blue flame. When cold and hot gases are injected along the same direction from the annular channel, the flow pattern and isotherms are affected by the velocity ratio. The streamwise velocity gradient along the r axis was intensified with an increase in N. The trend became larger in the vicinity of the injection nozzle. 15 refs., 9 figs.

  16. Investigation of periodical instabilities of confined turbulent swirl flames with laser based measurement techniques; Untersuchung periodischer Instabilitaeten von eingeschlossenen turbulenten Drallflammen mit Lasermessverfahren

    Energy Technology Data Exchange (ETDEWEB)

    Weigand, P.

    2007-07-01

    Swirl flames tend under certain operating conditions to exhibit strong pressure oscillations known as 'thermo-acoustic oscillations'. In this thesis a non-premixed, globally lean swirl flame that was close to industrial gas turbine design, was investigated with phase-resolution over an oscillation cycle using different laser based measurement techniques. Microphone probes were used to characterize the acoustic behaviour of the flame. Measurement of the Laser induced fluorescence of the CH-radical provided information of the structure of the flame zone and of the varying position and intensity of the heat release rate. The velocity field was measured by 3D Laser Doppler Anemometry and analysed with phase resolution. For the first time spontaneous Laser Raman Scattering was applied phase-resolved in an oscillating swirl flame to gain quantitatively correlated information of the concentrations of the main species, the temperature and the mixture fraction. The results give for the first time a quantitative insight of the changes and interactions in an oscillating swirl flame during an oscillation cycle. The data are so far unique with respect to the quantity and quality of the measured data and are thus of high value for the validation of numerical simulation programs. (orig.)

  17. The CO/NOx emissions of swirled, strongly pulsed jet diffusion flames

    KAUST Repository

    Liao, Ying-Hao

    2014-05-28

    The CO and NOx exhaust emissions of swirled, strongly pulsed, turbulent jet diffusion flames were studied experimentally in a coflow swirl combustor. Measurements of emissions were performed on the combustor centerline using standard emission analyzers combined with an aspirated sampling probe located downstream of the visible flame tip. The highest levels of CO emissions are generally found for compact, isolated flame puffs, which is consistent with the quenching due to rapid dilution with excess air. The imposition of swirl generally results in a decrease in CO levels by up to a factor of 2.5, suggesting more rapid and compete fuel/air mixing by imposing swirl in the coflow stream. The levels of NO emissions for most cases are generally below the steady-flame value. The NO levels become comparable to the steady-flame value for sufficiently short jet-off times. The swirled coflow air can, in some cases, increase the NO emissions due to a longer combustion residence time due to the flow recirculation within the swirl-induced recirculation zone. Scaling relations, when taking into account the impact of air dilution over an injection cycle on the flame length, reveal a strong correlation between the CO emissions and the global residence time. However, the NO emissions do not successfully correlate with the global residence time. For some specific cases, a compact flame with a simultaneous decrease in both CO and NO emissions compared to the steady flames was observed. © Copyright © Taylor & Francis Group, LLC.

  18. The Rayleigh-Taylor instability in inertial fusion, astrophysical plasma and flames

    International Nuclear Information System (INIS)

    Bychkov, V; Modestov, M; Akkerman, V; Eriksson, L-E

    2007-01-01

    Previous results are reviewed and new results are presented on the Rayleigh-Taylor instability in inertial confined fusion, flames and supernovae including gravitational and thermonuclear explosion mechanisms. The instability couples micro-scale plasma effects to large-scale hydrodynamic phenomena. In inertial fusion the instability reduces target compression. In supernovae the instability produces large-scale convection, which determines the fate of the star. The instability is often accompanied by mass flux through the unstable interface, which may have either a stabilizing or a destabilizing influence. Destabilization happens due to the Darrieus-Landau instability of a deflagration front. Still, it is unclear whether the instabilities lead to well-organized large-scale structures (bubbles) or to relatively isotropic turbulence (mixing layer)

  19. 5 kHz thermometry in a swirl-stabilized gas turbine model combustor using chirped probe pulse femtosecond CARS. Part 1: Temporally resolved swirl-flame thermometry

    KAUST Repository

    Dennis, Claresta N.

    2016-06-20

    Single-laser-shot temperature measurements at 5 kHz were performed in a gas turbine model combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at two conditions; one exhibiting a low level of thermoacoustic instability and the other a high level of instability. Measurements were performed at 73 locations within each flame in order to resolve the spatial flame structure and compare to previously published studies. The measurement procedures, including the procedure for calibrating the laser system parameters, are discussed in detail. Despite the high turbulence levels in the combustor, signals were obtained on virtually every laser shot, and these signals were strong enough for spectral fitting analysis for determination of flames temperatures. The spatial resolution of the single-laser shot temperature measurements was approximately 600 µm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2200 K, although some detector saturation was observed for low temperature spectra. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. In a companion paper, the time-resolved fs CARS data are analyzed to provide insight into the temporal dynamics of the gas turbine model combustor flow field.

  20. 5 kHz thermometry in a swirl-stabilized gas turbine model combustor using chirped probe pulse femtosecond CARS. Part 1: Temporally resolved swirl-flame thermometry

    KAUST Repository

    Dennis, Claresta N.; Slabaugh, Carson D.; Boxx, Isaac G.; Meier, Wolfgang; Lucht, Robert P.

    2016-01-01

    Single-laser-shot temperature measurements at 5 kHz were performed in a gas turbine model combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at two conditions; one exhibiting a low level of thermoacoustic instability and the other a high level of instability. Measurements were performed at 73 locations within each flame in order to resolve the spatial flame structure and compare to previously published studies. The measurement procedures, including the procedure for calibrating the laser system parameters, are discussed in detail. Despite the high turbulence levels in the combustor, signals were obtained on virtually every laser shot, and these signals were strong enough for spectral fitting analysis for determination of flames temperatures. The spatial resolution of the single-laser shot temperature measurements was approximately 600 µm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2200 K, although some detector saturation was observed for low temperature spectra. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. In a companion paper, the time-resolved fs CARS data are analyzed to provide insight into the temporal dynamics of the gas turbine model combustor flow field.

  1. Tracking coherent structures in massively-separated and turbulent flows

    Science.gov (United States)

    Rockwood, Matthew; Huang, Yangzi; Green, Melissa

    2018-01-01

    Coherent vortex structures are tracked in simulations of massively-separated and turbulent flows. Topological Lagrangian saddle points are found using intersections of the positive and negative finite-time Lyapunov exponent ridges, and these points are then followed in order to track individual coherent structure motion both in a complex interacting three-dimensional flow (turbulent channel) and during vortex formation (two-dimensional bluff body shedding). For a simulation of wall-bounded turbulence in a channel flow, tracking Lagrangian saddles shows that the average structure convection speed exhibits a similar trend as a previously published result based on velocity and pressure correlations, giving validity to the method. When this tracking method is applied in a study of a circular cylinder in cross-flow it shows that Lagrangian saddles rapidly accelerate away from the cylinder surface as the vortex sheds. This saddle behavior is compared with the time-resolved static pressure distribution on the circular cylinder, yielding locations on a cylinder surface where common sensors could detect this phenomenon, which is not available from force measurements or vortex circulation calculations. The current method of tracking coherent structures yields insight into the behavior of the coherent structures in both of the diverse flows presented, highlighting the breadth of its potential application.

  2. The effects of local blowing perturbations on thermal turbulent structures

    Science.gov (United States)

    Liu, Can; Araya, Guillermo; Leonardi, Stefano; Castillo, Luciano

    2013-11-01

    Blowing is an active flow control technique with several industrial applications, particularly in film cooling of turbine blades. In the past, the effects of localized blowing have been mostly analyzed on the velocity field and its influence of the flow parameters and turbulence structures (Krogstad and Kourakine, 2000). However, little literature can be found on the effects of blowing on the coherent thermal structures. In the present study, an incompressible turbulent channel flow with given steady blowing at the wall is simulated via DNS by means of five spanwise holes. The Reynolds number based on the friction velocity and half channel height is approximately Re = 394 and the molecular Prandtl number is Pr = 0.71. Temperature is considered a passive scalar with isothermal conditions at the wall. Different blowing amplitudes and perturbing angles (with respect to the streamwise direction) are applied to find out their effects on the turbulent thermal structures by means of a two-point correlation analysis. In addition, local reduction and increase of drag are connected to vorticity. The corresponding influence of perturbing amplitudes and angles on the energy budget of thermal fluctuations and turbulent Prandtl numbers are also shown and discussed.

  3. Modelling the average velocity of propagation of the flame front in a gasoline engine with hydrogen additives

    Science.gov (United States)

    Smolenskaya, N. M.; Smolenskii, V. V.

    2018-01-01

    The paper presents models for calculating the average velocity of propagation of the flame front, obtained from the results of experimental studies. Experimental studies were carried out on a single-cylinder gasoline engine UIT-85 with hydrogen additives up to 6% of the mass of fuel. The article shows the influence of hydrogen addition on the average velocity propagation of the flame front in the main combustion phase. The dependences of the turbulent propagation velocity of the flame front in the second combustion phase on the composition of the mixture and operating modes. The article shows the influence of the normal combustion rate on the average flame propagation velocity in the third combustion phase.

  4. Structures and Intermittency in Small Scales Solar Wind Turbulence

    International Nuclear Information System (INIS)

    Sahraoui, Fouad; Goldstein, Melvyn

    2010-01-01

    Several observations in space plasmas have reported the presence of coherent structures at different plasma scales. Structure formation is believed to result from nonlinear interactions between the plasma modes, which depend strongly on their phase synchronization. Despite this important role of the phases in turbulence, very limited work has been devoted to study the phases as potential tracers of nonlinearities in comparison with the wealth of literature on power spectra of turbulence where phases are totally missed. The reason why the phases are seldom used is probably because they usually appear to be completely mixed (due to their dependence on an arbitrary time origin and to 2π periodicity). To handle the phases properly, a new method based on using surrogate data has been developed recently to detect coherent structures in magnetized plasmas [Sahraoui, PRE, 2008]. Here, we show new applications of the technique to study the nature (weak vs strong, self-similar vs intermittent) of the small scale turbulence in the solar wind using the Cluster observations.

  5. Evaluation of scalar mixing and time scale models in PDF simulations of a turbulent premixed flame

    Energy Technology Data Exchange (ETDEWEB)

    Stoellinger, Michael; Heinz, Stefan [Department of Mathematics, University of Wyoming, Laramie, WY (United States)

    2010-09-15

    Numerical simulation results obtained with a transported scalar probability density function (PDF) method are presented for a piloted turbulent premixed flame. The accuracy of the PDF method depends on the scalar mixing model and the scalar time scale model. Three widely used scalar mixing models are evaluated: the interaction by exchange with the mean (IEM) model, the modified Curl's coalescence/dispersion (CD) model and the Euclidean minimum spanning tree (EMST) model. The three scalar mixing models are combined with a simple model for the scalar time scale which assumes a constant C{sub {phi}}=12 value. A comparison of the simulation results with available measurements shows that only the EMST model calculates accurately the mean and variance of the reaction progress variable. An evaluation of the structure of the PDF's of the reaction progress variable predicted by the three scalar mixing models confirms this conclusion: the IEM and CD models predict an unrealistic shape of the PDF. Simulations using various C{sub {phi}} values ranging from 2 to 50 combined with the three scalar mixing models have been performed. The observed deficiencies of the IEM and CD models persisted for all C{sub {phi}} values considered. The value C{sub {phi}}=12 combined with the EMST model was found to be an optimal choice. To avoid the ad hoc choice for C{sub {phi}}, more sophisticated models for the scalar time scale have been used in simulations using the EMST model. A new model for the scalar time scale which is based on a linear blending between a model for flamelet combustion and a model for distributed combustion is developed. The new model has proven to be very promising as a scalar time scale model which can be applied from flamelet to distributed combustion. (author)

  6. LES/FMDF of turbulent jet ignition in a rapid compression machine

    Science.gov (United States)

    Validi, Abdoulahad; Schock, Harold; Toulson, Elisa; Jaberi, Farhad; CFD; Engine Research Labs, Michigan State University Collaboration

    2015-11-01

    Turbulent Jet Ignition (TJI) is an efficient method for initiating and controlling combustion in combustion systems, e.g. internal combustion engines. It enables combustion in ultra-lean mixtures by utilizing hot product turbulent jets emerging from a pre-chamber combustor as the ignition source for the main combustion chamber. Here, we study the TJI-assisted ignition and combustion of lean methane-air mixtures in a Rapid Compression Machine (RCM) for various flow/combustion conditions with the hybrid large eddy simulation/filtered mass density function (LES/FMDF) computational model. In the LES/FMDF model, the filtered form of compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity, while the FMDF transport equation is solved with a Lagrangian stochastic method to obtain the scalar (species mass fraction and temperature) field. The LES/FMDF data are used to study the physics of TJI and combustion in RCM. The results show the very complex behavior of the reacting flow and the flame structure in the pre-chamber and RCM.

  7. Flames in vortices & tulip-flame inversion

    Science.gov (United States)

    Dold, J. W.

    This article summarises two areas of research regarding the propagation of flames in flows which involve significant fluid-dynamical motion [1]-[3]. The major difference between the two is that in the first study the fluid motion is present before the arrival of any flame and remains unaffected by the flame [1, 2] while, in the second study it is the flame that is responsible for all of the fluid dynamical effects [3]. It is currently very difficult to study flame-motion in which the medium is both highly disturbed before the arrival of a flame and is further influenced by the passage of the flame.

  8. Coherent Structures in Numerically Simulated Plasma Turbulence

    DEFF Research Database (Denmark)

    Kofoed-Hansen, O.; Pécseli, H.L.; Trulsen, J.

    1989-01-01

    Low level electrostatic ion acoustic turbulence generated by the ion-ion beam instability was investigated numerically. The fluctuations in potential were investigated by a conditional statistical analysis revealing propagating coherent structures having the form of negative potential wells which...

  9. Instantaneous three-dimensional visualization of concentration distributions in turbulent flows with crossed-plane laser-induced fluorescence imaging

    Science.gov (United States)

    Hoffmann, A.; Zimmermann, F.; Scharr, H.; Krömker, S.; Schulz, C.

    2005-01-01

    A laser-based technique for measuring instantaneous three-dimensional species concentration distributions in turbulent flows is presented. The laser beam from a single laser is formed into two crossed light sheets that illuminate the area of interest. The laser-induced fluorescence (LIF) signal emitted from excited species within both planes is detected with a single camera via a mirror arrangement. Image processing enables the reconstruction of the three-dimensional data set in close proximity to the cutting line of the two light sheets. Three-dimensional intensity gradients are computed and compared to the two-dimensional projections obtained from the two directly observed planes. Volume visualization by digital image processing gives unique insight into the three-dimensional structures within the turbulent processes. We apply this technique to measurements of toluene-LIF in a turbulent, non-reactive mixing process of toluene and air and to hydroxyl (OH) LIF in a turbulent methane-air flame upon excitation at 248 nm with a tunable KrF excimer laser.

  10. Vorticity and turbulence effects in fluid structure interaction an application to hydraulic structure design

    CERN Document Server

    Brocchini, M

    2006-01-01

    This book contains a collection of 11 research and review papers devoted to the topic of fluid-structure interaction.The subject matter is divided into chapters covering a wide spectrum of recognized areas of research, such as: wall bounded turbulence; quasi 2-D turbulence; canopy turbulence; large eddy simulation; lake hydrodynamics; hydraulic hysteresis; liquid impacts; flow induced vibrations; sloshing flows; transient pipe flow and air entrainment in dropshaft.The purpose of each chapter is to summarize the main results obtained by the individual research unit through a year-long activity on a specific issue of the above list. The main feature of the book is to bring state of the art research on fluid structure interaction to the attention of the broad international community.This book is primarily aimed at fluid mechanics scientists, but it will also be of value to postgraduate students and practitioners in the field of fluid structure interaction.

  11. Quantitative structure-activity relationship modeling on in vitro endocrine effects and metabolic stability involving 26 selected brominated flame retardants

    NARCIS (Netherlands)

    Harju, M.; Hamers, T.; Kamstra, J.H.; Sonneveld, E.; Boon, J.P.

    2007-01-01

    In this work, quantitative structure-activity relationships (QSARs) were developed to aid human and environmental risk assessment processes for brominated flame retardants (BFRs). Brominated flame retardants, such as the high-production-volume chemicals polybrominated diphenyl ethers (PBDEs),

  12. [Statistical modeling studies of turbulent reacting flows

    International Nuclear Information System (INIS)

    Dwyer, H.A.

    1987-01-01

    This paper discusses the study of turbulent wall shear flows, and we feel that this problem is both more difficult and a better challenge for the new methods we are developing. Turbulent wall flows have a wide variety of length and time scales which interact with the transport processes to produce very large fluxes of mass, heat, and momentum. At the present time we have completed the first calculation of a wall diffusion flame, and we have begun a velocity PDF calculation for the flat plate boundary layer. A summary of the various activities is contained in this report

  13. Structure formation in turbulent plasmas - test of nonlinear processes in plasma experiments

    International Nuclear Information System (INIS)

    Itoh, S.-I.; Yagi, Masatoshi; Inagaki, Shigeru

    2009-01-01

    Full text: Recent developments in plasma physics, either in the fusion research in a new era of ITER, or in space and in astro-physics, the world-wide and focused research has been developed on the subject of structural formation in turbulent plasma being associated with electro-magnetic field formation. Keys for the progress were a change of the physics view from the 'linear, local and deterministic' picture to the description based on 'nonlinear instability, nonlocal interaction and probabilistic excitation' for the turbulent state, and the integration of the theory-simulation-experiment. In this presentation, we first briefly summarize the theory of microscopic turbulence and mesoscale fluctuations and selection rules. In addition, the statistical formation of large-scale structure/deformation by turbulence is addressed. Then, the experimental measurements of the mesoscale structures (e.g., zonal flows, zonal fields, streamer and transport interface) and of the nonlinear interactions among them in turbulent plasmas are reported. Confirmations by, and new challenges from, the experiments are overviewed. Work supported by the Grant-in-Aid for Specially-Promoted Research (16002005). (author)

  14. Direct Numerical Simulations of Statistically Stationary Turbulent Premixed Flames

    KAUST Repository

    Im, Hong G.; Arias, Paul G.; Chaudhuri, Swetaprovo; Uranakara, Harshavardhana A.

    2016-01-01

    Direct numerical simulations (DNS) of turbulent combustion have evolved tremendously in the past decades, thanks to the rapid advances in high performance computing technology. Today’s DNS is capable of incorporating detailed reaction mechanisms

  15. CFD modeling using PDF approach for investigating the flame length in rotary kilns

    Science.gov (United States)

    Elattar, H. F.; Specht, E.; Fouda, A.; Bin-Mahfouz, Abdullah S.

    2016-12-01

    Numerical simulations using computational fluid dynamics (CFD) are performed to investigate the flame length characteristics in rotary kilns using probability density function (PDF) approach. A commercial CFD package (ANSYS-Fluent) is employed for this objective. A 2-D axisymmetric model is applied to study the effect of both operating and geometric parameters of rotary kiln on the characteristics of the flame length. Three types of gaseous fuel are used in the present work; methane (CH4), carbon monoxide (CO) and biogas (50 % CH4 + 50 % CO2). Preliminary comparison study of 2-D modeling outputs of free jet flames with available experimental data is carried out to choose and validate the proper turbulence model for the present numerical simulations. The results showed that the excess air number, diameter of kiln air entrance, radiation modeling consideration and fuel type have remarkable effects on the flame length characteristics. Numerical correlations for the rotary kiln flame length are presented in terms of the studied kiln operating and geometric parameters within acceptable error.

  16. Electron Scale Structures and Magnetic Reconnection Signatures in the Turbulent Magnetosheath

    Science.gov (United States)

    Yordanova, E.; Voros, Z.; Varsani, A.; Graham, D. B.; Norgren, C.; Khotyaintsev, Yu. V.; Vaivads, A.; Eriksson, E.; Nakamura, R.; Lindqvist, P.-A.;