Strain rate effect on sooting characteristics in laminar counterflow diffusion flames

KAUST Repository

Wang, Yu

2016-01-20

The effects of strain rate, oxygen enrichment and fuel type on the sooting characteristics of counterflow diffusion flames were studied. The sooting structures and relative PAH concentrations were measured with laser diagnostics. Detailed soot modeling using recently developed PAH chemistry and surface reaction mechanism was performed and the results were compared with experimental data for ethylene flames, focusing on the effects of strain rates. The results showed that increase in strain rate reduced soot volume fraction, average size and peak number density. Increase in oxygen mole fraction increased soot loading and decreased its sensitivity on strain rate. The soot volume fractions of ethane, propene and propane flames were also measured as a function of global strain rate. The sensitivity of soot volume fraction to strain rate was observed to be fuel dependent at a fixed oxygen mole fraction, with the sensitivity being higher for more sooting fuels. However, when the soot loadings were matched at a reference strain rate for different fuels by adjusting oxygen mole fraction, the dependence of soot loading on strain rate became comparable among the tested fuels. PAH concentrations were shown to decrease with increase in strain rate and the dependence on strain rate is more pronounced for larger PAHs. Soot modeling was performed using detailed PAH growth chemistry with molecular growth up to coronene. A qualitative agreement was obtained between experimental and simulation results, which was then used to explain the experimentally observed strain rate effect on soot growth. However, quantitatively, the simulation result exhibits higher sensitivity to strain rate, especially for large PAHs and soot volume fractions.

Nonpremixed flame in a counterflow under electric fields

KAUST Repository

Park, Daegeun

2016-05-08

Electrically assisted combustion has been studied in order to control or improve flame characteristics, and emphasizing efficiency and emission regulation. Many phenomenological observations have been reported on the positive impact of electric fields on flame, however there is a lack of detailed physical mechanisms for interpreting these. To clarify the effects of electric fields on flame, I have investigated flame structure, soot formation, and flow field with ionic wind electrical current responses in nonpremixed counterflow flames. The effects of direct current (DC) electric field on flame movement and flow field was also demonstrated in premixed Bunsen flames. When a DC electric field was applied to a lower nozzle, the flames moved toward the cathode side due to Lorentz force action on the positive ions, soot particles simultaneously disappeared completely and laser diagnostics was used to identify the results from the soot particles. To understand the effects of an electric field on flames, flow visualization was performed by Mie scattering to check the ionic wind effect, which is considered to play an important role in electric field assisted combustion. Results showed a bidirectional ionic wind, with a double-stagnant flow configuration, which blew from the flame (ionic source) toward both the cathode and the anode. This implies that the electric field affects strain rate and the axial location of stoichiometry, important factors in maintaining nonpremixed counterflow flames; thus, soot formation of the counterflow flame can also be affected by the electric field. In a test of premixed Bunsen flames having parallel electrodes, flame movement toward the cathode and bidirectional ionic wind were observed. Using PIV measurement it was found that a created radial velocity caused by positive ions (i.e. toward a cathode), was much faster than the velocity toward the anode. Even in a study of alternating current (AC) electric fields, bidirectional ionic wind could

Investigating Soot Morphology in Counterflow Flames at Elevated Pressures

KAUST Repository

Amin, Hafiz Muhammad Fahid

2018-01-01

Practical combustion devices such as gas turbines and diesel engines operate at high pressures to increase their efficiency. Pressure significantly increases the overall soot yield. Morphology of these ultra-fine particles determines their airborne lifetime and their interaction with the human respiratory system. Therefore, investigating soot morphology at high pressure is of practical relevance. In this work, a novel experimental setup has been designed and built to study the soot morphology at elevated pressures. The experimental setup consists of a pressure vessel, which can provide optical access from 10° to 165° for multi-angle light scattering, and a counterflow burner which produces laminar flames at elevated pressures. In the first part of the study, N2-diluted ethylene/air and ethane air counterflow flames are stabilized from 2 to 5 atm. Two-angle light scattering and extinction technique have been used to study the effects of pressure on soot parameters. Path averaged soot volume fraction is found to be very sensitive to pressure and increased significantly from 2 to 5 atm. Primary particle size and aggregate size also increased with pressure. Multi-angle light scattering is also performed and flames are investigated from 3 to 5 atm. Scattering to absorption ratio is calculated from multi-angle light scattering and extinction data. Scattering to absorption ratio increased with pressure whereas the number of primary particles in an aggregate decreased with increasing pressure. In the next part of the study, Thermophoretic Sampling of soot is performed, in counterflow flames from 3 to 10 atm, followed by transmission electron microscopy. Mean primary particle size increased with pressure and these trends are consistent withour light scattering measurements. Fractal properties of soot aggregates are found to be insensitive to pressure. 2D diffused light line of sight attenuation (LOSA) and Laser Induced Incandescence (LII) are used to measure local soot

Quantification of extinction mechanism in counterflow premixed flames

KAUST Repository

Choi, Sangkyu; Cho, Eunseong; Chung, Suk-Ho

2014-01-01

The extinction mechanisms of stretched premixed flames have been investigated numerically for the fuels of CH4, C3H8, H2, CO and for the mixture fuels of CH4+H2 and CO+H2 by adopting symmetric double premixed flames in a counterflow configuration

Role of the outer-edge flame on flame extinction in nitrogen-diluted non-premixed counterflow flames with finite burner diameters

KAUST Repository

Chung, Yong Ho; Park, Daegeun; Park, Jeong; Kwon, Oh Boong; Yun, Jin Han; Keel, Sang In

2013-01-01

This study of nitrogen-diluted non-premixed counterflow flames with finite burner diameters investigates the important role of the outer-edge flame on flame extinction through experimental and numerical analyses. It explores flame stability diagrams

A PAH growth mechanism and synergistic effect on PAH formation in counterflow diffusion flames

KAUST Repository

Wang, Yu

2013-09-01

A reaction mechanism having molecular growth up to benzene for hydrocarbon fuels with up to four carbon-atoms was extended to include the formation and growth of polycyclic aromatic hydrocarbons (PAHs) up to coronene (C24H12). The new mechanism was tested for ethylene premixed flames at low (20torr) and atmospheric pressures by comparing experimentally observed species concentrations with those of the computed ones for small chemical species and PAHs. As compared to several existing mechanisms in the literature, the newly developed mechanism showed an appreciable improvement in the predicted profiles of PAHs. The new mechanism was also used to simulate PAH formation in counterflow diffusion flames of ethylene to study the effects of mixing propane and benzene in the fuel stream. In the ethylene-propane flames, existing experimental results showed a synergistic effect in PAH concentrations, i.e. PAH concentrations first increased and then decreased with increasing propane mixing. This PAH behavior was successfully captured by the new mechanism. The synergistic effect was predicted to be more pronounced for larger PAH molecules as compared to the smaller ones, which is in agreement with experimental observations. In the experimental study in which the fuel stream of ethylene-propane flames was doped with benzene, a synergistic effect was mitigated for benzene, but was observed for large PAHs. This effect was also predicted in the computed PAH profiles for these flames. To explain these responses of PAHs in the flames of mixture fuels, a pathway analysis has been conducted, which show that several resonantly stabilized species as well as C4H4 and H atom contribute to the enhanced synergistic behaviors of larger PAHs as compared to the small ones in the flames of mixture fuels. © 2013 The Combustion Institute.

NO emission characteristics in counterflow diffusion flame of blended fuel of H2/CO2/Ar

International Nuclear Information System (INIS)

Jeong Park; Kyunghwan Lee; Keeman Lee

2002-01-01

Flame structure and NO emission characteristics in counterflow diffusion flame of blended fuel of H 2 /CO 2 /Ar have been numerically simulated with detailed chemistry. The combination of H 2 , CO 2 and Ar as fuel is selected to clearly display the contribution of hydrocarbon products to flame structure and NO emission characteristics due to the breakdown of CO 2 . A radiative heat loss term is involved to correctly describe the flame dynamics especially at low strain rates. The detailed chemistry adopts the reaction mechanism of GRI 2.11, which consists of 49 species and 279 elementary reactions. All mechanisms including thermal, NO 2 , N 2 O and Fenimore are taken into account to separately evaluate the effects of CO 2 addition on NO emission characteristics. The increase of added CO 2 quantity causes flame temperature to fall since at high strain rates a diluent effect is prevailing and at low strain rates the breakdown of CO 2 produces relatively populous hydrocarbon products and thus the existence of hydrocarbon products inhibits chain branching. It is also found that the contribution of NO production by N 2 O and NO 2 mechanisms are negligible and that thermal mechanism is concentrated on only the reaction zone. As strain rate and CO 2 quantity increase, NO production is remarkably augmented. (Author)

Soot measurements by two angle scattering and extinction in an N 2 -diluted ethylene/air counterflow diffusion flame from 2 to 5 atm

KAUST Repository

Amin, Hafiz M.F.

2016-06-27

The soot formed in an N-diluted ethylene/air counterflow diffusion flame at elevated pressure was investigated using two angle light scattering/extinction technique. To provide a well-controlled pressurized environment for the flame, a novel pressure vessel was built with the required optical access. The soot parameters were measured along the centerline of the counterflow flame. These properties included soot volume fraction (f ), primary particle diameter (d ), population averaged radius of gyration (R ) and number density of primary particles (n ). The Rayleigh-Debye-Gans theory for Fractal Aggregates (RDG-FA) was used to retrieve these properties from scattering and extinction measurements. Soot volume fraction was measured via light extinction from 2 to 5atm while maintaining the same global strain rate at all pressures. Scattered light from soot particles was measured at 45° and 135° and primary particle diameter was calculated using scattering/extinction ratio and the radius of gyration was determined from the dissymmetry ratio. Soot volume fraction, primary particle diameter and radius of gyration all increased with pressure while the number density of primary particles decreased with increasing pressure.

Soot measurements by two angle scattering and extinction in an N 2 -diluted ethylene/air counterflow diffusion flame from 2 to 5 atm

KAUST Repository

Amin, Hafiz M.F.; Roberts, William L.

2016-01-01

The soot formed in an N-diluted ethylene/air counterflow diffusion flame at elevated pressure was investigated using two angle light scattering/extinction technique. To provide a well-controlled pressurized environment for the flame, a novel pressure vessel was built with the required optical access. The soot parameters were measured along the centerline of the counterflow flame. These properties included soot volume fraction (f ), primary particle diameter (d ), population averaged radius of gyration (R ) and number density of primary particles (n ). The Rayleigh-Debye-Gans theory for Fractal Aggregates (RDG-FA) was used to retrieve these properties from scattering and extinction measurements. Soot volume fraction was measured via light extinction from 2 to 5atm while maintaining the same global strain rate at all pressures. Scattered light from soot particles was measured at 45° and 135° and primary particle diameter was calculated using scattering/extinction ratio and the radius of gyration was determined from the dissymmetry ratio. Soot volume fraction, primary particle diameter and radius of gyration all increased with pressure while the number density of primary particles decreased with increasing pressure.

Stochastic Simulation of Soot Formation Evolution in Counterflow Diffusion Flames

Directory of Open Access Journals (Sweden)

Xiao Jiang

2018-01-01

Full Text Available Soot generally refers to carbonaceous particles formed during incomplete combustion of hydrocarbon fuels. A typical simulation of soot formation and evolution contains two parts: gas chemical kinetics, which models the chemical reaction from hydrocarbon fuels to soot precursors, that is, polycyclic aromatic hydrocarbons or PAHs, and soot dynamics, which models the soot formation from PAHs and evolution due to gas-soot and soot-soot interactions. In this study, two detailed gas kinetic mechanisms (ABF and KM2 have been compared during the simulation (using the solver Chemkin II of ethylene combustion in counterflow diffusion flames. Subsequently, the operator splitting Monte Carlo method is used to simulate the soot dynamics. Both the simulated data from the two mechanisms for gas and soot particles are compared with experimental data available in the literature. It is found that both mechanisms predict similar profiles for the gas temperature and velocity, agreeing well with measurements. However, KM2 mechanism provides much closer prediction compared to measurements for soot gas precursors. Furthermore, KM2 also shows much better predictions for soot number density and volume fraction than ABF. The effect of nozzle exit velocity on soot dynamics has also been investigated. Higher nozzle exit velocity renders shorter residence time for soot particles, which reduces the soot number density and volume fraction accordingly.

Cool diffusion flames of butane isomers activated by ozone in the counterflow

KAUST Repository

Alfazazi, Adamu; Al Omier, Abdullah Abdulaziz; Secco, Andrea; Selim, Hatem; Ju, Yiguang; Sarathy, Mani

2018-01-01

and sustenance of cool diffusion flames; as ozone-less cool diffusion flame of butane isomers could not be established even at high fuel mole fractions. The structure of a stable n-butane cool diffusion flame was qualitatively examined using a time of flight mass

Bidirectional ionic wind in nonpremixed counterflow flames with DC electric fields

KAUST Repository

Park, Daegeun

2016-05-05

Under an electric field, ions in the reaction zone of a flame generate a bulk flow motion called ionic wind. Because the majority of ions are positive, ionic wind is commonly considered to be unidirectional toward the cathode. A more thorough understanding of the effects of electric fields on flames could be obtained by clarifying the role of minor negative ions in the ionic wind. Here, we report on the effects of direct current on nonpremixed counterflow flames by visualizing the ionic wind. We found that the original flow field separates near the flame when it locates at a flow stagnation plane, resulting in a double-stagnant flow configuration. This evidences a bidirectional ionic wind blowing from the flame to both the cathode and the anode due to the positive and the negative ions, respectively. Meanwhile, an electric body force pulls the flame toward the cathode. Thus, the electric field affects the strain rate and the axial location of the stoichiometry, which are important for characterizing nonpremixed counterflow flames. In addition, measurement of the electric current density roughly showed a nearly saturated current when these flames restabilized under relatively high voltage. Detailed explanations of flame behavior, electric currents, and flow characteristics of various fuels are discussed in this study.

On the dynamics of flame edges in diffusion-flame/vortex interactions

Energy Technology Data Exchange (ETDEWEB)

Hermanns, Miguel; Linan, Amable [Departamento de Motopropulsion y Termofluidodinamica, Universidad Politecnica de Madrid, Pza. Cardenal Cisneros 3, 28040 Madrid (Spain); Vera, Marcos [Area de Mecanica de Fluidos, Universidad Carlos III de Madrid, 28911 Leganes (Spain)

2007-04-15

We analyze the local flame extinction and reignition of a counterflow diffusion flame perturbed by a laminar vortex ring. Local flame extinction leads to the appearance of flame edges separating the burning and extinguished regions of the distorted mixing layer. The dynamics of these edges is modeled based on previous numerical results, with heat release effects fully taken into account, which provide the propagation velocity of triple and edge flames in terms of the upstream unperturbed value of the scalar dissipation. The temporal evolution of the mixing layer is determined using the classical mixture fraction approach, with both unsteady and curvature effects taken into account. Although variable density effects play an important role in exothermic reacting mixing layers, in this paper the description of the mixing layer is carried out using the constant density approximation, leading to a simplified analytical description of the flow field. The mathematical model reveals the relevant nondimensional parameters governing diffusion-flame/vortex interactions and provides the parameter range for the more relevant regime of local flame extinction followed by reignition via flame edges. Despite the simplicity of the model, the results show very good agreement with previously published experimental results. (author)

Sooting Characteristics and Modeling in Counterflow Diffusion Flames

KAUST Repository

Wang, Yu

2013-11-01

Soot formation is one of the most complex phenomena in combustion science and an understanding of the underlying physico-chemical mechanisms is important. This work adopted both experimental and numerical approaches to study soot formation in laminar counterfl ow diffusion flames. As polycyclic aromatic hydrocarbons (PAHs) are the precursors of soot particles, a detailed gas-phase chemical mechanism describing PAH growth upto coronene for fuels with 1 to 4 carbon atoms was validated against laminar premixed and counter- flow diffusion fl ames. Built upon this gas-phase mechanism, a soot model was then developed to describe soot inception and surface growth. This soot model was sub- sequently used to study fuel mixing effect on soot formation in counterfl ow diffusion flames. Simulation results showed that compared to the baseline case of the ethylene flame, the doping of 5% (by volume) propane or ethane in ethylene tends to increase the soot volume fraction and number density while keeping the average soot size almost unchanged. These results are in agreement with experimental observations. Laser light extinction/scattering as well as laser induced fluorescence techniques were used to study the effect of strain rate on soot and PAH formation in counterfl ow diffusion ames. The results showed that as strain rate increased both soot volume fraction and PAH concentrations decreased. The concentrations of larger PAH were more sensitive to strain rate compared to smaller ones. The effect of CO2 addition on soot formation was also studied using similar experimental techniques. Soot loading was reduced with CO2 dilution. Subsequent numerical modeling studies were able to reproduce the experimental trend. In addition, the chemical effect of CO2 addition was analyzed using numerical data. Critical conditions for the onset of soot were systematically studied in counterfl ow diffusion ames for various gaseous hydrocarbon fuels and at different strain rates. A sooting

Diffusion Flame Extinction in a Low Strain Flow

Science.gov (United States)

Sutula, Jason; Jones, Joshua; Torero, Jose L.; Borlik, Jeffrey; Ezekoye, Ofodike A.

1997-01-01

Diffusion flames are of great interest in fire safety and many industrial processes. Many parameters significantly affect the flame structure, shape and stability, of particular importance are the constraints imposed by geometrical boundaries. Physical boundaries determine the characteristics of the flow, affect heat, fuel, and oxidizer transport from and towards the flame and can act as heat sinks or heat sources. As a result, the existence of a flame, its shape and nature are intimately related to the geometrical characteristics of the environment that surrounds it. The counter-flow configuration provides a constant strain flow, therefore, is ideal to study the structure of diffusion flames. Most studies have concentrated on the high velocity, high strain limit, since buoyantly induced instabilities will disintegrate the planar flame as the velocity decreases. Only recently, experimental studies in micro-gravity conditions have begun to explore the low strain regimes. The main objective of these on-going studies is to determine the effect of radiative heat losses and variable strain on the structure and radiation-induced extinction of diffusion flames. For these programs, size, geometry, and experimental conditions have been chosen to keep the flame unaffected by the physical boundaries. Whether is the burning of condensed or gaseous fuels, for most real situations the boundaries impose a significant effect on the nature of the flame. There is, therefore, a need to better understand the effect that geometrical constraints (i.e. flow nonperpendicular to a fuel surface, heat losses to the boundaries, etc.) might have on the final characteristics of a diffusion flame. Preliminary experiments have shown that, in the absence of gravity, and depending on the distance from the flame to the boundary, three characteristically different regimes can be observed. Close to the boundary, the flame is parabolic, very thin and blue, almost soot-less. Diffusion is the main

Soot modeling of counterflow diffusion flames of ethylene-based binary mixture fuels

KAUST Repository

Wang, Yu

2015-03-01

A soot model was developed based on the recently proposed PAH growth mechanism for C1-C4 gaseous fuels (KAUST PAH Mechanism 2, KM2) that included molecular growth up to coronene (A7) to simulate soot formation in counterflow diffusion flames of ethylene and its binary mixtures with methane, ethane and propane based on the method of moments. The soot model has 36 soot nucleation reactions from 8 PAH molecules including pyrene and larger PAHs. Soot surface growth reactions were based on a modified hydrogen-abstraction-acetylene-addition (HACA) mechanism in which CH3, C3H3 and C2H radicals were included in the hydrogen abstraction reactions in addition to H atoms. PAH condensation on soot particles was also considered. The experimentally measured profiles of soot volume fraction, number density, and particle size were well captured by the model for the baseline case of ethylene along with the cases involving mixtures of fuels. The simulation results, which were in qualitative agreement with the experimental data in the effects of binary fuel mixing on the sooting structures of the measured flames, showed in particular that 5% addition of propane (ethane) led to an increase in the soot volume fraction of the ethylene flame by 32% (6%), despite the fact that propane and ethane are less sooting fuels than is ethylene, which is in reasonable agreement with experiments of 37% (14%). The model revealed that with 5% addition of methane, there was an increase of 6% in the soot volume fraction. The average soot particle sizes were only minimally influenced while the soot number densities were increased by the fuel mixing. Further analysis of the numerical data indicated that the chemical cross-linking effect between ethylene and the dopant fuels resulted in an increase in PAH formation, which led to higher soot nucleation rates and therefore higher soot number densities. On the other hand, the rates of soot surface growth per unit surface area through the HACA mechanism were

Dynamics and Suppression Effectiveness of Monodisperse Water Droplets in Non-Premixed Counterflow Flames

National Research Council Canada - National Science Library

Zegers, E. J; Williams, B. A; Sheinson, R. S; Fleming, J. W

2000-01-01

...-premixed propane/air counterflow flames are reported. Droplets were generated piezoelectrically, and the size and velocity distributions and the number density were determined by phase-Doppler particle anemometry...

Characterization of physical, thermal and chemical contributions of sodium bicarbonate particles in extinguishing counterflow nonpremixed flames

Energy Technology Data Exchange (ETDEWEB)

Chelliah, H.K.; Krauss, R.H.; Zhou, H.; Lentati, A.M.

1999-07-01

Based on laminar, nonpremixed methane-air flames established in a counterflow field, the flame extinction effectiveness of sodium bicarbonate particles is investigated here, both experimentally and numerically. In experiments, particles are separated into varying sizes (with the range of each size group approximately 10 {micro}m), and are introduced with the air stream. Flame extinction strain rates estimated using the measured nozzle exit velocities and separation distance are reported, as well as limited comparisons with LDV data (latter are mainly for calibration of the system). Numerical flame extinction results are also reported using a hybrid Eulerian-Lagrangian model previously developed for characterization of the flame extinction mechanism of fine-water droplets in a counterflow field. Comparison of the experimental and numerical results indicates a similar trend with particular size variation, but uncertainties in the particle decomposition model employed precludes any absolute comparisons at this time.

Time evolution of propagating nonpremixed flames in a counterflow, annular slot burner under AC electric fields

KAUST Repository

Tran, Vu Manh; Cha, Min

2016-01-01

alternating current electric fields to a gap between the upper and lower parts of a counterflow, annular slot burner and present the characteristics of the propagating nonpremixed edge-flames produced. Contrary to many other previous studies, flame

Compositional effects on PAH and soot formation in counterflow diffusion flames of gasoline surrogate fuels

KAUST Repository

Park, Sungwoo

2017-02-05

Gasoline surrogate fuels are widely used to understand the fundamental combustion properties of complex refinery gasoline fuels. In this study, the compositional effects on polycyclic aromatic hydrocarbons (PAHs) and soot formation were investigated experimentally for gasoline surrogate mixtures comprising n-heptane, iso-octane, and toluene in counterflow diffusion flames. A comprehensive kinetic model for the gasoline surrogate mixtures was developed to accurately predict the fuel oxidation along with the formation of PAHs and soot in flames. This combined model was first tested against ignition delay times and laminar burning velocities data. The proposed model for the formation and growth of PAHs up to coronene (C24H12) was based on previous studies and was tested against existing and present new experimental data. Additionally, in the accompanied soot model, PAHs with sizes larger than (including) pyrene were used for the inception of soot particles, followed by particle coagulations and PAH condensation/chemical reactions on soot surfaces. The major pathways for the formation of PAHs were also identified for the surrogate mixtures. The model accurately captures the synergistic PAH formation characteristics observed experimentally for n-heptane/toluene and iso-octane/toluene binary mixtures. Furthermore, the present experimental and modeling results also elucidated different trends in the formation of larger PAHs and soot between binary n-heptane/iso-octane and ternary n-heptane/iso-octane/toluene mixtures. Propargyl radicals (C3H3) were shown to be important in the formation and growth of PAHs for n-heptane/iso-octane mixtures when the iso-octane concentration increased; however, reactions involving benzyl radicals (C6H5CH2) played a significant role in the formation of PAHs for n-heptane/iso-octane/toluene mixtures. These results indicated that the formation of PAHs and subsequently soot was strongly affected by the composition of gasoline surrogate mixtures.

Compositional effects on PAH and soot formation in counterflow diffusion flames of gasoline surrogate fuels

KAUST Repository

Park, Sungwoo; Wang, Yu; Chung, Suk-Ho; Sarathy, Mani

2017-01-01

Gasoline surrogate fuels are widely used to understand the fundamental combustion properties of complex refinery gasoline fuels. In this study, the compositional effects on polycyclic aromatic hydrocarbons (PAHs) and soot formation were investigated experimentally for gasoline surrogate mixtures comprising n-heptane, iso-octane, and toluene in counterflow diffusion flames. A comprehensive kinetic model for the gasoline surrogate mixtures was developed to accurately predict the fuel oxidation along with the formation of PAHs and soot in flames. This combined model was first tested against ignition delay times and laminar burning velocities data. The proposed model for the formation and growth of PAHs up to coronene (C24H12) was based on previous studies and was tested against existing and present new experimental data. Additionally, in the accompanied soot model, PAHs with sizes larger than (including) pyrene were used for the inception of soot particles, followed by particle coagulations and PAH condensation/chemical reactions on soot surfaces. The major pathways for the formation of PAHs were also identified for the surrogate mixtures. The model accurately captures the synergistic PAH formation characteristics observed experimentally for n-heptane/toluene and iso-octane/toluene binary mixtures. Furthermore, the present experimental and modeling results also elucidated different trends in the formation of larger PAHs and soot between binary n-heptane/iso-octane and ternary n-heptane/iso-octane/toluene mixtures. Propargyl radicals (C3H3) were shown to be important in the formation and growth of PAHs for n-heptane/iso-octane mixtures when the iso-octane concentration increased; however, reactions involving benzyl radicals (C6H5CH2) played a significant role in the formation of PAHs for n-heptane/iso-octane/toluene mixtures. These results indicated that the formation of PAHs and subsequently soot was strongly affected by the composition of gasoline surrogate mixtures.

Extinction of counterflow premixed laminar flames

International Nuclear Information System (INIS)

Smooke, M.D.; Giovangigli, V.

1987-01-01

Problems in combustion and heat and mass transfer often depend upon one or more physical/chemical parameters. In many cases the combustion scientist is interested in knowing how the solution will behave if one or more of these parameters is varied. For some parameter regimes the governing equations can produce multiple solutions and the branches of the solution curve are linked via singular points. It is at these singular points, however that the system exhibits special behavior. To be able to predict the solution structure in the neighborhood of these points, the authors employ a phase-space, pseudo arclength, continuation method that utilizes Newton-like iterations and adaptive gridding techniques. The authors apply the method in the solution of counterflow premixed laminar flames

Correction of edge-flame propagation speed in a counterflow, annular slot burner

KAUST Repository

Tran, Vu Manh

2015-10-22

To characterize the propagation modes of flames, flame propagation speed must be accurately calculated. The impact of propagating edge-flames on the flow fields of unburned gases is limited experimentally. Thus, few studies have evaluated true propagation speeds by subtracting the flow velocities of unburned gases from flame displacement speeds. Here, we present a counterflow, annular slot burner that provides an ideal one-dimensional strain rate and lengthwise zero flow velocity that allowed us to study the fundamental behaviors of edge-flames. In addition, our burner has easy optical access for detailed laser diagnostics. Flame displacement speeds were measured using a high-speed camera and related flow fields of unburned gases were visualized by particle image velocimetry. These techniques allowed us to identify significant modifications to the flow fields of unburned gases caused by thermal expansion of the propagating edges, which enabled us to calculate true flame propagation speeds that took into account the flow velocities of unburned gases.

Effects of H2O, CO2, and N2 Air Contaminants on Critical Airside Strain Rates for Extinction of Hydrogen-Air Counterflow Diffusion Flames

Science.gov (United States)

Pellett, G. L.; Wilson, L. G.; Northam, G. B.; Guerra, Rosemary

1989-01-01

Coaxial tubular opposed jet burners (OJB) were used to form dish shaped counterflow diffusion flames (CFDF), centered by opposing laminar jets of H2, N2 and both clean and contaminated air (O2/N2 mixtures) in an argon bath at 1 atm. Jet velocities for flame extinction and restoration limits are shown versus wide ranges of contaminant and O2 concentrations in the air jet, and also input H2 concentration. Blowoff, a sudden breaking of CFDF to a stable ring shape, occurs in highly stretched stagnation flows and is generally believed to measure kinetically limited flame reactivity. Restore, a sudden restoration of central flame, is a relatively new phenomenon which exhibits a H2 dependent hysteresis from Blowoff. For 25 percent O2 air mixtures, mole for mole replacement of 25 percent N2 contaminant by steam increased U(air) or flame strength at Blowoff by about 5 percent. This result is consistent with laminar burning velocity results from analogous substitution of steam for N2 in a premixed stoichiometric H2-O2-N2 (or steam) flame, shown by Koroll and Mulpuru to promote a 10 percent increase in experimental and calculated laminar burning velocity, due to enhanced third body efficiency of water in: H + O2 + M yields HO2 + M. When the OJB results were compared with Liu and MacFarlane's experimental laminar burning velocity of premixed stoichiometric H2 + air + steam, a crossover occurred, i.e., steam enhanced OJB flame strength at extinction relative to laminar burning velocity.

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.

Effect of strain rate on sooting limits in counterflow diffusion flames of gaseous hydrocarbon fuels: Sooting temperature index and sooting sensitivity index

KAUST Repository

Wang, Yu

2014-05-01

The effect of the strain rate on the sooting limits in counterflow diffusion flames was investigated in various gaseous hydrocarbon fuels by varying the nitrogen dilution in the fuel and oxidizer streams. The sooting limit was defined as the critical fuel and oxygen mole fraction at which soot started to appear in the elastic light scattering signal. The sooting region for normal alkane fuels at a specified strain rate, in terms of the fuel and oxygen mole fraction, expanded as the number of carbon atoms increased. The alkene fuels (ethylene, propene) tested had a higher propensity for sooting as compared with alkane fuels with the same carbon numbers (ethane, propane). Branched iso-butane had a higher propensity for sooting than did n-butane. An increase in the strain rate reduced the tendency for sooting in all the fuels tested. The sensitivity of the sooting limit to the strain rate was more pronounced for less sooting fuels. When plotted in terms of calculated flame temperature, the critical oxygen mole fraction exhibited an Arrhenius form under sooting limit conditions, which can be utilized to significantly reduce the effort required to determine sooting limits at different strain rates. We found that the limiting temperatures of soot formation flames are viable sooting metrics for quantitatively rating the sooting tendency of various fuels, based on comparisons with threshold soot index and normalized smoke point data. We also introduce a sooting temperature index and a sooting sensitivity index, two quantitative measures to describe sooting propensity and its dependence on strain rate. © 2013 The Combustion Institute.

Simulation of soot size distribution in an ethylene counterflow flame

KAUST Repository

Zhou, Kun

2014-01-06

Soot, an aggregate of carbonaceous particles produced during the rich combustion of fossil fuels, is an undesirable pollutant and health hazard. Soot evolution involves various dynamic processes: nucleation soot formation from polycyclic aromatic hydrocarbons (PAHs) condensation PAHs condensing on soot particle surface surface processes hydrogen-abstraction-C2H2-addition, oxidation coagulation two soot particles coagulating to form a bigger particle This simulation work investigates soot size distribution and morphology in an ethylene counterflow flame, using i). Chemkin with a method of moments to deal with the coupling between vapor consumption and soot formation; ii). Monte Carlo simulation of soot dynamics.

A Numerical Study on Effect of Gas-Phase Radiative Heat Loss on Extinction of Hydrogen Diffusion Flames

International Nuclear Information System (INIS)

Sohn, Chae Hoon

2007-01-01

Extinction characteristics of hydrogen-air diffusion flames are investigated numerically by adopting counterflow flame configuration. At various pressures, effect of radiative heat loss on flame extinction is examined. Only gas-phase radiation is considered here. Radiative heat loss depends on flame thickness, temperature, H 2 O concentration, and pressure. From flame structures at various pressures, flame thickness decreases with pressure, but its gradient decreases at high pressure. Flame temperature and mole fraction of H 2 O increase slightly with pressure. Accordingly, as pressure increases, radiative heat loss becomes dominant. When radiative heat loss is considered, radiation-induced extinction is observed at low strain rate in addition to transport-induced extinction. As pressure increases, flammable region shifts to the high-temperature region and then, shrunk to the point on the coordinate plane of flame temperature and strain rate

Flame propagation and counterflow nonpremixed ignition of mixtures of methane and ethylene

Energy Technology Data Exchange (ETDEWEB)

Liu, W.; Kelley, A.P.; Law, C.K. [Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544 (United States)

2010-05-15

The ignition temperature of nitrogen-diluted mixtures of methane and ethylene counterflowing against heated air was measured up to five atmospheres. In addition, the stretch-corrected laminar flame speeds of mixtures of air, methane and ethylene were determined from outwardly-propagating spherical flames up to 10 atmospheres, for extensive range of the lean-to-rich equivalence ratio. These experimental data, relevant to low- to moderately-high-temperature ignition chemistry and high-temperature flame chemistry, respectively, were subsequently compared with calculations using two detailed kinetic mechanisms. A chemical explosive mode analysis (CEMA) was then conducted to identify the dominant ignition chemistry and the role of ethylene addition in facilitating nonpremixed ignition. Furthermore, the hierarchical structure of the associated oxidation kinetics was examined by comparing the sizes and constituents of the skeletal mechanisms of the pure fuels and their mixtures, derived using the method of directed relation graph (DRG). The skeletal mechanism was further reduced by time-scale analysis, leading to a 24-species reduced mechanism from the detailed mechanism of USC Mech II, validated within the parameter space of the conducted experiments. (author)

Propagating nonpremixed edge-flames in a counterflow, annular slot burner under DC electric fields

KAUST Repository

Tran, Vu Manh

2016-09-11

Characteristics of propagating nonpremixed edge-flames were investigated in a counterflow, annular slot burner. A high-voltage direct current (DC) was applied to the lower part of the burner and the upper part was grounded, creating electric field lines perpendicular to the direction of edge-flame propagation. Upon application of an electric field, an ionic wind is caused by the migration of positive and negative ions to lower and higher electrical potential sides of a flame, respectively. Under an applied DC, we found a significant decrease in edge-flame displacement speeds unlike several previous studies, which showed an increase in displacement speed. Within a moderate range of field intensity, we found effects on flame propagation speeds to be negligible after correcting the flame displacement speed with respect to the unburned flow velocity ahead of the flame edge. This indicates that the displacement speed of an edge-flame strongly depends on ionic wind and that an electric field has little or no impact on propagation speed. The ionic wind also influenced the location of the stoichiometric contour in front of the propagating edge in a given configuration such that a propagating edge was relocated to the higher potential side due to an imbalance between ionic winds originating from positive and negative ions. In addition, we observed a steadily wrinkled flame following transient propagation of the edge-flame, a topic for future research. © 2016 The Combustion Institute

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.

Time evolution of propagating nonpremixed flames in a counterflow, annular slot burner under AC electric fields

KAUST Repository

Tran, Vu Manh

2016-06-19

The mechanism behind improved flame propagation speeds under electric fields is not yet fully understood. Although evidence supports that ion movements cause ionic wind, how this wind affects flame propagation has not been addressed. Here, we apply alternating current electric fields to a gap between the upper and lower parts of a counterflow, annular slot burner and present the characteristics of the propagating nonpremixed edge-flames produced. Contrary to many other previous studies, flame displacement speed decreased with applied AC voltage, and, depending on the applied AC frequency, the trailing flame body took on an oscillatory wavy motion. When flame displacement speeds were corrected using measured unburned flow velocities, we found no significant difference in flame propagation speeds, indicating no thermal or chemical effects by electric fields on the burning velocity. Thus, we conclude that the generation of bidirectional ionic wind is responsible for the impact of electric fields on flames and that an interaction between this bidirectional ionic wind and the flame parameters creates visible and/or measurable phenomenological effects. We also explain that the presence of trailing flame bodies is a dynamic response to an electric body force on a reaction zone, an area that can be considered to have a net positively charged volume. In addition, we characterize the wavy motion of the transient flame as a relaxation time independent of mixture strength, strain rate, and Lewis number.

A comparative study on the sooting tendencies of various 1-alkene fuels in counterflow diffusion flames

KAUST Repository

Wang, Yu

2018-02-19

Alkenes are important components in transportation fuels, and are known to have increased sooting tendencies compared to analogous saturated hydrocarbons with the same carbon number. This work aims to understand the sooting tendencies of various 1-alkenes through experiments and numerical simulations for counterflow diffusion flames. Soot and PAH formation tendencies of 1-alkene fuels, including ethylene (C2H4), propene (C3H6), 1-butene (1-C4H8), 1-pentene (1-C5H10), 1-hexene (1-C6H12) and 1-octene (1-C8H16), were experimentally studied using laser induced-incandescence (LII) and laser-induced fluorescence (LIF) techniques, respectively. From the LII results, 1-C4H8 was found to be the most sooting fuel, followed by C3H6 > 1-C5H10 > 1-C6H12 > 1-C8H16 > C2H4. The LIF data with a detection wavelength of 500 nm indicated the PAH formation tendencies followed the order of 1-C4H8 > 1-C5H10 ∼1-C6H12 > C3H6 > 1-C8H16 > C2H4, which were different from the order of sooting tendencies. Numerical simulations with a comprehensive chemical kinetic model including PAH growth chemistry for the tested 1-alkene fuels were conducted to elucidate the aromatic formation pathways and rationalize the experimentally observed trends. The numerical results highlighted the importance of intermediate species with odd carbon numbers in aromatic species formation, such as propargyl, allyl, cyclopentadienyl and indenyl radicals. Their concentration differences, which could be traced back to the parent fuel molecules through rate of production analysis, rationalize the experimentally observed differences in soot and PAH formation tendencies.

Soot reduction under DC electric fields in counterflow non-premixed laminar ethylene flames

KAUST Repository

Park, Daegeun

2014-04-23

The effects of DC electric fields on non-premixed ethylene flames in a counterflow burner were studied experimentally with a focus on the reduction of soot particles. The experiment was conducted by connecting a high voltage terminal and a ground terminal to a lower (fuel) and upper (oxidizer) nozzle, respectively. We applied direct current (DC) potentials in a range of -5 kV < Vdc < 5 kV. Uniform electric fields were then generated in the gap between the two nozzles. The experimental conditions were selected to cover both soot formation (SF) and soot formation oxidation (SFO) flames. The flames subjected to the negative electric fields moved toward the fuel nozzle because of an ionic wind due to the Lorentz force acting on the positive ions in the flames. In addition, the yellow luminosity significantly decreased, indicating changes in the sooting characteristics. To analyze the sooting characteristics under the electric fields, planar laser induced incandescence (PLII) and fluorescence (PLIF) techniques were used to visualize the soot, polycyclic aromatic hydrocarbons (PAHs), and OH radicals. The sooting limits in terms of the fuel and oxygen mole fractions were measured. No substantial soot formation due to the effects of the DC electric fields for the tested range of voltages and reactant mole fractions could be identified. The detailed flame behaviors and sooting characteristics under the DC electric fields are discussed. Copyright © Taylor & Francis Group, LLC.

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

KAUST Repository

Belhi, Memdouh

2017-10-19

Two-dimensional axisymmetric simulations for counterflow nonpremixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current-voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current-voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.

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)

Charge-induced secondary atomization in diffusion flames of electrostatic sprays

Science.gov (United States)

Gomez, Alessandro; Chen, Gung

1994-01-01

The combustion of electrostatic sprays of heptane in laminar counterflow diffusion flames was experimentally studied by measuring droplet size and velocity distributions, as well as the gas-phase temperature. A detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets interact with the flame, an initially monodisperse size distribution becomes bimodal. A secondary sharp peak in the size histogram develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than that of the parent. Other evidence in support of this interpretation is offered by the measurements of droplet size-velocity correlation and velocity component distributions, showing that, as a consequence of the ejection process, the droplets responsible for the secondary peak have velocities uncorrelated with the mean flow. The fission is induced by the electric charge. When a droplet evaporates, in fact, the electric charge density on the droplet surface increases while the droplet shrinks, until the so-called Rayleigh limit is reached at which point the repulsion of electric charges overcomes the surface tension cohesive force, ultimately leading to a disintegraton into finer fragments. We report on the first observation of such fissions in combustion environments. If, on the other hand, insufficient evaporation has occurred before droplets enter the high temperature region, there appears to be no significant evidence of bimodality in their size distribution. In this case, in fact, the concentration of flame chemi-ions or, in the case of positively charged droplets, electrons may be sufficient for them to neutralize the charge on the droplets and to prevent disruption.

Ozone Activated Cool Diffusion Flames of Butane Isomers in a Counterflow Facility

KAUST Repository

Al Omier, Abdullah Abdulaziz

2017-01-01

ignition engines (HCCI) have been developed. These new engines rely on the low temperature chemistry (LTC) combustion concept. A detailed investigation of the properties of cool flames, governed by LTC, is essential for the design of these new engines

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)

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

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.

Flame-in-gas-shield and miniature diffusion flame hydride atomizers for atomic fluorescence spectrometry: optimization and comparison

International Nuclear Information System (INIS)

Marschner, Karel; Musil, Stanislav; Dědina, Jiří

2015-01-01

A detailed optimization of relevant experimental parameters of two hydride atomizers for atomic fluorescence spectrometry: flame-in-gas-shield atomizer with a two-channel shielding unit and a standard atomizer for atomic fluorescence spectrometry, miniature diffusion flame, was performed. Arsine, generated by the reaction with NaBH 4 in a flow injection arrangement, was chosen as the model hydride. Analytical characteristics of both the atomizers (sensitivity, noise, limits of detection) were compared. Under optimum conditions sensitivity obtained with flame-in-gas-shield atomizer was approximately twice higher than with miniature diffusion flame. The additional advantage of flame-in-gas-shield atomizer is significantly lower flame emission resulting in a better signal to noise ratio. The resulting arsenic limits of detection for miniature diffusion flame and flame-in-gas-shield atomizer were 3.8 ng l −1 and 1.0 ng l −1 , respectively. - Highlights: • We optimized and compared two hydride atomizers for atomic fluorescence spectrometry. • Miniature diffusion flame and flame-in-gas-shield atomizer were optimized. • The limit of detection for arsenic was 1.0 ng l −1

Flame-in-gas-shield and miniature diffusion flame hydride atomizers for atomic fluorescence spectrometry: optimization and comparison

Energy Technology Data Exchange (ETDEWEB)

Marschner, Karel, E-mail: karel.marschner@biomed.cas.cz [Institute of Analytical Chemistry of the ASCR, v. v. i., Veveří 97, 602 00 Brno (Czech Republic); Charles University in Prague, Faculty of Science, Department of Analytical Chemistry, Albertov 8, 128 43 Prague (Czech Republic); Musil, Stanislav; Dědina, Jiří [Institute of Analytical Chemistry of the ASCR, v. v. i., Veveří 97, 602 00 Brno (Czech Republic)

2015-07-01

A detailed optimization of relevant experimental parameters of two hydride atomizers for atomic fluorescence spectrometry: flame-in-gas-shield atomizer with a two-channel shielding unit and a standard atomizer for atomic fluorescence spectrometry, miniature diffusion flame, was performed. Arsine, generated by the reaction with NaBH{sub 4} in a flow injection arrangement, was chosen as the model hydride. Analytical characteristics of both the atomizers (sensitivity, noise, limits of detection) were compared. Under optimum conditions sensitivity obtained with flame-in-gas-shield atomizer was approximately twice higher than with miniature diffusion flame. The additional advantage of flame-in-gas-shield atomizer is significantly lower flame emission resulting in a better signal to noise ratio. The resulting arsenic limits of detection for miniature diffusion flame and flame-in-gas-shield atomizer were 3.8 ng l{sup −1} and 1.0 ng l{sup −1}, respectively. - Highlights: • We optimized and compared two hydride atomizers for atomic fluorescence spectrometry. • Miniature diffusion flame and flame-in-gas-shield atomizer were optimized. • The limit of detection for arsenic was 1.0 ng l{sup −1}.

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.

Numerical investigation of biogas diffusion flames characteristics under several operation conditions in counter-flow configuration with an emphasis on thermal and chemical effects of CO2 in the fuel mixture

Science.gov (United States)

Mameri, A.; Tabet, F.; Hadef, A.

2017-08-01

This study addresses the influence of several operating conditions (composition and ambient pressure) on biogas diffusion flame structure and NO emissions with particular attention on thermal and chemical effect of CO2. The biogas flame is modeled by a counter flow diffusion flame and analyzed in mixture fraction space using flamelet approach. The GRI Mech-3.0 mechanism that involves 53 species and 325 reactions is adopted for the oxidation chemistry. It has been observed that flame properties are very sensitive to biogas composition and pressure. CO2 addition decreases flame temperature by both thermal and chemical effects. Added CO2 may participate in chemical reaction due to thermal dissociation (chemical effect). Excessively supplied CO2 plays the role of pure diluent (thermal effect). The ambient pressure rise increases temperature and reduces flame thickness, radiation losses and dissociation amount. At high pressure, recombination reactions coupled with chain carrier radicals reduction, diminishes NO mass fraction.

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

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.

Effect of Low Frequency Burner Vibrations on the Characteristics of Jet Diffusion Flames

Directory of Open Access Journals (Sweden)

C. Kanthasamy

2012-03-01

Full Text Available Mechanical vibrations introduced in diffusion flame burners significantly affect the flame characteristics. In this experimental study, the effects of axial vibrations on the characteristics of laminar diffusion flames are investigated systematically. The effect of the frequency and amplitude of the vibrations on the flame height oscillations and flame stability is brought out. The amplitude of flame height oscillations is found to increase with increase in both frequency and amplitude of burner vibrations. Vibrations are shown to enhance stability of diffusion flames. Although flame lifts-off sooner with vibrations, stability of the flame increases.

Effect of Oxygen Enrichment in Propane Laminar Diffusion Flames under Microgravity and Earth Gravity Conditions

Science.gov (United States)

Bhatia, Pramod; Singh, Ravinder

2017-06-01

Diffusion flames are the most common type of flame which we see in our daily life such as candle flame and match-stick flame. Also, they are the most used flames in practical combustion system such as industrial burner (coal fired, gas fired or oil fired), diesel engines, gas turbines, and solid fuel rockets. In the present study, steady-state global chemistry calculations for 24 different flames were performed using an axisymmetric computational fluid dynamics code (UNICORN). Computation involved simulations of inverse and normal diffusion flames of propane in earth and microgravity condition with varying oxidizer compositions (21, 30, 50, 100 % O2, by mole, in N2). 2 cases were compared with the experimental result for validating the computational model. These flames were stabilized on a 5.5 mm diameter burner with 10 mm of burner length. The effect of oxygen enrichment and variation in gravity (earth gravity and microgravity) on shape and size of diffusion flames, flame temperature, flame velocity have been studied from the computational result obtained. Oxygen enrichment resulted in significant increase in flame temperature for both types of diffusion flames. Also, oxygen enrichment and gravity variation have significant effect on the flame configuration of normal diffusion flames in comparison with inverse diffusion flames. Microgravity normal diffusion flames are spherical in shape and much wider in comparison to earth gravity normal diffusion flames. In inverse diffusion flames, microgravity flames were wider than earth gravity flames. However, microgravity inverse flames were not spherical in shape.

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.

Characteristics of diffusion flames with accelerated motion

Directory of Open Access Journals (Sweden)

Lou Bo

2016-01-01

Full Text Available The aim of this work is to present an experiment to study the characteristics of a laminar diffusion flame under acceleration. A Bunsen burner (nozzle diameter 8 mm, using liquefied petroleum gas as its fuel, was ignited under acceleration. The temperature field and the diffusion flame angle of inclination were visualised with the assistance of the visual display technology incorporated in MATLAB™. Results show that the 2-d temperature field under different accelerations matched the variation in average temperatures: they both experience three variations at different time and velocity stages. The greater acceleration has a faster change in average temperature with time, due to the accumulation of combustion heat: the smaller acceleration has a higher average temperature at the same speed. No matter what acceleration was used, in time, the flame angle of inclination increased, but the growth rate decreased until an angle of 90°: this could be explained by analysis of the force distribution within the flame. It is also found that, initially, the growth rate of angle with velocity under the greater acceleration was always smaller than that at lower accelerations; it was also different in flames with uniform velocity fire conditions.

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.

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.

Development of PIV for Microgravity Diffusion Flames

Science.gov (United States)

Greenberg, Paul S.; Wernet, Mark P.; Yanis, William; Urban, David L.; Sunderland, Peter B.

2003-01-01

Results are presented from the application of Particle Image Velocimetry(PIV) to the overfire region of a laminar gas jet diffusion flame in normal gravity. A methane flame burning in air at 0.98 bar was considered. The apparatus demonstrated here is packaged in a drop rig designed for use in the 2.2 second drop tower.

A PAH growth mechanism and synergistic effect on PAH formation in counterflow diffusion flames

KAUST Repository

Wang, Yu; Raj, Abhijeet Dhayal; Chung, Suk-Ho

2013-01-01

was tested for ethylene premixed flames at low (20torr) and atmospheric pressures by comparing experimentally observed species concentrations with those of the computed ones for small chemical species and PAHs. As compared to several existing mechanisms

Optical investigation of gas-phase KCl/KOH sulfation in post flame conditions

DEFF Research Database (Denmark)

Weng, Wubin; chen, Shuang; Wu, Hao

2018-01-01

A counter-flow reactor setup was designed to investigate the gas-phase sulfation and homogeneous nucleation of potassium salts. Gaseous KOH and KCl were introduced into the post-flame zone of a laminar flat flame. The hot flame products mixed in the counter-flow with cold N2, with or without....... Depending on the potassium speciation in the inlet and the presence of SO2, they consisted of K2SO4, KCl, or K2CO3, respectively. The experiments showed that KOH was sulphated more readily than KCl, resulting in larger quantities of aerosols. The sulfation process in the counter-flow setup was simulated...... using a chemical kinetic model including a detailed subset for the Cl/S/K chemistry. Similar to the experimental results, much more potassium sulfate was predicted when seeding KOH compared to seeding KCl. For both KOH and KCl, sulfation was predicted to occur primarily through the reactions among...

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.

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.

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...

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

KAUST Repository

Zhou, Zhen; Hernandez Perez, Francisco; Shoshin, Yuriy; van Oijen, Jeroen A.; de Goey, Laurentius P.H.

2017-01-01

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H2 while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H2 is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H2 on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H2 caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

KAUST Repository

Zhou, Zhen

2017-04-12

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H2 while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H2 is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H2 on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H2 caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.

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.

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

Impact of co-flow air on buoyant diffusion flames flicker

Energy Technology Data Exchange (ETDEWEB)

Gohari Darabkhani, H., E-mail: h.g.darabkhani@gmail.com [School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Sackville Street, Manchester M13 9PL (United Kingdom); Wang, Q.; Chen, L.; Zhang, Y. [Mechanical Engineering Department, University of Sheffield, Mapping Street, Sheffield S1 3JD (United Kingdom)

2011-08-15

Highlights: {yields} We present the co-flow effects on flickering behaviour of diffusion flames. {yields} Co-flow air is shown to fully suppress the buoyancy driven flame oscillations. {yields} Schlieren and PIV illustrate the shift of outer vortices beyond the flame zone. {yields} Stability controlling parameter as a ratio of air to fuel velocities is presented. {yields} Equation for linear increase in flickering frequency by co-flow air is presented. - Abstract: This paper describes experimental investigation of co-flow air velocity effects on the flickering behaviour of laminar non-lifted methane diffusion flames. Chemiluminescence, high-speed photography, schlieren and Particle Imaging Velocimetry (PIV), have been used to study the changes in the flame/vortex interactions as well as the flame flickering frequency and magnitude by the co-flow air. Four cases of methane flow rates at different co-flow air velocities are investigated. It has been observed that the flame dynamics and stability of co-flow diffusion flames are strongly affected by the co-flow air velocity. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The schlieren and PIV imaging have revealed that the co-flow of air is able to push the initiation point of the outer toroidal vortices beyond the visible flame to create a very steady laminar flow region in the reaction zone. Then the buoyancy driven instability is only effective in the plume of hot gases above the visible flame. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity, {gamma}, is a stability controlling parameter. The velocity ratio, {gamma}, was found to be 0.72 for the range of tested flow rates. The dominant flickering frequency was observed to increase linearly with the co-flow rate (a) as; f = 0.33a + 11. The frequency amplitudes

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

Investigation of soot formation and temperature field in laminar diffusion flames of LPG-air mixture

Energy Technology Data Exchange (ETDEWEB)

Shahad, Haroun A.K.; Mohammed, Yassar K.A. [Babylon Univ., Dept. of Mechanical Engineering, Babylon (Israel)

2000-11-01

Soot formation and burnout were studied at atmospheric pressure in co-flowing, axisymmetric buoyant laminar diffusion flames and double flames of liquefied petroleum gases (LPG)-air mixtures. In diffusion flames, two different fuel flow rates were examined. In double flames, three different primary air flow rates were examined. A soot sampling probe and a thermocouple were used to measure the local soot mass concentration and flame temperature, respectively. Flame residence time was predicted using a uniformly accelerated motion model as function of axial distance of the flame. The increase of primary air flow rate was found to suppress the energy transfer from the annular region, at which the soot is produced, to the flame axis. The time required to initiate soot formation at the flame axis becomes longer as the primary air is increased. The trend rate of soot formation was found to be similar along the flame axis in all tested diffusion flames. The increase of primary air by 10% of the stoichiometric air requirement of the fuel results in a 70% reduction in maximum soot concentration. The final exhaust of soot, which is determined by the net effect of soot formation and burnout, is much lower in double flames than that in diffusion flames. (Author)

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.

Experimental characterization of methane inverse diffusion flame

KAUST Repository

Elbaz, Ayman M.; Roberts, William L.

2014-01-01

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

Propagation and diffusion-limited extinction of nonadiabatic heterogeneous flame in the SHS process

International Nuclear Information System (INIS)

Makino, Atsushi

1994-01-01

Nonadiabatic heterogeneous flame propagation and extinction in self-propagating high-temperature synthesis (SHS) are analyzed based on a premixed mode of propagation for the bulk flame supported by the nonpremixed reaction of dispersed nonmetals in the liquid metal. The formulation allows for volumetric heat loss throughout the bulk flame, finite-rate Arrhenius reaction at the particle surface, and temperature-sensitive Arrhenius mass diffusion in the liquid. Results show that, subsequent to melting of the metal, the flame structure consists of a relatively thin diffusion-consumption/convection zone followed by a relatively thick convection-loss zone, that the flame propagation rate decreases with increasing heat loss, that at a critical heat-loss rate the flame extinguishes as indicated by the characteristic turning-point behavior, that the surface reaction is diffusion limited such that the nonlinear, temperature-sensitive nature of the system is actually a consequence of the Arrhenius mass diffusion, and that extinction is sensitively affected by the mixture ratio, the degree of dilution, the initial temperature of the compact, and the size of the nonmetal particles. An explicit expression is derived for the normalized mass burning rate, which exhibits the characteristic turning point and shows that extinction occurs when this value is reduced to e -1/2 , which is the same as that for the nonadiabatic gaseous premixed flame. It is further shown that the theoretical results agree well with available experimental data, indicating that the present formulation captures the essential features of the nonadiabatic heterogeneous SHS processes and its potential for extension to describe other SHS phenomena

Measurements and Experimental Database Review for Laminar Flame Speed Premixed Ch4/Air Flames

Science.gov (United States)

Zubrilin, I. A.; Matveev, S. S.; Matveev, S. G.; Idrisov, D. V.

2018-01-01

Laminar flame speed (SL ) of CH4 was determined at atmospheric pressure and initial gas temperatures in range from 298 to 358 K. The heat flux method was employed to measure the flame speed in non-stretched flames. The kinetic mechanism GRI 3.0 [1] were used to simulate SL . The measurements were compared with available literature results. The data determined with the heat flux method agree with some previous burner measurements and disagree with the data from some vessel closed method and counterflow method. The GRI 3.0 mechanism was able to reproduce the present experiments. Laminar flame speed was determined at pressures range from of 1 to 20 atmospheres through mechanism GRI 3.0. Based on experimental data and calculations was obtained SL dependence on pressure and temperature. The resulting of dependence recommended use during the numerical simulation of methane combustion.

Experimental studies of flame stability and emission characteristics of simple LPG jet diffusion flame

Energy Technology Data Exchange (ETDEWEB)

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

2007-07-15

In the present study, experiments were carried out to measure the lift-off height, H{sub L}; flame length, L{sub f} and blow-off velocity for a simple LPG (liquefied petroleum gas) jet diffusion flames. It is observed that lift-off height is proportional to the fuel exit velocity, U{sub f}. A semi-empirical correlation between lift-off height and global strain rate, U{sub f}/D{sub f} is proposed. Two regimes identified either as buoyancy or momentum dominated were characterized by Froude number, Fr. For momentum dominated jet diffusion flames, L{sub f}/D{sub f} remains almost constant and therefore is independent of the Froude number. The NOx emissions, expressed in terms of emission index, EINOx is found to decrease with U{sub f}. This decreasing trend is consistent with the concept that increasing jet velocity reduces the residence time as reported in the literature. The present data is also compared with the available data of propane gas and found to be in good agreement well particularly in trend wise. Besides these data, EINOx scaling law is also reported in the present study. 20 refs., 8 figs.

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.

Analysis of Flame Extinguishment and Height in Low Frequency Acoustically Excited Methane Jet Diffusion Flame

Science.gov (United States)

Zong, Ruowen; Kang, Ruxue; Liu, Chen; Zhang, Zhiyang; Zhi, Youran

2018-01-01

The exploration of microgravity conditions in space is increasing and existing fire extinguishing technology is often inadequate for fire safety in this special environment. As a result, improving the efficiency of portable extinguishers is of growing importance. In this work, a visual study of the effects on methane jet diffusion flames by low frequency sound waves is conducted to assess the extinguishing ability of sound waves. With a small-scale sound wave extinguishing bench, the extinguishing ability of certain frequencies of sound waves are identified, and the response of the flame height is observed and analyzed. Results show that the flame structure changes with disturbance due to low frequency sound waves of 60-100 Hz, and quenches at effective frequencies in the range of 60-90 Hz. In this range, 60 Hz is considered to be the quick extinguishing frequency, while 70-90 Hz is the stable extinguishing frequency range. For a fixed frequency, the flame height decreases with sound pressure level (SPL). The flame height exhibits the greatest sensitivity to the 60 Hz acoustic waves, and the least to the 100 Hz acoustic waves. The flame height decreases almost identically with disturbance by 70-90 Hz acoustic waves.

Nonpremixed flame in a counterflow under electric fields

KAUST Repository

Park, Daegeun

2016-01-01

Electrically assisted combustion has been studied in order to control or improve flame characteristics, and emphasizing efficiency and emission regulation. Many phenomenological observations have been reported on the positive impact of electric

Computational and Experimental Study of Energetic Materials in a Counterflow Microgravity Environment

Science.gov (United States)

Takahashi, Fumiaki (Technical Monitor); Urban, David (Technical Monitor); Smooke, M. D.; Parr, T. P.; Hanson-Parr, D. M.; Yetter, R. A.; Risha, G.

2004-01-01

Counterflow diffusion flames are studied for various fuels flowing against decomposition products from solid ammonium perchlorate (AP) pellets in order to obtain fundamental understanding of composite propellant flame structure and chemistry. We illustrate this approach through a combined experimental and numerical study of a fuel mixture consisting of C2H4 CO + H2, and C2H2 + C2H4 flowing against solid AP. For these particular AP-fuel systems, the resulting flame zone simulates the various flame structures that are ex+ to exist between reaction products from Ap crystals and a hydrocarbon binder. As in all our experimental studies, quantitative species and temperature profiles have been measured between the fuel exit and AP surface. Species measured included CN, NH, NO, OH, N2, CO2, CO, H2, CO, HCl, and H2O. Temperature was measured using a thermocouple at the exit, spontaneous Raman scattering measurements throughout the flame, OH rotational population distributions, and NO vibrational population distributions. The burning rate of AP was also measured as a function of strain rate, given by the separation distance between the AP surface and the gaseous hydrocarbon fuel tube exit plane. This distance was nominally set at 5 mm, although studies have been performed for variations in separation distance. The measured 12 scalars are compared with predictions from a detailed gas-phase kinetics model consisting of 86 species and 531 reactions. Model predictions are found to be in good agreement with experiment and illustrate the type of kinetic features that may be expected to occur in propellants when AP particle size distributions are varied. Furthermore, the results constitute the continued development of a necessary database and validation of a comprehensive model for studying more complex AP-solid fuel systems in microgravity. Exploratory studies have also been performed with liquid and solid fuels at normal gravity. Because of melting (and hence dripping) and deep

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)

Role of soot in the transport of chlorine in hydrocarbon-air diffusion flames

International Nuclear Information System (INIS)

Venkatesh, S.; Saito, K.; Stencel, J.M.; Majidi, V.; Owens, M.

1991-01-01

Soot is an inevitable product of incomplete combustion in many practical combustion systems such as automobiles, incinerators and furnaces. Recent studies on chlorinated hydrocarbon combustion have shown that soot and other praticulates (eg. fly ash) play an important role in secondary reactions leading to the formation of chlorine substituted polyaromatic hydrocarbons (PAHs). In order to attain very high destruction efficiencies the fundamental chemical and physical processes that are associated with combustion, and post-combustion cleanup must be well understood. In order to understand the effect of chlorine on the soot formed in a combustion system, fundamental studies using a coflow laminar hydrocarbon-air diffusion flame have been carried out. Phenomenological studies have revealed the effect of chlorine on the visible structure of the flame. Soot inception activation energies were estimated for methane, ethane and ethylene diffusion flames for the case of with and without chlorine addition. No significant difference in the activation energy was estimated for either case. The effect of chlorine on the soot escape rate of an acetylene diffusion flame was estimated. The soot formed in these diffusion flames was analyzed for chlorine using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and by laser induced plasma spectroscopy (LIPS). REsults from these techniques indicate the presence of chlorine in the soot formed. In this paper a chemical scheme to explain the chlorine found in the soot is proposed based on known theories of soot formation

A multi-probe thermophoretic soot sampling system for high-pressure diffusion flames

Energy Technology Data Exchange (ETDEWEB)

Vargas, Alex M.; Gülder, Ömer L. [Institute for Aerospace Studies, University of Toronto, Toronto, Ontario M3H 5T6 (Canada)

2016-05-15

Optical diagnostics and physical probing of the soot processes in high pressure combustion pose challenges that are not faced in atmospheric flames. One of the preferred methods of studying soot in atmospheric flames is in situ thermophoretic sampling followed by transmission electron microscopy imaging and analysis for soot sizing and morphology. The application of this method of sampling to high pressures has been held back by various operational and mechanical problems. In this work, we describe a rotating disk multi-probe thermophoretic soot sampling system, driven by a microstepping stepper motor, fitted into a high-pressure chamber capable of producing sooting laminar diffusion flames up to 100 atm. Innovative aspects of the sampling system design include an easy and precise control of the sampling time down to 2.6 ms, avoidance of the drawbacks of the pneumatic drivers used in conventional thermophoretic sampling systems, and the capability to collect ten consecutive samples in a single experimental run. Proof of principle experiments were performed using this system in a laminar diffusion flame of methane, and primary soot diameter distributions at various pressures up to 10 atm were determined. High-speed images of the flame during thermophoretic sampling were recorded to assess the influence of probe intrusion on the flow field of the flame.

Effects of Burner Configurations on the Natural Oscillation Characteristics of Laminar Jet Diffusion Flames

Directory of Open Access Journals (Sweden)

K. R. V. Manikantachari

2015-09-01

Full Text Available In this work, effects of burner configurations on the natural oscillations of methane laminar diffusion flames under atmospheric pressure and normal gravity conditions have been studied experimentally. Three regimes of laminar diffusion flames, namely, steady, intermittent flickering and continuous flickering have been investigated. Burner configurations such as straight pipe, contoured nozzle and that having an orifice plate at the exit have been considered. All burners have the same area of cross section at the exit and same burner lip thickness. Flame height data has been extracted from direct flame video using MATLAB. Shadowgraph videos have been captured to analyze the plume width characteristics. Results show that, the oscillation characteristics of the orifice burner is significantly different from the other two burners; orifice burner produces a shorter flame and wider thermal plume width in the steady flame regime and the onset of the oscillation/flickering regimes for the orifice burner occurs at a higher fuel flow rate. In the natural flickering regime, the dominating frequency of flame flickering remains within a small range, 12.5 Hz to 15 Hz, for all the burners and for all fuel flow rates. The time-averaged flame length-scale parameters, such as the maximum and the minimum flame heights, increase with respect to the fuel flow rate, however, the difference in the maximum and the minimum flame heights remains almost constant.

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)

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)

Multiple-diffusion flame synthesis of pure anatase and carbon-coated titanium dioxide nanoparticles

KAUST Repository

Memon, Nasir; Anjum, Dalaver H.; Chung, Suk-Ho

2013-01-01

A multi-element diffusion flame burner (MEDB) is useful in the study of flame synthesis of nanomaterials. Here, the growth of pure anatase and carbon-coated titanium dioxide (TiO2) using an MEDB is demonstrated. Hydrogen (H2), oxygen (O2), and argon

Diffusion air effects on the soot axial distribution concentration in a premixed acetylene/air flame

Energy Technology Data Exchange (ETDEWEB)

Fassani, Fabio Luis; Santos, Alex Alisson Bandeira; Goldstein Junior, Leonardo [Universidade Estadual de Campinas, SP (Brazil). Faculdade de Engenharia Mecanica. Dept. de Engenharia Termica e de Fluidos]. E-mails: fassani@fem.unicamp.br; absantos@fem.unicamp.br; leonardo@fem.unicamp.br; Ferrari, Carlos Alberto [Universidade Estadual de Campinas, SP (Brazil). Inst. de Fisica. Dept. de Eletronica Quantica]. E-mail: ferrari@ifi.unicamp.br

2000-07-01

Soot particles are produced during the high temperature pyrolysis or combustion of hydrocarbons. The emission of soot from a combustor, or from a flame, is determined by the competition between soot formation and its oxidation. Several factors affect these processes, including the type of fuel, the air-to-fuel ratio, flame temperature, pressure, and flow pattern. In this paper, the influence of the induced air diffusion on the soot axial distribution concentration in a premixed acetylene/air flame was studied. The flame was generated in a vertical axis burner in which the fuel - oxidant mixture flow was surrounded by a nitrogen discharge coming from the annular region between the burner tube and an external concentric tube. The nitrogen flow provided a shield that protected the flame from the diffusion of external air, enabling its control. The burner was mounted on a step-motor driven, vertical translation table. The use of several air-to-fuel ratios made possible to establish the sooting characteristics of this flame, by measuring soot concentration along the flame height with a non-intrusive laser light absorption technique. (author)

Response to acoustic forcing of laminar coflow jet diffusion flames

KAUST Repository

Chrystie, Robin

2014-04-23

Toward the goal of understanding and controlling instability in combustion systems, we present a fundamental characterization of the interaction of the buoyancy-induced instability in flickering flames with forced excitation of fuel supply. Laminar coflow diffusion flames were acoustically forced, whose frequency responses were recorded as a function of excitation frequency and amplitude. The evolving structure of such flames was also examined through the use of video analysis and particle imaging velocimetry (PIV). For specific combinations of excitation frequency and amplitude, the frequency response of the flames was found to couple to that of the forcing, where the contribution of natural puffing frequency disappears. Such instances of coupling exhibited many harmonics of the excitation frequency, related indirectly to the natural puffing frequency. We showed how such harmonics form, through application of PIV, and furthermore unveiled insight into the physics of how the flame couples to the forcing under certain conditions. Our frequency response characterization provides quantitative results, which are of utility for both modeling studies and active-control strategies. Copyright © Taylor & Francis Group, LLC.

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.

DC field response of one-dimensional flames using an ionized layer model

KAUST Repository

Xiong, Yuan

2015-11-18

We develop a simplified model to better explain electric current response when direct current (DC) is applied to a flame. In particular, different current responses have been observed by changing the polarity of the DC in a sub-saturated current regime that results from the presence of ions and electrons in the flame zone. A flame zone was modeled as a thin, ionized layer located in one-dimensional DC electric fields. We derived simplified model-governing equations from species equations by implementing mobility differences dependent on the type of charged particle, particularly between ions and electrons; we performed experiments to substantiate the model. Results showed that the sub-saturated current and local field intensity were significantly influenced by the polarity of the DC because of the combined effect of unequal mobility of charged particles and the position of the ionized layer in the gap relative to two electrodes. When an energized electrode is close to the ionized layer, applying a negative DC causes a more rapid increase in current than by applying a positive DC to the same electrode. Results from our experimental measurement of current using counterflow diffusion flames agreed qualitatively well with the model predictions. A sensitivity analysis using dimensional and non-dimensional parameters also supported the importance of the mobility difference and the relative location of the ionized layer on the electric current response.

Effect of diluents on soot precursor formation and temperature in ethylene laminar diffusion flames

KAUST Repository

Abhinavam Kailasanathan, Ranjith Kumar

2013-03-01

Soot precursor species concentrations and flame temperature were measured in a diluted laminar co-flow jet diffusion flame at pressures up to eight atmospheres while varying diluent type. The objective of this study was to gain a better understanding of soot production and oxidation mechanisms, which could potentially lead to a reduction in soot emissions from practical combustion devices. Gaseous samples were extracted from the centerline of an ethylene-air laminar diffusion flame, which was diluted individually with four diluents (argon, helium, nitrogen, and carbon dioxide) to manipulate flame temperature and transport properties. The diluted fuel and co-flow exit velocities (top-hat profiles) were matched at all pressures to minimize shear-layer effects, and the mass fluxes were fixed over the pressure range to maintain constant Reynolds number. The flame temperature was measured using a fine gauge R-type thermocouple at pressures up to four atmospheres. Centerline concentration profiles of major non-fuel hydrocarbons collected via extractive sampling with a quartz microprobe and quantification using GC/MS+FID are reported within. The measured hydrocarbon species concentrations are vary dramatically with pressure and diluent, with the helium and carbon dioxide diluted flames yielding the largest and smallest concentrations of soot precursors, respectively. In the case of C2H2 and C6H6, two key soot precursors, helium diluted flames had concentrations more than three times higher compared with the carbon dioxide diluted flame. The peak flame temperature vary with diluents tested, as expected, with carbon dioxide diluted flame being the coolest, with a peak temperature of 1760K at 1atm, and the helium diluted flame being the hottest, with a peak temperature of 2140K. At four atmospheres, the helium diluted flame increased to 2240K, but the CO2 flame temperature increased more, decreasing the difference to approximately 250K. © 2012 The Combustion Institute.

Role of Fluid-Dynamics in Soot Formation and Microstructure in Acetylene-Air Laminar Diffusion Flames

Directory of Open Access Journals (Sweden)

Praveen Pandey

2015-03-01

Full Text Available Residence time and thermo-chemical environment are important factors in the soot-formation process in flames. Studies have revealed that flow-dynamics plays a dominant role in soot formation process. For understanding the effect of flow dynamics on soot formation and physical structure of the soot formed in different combustion environments two types of laminar diffusion flames of Acetylene and air, a normal diffusion flame (NDF and an inverse diffusion flame (IDF have been investigated. The fuel and air supply in the reaction zone in two flame types were kept constant but the interchange of relative position of fuel and air altered the burner exit Reynolds and Froude numbers of gases, fuel/air velocity ratio and flame shape. Soot samples were collected using thermophoretic sampling on transmission electron microscope (TEM grids at different flame heights and were analyzed off-line in a Transmission Electron Microscope. Soot primary particle size, soot aggregate size and soot volume fraction were measured using an image analysis software. In NDF the maximum flame temperature was about 1525 K and 1230 K for IDF. The soot primary particles are distinctly smaller in size in IDF (between 19 – 26 nm compared to NDF (between 29–34 nm. Both NDF and IDF show chainlike branched structure of soot agglomerate with soot particles of a nearly spherical shape. The average number of soot primary particles per aggregate in NDF was in the range of 24 to 40 and in IDF it varied between 16 to 24. Soot volume fraction was between 0.6 to 1.5 ppm in NDF where as it was less than 0.2 ppm in IDF. The change in sooting characteristics of the two flame types is attributed to changed fuel/air velocity ratio, entrainment of gas molecules and thermophoresis on soot particles.

Turbulent jet in confined counterflow

Indian Academy of Sciences (India)

The mean flowfield of a turbulent jet issuing into a confined, uniform counterflow was investigated computationally. Based on dimensional analysis, the jet penetration length was shown to scale with jet-to-counterflow momentum flux ratio. This scaling and the computational results reproduce the well-known correct limit of ...

Correction of edge-flame propagation speed in a counterflow, annular slot burner

KAUST Repository

Tran, Vu Manh; Cha, Min

2015-01-01

to study the fundamental behaviors of edge-flames. In addition, our burner has easy optical access for detailed laser diagnostics. Flame displacement speeds were measured using a high-speed camera and related flow fields of unburned gases were visualized

Propagation of intense laser radiation through a diffusion flame of burning oil

Science.gov (United States)

Gvozdev, S. V.; Glova, A. F.; Dubrovskii, V. Yu; Durmanov, S. T.; Krasyukov, A. G.; Lysikov, A. Yu; Smirnov, G. V.; Pleshkov, V. M.

2015-06-01

We report the results of measuring the absorption coefficient of radiation from a cw ytterbium fibre single-mode laser with the power up to 1.5 kW by a diffusion flame of oil, burning in the atmosphere air at normal pressure on a free surface. For the constant length (30 mm) and width (30 mm) of the flame and the distance 10 mm between the laser beam axis and the oil surface the dependence of the absorption coefficient, averaged over the flame length, on the mean radiation intensity (varied from 4.5 × 103 to 1.2 × 106 W cm-2) entering the flame is obtained. The qualitative explanation of nonmonotonic behaviour of the absorption coefficient versus the intensity is presented.

Experimental study of the inverse diffusion flame using high repetition rate OH/acetone PLIF and PIV

KAUST Repository

Elbaz, Ayman M.; Roberts, William L.

2015-01-01

Most previous work on inverse diffusion flames (IDFs) has focused on laminar IDF emissions and the soot formation characteristics. Here, we investigate the characteristics and structure of methane IDFs using high speed planar laser-induced fluorescence (PLIF) images of OH, particle image velocimetry (PIV), and acetone PLIF imaging for non-reacting cases. First, the flame appearance was investigated with fixed methane loading (mass flux) but with varying airflow rates, yielding a central air jet Reynolds number (Re) of 1,000 to 6,000 (when blow-off occurs). Next, it was investigated a fixed central air jet Re of 4500, but with varied methane mass flux such that the global equivalence ratio spanned 0.5 to 4. It was observed that at Re smaller than 2000, the inner air jet promotes the establishment of an inverse diffusion flame surrounded by a normal diffusion flame. However, when the Re was increased to 2500, two distinct zones became apparent in the flame, a lower entrainment zone and an upper mixing and combustion zone. 10 kHz OH-PLIF images, and 2D PIV allow the identification of the fate and spatial flame structure. Many flame features were identified and further analyzed using simple but effective image processing methods, where three types of structure in all the flames investigated here: flame holes or breaks; closures; and growing kernels. Insights about the rate of evolution of these features, the dynamics of local extinction, and the sequence of events that lead to re-ignition are reported here. In the lower entrainment zone, the occurrence of the flame break events is counterbalanced by closure events, and the edge propagation appears to control the rate at which the flame holes and closures propagate. The rate of propagation of holes was found to be statistically faster than the rate of closure. As the flames approach blow-off, flame kernels become the main mechanism for flame re-ignition further downstream. The simultaneous OH-PLIF/Stereo PIV

Experimental study of the inverse diffusion flame using high repetition rate OH/acetone PLIF and PIV

KAUST Repository

Elbaz, Ayman M.

2015-10-29

Most previous work on inverse diffusion flames (IDFs) has focused on laminar IDF emissions and the soot formation characteristics. Here, we investigate the characteristics and structure of methane IDFs using high speed planar laser-induced fluorescence (PLIF) images of OH, particle image velocimetry (PIV), and acetone PLIF imaging for non-reacting cases. First, the flame appearance was investigated with fixed methane loading (mass flux) but with varying airflow rates, yielding a central air jet Reynolds number (Re) of 1,000 to 6,000 (when blow-off occurs). Next, it was investigated a fixed central air jet Re of 4500, but with varied methane mass flux such that the global equivalence ratio spanned 0.5 to 4. It was observed that at Re smaller than 2000, the inner air jet promotes the establishment of an inverse diffusion flame surrounded by a normal diffusion flame. However, when the Re was increased to 2500, two distinct zones became apparent in the flame, a lower entrainment zone and an upper mixing and combustion zone. 10 kHz OH-PLIF images, and 2D PIV allow the identification of the fate and spatial flame structure. Many flame features were identified and further analyzed using simple but effective image processing methods, where three types of structure in all the flames investigated here: flame holes or breaks; closures; and growing kernels. Insights about the rate of evolution of these features, the dynamics of local extinction, and the sequence of events that lead to re-ignition are reported here. In the lower entrainment zone, the occurrence of the flame break events is counterbalanced by closure events, and the edge propagation appears to control the rate at which the flame holes and closures propagate. The rate of propagation of holes was found to be statistically faster than the rate of closure. As the flames approach blow-off, flame kernels become the main mechanism for flame re-ignition further downstream. The simultaneous OH-PLIF/Stereo PIV

Numerical Investigation of Laminar Diffusion Flames Established on a Horizontal Flat Plate in a Parallel Air Stream

Directory of Open Access Journals (Sweden)

E. D. Gopalakrishnan

2011-06-01

Full Text Available Numerical investigation of laminar diffusion flames established on a flat plate in a parallel air stream is presented. A numerical model with a multi-step chemical kinetics mechanism, variable thermo-physical properties, multi-component species diffusion and a radiation sub-model is employed for this purpose. Both upward and downward injection of fuel has been considered in a normal gravity environment. The thermal and aerodynamic structure of the flame has been explained with the help of temperature and species contours, net reaction rate of fuel and streamlines. Flame characteristics and stability aspects for several air and fuel velocity combinations have been studied. An important characteristic of a laminar boundary layer diffusion flame with upward injection of fuel is the velocity overshoot that occurs near the flame zone. This is not observed when the fuel is injected in the downward direction. The flame standoff distance is slightly higher for the downward injection of fuel due to increase in displacement thickness of boundary layer. Influence of an obstacle, namely the backward facing step, on the flame characteristics and stability aspects is also investigated. Effects of air and fuel velocities, size and location of the step are studied in detail. Based on the air and fuel velocities, different types of flames are predicted. The use of a backward-facing step as a flame holding mechanism for upward injection of fuel, results in increased stability limits due to the formation of a recirculation zone behind the step. The predicted stability limits match with experimentally observed limits. The step location is seen to play a more important role as compared to the step height in influencing the stability aspects of flames.

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.

Propagation of intense laser radiation through a diffusion flame of burning oil

International Nuclear Information System (INIS)

Gvozdev, S V; Glova, A F; Dubrovskii, V Yu; Durmanov, S T; Krasyukov, A G; Lysikov, A Yu; Smirnov, G V; Pleshkov, V M

2015-01-01

We report the results of measuring the absorption coefficient of radiation from a cw ytterbium fibre single-mode laser with the power up to 1.5 kW by a diffusion flame of oil, burning in the atmosphere air at normal pressure on a free surface. For the constant length (30 mm) and width (30 mm) of the flame and the distance 10 mm between the laser beam axis and the oil surface the dependence of the absorption coefficient, averaged over the flame length, on the mean radiation intensity (varied from 4.5 × 10 3 to 1.2 × 10 6 W cm -2 ) entering the flame is obtained. The qualitative explanation of nonmonotonic behaviour of the absorption coefficient versus the intensity is presented. (laser applications and other topics in quantum electronics)

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.

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...

Extinction of laminar partially premixed flames

Energy Technology Data Exchange (ETDEWEB)

Aggarwal, Suresh K. [Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor Street, Room 2039, MC-251, Chicago, IL 60607-7022 (United States)

2009-12-15

Flame extinction represents one of the classical phenomena in combustion science. It is important to a variety of combustion systems in transportation and power generation applications. Flame extinguishment studies are also motivated from the consideration of fire safety and suppression. Such studies have generally considered non-premixed and premixed flames, although fires can often originate in a partially premixed mode, i.e., fuel and oxidizer are partially premixed as they are transported to the reaction zone. Several recent investigations have considered this scenario and focused on the extinction of partially premixed flames (PPFs). Such flames have been described as hybrid flames possessing characteristics of both premixed and non-premixed flames. This paper provides a review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames. Theoretical, numerical and experimental studies dealing with the extinction of such flames in coflow and counterflow configurations are discussed. Since these flames contain both premixed and non-premixed burning zones, a brief review of the dilution-induced extinction of premixed and non-premixed flames is also provided. For the coflow configuration, processes associated with flame liftoff and blowout are described. Since lifted non-premixed jet flames often contain a partially premixed or an edge-flame structure prior to blowout, the review also considers such flames. While the perspective of this review is broad focusing on the fundamental aspects of flame extinction and blowout, results mostly consider flame extinction caused by the addition of a flame suppressant, with relevance to fire suppression on earth and in space environment. With respect to the latter, the effect of gravity on the extinction of PPFs is discussed. Future research needs are identified. (author)

Soot particle size measurements in ethylene diffusion flames at elevated pressures

KAUST Repository

Steinmetz, Scott

2016-05-07

Soot particle size is investigated in laminar nitrogen-diluted ethylene coflow diffusion flames at 4, 8, 12 and 16 atm. Line of sight attenuation and scattering are used to measure two-dimensional soot volume fraction and particle size fields for the first time at elevated pressures. Soot volume fraction dependence on pressure is consistent with the observations of similar studies, scaling approximately with the square of pressure. Scattering intensity is analyzed through Rayleigh and Rayleigh-Debye-Gans polydisperse fractal aggregate theories to provide two estimates of particle size. An increase in overall particle sizes with pressure is found, consistent with similar one-dimensional studies. Particle diameters in the annulus of the flame increase faster with pressure than those on centerline. Contrary to previous studies, the dependence of particle size on pressure was found to taper off between 8 and 12 atm, with little observed growth beyond 12 atm. The measurements provide additional data for one of the International Sooting Flame (ISF) workshop\\'s target pressurized flames.

Turbulent structure and emissions of strongly-pulsed jet diffusion flames

Science.gov (United States)

Fregeau, Mathieu

-pulsed flames was not strongly impacted by buoyancy. This lack of sensitivity to buoyancy was consistent with offsetting changes in flame puff celerity and time to burnout for the microgravity versus normal-gravity cases. The emissions of CO and NO were examined in the vicinity of the visible flame tip and at the combustor exit for strongly-pulsed flames. The highest exhaust-point emission indices of CO for compact, isolated puffs were as much as a factor of six higher than those of elongated flames with longer injection times. The amount of CO decreased substantially with a decreased amount of flame puff interaction. The higher CO levels for pulsed flames with the shortest injection times were consistent with quenching due to the very rapid mixing and dilution with excess air for the most compact flame puffs. The injection time for which steady-flame emission levels were attained was comparable to the injection time for which the visible flame length approached the flame length of steady flames. The CO emissions, for a given fuelling rate, were strongly dependent on both the injection time and jet-off time for a jet-on fraction less than approximately 50%. The NO levels were generally proportional to the fuelling rate. This work indicates that there are specific combinations of injection time and jet-off time that considerably change the fuel/air mixing, resulting in emissions comparable to those of the steady flame while the flame length is significantly shorter. This points the potential utility of the strongly-pulsed injection technique in the development of compact, low emissions combustors involving turbulent diffusion flames. (Abstract shortened by UMI.)

Effects of preheated combustion air on laminar coflow diffusion flames under normal and microgravity conditions

Science.gov (United States)

Ghaderi Yeganeh, Mohammad

Global energy consumption has been increasing around the world, owing to the rapid growth of industrialization and improvements in the standard of living. As a result, more carbon dioxide and nitrogen oxide are being released into the environment. Therefore, techniques for achieving combustion at reduced carbon dioxide and nitric oxide emission levels have drawn increased attention. Combustion with a highly preheated air and low-oxygen concentration has been shown to provide significant energy savings, reduce pollution and equipment size, and uniform thermal characteristics within the combustion chamber. However, the fundamental understanding of this technique is limited. The motivation of the present study is to identify the effects of preheated combustion air on laminar coflow diffusion flames. Combustion characteristics of laminar coflow diffusion flames are evaluated for the effects of preheated combustion air temperature under normal and low-gravity conditions. Experimental measurements are conducted using direct flame photography, particle image velocimetry (PIV) and optical emission spectroscopy diagnostics. Laminar coflow diffusion flames are examined under four experimental conditions: normal-temperature/normal-gravity (case I), preheated-temperature/normal gravity (case II), normal-temperature/low-gravity (case III), and preheated-temperature/low-gravity (case IV). Comparisons between these four cases yield significant insights. In our studies, increasing the combustion air temperature by 400 K (from 300 K to 700 K), causes a 37.1% reduction in the flame length and about a 25% increase in peak flame temperature. The results also show that a 400 K increase in the preheated air temperature increases CH concentration of the flame by about 83.3% (CH is a marker for the rate of chemical reaction), and also increases the C2 concentration by about 60% (C2 is a marker for the soot precursor). It can therefore be concluded that preheating the combustion air

Plasma Assisted Combustion: Flame Regimes and Kinetic Studies

Science.gov (United States)

2015-01-05

Schauer, Yiguang Ju, Schlieren Imaging and Pulsed Detonation Engine Testing of Ignition by a Nanosecond Repetitively Pulsed Discharge , submitted to...diffusional cool flames • A heated counterflow burner integrated with vaporization system1 • n-heptane/nitrogen vs. oxygen/ ozone • Ozone generator...micro-DBD) produces 2- 5 % of ozone in oxygen stream, depending on oxygen flow rate • Speciation profiles by using a micro-probe sampling with a

Characterization of bluff-body stabilized LPG jet diffusion flame with N2 dilution

International Nuclear Information System (INIS)

Kumar, P.; Mishra, D.P.

2008-01-01

Measurements of several parameters like flame length, soot free length fraction (SFLF), radiant fraction, gas temperature and emission levels are performed in coaxial LPG jet diffusion flame to provide an insight into the effect of lip thickness and inert addition on flame characteristics. The present measurements reveal that the visible flame length is found to be reduced with increase in lip thickness. In contrast, the flame length for all lip thicknesses gets enhanced by inert addition to the fuel stream, which can be attributed to the reduced diffusivity of the mixture gases. The SFLF is seen to be enhanced with N 2 addition due to decrease in the fuel concentration and flame temperature. However, the SFLF is reduced with increasing lip thickness of the bluff-body which is caused due to the reduction in induction period of soot formation. For all lip thickness, the NO x emission level is observed to be attenuated with inert (N 2 ) addition, which can be ascribed to reduction in residence time of gas mixture. Besides this, NO x emission level is enhanced with increasing lip thickness. The emission index of CO is found to be increased with nitrogen addition; in contrast, the EICO 2 is seen to be reduced, for all lip thickness. This can be attributed to attrition in residence time with inert addition that tends to inhibit the conversion of CO to CO 2 . Moreover, EICO is also observed to be attenuated while EICO 2 gets enhanced, with increasing lip thickness which might be due to the increased residence time of mixture gases

Propagating nonpremixed edge-flames in a counterflow, annular slot burner under DC electric fields

KAUST Repository

Tran, Vu Manh; Cha, Min

2016-01-01

to be negligible after correcting the flame displacement speed with respect to the unburned flow velocity ahead of the flame edge. This indicates that the displacement speed of an edge-flame strongly depends on ionic wind and that an electric field has little

Propagation of intense laser radiation through a diffusion flame of burning oil

Energy Technology Data Exchange (ETDEWEB)

Gvozdev, S V; Glova, A F; Dubrovskii, V Yu; Durmanov, S T; Krasyukov, A G; Lysikov, A Yu; Smirnov, G V; Pleshkov, V M [State Research Center of Russian Federation ' Troitsk Institute for Innovation and Fusion Research' , Troitsk, Moscow Region (Russian Federation)

2015-06-30

We report the results of measuring the absorption coefficient of radiation from a cw ytterbium fibre single-mode laser with the power up to 1.5 kW by a diffusion flame of oil, burning in the atmosphere air at normal pressure on a free surface. For the constant length (30 mm) and width (30 mm) of the flame and the distance 10 mm between the laser beam axis and the oil surface the dependence of the absorption coefficient, averaged over the flame length, on the mean radiation intensity (varied from 4.5 × 10{sup 3} to 1.2 × 10{sup 6} W cm{sup -2}) entering the flame is obtained. The qualitative explanation of nonmonotonic behaviour of the absorption coefficient versus the intensity is presented. (laser applications and other topics in quantum electronics)

Influence of thermal radiation on soot production in Laminar axisymmetric diffusion flames

International Nuclear Information System (INIS)

Demarco, R.; Nmira, F.; Consalvi, J.L.

2013-01-01

The aim of this paper is to study the effect of radiative heat transfer on soot production in laminar axisymmetric diffusion flames. Twenty-four C 1 –C 3 hydrocarbon–air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames at both normal gravity (1 g) and microgravity (0 g), and covering a wide range of conditions affecting radiative heat transfer, were simulated. The numerical model is based on the Steady Laminar Flamelet (SLF) model, a semi-empirical two-equation acetylene/benzene based soot model and the Statistical Narrow Band Correlated K (SNBCK) model coupled to the Finite Volume Method (FVM) to compute thermal radiation. Predictions relative to velocity, temperature, soot volume fraction and radiative losses are on the whole in good agreement with the available experimental data. Model results show that, for all the flames considered, thermal radiation is a crucial process with a view to providing accurate predictions for temperatures and soot concentrations. It becomes increasingly significant from IDFs to NDFs and its influence is much greater as gravity is reduced. The radiative contribution of gas prevails in the weakly-sooting IDFs and in the methane and ethane NDFs, whereas soot radiation dominates in the other flames. However, both contributions are significant in all cases, with the exception of the 1 g IDFs investigated where soot radiation can be ignored. The optically-thin approximation (OTA) was also tested and found to be applicable as long as the optical thickness, based on flame radius and Planck mean absorption coefficient, is less than 0.05. The OTA is reasonable for the IDFs and for most of the 1 g NDFs, but it fails to predict the radiative heat transfer for the 0 g NDFs. The accuracy of radiative-property models was then assessed in the latter cases. Simulations show that the gray approximation can be applied to soot but not to combustion gases. Both the non-gray and gray soot versions of the Full Spectrum Correlated

Absorption and scattering of laser radiation by the diffusion flame of aviation kerosene

Energy Technology Data Exchange (ETDEWEB)

Gvozdev, S V; Glova, A F; Dubrovskii, V Yu; Durmanov, S T; Krasyukov, A G; Lysikov, A Yu; Smirnov, G V; Solomakhin, V B

2012-04-30

The absorption coefficient of the radiation of a repetitively pulsed Nd : YAG laser with an average output power up to 6 W and of a cw ytterbium optical fibre laser with an output power up to 3 kW was measured in the diffusion flame of aviation kerosene burning on a free surface in the atmospheric air. The absorption coefficient as a function of flame length, radiation power, and radiation intensity, which was varied in the {approx}10{sup 3} - 5 Multiplication-Sign 10{sup 4} W cm{sup -2} range, was obtained for two distances (1 and 2 cm) between the laser beam axis and the surface. The coefficient of radiation absorption by kerosene flame was compared with that in ethanol and kerosene - ethanol mixture flames. The radiation power scattered by a small segment of the kerosene flame irradiated by Nd : YAG laser radiation was measured as a function of longitudinal and azimuthal coordinates. An estimate was made of the total scattered radiation power.

Absorption and scattering of laser radiation by the diffusion flame of aviation kerosene

International Nuclear Information System (INIS)

Gvozdev, S V; Glova, A F; Dubrovskii, V Yu; Durmanov, S T; Krasyukov, A G; Lysikov, A Yu; Smirnov, G V; Solomakhin, V B

2012-01-01

The absorption coefficient of the radiation of a repetitively pulsed Nd : YAG laser with an average output power up to 6 W and of a cw ytterbium optical fibre laser with an output power up to 3 kW was measured in the diffusion flame of aviation kerosene burning on a free surface in the atmospheric air. The absorption coefficient as a function of flame length, radiation power, and radiation intensity, which was varied in the ∼10 3 - 5×10 4 W cm -2 range, was obtained for two distances (1 and 2 cm) between the laser beam axis and the surface. The coefficient of radiation absorption by kerosene flame was compared with that in ethanol and kerosene - ethanol mixture flames. The radiation power scattered by a small segment of the kerosene flame irradiated by Nd : YAG laser radiation was measured as a function of longitudinal and azimuthal coordinates. An estimate was made of the total scattered radiation power.

Absorption and scattering of laser radiation by the diffusion flame of aviation kerosene

Science.gov (United States)

Gvozdev, S. V.; Glova, A. F.; Dubrovskii, V. Yu; Durmanov, S. T.; Krasyukov, A. G.; Lysikov, A. Yu; Smirnov, G. V.; Solomakhin, V. B.

2012-04-01

The absorption coefficient of the radiation of a repetitively pulsed Nd : YAG laser with an average output power up to 6 W and of a cw ytterbium optical fibre laser with an output power up to 3 kW was measured in the diffusion flame of aviation kerosene burning on a free surface in the atmospheric air. The absorption coefficient as a function of flame length, radiation power, and radiation intensity, which was varied in the ~103 — 5×104 W cm-2 range, was obtained for two distances (1 and 2 cm) between the laser beam axis and the surface. The coefficient of radiation absorption by kerosene flame was compared with that in ethanol and kerosene — ethanol mixture flames. The radiation power scattered by a small segment of the kerosene flame irradiated by Nd : YAG laser radiation was measured as a function of longitudinal and azimuthal coordinates. An estimate was made of the total scattered radiation power.

Laminar oxy-fuel diffusion flame supported by an oxygen-permeable-ion-transport membrane

KAUST Repository

Hong, Jongsup

2013-03-01

A numerical model with detailed gas-phase chemistry and transport was used to predict homogeneous fuel conversion processes and to capture the important features (e.g., the location, temperature, thickness and structure of a flame) of laminar oxy-fuel diffusion flames stabilized on the sweep side of an oxygen permeable ion transport membrane (ITM). We assume that the membrane surface is not catalytic to hydrocarbon or syngas oxidation. It has been demonstrated that an ITM can be used for hydrocarbon conversion with enhanced reaction selectivity such as oxy-fuel combustion for carbon capture technologies and syngas production. Within an ITM unit, the oxidizer flow rate, i.e., the oxygen permeation flux, is not a pre-determined quantity, since it depends on the oxygen partial pressures on the feed and sweep sides and the membrane temperature. Instead, it is influenced by the oxidation reactions that are also dependent on the oxygen permeation rate, the initial conditions of the sweep gas, i.e., the fuel concentration, flow rate and temperature, and the diluent. In oxy-fuel combustion applications, the sweep side is fuel-diluted with CO2, and the entire unit is preheated to achieve a high oxygen permeation flux. This study focuses on the flame structure under these conditions and specifically on the chemical effect of CO2 dilution. Results show that, when the fuel diluent is CO2, a diffusion flame with a lower temperature and a larger thickness is established in the vicinity of the membrane, in comparison with the case in which N2 is used as a diluent. Enhanced OH-driven reactions and suppressed H radical chemistry result in the formation of products with larger CO and H2O and smaller H2 concentrations. Moreover, radical concentrations are reduced due to the high CO2 fraction in the sweep gas. CO2 dilution reduces CH3 formation and slows down the formation of soot precursors, C2H2 and C2H4. The flame location impacts the species diffusion and heat transfer from the

Instability and electrical response of small laminar coflow diffusion flames under AC electric fields: Toroidal vortex formation and oscillating and spinning flames

KAUST Repository

Xiong, Yuan; Chung, Suk-Ho; Cha, Min

2016-01-01

Dynamical and electrical responses of a small coflow diffusion flame were investigated by applying a high-voltage alternating current (AC), to a fuel jet nozzle. High-speed imaging and electrical diagnostics were adopted to capture flame dynamics and electrical signals, such as voltage (V ), frequency (f ) and current (I ). In the V -f domain of 0-5kV and 0-5kHz, AC-driven instabilities, resulting in various flame modes such as an oscillation, pinch-off and spinning of flames were identified. Characteristic frequency of each mode was determined and a visualization of near-nozzle flow structures suggested a close causality of initial counter-rotating vortices (inner and outer toroidal vortices - ITV and OTV), to the other observed flame. An axisymmetric ITV shedding was identified within oscillating and pinch-off modes, while asymmetric ITV shedding was identified with the spinning mode. Integrated electric power over several AC periods correlated well with variation in the flame surface area for these instabilities, demonstrating that measured electric power is a potential indicator of combustion instabilities in electric-field-assisted combustion.

Instability and electrical response of small laminar coflow diffusion flames under AC electric fields: Toroidal vortex formation and oscillating and spinning flames

KAUST Repository

Xiong, Yuan

2016-06-24

Dynamical and electrical responses of a small coflow diffusion flame were investigated by applying a high-voltage alternating current (AC), to a fuel jet nozzle. High-speed imaging and electrical diagnostics were adopted to capture flame dynamics and electrical signals, such as voltage (V ), frequency (f ) and current (I ). In the V -f domain of 0-5kV and 0-5kHz, AC-driven instabilities, resulting in various flame modes such as an oscillation, pinch-off and spinning of flames were identified. Characteristic frequency of each mode was determined and a visualization of near-nozzle flow structures suggested a close causality of initial counter-rotating vortices (inner and outer toroidal vortices - ITV and OTV), to the other observed flame. An axisymmetric ITV shedding was identified within oscillating and pinch-off modes, while asymmetric ITV shedding was identified with the spinning mode. Integrated electric power over several AC periods correlated well with variation in the flame surface area for these instabilities, demonstrating that measured electric power is a potential indicator of combustion instabilities in electric-field-assisted combustion.

Theory of the propagation dynamics of spiral edges of diffusion flames in von Karman swirling flows

Energy Technology Data Exchange (ETDEWEB)

Urzay, Javier; Williams, Forman A. [Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, CA 92093-0411 (United States); Nayagam, Vedha [National Center for Space Exploration Research, NASA Glenn Research Center, Cleveland, OH 44135 (United States)

2011-02-15

This analysis addresses the propagation of spiral edge flames found in von Karman swirling flows induced in rotating porous-disk burners. In this configuration, a porous disk is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame adjacent to the disk. Among other flame patterns experimentally found, a stable, rotating spiral flame is observed for sufficiently large rotation velocities and small fuel flow rates as a result of partial extinction of the underlying diffusion flame. The tip of the spiral can undergo a steady rotation for sufficiently large rotational velocities or small fuel flow rates, whereas a meandering tip in an epicycloidal trajectory is observed for smaller rotational velocities and larger fuel flow rates. A formulation of this problem is presented in the equidiffusional and thermodiffusive limits within the framework of one-step chemistry with large activation energies. Edge-flame propagation regimes are obtained by scaling analyses of the conservation equations and exemplified by numerical simulations of straight two-dimensional edge flames near a cold porous wall, for which lateral heat losses to the disk and large strains induce extinction of the trailing diffusion flame but are relatively unimportant in the front region, consistent with the existence of the cooling tail found in the experiments. The propagation dynamics of a steadily rotating spiral edge is studied in the large-core limit, for which the characteristic Markstein length is much smaller than the distance from the center at which the spiral tip is anchored. An asymptotic description of the edge tangential structure is obtained, spiral edge shapes are calculated, and an expression is found that relates the spiral rotational velocity to the rest of the parameters. A quasiestatic stability analysis of the edge shows that the edge curvature at extinction in the tip

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.

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

An assessment of radiation modeling strategies in simulations of laminar to transitional, oxy-methane, diffusion flames

International Nuclear Information System (INIS)

Abdul-Sater, Hassan; Krishnamoorthy, Gautham

2013-01-01

Twenty four, laboratory scale, laminar to transitional, diffusion oxy-methane flames were simulated employing different radiation modeling options and their predictions compared against experimental measurements of: temperature, flame length and radiant fraction. The models employed were: gray and non-gray formulations of a recently proposed weighted-sum-of-gray gas model, non-adiabatic extension of the equilibrium based mixture fraction model and investigations into the effects of: the thermal boundary conditions, soot and turbulence radiation interactions (TRI). Predictions of gas, wall temperatures and flame lengths were in good agreement with experimental measurements. Flame lengths determined through the axial profiles of OH confirmed with the experimental trends by increasing with increase in fuel-inlet Reynolds numbers and decreasing with the increase in O 2 composition in oxidizer. The temperature and flame length predictions were not sensitive to the radiative property model employed. There were significant variations between the gray and non-gray model radiant fraction predictions with the variations in general increasing with decrease in Reynolds numbers possibly attributed to shorter flames and steeper temperature gradients. The inclusion of soot model and TRI model did not affect our predictions as a result of low soot volume fractions and the radiation emission enhancement to the temperature fluctuations being localized to the flame sheet. -- Highlights: • Twenty four, lab scale, laminar to transitional, diffusion, oxy-methane flames were simulated. • Equilibrium model adequately predicted the temperature and flame lengths. • The experimental trends in radiant fractions were replicated. • Gray and non-gray model differences in radiant fractions were amplified at low Re. • Inclusion of soot and TRI models did not affect our predictions

Characterization of bluff-body stabilized LPG jet diffusion flame with N{sub 2} dilution

Energy Technology Data Exchange (ETDEWEB)

Kumar, P.; Mishra, D.P. [Combustion Lab, Department of Aerospace Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh 208 016 (India)

2008-10-15

Measurements of several parameters like flame length, soot free length fraction (SFLF), radiant fraction, gas temperature and emission levels are performed in coaxial LPG jet diffusion flame to provide an insight into the effect of lip thickness and inert addition on flame characteristics. The present measurements reveal that the visible flame length is found to be reduced with increase in lip thickness. In contrast, the flame length for all lip thicknesses gets enhanced by inert addition to the fuel stream, which can be attributed to the reduced diffusivity of the mixture gases. The SFLF is seen to be enhanced with N{sub 2} addition due to decrease in the fuel concentration and flame temperature. However, the SFLF is reduced with increasing lip thickness of the bluff-body which is caused due to the reduction in induction period of soot formation. For all lip thickness, the NO{sub x} emission level is observed to be attenuated with inert (N{sub 2}) addition, which can be ascribed to reduction in residence time of gas mixture. Besides this, NO{sub x} emission level is enhanced with increasing lip thickness. The emission index of CO is found to be increased with nitrogen addition; in contrast, the EICO{sub 2} is seen to be reduced, for all lip thickness. This can be attributed to attrition in residence time with inert addition that tends to inhibit the conversion of CO to CO{sub 2}. Moreover, EICO is also observed to be attenuated while EICO{sub 2} gets enhanced, with increasing lip thickness which might be due to the increased residence time of mixture gases. (author)

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

POLYCYCLIC AROMATIC HYDROCARBON FORMATION IN OPPOSED FLOW DIFFUSION FLAMES OF ETHANE. (R825412)

Science.gov (United States)

AbstractThe effect of fuel-side carbon density on the levels of polycyclic aromatic hydrocarbon (PAH) formation in atmospheric pressure, opposed flow, ethane diffusion flames has been studied using heated micro-probe sampling and gas chromatography/mass spectrometry (...

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)

Influence of thermal radiation on soot production in Laminar axisymmetric diffusion flames

Science.gov (United States)

Demarco, R.; Nmira, F.; Consalvi, J. L.

2013-05-01

The aim of this paper is to study the effect of radiative heat transfer on soot production in laminar axisymmetric diffusion flames. Twenty-four C1-C3 hydrocarbon-air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames at both normal gravity (1 g) and microgravity (0 g), and covering a wide range of conditions affecting radiative heat transfer, were simulated. The numerical model is based on the Steady Laminar Flamelet (SLF) model, a semi-empirical two-equation acetylene/benzene based soot model and the Statistical Narrow Band Correlated K (SNBCK) model coupled to the Finite Volume Method (FVM) to compute thermal radiation. Predictions relative to velocity, temperature, soot volume fraction and radiative losses are on the whole in good agreement with the available experimental data. Model results show that, for all the flames considered, thermal radiation is a crucial process with a view to providing accurate predictions for temperatures and soot concentrations. It becomes increasingly significant from IDFs to NDFs and its influence is much greater as gravity is reduced. The radiative contribution of gas prevails in the weakly-sooting IDFs and in the methane and ethane NDFs, whereas soot radiation dominates in the other flames. However, both contributions are significant in all cases, with the exception of the 1 g IDFs investigated where soot radiation can be ignored. The optically-thin approximation (OTA) was also tested and found to be applicable as long as the optical thickness, based on flame radius and Planck mean absorption coefficient, is less than 0.05. The OTA is reasonable for the IDFs and for most of the 1 g NDFs, but it fails to predict the radiative heat transfer for the 0 g NDFs. The accuracy of radiative-property models was then assessed in the latter cases. Simulations show that the gray approximation can be applied to soot but not to combustion gases. Both the non-gray and gray soot versions of the Full Spectrum Correlated k (FSCK

Bidirectional ionic wind in nonpremixed counterflow flames with DC electric fields

KAUST Repository

Park, Daegeun; Chung, Suk-Ho; Cha, Min

2016-01-01

Under an electric field, ions in the reaction zone of a flame generate a bulk flow motion called ionic wind. Because the majority of ions are positive, ionic wind is commonly considered to be unidirectional toward the cathode. A more thorough

Liftoff and blowoff of a diffusion flame between parallel streams of fuel and air

Energy Technology Data Exchange (ETDEWEB)

Fernandez-Tarrazo, Eduardo [I.N.T.A. Area de Propulsion-Edificio R02, Ctra. Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid (Spain); Vera, Marcos [Area de Mecanica de Fluidos, Universidad Carlos III de Madrid, 28911 Leganes (Spain); Linan, Amable [Departamento de Motopropulsion y Termofluidodinamica, Universidad Politecnica de Madrid, Pza. Cardenal Cisneros 3, 28040 Madrid (Spain)

2006-01-01

A numerical analysis is presented to describe the liftoff and blowoff of a diffusion flame in the mixing layer between two parallel streams of fuel (mainly methane diluted with nitrogen) and air emerging from porous walls. The analysis, which takes into account the effects of thermal expansion, assumes a one-step overall Arrhenius reaction, where the activation energy E is allowed to vary to reproduce the variations of the planar flame propagation velocity with the equivalence ratio. First, we describe the steady flame-front structure when stabilized close to the porous wall (attached flame regime). Then, we analyze the case where the flame front is located far away from the porous wall, at a distance x{sub f}' such that, upstream of the flame front, the mixing layer has a self-similar structure (lifted flame regime). For steady lifted flames, the results, given here in the case when the fuel and air streams are injected with the same velocity, relate U{sub f}'/S{sub L}, the front velocity (relative to the upstream flow) measured with the planar stoichiometric flame velocity, with the Damkohler number D{sub m}=({delta}{sub m}/{delta}{sub L}){sup 2}, based on the thickness, {delta}{sub m}, of the nonreacting mixing layer at the flame-front position and the laminar flame thickness, {delta}{sub L}. For large values of D{sub m}, the results, presented here for a wide range of dilutions of the fuel stream, provide values of the front propagation velocity that are in good agreement with previous experimental results, yielding well-defined conditions for blowoff. The calculated flame-front velocity can also be used to describe the transient flame-front dynamics after ignition by an external energy source.

Pollutant emission and noise radiation from open and impinging inverse diffusion flames

International Nuclear Information System (INIS)

Choy, Y.S.; Zhen, H.S.; Leung, C.W.; Li, H.B.

2012-01-01

Highlights: ► The effect of burner geometry (d air and S) on inverse diffusion flames is studied. ► With fixed air/fuel supplies, a smaller d air curtails NO x emission but augments noise radiation. ► With fixed air/fuel supplies, a larger S reduces NO x emission but increases noise radiation. ► Both NO x emission and noise radiation are maximum under stoichiometric combustion. ► Impinging flames are nosier than corresponding open flames due to the mirror effect of the plate. -- Abstract: This paper reports an experimental investigation of the pollutant emission and noise radiation characteristics of both open and impinging inverse diffusion flames (IDFs), produced by five burners of different air port diameter (d air = 5, 6 and 6.84 mm) and air-to-fuel spacing (S = 8, 11.5 and 15 mm). The effects of d air , S, overall equivalence ratio φ and nozzle-to-plate spacing H on the pollutant emissions of CO and NO x and the noise radiation are examined. The results show that at fixed air flow rate, a smaller d air curtails NO x emission but augments noise radiation, indicative of a role played by turbulence, which tends to decrease pollutant emission and increase noise radiation. A larger S reduces NO x emission but increases noise radiation, indicating that different flame zones may be responsible for pollutant emission and noise radiation. When the IDF is under stoichiometric φ = 1.6, both the NO x emission and noise radiation are highest, as a result of maximum heat release rate. A comparison of EINO x for the open and impinging IDFs shows that the impinging IDFs emit more NO x probably due to the absence of NO reburning. The impinging IDFs have higher noise radiation than the corresponding open IDFs. A higher level of noise radiation from the impinging IDFs is observed as the target plate is brought closer to the burner.

A comparative experimental and computational study of methanol, ethanol, and n-butanol flames

Energy Technology Data Exchange (ETDEWEB)

Veloo, Peter S.; Wang, Yang L.; Egolfopoulos, Fokion N. [Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453 (United States); Westbrook, Charles K. [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States)

2010-10-15

Laminar flame speeds and extinction strain rates of premixed methanol, ethanol, and n-butanol flames were determined experimentally in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Additional measurements were conducted also to determine the laminar flame speeds of their n-alkane/air counterparts, namely methane, ethane, and n-butane in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. For both propagation and extinction experiments the flow velocities were determined using the digital particle image velocimetry method. Laminar flame speeds were derived through a non-linear extrapolation approach based on direct numerical simulations of the experiments. Two recently developed detailed kinetics models of n-butanol oxidation were used to simulate the experiments. The experimental results revealed that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane/air counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. The laminar flame speeds of methanol/air flames are considerably higher compared to both ethanol/air and n-butanol/air flames under fuel-rich conditions. Numerical simulations of n-butanol/air freely propagating flames, revealed discrepancies between the two kinetic models regarding the consumption pathways of n-butanol and its intermediates. (author)

Stability and Behaviors of Methane/Propane and Hydrogen Micro Flames

Science.gov (United States)

Yoshimoto, Takamitsu; Kinoshita, Koichiro; Kitamura, Hideki; Tanigawa, Ryoichi

The flame stability limits essentially define the fundamental operation of the combustion system. Recently the micro diffusion flame has been remarked. The critical conditions of the flame stability limit are highly dependent on nozzle diameter, species of fuel and so on. The micro diffusion flame of Methane/Propane and Hydrogen is formed by using the micro-scale nozzle of which inner diameter is less than 1mm. The configurations and behaviors of the flame are observed directly and visualized by the high speed video camera The criteria of stability limits are proposed for the micro diffusion flame. The objectives of the present study are to get further understanding of lifting/blow-off for the micro diffusion flame. The results obtained are as follows. (1) The behaviors of the flames are classified into some regions for each diffusion flame. (2) The micro diffusion flame of Methane/Propane cannot be sustained, when the nozzle diameter is less than 0.14 mm. (3) The diffusion flame cannot be sustained below the critical fuel flow rate. (4) The minimum flow which is formed does not depends on the average jet velocity, but on the fuel flow rate. (5) the micro flame is laminar. The flame length is decided by fuel flow rate.

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

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.

Tabulated Combustion Model Development For Non-Premixed Flames

Science.gov (United States)

Kundu, Prithwish

D diffusion flame solver. The proposed model did not use progress variables like the traditional chemistry tabulation methods. The resulting model demonstrated an order of magnitude computational speed up over the RIF model. The results were validated across a wide range of operating conditions for diesel injections and the results were in close agreement to those of the experimental data. History of scalar dissipation rates plays a very important role in non premixed flames. However, tabulated methods have not been able to incorporate this physics in their models. A comparative approach is developed that can quantify these effects and find correlations with flow variables. A new model is proposed to include these effects in tabulated combustion models. The model is initially validated for 1D counterflow diffusion flame problems at engine conditions. The model is further implemented and validated in a 3D RANS code across a range of operating conditions for spray flames.

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.

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...

Effects of N2 gas on preheated laminar LPG jet diffusion flame

International Nuclear Information System (INIS)

Mishra, D.P.; Kumar, P.

2010-01-01

This paper presents an experimental investigation of the inert gas effect on flame length, NO x and soot free length fraction (SFLF) in a laminar LPG diffusion flame. Besides this, flame radiant fraction and temperature are also measured to explain observed NO x emission and SFLF. The inert is added to both air and fuel stream at each base line condition by maintaining a constant mass flow rate in each stream. Results indicate that inert addition leads to a significant enhancement in flame length for air-diluted stream than fuel-diluted stream. However, the flame length is observed to reduce with increasing reactant temperature. It is also observed that the SFLF increases with addition of N 2 for fuel-diluted stream. In contrast, SFLF remains almost constant when N 2 is added to air stream. The decrease in fuel concentration and gas temperature caused by inert addition leads to reduction in soot volume fraction and hence enhances SFLF. Interestingly, the SFLF reduces with increasing reactant temperature, due to reduction in induction period of soot formation caused by enhanced flame temperature. Besides this, the reduction in NO x emission level with inert addition is also observed. For all the three cases, the air dilution proved to be much efficient in reducing NO x emission level as compared to fuel dilution. This can be attributed to the differences in reduced gas temperature and residence time between air and fuel-diluted streams. On the contrary, NO x emission level enhances significantly with increasing reactant temperature as a result of increase in thermal NO x through Zeldovich mechanism.

Experimental investigation of laminar LPG-H{sub 2} jet diffusion flame with preheated reactants

Energy Technology Data Exchange (ETDEWEB)

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

2008-10-15

This paper presents an experimental investigation of the effect of H{sub 2} addition on flame length, soot free length fraction (SFLF), flame radiant fraction, gas temperature and emission level in LPG-H{sub 2} composite fuel jet diffusion flame for two preheated cases namely, (i) preheated air and (ii) preheated air and fuel. Results show that the H{sub 2} addition leads to a reduction in flame length which may be caused due to an increased gas temperature. Besides this, the flame length is also observed to be reduced with increasing reactants temperature. The soot free length fraction (SFLF) increases as H{sub 2} is added to fuel stream. This might have been caused by decrease in the C/H ratio in the flame and is favorable to attenuate PAH formation rate. Interestingly, the SFLF is observed to be reduced with increasing reactants temperature that may be due to reduction in induction period of soot formation caused by enhanced flame temperature. Moreover, the decreased radiant heat fraction with hydrogen addition is pertinent with the reduction in soot concentration level. The reduction in NOx emission level with H{sub 2} addition to the fuel stream is also observed. On the contrary, NOx emission level is found to be enhanced significantly with reactant temperature that can be attributed to the increase in thermal NOx through Zeldovich mechanism. 31 refs., 4 figs., 2 tabs.

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

Investigations of Sooting Laminar Coflow Diffusion Flames at Elevated Pressures

KAUST Repository

Steinmetz, Scott A.

2016-12-01

Soot is a common byproduct of hydrocarbon based combustion systems. It poses a risk to human and environmental health, and can negatively or positively affect combustor performance. As a result, there is significant interest in understanding soot formation in order to better control it. More recently, the need to study soot formation in engine relevant conditions has become apparent. One engine relevant parameter that has had little focus is the ambient pressure. This body of work focuses on the formation of soot in elevated pressure environments, and a number of investigations are carried out with this purpose. Laminar coflow diffusion flames are used as steady, simple soot producers. First, a commonly studied flame configuration is further characterized. Coflow flames are frequently used for fundamental flame studies, particularly at elevated pressures. However, they are more susceptible to buoyancy induced instabilities at elevated pressures. The velocity of the coflow is known to have an effect on flame stability and soot formation, though these have not been characterized at elevated pressures. A series of flames are investigated covering a range of flowrates, pressures, and nozzle diameters. The stability limits of coflow flames in this range is investigated. Additionally, an alternative strategy for scaling these flames to elevated pressures is proposed. Finally, the effect of coflow rate on soot formation is evaluated. Identification of fundamental flames for coordinated research can facilitate our understanding of soot formation. The next study of this work focuses on adding soot concentration and particle size information to an existing fundamental flame dataset for the purpose of numerical model validation. Soot volume fraction and average particle diameters are successfully measured in nitrogen-diluted ethylene-air laminar coflow flames at pressures of 4, 8, 12, and 16 atm. An increase in particle size with pressure is found up to 12 atm, where particle

Effects of diluents on soot surface temperature and volume fraction in diluted ethylene diffusion flames at pressure

KAUST Repository

Kailasanathan, Ranjith Kumar Abhinavam

2014-05-20

Soot surface temperature and volume fraction are measured in ethylene/air coflowing laminar diffusion flames at high pressures, diluted with one of four diluents (argon, helium, nitrogen, and carbon dioxide) using a two-color technique. Both temperature and soot measurements presented are line-of-sight averages. The results aid in understanding the kinetic and thermodynamic behavior of the soot formation and oxidation chemistry with changes in diluents, ultimately leading to possible methods of reducing soot emission from practical combustion hardware. The diluted fuel and coflow exit velocities (top-hat profiles) were matched at all pressures to minimize shear effects. In addition to the velocity-matched flow rates, the mass fluxes were held constant for all pressures. Addition of a diluent has a pronounced effect on both the soot surface temperature and volume fraction, with the helium diluted flame yielding the maximum and carbon dioxide diluted flame yielding minimum soot surface temperature and volume fraction. At low pressures, peak soot volume fraction exists at the tip of the flame, and with an increase in pressure, the location shifts lower to the wings of the flame. Due to the very high diffusivity of helium, significantly higher temperature and volume fraction are measured and explained. Carbon dioxide has the most dramatic soot suppression effect. By comparing the soot yield with previously measured soot precursor concentrations in the same flame, it is clear that the lower soot yield is a result of enhanced oxidation rates rather than a reduction in precursor formation. Copyright © 2014 Taylor & Francis Group, LLC.

Quantitative determination of flame color and its determining factor in hydrocarbon/air laminar diffusion flames; Soryu kakusan kaen ni okeru kaenshoku no teiryoka to sono kettei yoin

Energy Technology Data Exchange (ETDEWEB)

Tatsuta, S. [Asahikawa National College of Technology, Hokkaido (Japan); Fujita, O.; Ito, K. [Hokkaido University, Sapporo (Japan)

1998-08-25

The color of laminar diffusion flames burning propane, methane and ethylene was determined by chromaticity coordinates (x, y) defined by the CIE 1931 standard colorimetric system. The differences in flame color attributed to burning condition and fuel types were examined with a colorimeter. Spectroscopic measurement and numerical analysis using a simplified radiation model were also carried out to discuss the determining factors of the flame color. The relation between x and y measured on the central axis of the flames was expressed in the experimental equations. The (x, y) in the luminous region plotted on a chromaticity diagram changed along Planckian locus with the burning conditions. The contribution of the thermal radiation of soot particles and the chemiluminescence to the flame color was successfully evaluated by introducing the concept of additive mixture of color stimuli. The (x, y) profiles from the numerical analysis agreed well with the experimental results. 17 refs., 14 figs., 1 tab.

Particle Sampling and Real Time Size Distribution Measurement in H2/O2/TEOS Diffusion Flame

International Nuclear Information System (INIS)

Ahn, K.H.; Jung, C.H.; Choi, M.; Lee, J.S.

2001-01-01

Growth characteristics of silica particles have been studied experimentally using in situ particle sampling technique from H 2 /O 2 /Tetraethylorthosilicate (TEOS) diffusion flame with carefully devised sampling probe. The particle morphology and the size comparisons are made between the particles sampled by the local thermophoretic method from the inside of the flame and by the electrostatic collector sampling method after the dilution sampling probe. The Transmission Electron Microscope (TEM) image processed data of these two sampling techniques are compared with Scanning Mobility Particle Sizer (SMPS) measurement. TEM image analysis of two sampling methods showed a good agreement with SMPS measurement. The effects of flame conditions and TEOS flow rates on silica particle size distributions are also investigated using the new particle dilution sampling probe. It is found that the particle size distribution characteristics and morphology are mostly governed by the coagulation process and sintering process in the flame. As the flame temperature increases, the effect of coalescence or sintering becomes an important particle growth mechanism which reduces the coagulation process. However, if the flame temperature is not high enough to sinter the aggregated particles then the coagulation process is a dominant particle growth mechanism. In a certain flame condition a secondary particle formation is observed which results in a bimodal particle size distribution

A reaction mechanism for gasoline surrogate fuels for large polycyclic aromatic hydrocarbons

KAUST Repository

Raj, Abhijeet

2012-02-01

This work aims to develop a reaction mechanism for gasoline surrogate fuels (n-heptane, iso-octane and toluene) with an emphasis on the formation of large polycyclic aromatic hydrocarbons (PAHs). Starting from an existing base mechanism for gasoline surrogate fuels with the largest chemical species being pyrene (C 16H 10), this new mechanism is generated by adding PAH sub-mechanisms to account for the formation and growth of PAHs up to coronene (C 24H 12). The density functional theory (DFT) and the transition state theory (TST) have been adopted to evaluate the rate constants for several PAH reactions. The mechanism is validated in the premixed laminar flames of n-heptane, iso-octane, benzene and ethylene. The characteristics of PAH formation in the counterflow diffusion flames of iso-octane/toluene and n-heptane/toluene mixtures have also been tested for both the soot formation and soot formation/oxidation flame conditions. The predictions of the concentrations of large PAHs in the premixed flames having available experimental data are significantly improved with the new mechanism as compared to the base mechanism. The major pathways for the formation of large PAHs are identified. The test of the counterflow diffusion flames successfully predicts the PAH behavior exhibiting a synergistic effect observed experimentally for the mixture fuels, irrespective of the type of flame (soot formation flame or soot formation/oxidation flame). The reactions that lead to this synergistic effect in PAH formation are identified through the rate-of-production analysis. © 2011 The Combustion Institute.

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

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

Effects of N{sub 2} gas on preheated laminar LPG jet diffusion flame

Energy Technology Data Exchange (ETDEWEB)

Mishra, D.P.; Kumar, P. [Department of Aerospace Engineering, Indian Institute of Technology, Kanpur 208 016 (India)

2010-11-15

This paper presents an experimental investigation of the inert gas effect on flame length, NO{sub x} and soot free length fraction (SFLF) in a laminar LPG diffusion flame. Besides this, flame radiant fraction and temperature are also measured to explain observed NO{sub x} emission and SFLF. The inert is added to both air and fuel stream at each base line condition by maintaining a constant mass flow rate in each stream. Results indicate that inert addition leads to a significant enhancement in flame length for air-diluted stream than fuel-diluted stream. However, the flame length is observed to reduce with increasing reactant temperature. It is also observed that the SFLF increases with addition of N{sub 2} for fuel-diluted stream. In contrast, SFLF remains almost constant when N{sub 2} is added to air stream. The decrease in fuel concentration and gas temperature caused by inert addition leads to reduction in soot volume fraction and hence enhances SFLF. Interestingly, the SFLF reduces with increasing reactant temperature, due to reduction in induction period of soot formation caused by enhanced flame temperature. Besides this, the reduction in NO{sub x} emission level with inert addition is also observed. For all the three cases, the air dilution proved to be much efficient in reducing NO{sub x} emission level as compared to fuel dilution. This can be attributed to the differences in reduced gas temperature and residence time between air and fuel-diluted streams. On the contrary, NO{sub x} emission level enhances significantly with increasing reactant temperature as a result of increase in thermal NO{sub x} through Zeldovich mechanism. (author)

Quantitative Measurements of CH* Concentration in Normal Gravity and Microgravity Coflow Laminar Diffusion Flames

Science.gov (United States)

Giassi, D.; Cao, S.; Stocker, D. P.; Takahashi, F.; Bennett, B. A.; Smooke, M. D.; Long, M. B.

2015-01-01

With the conclusion of the SLICE campaign aboard the ISS in 2012, a large amount of data was made available for the analysis of the effect of microgravity on laminar coflow diffusion flames. Previous work focused on the study of sooty flames in microgravity as well as the ability of numerical models to predict its formation in a simplified buoyancy-free environment. The current work shifts the investigation to soot-free flames, putting an emphasis on the chemiluminescence emission from electronically excited CH (CH*). This radical species is of significant interest in combustion studies: it has been shown that the CH* spatial distribution is indicative of the flame front position and, given the relatively simple diagnostic involved with its measurement, several works have been done trying to understand the ability of CH* chemiluminescence to predict the total and local flame heat release rate. In this work, a subset of the SLICE nitrogen-diluted methane flames has been considered, and the effect of fuel and coflow velocity on CH* concentration is discussed and compared with both normal gravity results and numerical simulations. Experimentally, the spectral characterization of the DSLR color camera used to acquire the flame images allowed the signal collected by the blue channel to be considered representative of the CH* emission centered around 431 nm. Due to the axisymmetric flame structure, an Abel deconvolution of the line-of-sight chemiluminescence was used to obtain the radial intensity profile and, thanks to an absolute light intensity calibration, a quantification of the CH* concentration was possible. Results show that, in microgravity, the maximum flame CH* concentration increases with the coflow velocity, but it is weakly dependent on the fuel velocity; normal gravity flames, if not lifted, tend to follow the same trend, albeit with different peak concentrations. Comparisons with numerical simulations display reasonably good agreement between measured and

Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels

Energy Technology Data Exchange (ETDEWEB)

Matti Maricq, M. [Research and Advanced Engineering, Ford Motor Company, Dearborn, MI (United States)

2011-01-15

Data are presented to compare soot formation in both surrogate and practical fatty acid methyl ester biodiesel and petroleum fuel diffusion flames. The approach here uses differential mobility analysis to follow the size distributions and electrical charge of soot particles as they evolve in the flame, and laser ablation particle mass spectrometry to elucidate their composition. Qualitatively, these soot properties exhibit a remarkably similar development along the flames. The size distributions begin as a single mode of precursor nanoparticles, evolve through a bimodal phase marking the onset of aggregate formation, and end in a self preserving mode of fractal-like particles. Both biodiesel and hydrocarbon fuels yield a common soot composition dominated by C{sub x}H{sub y}{sup +} ions, stabilomer PAHs, and fullerenes in the positive ion mass spectrum, and C{sub x}{sup -} and C{sub 2x}H{sup -} in the negative ion spectrum. These ion intensities initially grow with height in the diffusion flames, but then decline during later stages, consistent with soot carbonization. There are important quantitative differences between fuels. The surrogate biodiesel fuel methyl butanoate substantially reduces soot levels, but soot formation and evolution in this flame are delayed relative to both soy and petroleum fuels. In contrast, soots from soy and hexadecane flames exhibit nearly quantitative agreement in their size distribution and composition profiles with height, suggesting similar soot precursor chemistry. (author)

Calculation and analysis of the mobility and diffusion coefficient of thermal electrons in methane/air premixed flames

KAUST Repository

Bisetti, Fabrizio

2012-12-01

Simulations of ion and electron transport in flames routinely adopt plasma fluid models, which require transport coefficients to compute the mass flux of charged species. In this work, the mobility and diffusion coefficient of thermal electrons in atmospheric premixed methane/air flames are calculated and analyzed. The electron mobility is highest in the unburnt region, decreasing more than threefold across the flame due to mixture composition effects related to the presence of water vapor. Mobility is found to be largely independent of equivalence ratio and approximately equal to 0.4m 2V -1s -1 in the reaction zone and burnt region. The methodology and results presented enable accurate and computationally inexpensive calculations of transport properties of thermal electrons for use in numerical simulations of charged species transport in flames. © 2012 The Combustion Institute.

Flame synthesis of zinc oxide nanocrystals

Energy Technology Data Exchange (ETDEWEB)

Merchan-Merchan, Wilson, E-mail: wmerchan-merchan@ou.edu [School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019 (United States); Farahani, Moien Farmahini [School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019 (United States)

2013-02-01

Highlights: Black-Right-Pointing-Pointer We report a single-step flame method for the synthesis of Zn oxide nanocrystals. Black-Right-Pointing-Pointer Diverse flame positions lead to a variation of Zn oxide nanocrystal growth. Black-Right-Pointing-Pointer The synthesized crystals have polyhedral, pipet- and needle-like shape. Black-Right-Pointing-Pointer High length-to-diameter aspect-ratio crystals appear in a higher temperature flame. Black-Right-Pointing-Pointer The crystal growth mechanism corresponds to vapor-to-solid conversion. - Abstract: Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counter-flow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of {approx}99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 {mu}m. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of {approx}0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells

Experimental and detailed kinetic modeling study of PAH formation in laminar co-flow methane diffusion flames

DEFF Research Database (Denmark)

Cuoci, Alberto; Frassoldati, Alessio; Faravelli, Tiziano

2013-01-01

In the present paper, synchrotron VUV photoionization mass spectrometry is used to study the detailed chemistry of co-flow methane diffusion flames with different dilution ratios. The experimental results constitute a comprehensive characterization of species important for PAH and soot formation...

Aromatic ring formation in opposed-flow diffusive 1,3-butadiene flames

KAUST Repository

Moshammer, Kai

2016-10-17

This paper is concerned with the formation of one- and two-ring aromatic species in near atmospheric-pressure opposed-flow diffusion flames of 1,3-butadiene (1,3-CH). The chemical structures of two different 1,3-CH/Ar-O/Ar flames were explored using flame-sampling molecular-beam mass spectrometry with both electron and single-photon ionization. We provide mole fraction profiles of 47 components as function of distance from the fuel outlet and compare them to chemically detailed modeling results. To this end, the hierarchically developed model described by Seidel et al. [16] has been updated to accurately comprise the chemistry of 1,3-butadiene. Generally a very good agreement is observed between the experimental and modeling data, allowing for a meaningful reaction path analysis. With regard to the formation of aromatic species up to naphthalene, it was essential to improve the fulvene and the C chemistry description in the mechanism. In particular, benzene is found to be formed mainly via fulvene through the reactions of the CH isomers with CH The n-CH radical reacts with CH forming 1,3-pentadiene (CH), which is subsequently oxidized to form the naphthalene precursor cyclopentadienyl (CH). Oxidation of naphthalene is predicted to be a contributor to the formation of phenylacetylene (CH), indicating that consumption reactions can be of similar importance as molecular growth reactions.

Aromatic ring formation in opposed-flow diffusive 1,3-butadiene flames

KAUST Repository

Moshammer, Kai; Seidel, Lars; Wang, Yu; Selim, Hatem; Sarathy, Mani; Mauss, Fabian; Hansen, Nils

2016-01-01

This paper is concerned with the formation of one- and two-ring aromatic species in near atmospheric-pressure opposed-flow diffusion flames of 1,3-butadiene (1,3-CH). The chemical structures of two different 1,3-CH/Ar-O/Ar flames were explored using flame-sampling molecular-beam mass spectrometry with both electron and single-photon ionization. We provide mole fraction profiles of 47 components as function of distance from the fuel outlet and compare them to chemically detailed modeling results. To this end, the hierarchically developed model described by Seidel et al. [16] has been updated to accurately comprise the chemistry of 1,3-butadiene. Generally a very good agreement is observed between the experimental and modeling data, allowing for a meaningful reaction path analysis. With regard to the formation of aromatic species up to naphthalene, it was essential to improve the fulvene and the C chemistry description in the mechanism. In particular, benzene is found to be formed mainly via fulvene through the reactions of the CH isomers with CH The n-CH radical reacts with CH forming 1,3-pentadiene (CH), which is subsequently oxidized to form the naphthalene precursor cyclopentadienyl (CH). Oxidation of naphthalene is predicted to be a contributor to the formation of phenylacetylene (CH), indicating that consumption reactions can be of similar importance as molecular growth reactions.

Calculation and analysis of the mobility and diffusion coefficient of thermal electrons in methane/air premixed flames

KAUST Repository

Bisetti, Fabrizio; El Morsli, Mbark

2012-01-01

Simulations of ion and electron transport in flames routinely adopt plasma fluid models, which require transport coefficients to compute the mass flux of charged species. In this work, the mobility and diffusion coefficient of thermal electrons

Effects of diluents on soot surface temperature and volume fraction in diluted ethylene diffusion flames at pressure

KAUST Repository

Kailasanathan, Ranjith Kumar Abhinavam; Zhang, Ji; Fang, Tiegang; Roberts, William L.

2014-01-01

Soot surface temperature and volume fraction are measured in ethylene/air coflowing laminar diffusion flames at high pressures, diluted with one of four diluents (argon, helium, nitrogen, and carbon dioxide) using a two-color technique. Both

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

KAUST Repository

Belhi, Memdouh; Lee, Bok Jik; Bisetti, Fabrizio; Im, Hon G.

2017-01-01

and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current-voltage curve. The flame response to the electric field with positive and negative polarity exhibited

AC electric field induced vortex in laminar coflow diffusion flames

KAUST Repository

Xiong, Yuan

2014-09-22

Experiments were performed by applying sub-critical high-voltage alternating current (AC) to the nozzle of laminar propane coflow diffusion flames. Light scattering, laser-induced incandescence and laser-induced fluorescence techniques were used to identify the soot zone, and the structures of OH and polycyclic aromatic hydrocarbons (PAHs). Particle image velocimetry was adopted to quantify the velocity field. Under certain AC conditions of applied voltage and frequency, the distribution of PAHs and the flow field near the nozzle exit were drastically altered, leading to the formation of toroidal vortices. Increased residence time and heat recirculation inside the vortex resulted in appreciable formation of PAHs and soot near the nozzle exit. Decreased residence time along the jet axis through flow acceleration by the vortex led to a reduction in the soot volume fraction in the downstream sooting zone. Electromagnetic force generated by AC was proposed as a viable mechanism for the formation of the toroidal vortex. The onset conditions for the vortex formation supported the role of an electromagnetic force acting on charged particles in the flame zone. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

AC electric field induced vortex in laminar coflow diffusion flames

KAUST Repository

Xiong, Yuan; Cha, Min; Chung, Suk-Ho

2014-01-01

Experiments were performed by applying sub-critical high-voltage alternating current (AC) to the nozzle of laminar propane coflow diffusion flames. Light scattering, laser-induced incandescence and laser-induced fluorescence techniques were used to identify the soot zone, and the structures of OH and polycyclic aromatic hydrocarbons (PAHs). Particle image velocimetry was adopted to quantify the velocity field. Under certain AC conditions of applied voltage and frequency, the distribution of PAHs and the flow field near the nozzle exit were drastically altered, leading to the formation of toroidal vortices. Increased residence time and heat recirculation inside the vortex resulted in appreciable formation of PAHs and soot near the nozzle exit. Decreased residence time along the jet axis through flow acceleration by the vortex led to a reduction in the soot volume fraction in the downstream sooting zone. Electromagnetic force generated by AC was proposed as a viable mechanism for the formation of the toroidal vortex. The onset conditions for the vortex formation supported the role of an electromagnetic force acting on charged particles in the flame zone. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Quantitative characterization of steady and time-varying, sooting, laminar diffusion flames using optical techniques

Science.gov (United States)

Connelly, Blair C.

In order to reduce the emission of pollutants such as soot and NO x from combustion systems, a detailed understanding of pollutant formation is required. In addition to environmental concerns, this is important for a fundamental understanding of flame behavior as significant quantities of soot lower local flame temperatures, increase overall flame length and affect the formation of such temperature-dependent species as NOx. This problem is investigated by carrying out coupled computational and experimental studies of steady and time-varying sooting, coflow diffusion flames. Optical diagnostic techniques are a powerful tool for characterizing combustion systems, as they provide a noninvasive method of probing the environment. Laser diagnostic techniques have added advantages, as systems can be probed with high spectral, temporal and spatial resolution, and with species selectivity. Experimental soot volume fractions were determined by using two-dimensional laser-induced incandescence (LII), calibrated with an on-line extinction measurement, and soot pyrometry. Measurements of soot particle size distributions are made using time-resolved LII (TR-LII). Laser-induced fluorescence measurements are made of NO and formaldehyde. These experimental measurements, and others, are compared with computational results in an effort to understand and model soot formation and to examine the coupled relationship of soot and NO x formation.

Transition of carbon nanostructures in heptane diffusion flames

Energy Technology Data Exchange (ETDEWEB)

Hu, Wei-Chieh [National Cheng Kung University, Department of Mechanical Engineering (China); Hou, Shuhn-Shyurng [Kun Shan University, Department of Mechanical Engineering (China); Lin, Ta-Hui, E-mail: thlin@mail.ncku.edu.tw [National Cheng Kung University, Department of Mechanical Engineering (China)

2017-02-15

The flame synthesis has high potential in industrial production of carbon nanostructure (CNS). Unfortunately, the complexity of combustion chemistry leads to less controlling of synthesized products. In order to improve the understanding of the relation between flames and CNSs synthesized within, experiments were conducted through heptane flames in a stagnation-point liquid-pool system. The operating parameters for the synthesis include oxygen supply, sampling position, and sampling time. Two kinds of nanostructures were observed, carbon nanotube (CNT) and carbon nano-onion (CNO). CNTs were synthesized in a weaker flame near extinction. CNOs were synthesized in a more sooty flame. The average diameter of CNTs formed at oxygen concentration of 15% was in the range of 20–30 nm. For oxygen concentration of 17%, the average diameter of CNTs ranged from 24 to 27 nm, while that of CNOs was around 28 nm. For oxygen concentration of 19%, the average diameter of CNOs produced at the sampling position 0.5 mm below the flame front was about 57 nm, while the average diameters of CNOs formed at the sampling positions 1–2.5 mm below the flame front were in the range of 20–25 nm. A transition from CNT to CNO was observed by variation of sampling position in a flame. We found that the morphology of CNS is directly affected by the presence of soot layer due to the carbonaceous environment and the growth mechanisms of CNT and CNO. The sampling time can alter the yield of CNSs depending on the temperature of sampling position, but the morphology of products is not affected.

Effect of H2 addition on combustion characteristics of dimethyl ether jet diffusion flame

International Nuclear Information System (INIS)

Kang, Yinhu; Lu, Xiaofeng; Wang, Quanhai; Gan, Lu; Ji, Xuanyu; Wang, Hu; Guo, Qiang; Song, Decai; Ji, Pengyu

2015-01-01

Highlights: • DME- and H 2 -dominated combustion regimes were quantitatively characterized. • The flame structure changed significantly when H 2 addition was above 60 vol.%. • An empirical correlation for normalized flame entrainment rate was developed. • The optimal H 2 addition to DME was 60 vol.% in the practical engineering. - Abstract: In this paper, experiments and numerical calculations were conducted to investigate the effect of H 2 addition on dimethyl ether (DME) jet diffusion flame behaviors, in terms of thermal and chemical structures, reaction zone size, flame entrainment, and NOx and CO emission indices. A wide range of H 2 additions from pure DME to pure H 2 were involved herein, while maintaining the volumetric flow rate of fuel mixture constant. The results indicate that when H 2 mole fraction in the fuel mixture exceeded 60%, the blended fuel was converted to H 2 -dominated. Besides, the flames behaved rather distinctly at the DME- and H 2 -dominated regimes. With the increment in H 2 addition, flame temperature, H 2 , H, O, and OH concentrations increased gradually, but concentrations of the intermediate hydrocarbons (such as CO, CH 2 O, CH 2 , and CH 3 ) decreased on the contrary. Additionally, after the flame became H 2 -dominated, the species concentrations varied increasingly quickly with H 2 addition. The reaction zone length and width decreased nearly linearly with H 2 addition at the DME- and H 2 -dominated regimes. But the decreasing speed of reaction zone length became faster after the flame was converted to H 2 -dominated. At the DME-dominated regime, the dependence of flame entrainment coefficient (C e ) on H 2 addition was rather small. While at the H 2 -dominated regime, C e increased increasingly quickly with H 2 addition. Moreover, with the increment in H 2 addition, NOx emission index increased and CO emission index decreased gradually. In addition, at the DME-dominated regime, NOx emission index increased fairly slowly

Extinction of corrugated hydrogen/air flames

International Nuclear Information System (INIS)

Mizomoto, M.; Asaka, Y.; Ikai, S.; Law, C.K.

1982-01-01

Recent studies on flammability limits reveal the importance of flow nonuniformity, flame curvature, and molecular and thermal diffusivities in determining the extinguishability and the associated limits of premixed fuel/air flames. In particular, it is found that conditions which favor extinction of a lean flame may cause intensification of a rich flame. In the present study the authors have experimentally determined the extinction characteristics and limits of highly curved hydrogen/air flames as represented by the opening of bunsen flame tips. Results show that the tip opens at a constant fuel equivalence ratio of phi = 1.15, regardless of the velocity and uniformity of the upstream flow. This critical mixture concentration, while being rich, is still on the lean side of that corresponding to the maximum burning velocity (phi = 1.8), implying that for highly diffusive systems, the relevant reference concentration is that for maximum burning velocity instead of stoichiometry

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.

Investigation of noise radiation from a swirl stabilized diffusion flame with an array of microphones

International Nuclear Information System (INIS)

Singh, A.V.; Yu, M.; Gupta, A.K.; Bryden, K.M.

2013-01-01

Highlights: • Acoustic spectral characteristics independent of equivalence ratio and flow velocity. • Combustion noise dependent on global equivalence ratio and flow velocity. • Increased global equivalence ratio decreased the frequency of peak. • Decay and growth coefficients largely independent of different flow conditions. • Acoustic radiation coherent up to 1.5 kHz for spatially separated microphones. - Abstract: Next generation of combustors are expected to provide significant improvement on efficiency and reduced pollutants emission. In such combustors, the challenges of local flow, pressure, chemical composition and thermal signatures as well as their interactions will require detailed investigation for seeking optimum performance. Sensor networks with a large number of sensors will be employed in future smart combustors, which will allow one to obtain fast and comprehensive information on the various ongoing processes within the system. In this paper sensor networks with specific focus on an array of homogeneous microphones are used examine the spectral characteristics of combustion noise from a non-premixed combustor. A non-premixed double concentric swirl-flame burner was used. Noise spectra were determined experimentally for the non-premixed swirl flame at various fuel–air ratios using an array of homogeneous condenser microphones. Multiple microphones positioned at discrete locations around the turbulent diffusion flame, provided an understanding of the total sound power and their spectral characteristics. The growth and decay coefficients of total sound power were investigated at different test conditions. The signal coherence between different microphone pairs was also carried out to determine the acoustic behavior of a swirl stabilized turbulent diffusion flame. The localization of acoustic sources from the multiple microphones was examined using the noise spectra. The results revealed that integration of multiple sensors in combustors

PIV Measurements of He II Counterflow Around a Cylinder

International Nuclear Information System (INIS)

Fuzier, S.; Van Stiver, S. W.; Zhang, T.

2006-01-01

The induced flow field of counterflow He II across a circular cylinder has been quantitatively studied using the particle image velocimetry (PIV) technique. Two different size cylinders (6.35 mm and 2 mm in diameter) were used and placed in a 20 mm wide rectangular channel. In these experiments, large-scale eddy motion generated by the He II counterflow was observed both in front of and behind the cylinder, an effect which has no analogue in classical fluids

Application of a primitive variable Newton's method for the calculation of an axisymmetric laminar diffusion flame

International Nuclear Information System (INIS)

Xu, Yuenong; Smooke, M.D.

1993-01-01

In this paper we present a primitive variable Newton-based solution method with a block-line linear equation solver for the calculation of reacting flows. The present approach is compared with the stream function-vorticity Newton's method and the SIMPLER algorithm on the calculation of a system of fully elliptic equations governing an axisymmetric methane-air laminar diffusion flame. The chemical reaction is modeled by the flame sheet approximation. The numerical solution agrees well with experimental data in the major chemical species. The comparison of three sets of numerical results indicates that the stream function-vorticity solution using the approximate boundary conditions reported in the previous calculations predicts a longer flame length and a broader flame shape. With a new set of modified vorticity boundary conditions, we obtain agreement between the primitive variable and stream function-vorticity solutions. The primitive variable Newton's method converges much faster than the other two methods. Because of much less computer memory required for the block-line tridiagonal solver compared to a direct solver, the present approach makes it possible to calculate multidimensional flames with detailed reaction mechanisms. The SIMPLER algorithm shows a slow convergence rate compared to the other two methods in the present calculation

Polydisperse effects in jet spray flames

Science.gov (United States)

Weinberg, Noam; Greenberg, J. Barry

2018-01-01

A laminar jet polydisperse spray diffusion flame is analysed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, droplet size and initial spray polydispersity play a critical role in determining potential flame scenarios. Some qualitative agreement for lift-off height is found when comparing predictions of the theory and sparse independent experimental evidence from the literature.

Multiple-diffusion flame synthesis of pure anatase and carbon-coated titanium dioxide nanoparticles

KAUST Repository

Memon, Nasir

2013-09-01

A multi-element diffusion flame burner (MEDB) is useful in the study of flame synthesis of nanomaterials. Here, the growth of pure anatase and carbon-coated titanium dioxide (TiO2) using an MEDB is demonstrated. Hydrogen (H2), oxygen (O2), and argon (Ar) are utilized to establish the flame, whereas titanium tetraisopropoxide is used as the precursor for TiO2. The nanoparticles are characterized using high-resolution transmission electron microscopy, with elemental mapping (of C, O, and Ti), X-ray diffraction, Raman spectroscopy, and thermogravimetric analysis. The growth of pure anatase TiO2 nanoparticles occurs when Ar and H2 are used as the precursor carrier gas, while the growth of carbon-coated nanoparticles ensues when Ar and ethylene (C2H4) are used as the precursor carrier gas. A uniform coating of 3-5nm of carbon is observed around TiO2 particles. The growth of highly crystalline TiO2 nanoparticles is dependent on the gas flow rate of the precursor carrier and amorphous particles are observed at high flow rates. Carbon coating occurs only on crystalline nanoparticles, suggesting a possible growth mechanism of carbon-coated TiO2 nanoparticles. © 2013 The Combustion Institute.

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.

Quantitative Measurements of Electronically Excited CH Concentration in Normal Gravity and Microgravity Coflow Laminar Diffusion Flames

Science.gov (United States)

Giassi, D.; Cao, S.; Stocker, D. P.; Takahashi, F.; Bennett, B. A. V.; Smooke, M. D.; Long, M. B.

2015-01-01

With the conclusion of the SLICE campaign aboard the ISS in 2012, a large amount of data was made available for the analysis of the effect of microgravity on laminar coflow diffusion flames. Previous work focused on the study of sooty flames in microgravity as well as the ability of numerical models to predict its formation in a simplified buoyancy-free environment. The current work shifts the investigation to soot-free flames, putting an emphasis on the chemiluminescence emission from electronically excited CH (CH*). This radical species is of significant interest in combustion studies: it has been shown that the electronically excited CH spatial distribution is indicative of the flame front position and, given the relatively simple diagnostic involved with its measurement, several works have been done trying to understand the ability of electronically excited CH chemiluminescence to predict the total and local flame heat release rate. In this work, a subset of the SLICE nitrogen-diluted methane flames has been considered, and the effect of fuel and coflow velocity on electronically excited CH concentration is discussed and compared with both normal gravity results and numerical simulations. Experimentally, the spectral characterization of the DSLR color camera used to acquire the flame images allowed the signal collected by the blue channel to be considered representative of the electronically excited CH emission centered around 431 nm. Due to the axisymmetric flame structure, an Abel deconvolution of the line-of-sight chemiluminescence was used to obtain the radial intensity profile and, thanks to an absolute light intensity calibration, a quantification of the electronically excited CH concentration was possible. Results show that, in microgravity, the maximum flame electronically excited CH concentration increases with the coflow velocity, but it is weakly dependent on the fuel velocity; normal gravity flames, if not lifted, tend to follow the same trend

Turbulent jet in confined counterflow

Indian Academy of Sciences (India)

framework for presenting the results of the flowfield and jet penetration length. ... A turbulent jet is a basic free shear flow and has received research attention (see, .... MBE76 identify this to be a transitional zone and for. √ .... higher return flow and also higher velocity from counterflow due to a narrower gap thus leading.

Effects of substitution on counterflow ignition and extinction of C3 and C4 alcohols

KAUST Repository

Alfazazi, Adamu

2016-06-17

Dwindling reserves and inherent uncertainty in the price of conventional fuels necessitates a search for alternative fuels. Alcohols represent a potential source of energy for the future. The structural features of an alcohol fuel have a direct impact on combustion properties. In particular, substitution in alcohols can alter the global combustion reactivity. In this study, experiments and numerical simulations were conducted to investigate the critical conditions of extinction and autoignition of n-propanol, 1-butanol, iso-propanol and iso-butanol in non-premixed diffusion flames. Experiments were carried out in the counterflow configuration, while simulations were conducted using a skeletal chemical kinetic model for the C3 and C4 alcohols. The fuel stream consists of the pre-vaporized fuel diluted with nitrogen, while the oxidizer stream is air. The experimental results show that autoignition temperatures of the tested alcohols increase in the following order: iso-propanol > iso-butanol > 1-butanol ≈ n-propanol. The simulated results for the branched alcohols agree with the experiments, while the autoignition temperature of 1-butanol is slightly higher than that of n-propanol. For extinction, the experiments show that the extinction limits of the tested fuels increase in the following order: n-propanol ≈ 1-butanol > iso-butanol > iso-propanol. The model suggests that the extinction limits of 1-butanol is slightly higher than n-propanol with extinction strain rate of iso-butanol and iso-propanol maintaining the experimentally observed trend. The transport weighted enthalpy (TWE) and radical index (Ri) concepts were utilized to rationalize the observed reactivity trends for these fuels.

Effects of substitution on counterflow ignition and extinction of C3 and C4 alcohols

KAUST Repository

Alfazazi, Adamu; Niemann, Ulrich; Selim, Hatem; Cattolica, Robert J.; Sarathy, Mani

2016-01-01

Dwindling reserves and inherent uncertainty in the price of conventional fuels necessitates a search for alternative fuels. Alcohols represent a potential source of energy for the future. The structural features of an alcohol fuel have a direct impact on combustion properties. In particular, substitution in alcohols can alter the global combustion reactivity. In this study, experiments and numerical simulations were conducted to investigate the critical conditions of extinction and autoignition of n-propanol, 1-butanol, iso-propanol and iso-butanol in non-premixed diffusion flames. Experiments were carried out in the counterflow configuration, while simulations were conducted using a skeletal chemical kinetic model for the C3 and C4 alcohols. The fuel stream consists of the pre-vaporized fuel diluted with nitrogen, while the oxidizer stream is air. The experimental results show that autoignition temperatures of the tested alcohols increase in the following order: iso-propanol > iso-butanol > 1-butanol ≈ n-propanol. The simulated results for the branched alcohols agree with the experiments, while the autoignition temperature of 1-butanol is slightly higher than that of n-propanol. For extinction, the experiments show that the extinction limits of the tested fuels increase in the following order: n-propanol ≈ 1-butanol > iso-butanol > iso-propanol. The model suggests that the extinction limits of 1-butanol is slightly higher than n-propanol with extinction strain rate of iso-butanol and iso-propanol maintaining the experimentally observed trend. The transport weighted enthalpy (TWE) and radical index (Ri) concepts were utilized to rationalize the observed reactivity trends for these fuels.

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.

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.

Experimental study of an oxygen-hydrogen diffusion flame laden with solid alumina particles; Etude experimentale d'une flamme de diffusion oxygene-hydrogene ensemencee en particules solides d'alumine

Energy Technology Data Exchange (ETDEWEB)

Labor, S.

2003-07-15

Monocrystalline sapphire microspheres are generated through the melting of alumina (AL{sub 2}O{sub 3}) particles in a flame. The alumina particles are injected in a very peculiar O{sub 2}/H{sub 2} confined diffusion flame as it is a downwards vertical flame having fuel in periphery of a central powdered oxygen jet. Quantitative measurements were carried out (ADL, PIV) and supplemented by a numerical study (N3S-Natur). (1) The laminar behavior of the isothermal conditions is kept through reactive flow. Therefore, particles will mainly collide due to speed gradients. (2) It has been shown that an axial particle will have a transit time int the high temperature zone very different to that of an off-line one. (3) The PIV date proved that the particle density was not homogeneous. (4) The hydrogen jet hardly influences the flame aerodynamic structure. Conversely, the central oxygen jet is at premium due to its effect on both the flame speed and temperature distribution. (author)

Computational Analysis of Spray Jet Flames

Science.gov (United States)

Jain, Utsav

droplet parameters needed, a rigorous parametric study is conducted for five different parameters in both physical as well as mixing variable space. The parametric study is conducted for a counterflow setup with n-heptane and inert nitrogen on the fuel side and oxygen with inert nitrogen on the oxidizer side. The computational setup (the temperature and velocity field) is validated against the experimental data from the Yale heptane counterflow flame. The five parameters that are investigated are: aerodynamic strain rate, initial droplet diameter, number of fuel droplets, droplet velocity slip ratio and pre-vaporization ratio. It is not the first time such a study has been accomplished but not a lot of research has been done for heavier fuels such as n-heptane (a very crucial reference fuel for the octane ratings in various applications). Also parameters such as droplet slip ratio and pre-vaporization ratio have not been prudently studied in the past. It is observed that though the slip ratio is not very significant in spray flamelet characterization, the pre-vaporization ratio is important to study and has an interesting influence on spray flamelet structure. In future, based on the current parametric study, the laminar spray flamelet library can be generated which will eventually be integrated to predict turbulent spray flames.

A Computational and Experimental Study of Coflow Laminar Methane/Air Diffusion Flames: Effects of Fuel Dilution, Inlet Velocity, and Gravity

Science.gov (United States)

Cao, S.; Ma, B.; Bennett, B. A. V.; Giassi, D.; Stocker, D. P.; Takahashi, F.; Long, M. B.; Smooke, M. D.

2014-01-01

The influences of fuel dilution, inlet velocity, and gravity on the shape and structure of laminar coflow CH4-air diffusion flames were investigated computationally and experimentally. A series of nitrogen-diluted flames measured in the Structure and Liftoff in Combustion Experiment (SLICE) on board the International Space Station was assessed numerically under microgravity (mu g) and normal gravity (1g) conditions with CH4 mole fraction ranging from 0.4 to 1.0 and average inlet velocity ranging from 23 to 90 cm/s. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modeled by sectional aerosol equations. The governing equations and boundary conditions were discretized on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, flame shape and soot temperature were determined by flame emission images recorded by a digital color camera. Very good agreement between computation and measurement was obtained, and the conclusions were as follows. (1) Buoyant and nonbuoyant luminous flame lengths are proportional to the mass flow rate of the fuel mixture; computed and measured nonbuoyant flames are noticeably longer than their 1g counterparts; the effect of fuel dilution on flame shape (i.e., flame length and flame radius) is negligible when the flame shape is normalized by the methane flow rate. (2) Buoyancy-induced reduction of the flame radius through radially inward convection near the flame front is demonstrated. (3) Buoyant and nonbuoyant flame structure is mainly controlled by the fuel mass flow rate, and the effects from fuel dilution and inlet velocity are secondary.

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

Numerical and experimental studies of ethanol flames and autoignition theory for higher alkanes

Science.gov (United States)

Saxena, Priyank

elementary steps and the addition and deletion of a few key steps relevant to these tests. A mechanism developed in such a hierarchical way starting with simpler fuels such as hydrogen and carbon monoxide to the fuels with one and two carbon atoms has reduced uncertainties in the combustion chemistry of a fuel. This reaction mechanism, consisting of 137 reactions among 30 species, provides a robust building block upon which an ethanol mechanism is developed. The San Diego Mech is extended for ethanol combustion by adding 55 new reactions and 6 new species. Specifically, 33 reactions are added that involve C 2H5OH or one of the three isomers produced by abstraction of an H atom from it, CH3CHOH, CH2CH2OH and CH3CH2O, and 22 reactions are added that involve acetaldehyde or one of the two isomers produced by abstraction of H from it, CH2CHO and CH3CO. Ethanol combustion is investigated on the basis of a new reaction mechanism, thus developed, consisting of 192 elementary steps among 36 species, augmented by 53 additional steps and 14 additional species to address the formation of the oxides of nitrogen and 43 steps and 7 species to address formation of compounds involving three carbon atoms. The mechanism is tested against shock-tube autoignition-delay data, laminar burning velocities, counterflow diffusion-flame extinction and measurements of structures of counterflow partially premixed and diffusion flames. Measurements on ethanol-air flames at a strain rate of 100 s-1, employing prevaporized ethanol with a mole fraction of 0.3 in a nitrogen carrier stream, were made for the pure diffusion flame and for a partially premixed flame with a fuel-side equivalence ratio of 2.3 and involved thermocouple measurements of temperature profiles and determination of concentration profiles of C2H5OH, CO, CO2, H2, H2O, O2, N2, CH4, C2H6 and C2H2+C 2H4 by gas chromatographic analysis of samples withdrawn through fine quartz probes. Computational investigations also were made of profiles of

Fuel density effect on near nozzle flow field in small laminar coflow diffusion flames

KAUST Repository

Xiong, Yuan

2015-01-01

Flow characteristics in small coflow diffusion flames were investigated with a particular focus on the near-nozzle region and on the buoyancy force exerted on fuels with densities lighter and heavier than air (methane, ethylene, propane, and n-butane). The flow-fields were visualized through the trajectories of seed particles. The particle image velocimetry technique was also adopted for quantitative velocity field measurements. The results showed that the buoyancy force exerted on the fuel as well as on burnt gas significantly distorted the near-nozzle flow-fields. In the fuels with densities heavier than air, recirculation zones were formed very close to the nozzle, emphasizing the importance of the relative density of the fuel to that of the air on the flow-field. Nozzle heating influenced the near-nozzle flow-field particularly among lighter fuels (methane and ethylene). Numerical simulations were also conducted, focusing specifically on the effect of specifying inlet boundary conditions for fuel. The results showed that a fuel inlet boundary with a fully developed velocity profile for cases with long tubes should be specified inside the fuel tube to permit satisfactory prediction of the flow-field. The calculated temperature fields also indicated the importance of the selection of the location of the inlet boundary, especially in testing various combustion models that include soot in small coflow diffusion flames. © 2014 The Combustion Institute.

Bifurcation and extinction limit of stretched premixed flames with chain-branching intermediate kinetics and radiative loss

Science.gov (United States)

Zhang, Huangwei; Chen, Zheng

2018-05-01

Premixed counterflow flames with thermally sensitive intermediate kinetics and radiation heat loss are analysed within the framework of large activation energy. Unlike previous studies considering one-step global reaction, two-step chemistry consisting of a chain branching reaction and a recombination reaction is considered here. The correlation between the flame front location and stretch rate is derived. Based on this correlation, the extinction limit and bifurcation characteristics of the strained premixed flame are studied, and the effects of fuel and radical Lewis numbers as well as radiation heat loss are examined. Different flame regimes and their extinction characteristics can be predicted by the present theory. It is found that fuel Lewis number affects the flame bifurcation qualitatively and quantitatively, whereas radical Lewis number only has a quantitative influence. Stretch rates at the stretch and radiation extinction limits respectively decrease and increase with fuel Lewis number before the flammability limit is reached, while the radical Lewis number shows the opposite tendency. In addition, the relation between the standard flammability limit and the limit derived from the strained near stagnation flame is affected by the fuel Lewis number, but not by the radical Lewis number. Meanwhile, the flammability limit increases with decreased fuel Lewis number, but with increased radical Lewis number. Radical behaviours at flame front corresponding to flame bifurcation and extinction are also analysed in this work. It is shown that radical concentration at the flame front, under extinction stretch rate condition, increases with radical Lewis number but decreases with fuel Lewis number. It decreases with increased radiation loss.

Characteristics of Oscillating Flames in a Coaxial Confined Jet

Directory of Open Access Journals (Sweden)

Min Suk Cha

2010-12-01

Full Text Available Flame characteristics when a non-premixed n-butane jet is ejected into a coaxial cylindrical tube are investigated experimentally. Flame stability depends mainly on the characteristics of flame propagation as well as air entrainment which depend on the jet momentum and on the distance between the nozzle exit and the base of a confined tube. As flow rate increases, the flame lifts off from a nozzle attached diffusion flame and a stationary lifted flame can be stabilized. The liftoff height increases nearly linearly with the average velocity at the nozzle exit. The lifted flame has a tribrachial flame structure, which consists of a rich premixed flame, a lean premixed flame, and a diffusion flame, all extending from a single location. As flow rate further increases, periodically oscillating flames are observed inside the confined tube. Once flame oscillation occurs, the flame undergoes relatively stable oscillation such that it has nearly constant oscillation amplitude and frequency. The criteria of flame oscillation are mapped as functions of nozzle diameter, the distance between nozzle and tube, and jet velocity. This type of flame oscillation can be characterized by Strouhal number in terms of flame oscillation amplitude, frequency, and jet velocity. Buoyancy driven flame oscillation which is one of the viable mechanism for flame oscillation is modeled and the results agrees qualitatively with experimental results, suggesting that the oscillation is due to periodic blowoff and flashback under the influence of buoyancy.

Effect of pointed and diffused air injection on premixed flame confined in a Rijke tube

Directory of Open Access Journals (Sweden)

Nilaj N. Deshmukh

2016-12-01

Full Text Available The coupling between pressure fluctuations and unsteady heat release in a combustion systems results in acoustic oscillations inside the combustion system. These acoustic oscillations, when grow sufficiently, may cause serious structural damage thereby reducing the lifespan of jet engines, gas turbines, and industrial burners. The aim of the first part of study is to define acoustically stable and unstable regions. The second part is focused on studying the effect of change in pressure field near the flame on the amplitude and frequency of the oscillations of instability. This study is carried out for three-burner positions and equivalence ratio of 0.7 by varying heat supply and total flow rate. The results show two acoustically unstable regions for 0.1 and 0.2 burner positions and only one acoustically unstable region for 0.25 burner position. The effect of pointed injection and diffused injection over a premixed flame on the sound pressure level was studied. The results show for burner position of x/L = 0.2 there is 25 dB suppression is possible using pointed injection at higher total flow rate. The experiment of diffused injection shows sound amplification more than 12 dB was observed.

The i-V curve characteristics of burner-stabilized premixed flames: detailed and reduced models

KAUST Repository

Han, Jie

2016-07-17

The i-V curve describes the current drawn from a flame as a function of the voltage difference applied across the reaction zone. Since combustion diagnostics and flame control strategies based on electric fields depend on the amount of current drawn from flames, there is significant interest in modeling and understanding i-V curves. We implement and apply a detailed model for the simulation of the production and transport of ions and electrons in one-dimensional premixed flames. An analytical reduced model is developed based on the detailed one, and analytical expressions are used to gain insight into the characteristics of the i-Vcurve for various flame configurations. In order for the reduced model to capture the spatial distribution of the electric field accurately, the concept of a dead zone region, where voltage is constant, is introduced, and a suitable closure for the spatial extent of the dead zone is proposed and validated. The results from the reduced modeling framework are found to be in good agreement with those from the detailed simulations. The saturation voltage is found to depend significantly on the flame location relative to the electrodes, and on the sign of the voltage difference applied. Furthermore, at sub-saturation conditions, the current is shown to increase linearly or quadratically with the applied voltage, depending on the flame location. These limiting behaviors exhibited by the reduced model elucidate the features of i-V curves observed experimentally. The reduced model relies on the existence of a thin layer where charges are produced, corresponding to the reaction zone of a flame. Consequently, the analytical model we propose is not limited to the study of premixed flames, and may be applied easily to others configurations, e.g.~nonpremixed counterflow flames.

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.

Impact of Equivalence Ratio on the Macrostructure of Premixed Swirling CH 4 /Air and CH 4 /O 2 /CO 2 Flames

KAUST Repository

Watanabe, Hirotatsu

2015-06-15

Premixed CH4/O2/CO2 flames (oxy-flames) and CH4/air flames (air-flames) were experimentally studied in a swirl-stabilized combustor. For comparing oxy and air flames, the same equivalence ratio and adiabatic flame temperature were used. CO2 dilution was adjusted to attain the same adiabatic temperature for the oxy-flame and the corresponding air-flame while keeping the equivalence ratio and Reynolds number (=20,000) the same. For high equivalence ratios, we observed flames stabilized along the inner and outer shear layers of the swirling flow and sudden expansion, respectively, in both flames. However, one notable difference between the two flames appears as the equivalence ratio reaches 0.60. At this point, the outer shear layer flame disappears in the air-flame while it persists in the oxy-flame, despite the lower burning velocity of the oxy-flame. Prior PIV measurements (Ref. 9) showed that the strains along the outer shear layer are higher than along the inner shear layer. Therefore, the extinction strain rates in both flames were calculated using a counter-flow premixed twin flame configuration. Calculations at the equivalence ratio of 0.60 show that the extinction strain rate is higher in the oxy than in the air flame, which help explain why it persists on the outer shear layer with higher strain rate. It is likely that extinction strain rates contribute to the oxy-flame stabilization when air flame extinguish in the outer shear layer. However, the trend reverses at higher equivalence ratio, and the cross point of the extinction strain rate appears at equivalence ratio of 0.64.

Experimental Characterization of Soot Formation in Diffusion Flames and Explosive Fireballs

Science.gov (United States)

2012-04-01

profiles for the opposed jet burner using Unicorn and Chemkin Pro, ethylene/air flame, Wang-Colket mechanism. .............................33 Figure...35 Figure 31. Flame simulations using UNICORN (Katta et al...two-dimensional (2-D) flame simulation computer code UNICORN (Katta et al., 2006) with those obtained using the one- dimensional (1-D) flame

Effects of pressure and fuel dilution on coflow laminar methane-air diffusion flames: A computational and experimental study

Science.gov (United States)

Cao, Su; Ma, Bin; Giassi, Davide; Bennett, Beth Anne V.; Long, Marshall B.; Smooke, Mitchell D.

2018-03-01

In this study, the influence of pressure and fuel dilution on the structure and geometry of coflow laminar methane-air diffusion flames is examined. A series of methane-fuelled, nitrogen-diluted flames has been investigated both computationally and experimentally, with pressure ranging from 1.0 to 2.7 atm and CH4 mole fraction ranging from 0.50 to 0.65. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the reactive gaseous mixture, and soot evolution was modelled by sectional aerosol equations. The governing equations and boundary conditions were discretised on a two-dimensional computational domain by finite differences, and the resulting set of fully coupled, strongly nonlinear equations was solved simultaneously at all points using a damped, modified Newton's method. Experimentally, chemiluminescence measurements of CH* were taken to determine its relative concentration profile and the structure of the flame front. A thin-filament ratio pyrometry method using a colour digital camera was employed to determine the temperature profiles of the non-sooty, atmospheric pressure flames, while soot volume fraction was quantified, after evaluation of soot temperature, through an absolute light calibration using a thermocouple. For a broad spectrum of flames in atmospheric and elevated pressures, the computed and measured flame quantities were examined to characterise the influence of pressure and fuel dilution, and the major conclusions were as follows: (1) maximum temperature increases with increasing pressure or CH4 concentration; (2) lift-off height decreases significantly with increasing pressure, modified flame length is roughly independent of pressure, and flame radius decreases with pressure approximately as P-1/2; and (3) pressure and fuel stream dilution significantly affect the spatial distribution and the peak value of the soot volume fraction.

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

A comparison of the thermal, emission and heat transfer characteristics of swirl-stabilized premixed and inverse diffusion flames

Energy Technology Data Exchange (ETDEWEB)

Zhen, H.S.; Leung, C.W.; Cheung, C.S. [Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong (China)

2011-02-15

Two swirl-stabilized flames, a premixed flame (PMF-s) and an inverse diffusion flame (IDF-s), were investigated experimentally in order to obtain information on their thermal, emission and heat transfer characteristics. The two flames, having different global air/fuel mixing mechanisms, were compared under identical air and fuel flow rates. Results showed that the two flames have similar visual features such as flame shape, size and structure because the Reynolds number and the swirl number which are important parameters representative of the aerodynamic characteristics of a swirling jet flow, are almost the same for the two flames. The minor dissimilarity in flame color and flame length indicates that the IDF-s is more diffusional. Both the PMF-s and IDF-s are stabilized by the internal recirculation zone (IRZ) and the IDF-s is more stable. Flame temperature is uniformly distributed in the IRZ due to the strong mixing caused by flow recirculation. The highest flame temperature is achieved at the main reaction zone and it is higher for the PMF-s due to more rapid and localized heat release. For the IDF-s, the thermal NO mechanism dominates the NO{sub x} formation. For the PMF-s, both the thermal and prompt mechanisms tend to play important roles in the global NO{sub x} emission under rich conditions. The comparison of EINO{sub x} and EICO shows that the PMF-s has lower level of NO{sub x} emission under lean combustion and lower level of CO emission under all conditions. The reason is that the air/fuel premixing in the PMF-s significantly enhances the mixedness of the supplied air/fuel mixture. The analysis of the behaviors of the impinging PMF-s and IDF-s heat transfer reveals that because the PMF-s has more rapid and localized heat release at the main reaction zone, the peak heat flux is higher than that of the IDF-s and the IDF-s has more uniform heating effect. A comparison of the overall heat transfer rates shows that, due to more complete combustion, the PMF

A comparison of the thermal, emission and heat transfer characteristics of swirl-stabilized premixed and inverse diffusion flames

International Nuclear Information System (INIS)

Zhen, H.S.; Leung, C.W.; Cheung, C.S.

2011-01-01

Two swirl-stabilized flames, a premixed flame (PMF-s) and an inverse diffusion flame (IDF-s), were investigated experimentally in order to obtain information on their thermal, emission and heat transfer characteristics. The two flames, having different global air/fuel mixing mechanisms, were compared under identical air and fuel flow rates. Results showed that the two flames have similar visual features such as flame shape, size and structure because the Reynolds number and the swirl number which are important parameters representative of the aerodynamic characteristics of a swirling jet flow, are almost the same for the two flames. The minor dissimilarity in flame color and flame length indicates that the IDF-s is more diffusional. Both the PMF-s and IDF-s are stabilized by the internal recirculation zone (IRZ) and the IDF-s is more stable. Flame temperature is uniformly distributed in the IRZ due to the strong mixing caused by flow recirculation. The highest flame temperature is achieved at the main reaction zone and it is higher for the PMF-s due to more rapid and localized heat release. For the IDF-s, the thermal NO mechanism dominates the NO x formation. For the PMF-s, both the thermal and prompt mechanisms tend to play important roles in the global NO x emission under rich conditions. The comparison of EINO x and EICO shows that the PMF-s has lower level of NO x emission under lean combustion and lower level of CO emission under all conditions. The reason is that the air/fuel premixing in the PMF-s significantly enhances the mixedness of the supplied air/fuel mixture. The analysis of the behaviors of the impinging PMF-s and IDF-s heat transfer reveals that because the PMF-s has more rapid and localized heat release at the main reaction zone, the peak heat flux is higher than that of the IDF-s and the IDF-s has more uniform heating effect. A comparison of the overall heat transfer rates shows that, due to more complete combustion, the PMF-s has higher overall

Effects of DME mixing on number density and size properties of soot particles in counterflow non-premixed ethylene flames

KAUST Repository

Choi, J. H.

2015-05-01

In order to investigate the effect of DME mixing on the number density and size of soot particles, DME was mixed in a counter flow non-premixed ethylene flame with mixture ratios of 5%, 14% and 30%. A laser extinction/scattering technique has been adopted to measure the volume fraction, number density, and mean size of soot particles. The experimental results showed that the highest soot concentrations were observed for flames with mixture ratios of 5% and 14%; however, for a mixture ratio of 30% the soot concentration decreased. Numerical results showed that the concentrations of propargyl radicals (C3H3) at the 5% and 14% ratios were higher than those measured in the ethylene-based flame, and the production of benzene (C6H6) in the 5% and 14% DME mixture flames was also increased. This indicates the crucial role of propargyl in benzene ring formation. These reactions generally become stronger with increased DME mixing, except for A1- + H2 → A1 + H (-R554) and n-C4H5 + C2H2 → A1 + H (R542). Therefore, it is indicated that adding DME to ethylene flames promotes benzene ring formation. Note that although the maximum C6H6 concentration is largest in the 30% DME mixing flame, the soot volume fraction is smaller than those for the 5% and 14% mixture ratios. This is because the local C6H6 concentration decreases in the relatively low temperature region in the fuel side where soot growth occurs. © 2015, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

Effects of DME mixing on number density and size properties of soot particles in counterflow non-premixed ethylene flames

KAUST Repository

Choi, J. H.; Choi, B. C.; Lee, S. M.; Chung, Suk-Ho; Jung, K. S.; Jeong, W. L.; Choi, S. K.; Park, S. K.

2015-01-01

In order to investigate the effect of DME mixing on the number density and size of soot particles, DME was mixed in a counter flow non-premixed ethylene flame with mixture ratios of 5%, 14% and 30%. A laser extinction/scattering technique has been adopted to measure the volume fraction, number density, and mean size of soot particles. The experimental results showed that the highest soot concentrations were observed for flames with mixture ratios of 5% and 14%; however, for a mixture ratio of 30% the soot concentration decreased. Numerical results showed that the concentrations of propargyl radicals (C3H3) at the 5% and 14% ratios were higher than those measured in the ethylene-based flame, and the production of benzene (C6H6) in the 5% and 14% DME mixture flames was also increased. This indicates the crucial role of propargyl in benzene ring formation. These reactions generally become stronger with increased DME mixing, except for A1- + H2 → A1 + H (-R554) and n-C4H5 + C2H2 → A1 + H (R542). Therefore, it is indicated that adding DME to ethylene flames promotes benzene ring formation. Note that although the maximum C6H6 concentration is largest in the 30% DME mixing flame, the soot volume fraction is smaller than those for the 5% and 14% mixture ratios. This is because the local C6H6 concentration decreases in the relatively low temperature region in the fuel side where soot growth occurs. © 2015, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

Performance Analyses of Counter-Flow Closed Wet Cooling Towers Based on a Simplified Calculation Method

Directory of Open Access Journals (Sweden)

Xiaoqing Wei

2017-02-01

Full Text Available As one of the most widely used units in water cooling systems, the closed wet cooling towers (CWCTs have two typical counter-flow constructions, in which the spray water flows from the top to the bottom, and the moist air and cooling water flow in the opposite direction vertically (parallel or horizontally (cross, respectively. This study aims to present a simplified calculation method for conveniently and accurately analyzing the thermal performance of the two types of counter-flow CWCTs, viz. the parallel counter-flow CWCT (PCFCWCT and the cross counter-flow CWCT (CCFCWCT. A simplified cooling capacity model that just includes two characteristic parameters is developed. The Levenberg–Marquardt method is employed to determine the model parameters by curve fitting of experimental data. Based on the proposed model, the predicted outlet temperatures of the process water are compared with the measurements of a PCFCWCT and a CCFCWCT, respectively, reported in the literature. The results indicate that the predicted values agree well with the experimental data in previous studies. The maximum absolute errors in predicting the process water outlet temperatures are 0.20 and 0.24 °C for the PCFCWCT and CCFCWCT, respectively. These results indicate that the simplified method is reliable for performance prediction of counter-flow CWCTs. Although the flow patterns of the two towers are different, the variation trends of thermal performance are similar to each other under various operating conditions. The inlet air wet-bulb temperature, inlet cooling water temperature, air flow rate, and cooling water flow rate are crucial for determining the cooling capacity of a counter-flow CWCT, while the cooling tower effectiveness is mainly determined by the flow rates of air and cooling water. Compared with the CCFCWCT, the PCFCWCT is much more applicable in a large-scale cooling water system, and the superiority would be amplified when the scale of water

Experimental and numerical reaction analysis on sodium-water chemical reaction field

International Nuclear Information System (INIS)

Deguchi, Yoshihiro; Takata, Takashi; Yamaguchi, Akira; Kikuchi, Shin; Ohshima, Hiroyuki

2015-01-01

In a sodium-cooled fast reactor (SFR), liquid sodium is used as a heat transfer fluid because of its excellent heat transport capability. On the other hand, it has strong chemical reactivity with water vapor. One of the design basis accidents of the SFR is the water leakage into the liquid sodium flow by a breach of heat transfer tubes. This process ends up damages on the heat transport equipment in the SFR. Therefore, the study on sodium-water chemical reactions is of paramount importance for security reasons. This study aims to clarify the sodium-water reaction mechanisms using an elementary reaction analysis. A quasi one-dimensional flame model is applied to a sodium-water counter-flow reaction field. The analysis contains 25 elementary reactions, which consist of 17 H_2-O_2 and 8 Na-H_2O reactions. Temperature and species concentrations in the counter-flow reaction field were measured using laser diagnostics such as LIF and CARS. The main reaction in the experimental conditions is Na+H_2O → NaOH+H and OH is produced by H_2O+H → H_2+OH. It is demonstrated that the reaction model in this study well explains the structure of the sodium-water counter-flow diffusion flame. (author)

Nucleation and growth mechanism for flame synthesis of MoO2 hollow microchannels with nanometer wall thickness.

Science.gov (United States)

Merchan-Merchan, Wilson; Saveliev, Alexei V; Taylor, Aaron M

2009-12-01

The growth and morphological evolution of molybdenum-oxide microstructures formed in the high temperature environment of a counter-flow oxy-fuel flame using molybdenum probes is studied. Experiments conducted using various probe retention times show the sequence of the morphological changes. The morphological row begins with micron size objects exhibiting polygonal cubic shape, develops into elongated channels, changes to large structures with leaf-like shape, and ends in dendritic structures. Time of probe-flame interaction is found to be a governing parameter controlling the wide variety of morphological patterns; a molecular level growth mechanism is attributed to their development. This study reveals that the structures are grown in several consecutive stages: material "evaporation and transportation", "transformation", "nucleation", "initial growth", "intermediate growth", and "final growth". XRD analysis shows that the chemical compositions of all structures correspond to MoO(2).

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.

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

PIV Measurements in Weakly Buoyant Gas Jet Flames

Science.gov (United States)

Sunderland, Peter B.; Greenbberg, Paul S.; Urban, David L.; Wernet, Mark P.; Yanis, William

2001-01-01

Despite numerous experimental investigations, the characterization of microgravity laminar jet diffusion flames remains incomplete. Measurements to date have included shapes, temperatures, soot properties, radiative emissions and compositions, but full-field quantitative measurements of velocity are lacking. Since the differences between normal-gravity and microgravity diffusion flames are fundamentally influenced by changes in velocities, it is imperative that the associated velocity fields be measured in microgravity flames. Velocity measurements in nonbuoyant flames will be helpful both in validating numerical models and in interpreting past microgravity combustion experiments. Pointwise velocity techniques are inadequate for full-field velocity measurements in microgravity facilities. In contrast, Particle Image Velocimetry (PIV) can capture the entire flow field in less than 1% of the time required with Laser Doppler Velocimetry (LDV). Although PIV is a mature diagnostic for normal-gravity flames , restrictions on size, power and data storage complicate these measurements in microgravity. Results from the application of PIV to gas jet flames in normal gravity are presented here. Ethane flames burning at 13, 25 and 50 kPa are considered. These results are presented in more detail in Wernet et al. (2000). The PIV system developed for these measurements recently has been adapted for on-rig use in the NASA Glenn 2.2-second drop tower.

Second-sound studies of coflow and counterflow of superfluid 4He in channels

International Nuclear Information System (INIS)

Varga, Emil; Skrbek, L.; Babuin, Simone

2015-01-01

We report a comprehensive study of turbulent superfluid 4 He flow through a channel of square cross section. We study for the first time two distinct flow configurations with the same apparatus: coflow (normal and superfluid components move in the same direction), and counterflow (normal and superfluid components move in opposite directions). We realise also a variation of counterflow with the same relative velocity, but where the superfluid component moves while there is no net flow of the normal component through the channel, i.e., pure superflow. We use the second-sound attenuation technique to measure the density of quantised vortex lines in the temperature range 1.2 K ≲ T ≲ T λ ≈ 2.18 K and for flow velocities from about 1 mm/s up to almost 1 m/s in fully developed turbulence. We find that both the steady-state and temporal decay of the turbulence significantly differ in the three flow configurations, yielding an interesting insight into two-fluid hydrodynamics. In both pure superflow and counterflow, the same scaling of vortex line density with counterflow velocity is observed, L∝V cf 2 , with a pronounced temperature dependence; in coflow instead, the vortex line density scales with velocity as L ∝ V 3/2 and is temperature independent; we provide theoretical explanations for these observations. Further, we develop a new promising technique to use different second-sound resonant modes to probe the spatial distribution of quantised vortices in the direction perpendicular to the flow. Preliminary measurements indicate that coflow is less homogeneous than counterflow/superflow, with a denser concentration of vortices between the centre of the channel and its walls

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

Behavioral Characteristics of the Non-Premixed Methane-Air Flame Oppositely Injected in a Narrow Channel

International Nuclear Information System (INIS)

Yun, Young Min; Lee, Min Jung; Cho, Sang Moon; Kim, Nam Il

2009-01-01

Characteristics of a counter flowing diffusion flame, which is formulated by an oppositely-injected methane-jet flow in a narrow channel of a uniform air flow. The location of the flame fronts and the flame lengths were compared by changing the flow rates of fuel. To distinguish the effects of the narrow channel on the diffusion flame, a numerical simulation for an ideal two-dimensional flame was conducted. Overall trends of the flame behavior were similar in both numerical and experimental results. With the increase of the ratio of jet velocity to air velocity flame front moved farther upstream. It is thought that the flow re-direction in the channel suppresses fuel momentum more significantly due to the higher temperature and increased viscosity of burned gas. Actual flames in a narrow channel suffer heat loss to the ambient and it has finite length of diffusion flame in contrast to the numerical results of infinite flame length. Thus a convective heat loss was additionally employed in numerical simulation and closer results were obtained. These results can be used as basic data in development of a small combustor of a nonpremixed flame

Behavioral Characteristics of the Non-Premixed Methane-Air Flame Oppositely Injected in a Narrow Channel

Energy Technology Data Exchange (ETDEWEB)

Yun, Young Min; Lee, Min Jung; Cho, Sang Moon; Kim, Nam Il [Chungang University, Seoul (Korea, Republic of)

2009-04-15

Characteristics of a counter flowing diffusion flame, which is formulated by an oppositely-injected methane-jet flow in a narrow channel of a uniform air flow. The location of the flame fronts and the flame lengths were compared by changing the flow rates of fuel. To distinguish the effects of the narrow channel on the diffusion flame, a numerical simulation for an ideal two-dimensional flame was conducted. Overall trends of the flame behavior were similar in both numerical and experimental results. With the increase of the ratio of jet velocity to air velocity flame front moved farther upstream. It is thought that the flow re-direction in the channel suppresses fuel momentum more significantly due to the higher temperature and increased viscosity of burned gas. Actual flames in a narrow channel suffer heat loss to the ambient and it has finite length of diffusion flame in contrast to the numerical results of infinite flame length. Thus a convective heat loss was additionally employed in numerical simulation and closer results were obtained. These results can be used as basic data in development of a small combustor of a nonpremixed flame.

Effects of Large Polycyclic Aromatic Hydrocarbons on the Soot Formation in Ethylene-Air Nonpremixed Flames

KAUST Repository

Prabhu, S.; Arias, P.G.; Wang, Y.; Gao, Y.; Park, S.; Im, Hong G.; Sarathy, Mani; Chung, Suk-Ho; Lu, T.

2015-01-01

This study presents updated comprehensive gas-phase kinetic mechanism and aerosol models to predict soot formation characteristics in ethylene-air nonpremixed flames. A main objective is to investigate the sensitivity of the soot formation rate to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph (DRG) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames of pure ethylene at low strain rate sooting conditions are considered, for which the sensitivity of soot formation characteristics with respect to hetrogeneous nucleation is investigated. Results show that higher PAH concentrations result in higher soot nucleation rate, and that the average size of the particles are in good agreement with experimental results. It is found that the nucleation processes (i.e., soot inception) from higher PAH precursors, coronene in particular, is critical for accurate prediction of the overall soot formation.

Effects of Large Polycyclic Aromatic Hydrocarbons on the Soot Formation in Ethylene-Air Nonpremixed Flames

KAUST Repository

Prabhu, S.

2015-03-30

This study presents updated comprehensive gas-phase kinetic mechanism and aerosol models to predict soot formation characteristics in ethylene-air nonpremixed flames. A main objective is to investigate the sensitivity of the soot formation rate to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph (DRG) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames of pure ethylene at low strain rate sooting conditions are considered, for which the sensitivity of soot formation characteristics with respect to hetrogeneous nucleation is investigated. Results show that higher PAH concentrations result in higher soot nucleation rate, and that the average size of the particles are in good agreement with experimental results. It is found that the nucleation processes (i.e., soot inception) from higher PAH precursors, coronene in particular, is critical for accurate prediction of the overall soot formation.

Modelling of a 400 kW natural gas diffusion flame using finite-rate chemistry schemes

International Nuclear Information System (INIS)

Mueller, Christian; Kremer, Hans; Brink, Anders; Kilpinen, Pia; Hupa, Mikko

1999-01-01

The Eddy-Dissipation Combustion Model combined with three different reaction mechanisms is applied to simulate a fuel-rich 400 kW natural gas diffusion flame. The chemical schemes include a global 2-step and a global 4-step approach as well as a reduced 4-step mechanism systematically derived from an elementary scheme. The species and temperature distributions resulting from the different schemes are studied in detail and compared to each other and to experiments. Furthermore the method of implementing finite-rate chemistry to the Eddy-Dissipation Combustion Model is discussed. (author)

Effect of unequal fuel and oxidizer Lewis numbers on flame dynamics

Energy Technology Data Exchange (ETDEWEB)

Shamim, Tariq [Department of Mechanical Engineering, The University of Michigan-Dearborn, Dearborn, MI 48128-1491 (United States)

2006-12-15

The interaction of non-unity Lewis number (due to preferential diffusion and/or unequal rates of heat and mass transfer) with the coupled effect of radiation, chemistry and unsteadiness alters several characteristics of a flame. The present study numerically investigates this interaction with a particular emphasis on the effect of unequal and non-unity fuel and oxidizer Lewis numbers in a transient diffusion flame. The unsteadiness is simulated by considering the flame subjected to modulations in reactant concentration. Flames with different Lewis numbers (ranging from 0.5 to 2) and subjected to different modulating frequencies are considered. The results show that the coupled effect of Lewis number and unsteadiness strongly influences the flame dynamics. The impact is stronger at high modulating frequencies and strain rates, particularly for large values of Lewis numbers. Compared to the oxidizer side Lewis number, the fuel side Lewis number has greater influence on flame dynamics. (author)

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.

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.

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.

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)

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.

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames.

Science.gov (United States)

Singh, Ajay V; Gollner, Michael J

2016-06-01

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

Nitric oxide concentration measurements in atmospheric pressure flames using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

Science.gov (United States)

Chai, N.; Kulatilaka, W. D.; Naik, S. V.; Laurendeau, N. M.; Lucht, R. P.; Kuehner, J. P.; Roy, S.; Katta, V. R.; Gord, J. R.

2007-06-01

We report the application of electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) for measurements of nitric oxide concentration ([NO]) in three different atmospheric pressure flames. Visible pump (532 nm) and Stokes (591 nm) beams are used to probe the Q-branch of the Raman transition. A significant resonance enhancement is obtained by tuning an ultraviolet probe beam (236 nm) into resonance with specific rotational transitions in the (v’=0, v”=1) vibrational band of the A2Σ+-X2Π electronic system of NO. ERE-CARS spectra are recorded at various heights within a hydrogen-air flame producing relatively low concentrations of NO over a Hencken burner. Good agreement is obtained between NO ERE-CARS measurements and the results of flame computations using UNICORN, a two-dimensional flame code. Excellent agreement between measured and calculated NO spectra is also obtained when using a modified version of the Sandia CARSFT code for heavily sooting acetylene-air flames (φ=0.8 to φ=1.6) on the same Hencken burner. Finally, NO concentration profiles are measured using ERE-CARS in a laminar, counter-flow, non-premixed hydrogen-air flame. Spectral scans are recorded by probing the Q1 (9.5), Q1 (13.5) and Q1 (17.5) Raman transitions. The measured shape of the [NO] profile is in good agreement with that predicted using the OPPDIF code, even without correcting for collisional effects. These comparisons between [NO] measurements and predictions establish the utility of ERE-CARS for detection of NO in flames with large temperature and concentration gradients as well as in sooting environments.

Two-dimensional temperature and carbon dioxide concentration profiles in atmospheric laminar diffusion flames measured by mid-infrared direct absorption spectroscopy at 4.2 μm

Science.gov (United States)

Liu, Xunchen; Zhang, Guoyong; Huang, Yan; Wang, Yizun; Qi, Fei

2018-04-01

We present a multi-line flame thermometry technique based on mid-infrared direct absorption spectroscopy of carbon dioxide at its v_3 fundamental around 4.2 μm that is particularly suitable for sooting flames. Temperature and concentration profiles of gas phase molecules in a flame are important characteristics to understand its flame structure and combustion chemistry. One of the standard laboratory flames to analyze polycyclic aromatic hydrocarbons (PAH) and soot formation is laminar non-premixed co-flow flame, but PAH and soot introduce artifact to most non-contact optical measurements. Here we report an accurate diagnostic method of the temperature and concentration profiles of CO2 in ethylene diffusion flames by measuring its v_3 vibrational fundamental. An interband cascade laser was used to probe the R-branch bandhead at 4.2 μm, which is highly sensitive to temperature change, free from soot interference and ambient background. Calibration measurement was carried out both in a low-pressure Herriott cell and an atmospheric pressure tube furnace up to 1550 K to obtain spectroscopic parameters for high-temperature spectra. In our co-flow flame measurement, two-dimensional line-of-sight optical depth of an ethylene/N2 laminar sooting flame was recorded by dual-beam absorption scheme. The axially symmetrical attenuation coefficient profile of CO2 in the co-flow flame was reconstructed from the optical depth by Abel inversion. Spatially resolved flame temperature and in situ CO2 volume fraction profiles were derived from the calibrated CO2 spectroscopic parameters and compared with temperature profiles measured by two-line atomic fluorescence.

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.

Visualization of He II counterflow around a cylinder

Czech Academy of Sciences Publication Activity Database

Chagovets, Tymofiy; Van Sciver, S.W.

2013-01-01

Roč. 25, č. 10 (2013), "105104-1"-"105104-6" ISSN 1070-6631 R&D Projects: GA ČR GP13-03806P Institutional support: RVO:68378271 Keywords : He II * thermal counterflow * visualization technique Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.040, year: 2013 http://dx.doi.org/10.1063/1.4824004

Aerosol counterflow two-jets unit for continuous measurement of the soluble fraction of atmospheric aerosols.

Science.gov (United States)

Mikuska, Pavel; Vecera, Zbynek

2005-09-01

A new type of aerosol collector employing a liquid at laboratory temperature for continuous sampling of atmospheric particles is described. The collector operates on the principle of a Venturi scrubber. Sampled air flows at high linear velocity through two Venturi nozzles "atomizing" the liquid to form two jets of a polydisperse aerosol of fine droplets situated against each other. Counterflow jets of droplets collide, and within this process, the aerosol particles are captured into dispersed liquid. Under optimum conditions (air flow rate of 5 L/min and water flow rate of 2 mL/min), aerosol particles down to 0.3 microm in diameter are quantitatively collected in the collector into deionized water while the collection efficiency of smaller particles decreases. There is very little loss of fine aerosol within the aerosol counterflow two-jets unit (ACTJU). Coupling of the aerosol collector with an annular diffusion denuder located upstream of the collector ensures an artifact-free sampling of atmospheric aerosols. Operation of the ACTJU in combination with on-line detection devices allows in situ automated analysis of water-soluble aerosol species (e.g., NO2-, NO3-)with high time resolution (as high as 1 s). Under the optimum conditions, the limit of detection for particulate nitrite and nitrate is 28 and 77 ng/m(3), respectively. The instrument is sufficiently rugged for its application at routine monitoring of aerosol composition in the real time.

Prediction of flame formation in highly preheated air combustion

International Nuclear Information System (INIS)

Yang, Jang Sik; Choi, Gyung Min; Kim, Duck Jool; Katsuki, Masashi

2008-01-01

Fundamental information about the ignition position and shape of a flame in highly preheated air combustion was obtained, and the suitability of the suggested reduced kinetic mechanism that reflects the characteristics of the highly preheated air combustion was demonstrated. Flame lift height and flame length with variations of premixed air temperature and oxygen concentration were measured by CH chemiluminescence intensity, and were computed with a reduced kinetic mechanism. Flame attached near a fuel nozzle started to lift when preheated air temperature became close to auto-ignition temperature and/or oxygen concentration reduced. The flame lift height increased but the flame length decreased with decreasing preheated air temperature and flame length reversed after a minimum value. Calculated results showed good agreement with those of experiment within tolerable error. Flame shape shifted from diffusion flame shape to partial premixed flame shape with increasing lift height and this tendency was also observed in the computation results

Prediction of flame formation in highly preheated air combustion

Energy Technology Data Exchange (ETDEWEB)

Yang, Jang Sik; Choi, Gyung Min; Kim, Duck Jool [Pusan National University, Busan (Korea, Republic of); Katsuki, Masashi [Osaka University, Osaka (Japan)

2008-11-15

Fundamental information about the ignition position and shape of a flame in highly preheated air combustion was obtained, and the suitability of the suggested reduced kinetic mechanism that reflects the characteristics of the highly preheated air combustion was demonstrated. Flame lift height and flame length with variations of premixed air temperature and oxygen concentration were measured by CH chemiluminescence intensity, and were computed with a reduced kinetic mechanism. Flame attached near a fuel nozzle started to lift when preheated air temperature became close to auto-ignition temperature and/or oxygen concentration reduced. The flame lift height increased but the flame length decreased with decreasing preheated air temperature and flame length reversed after a minimum value. Calculated results showed good agreement with those of experiment within tolerable error. Flame shape shifted from diffusion flame shape to partial premixed flame shape with increasing lift height and this tendency was also observed in the computation results

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.

Heat, mass, and momentum transport model for hydrogen diffusion flames in nuclear reactor containments

International Nuclear Information System (INIS)

Travis, J.R.

1985-01-01

It is now possible to analyze the time-dependent, fully three-dimensional behavior of hydrogen diffusion flames in nuclear reactor containments. This analysis involves coupling the full Navier-Stokes equations with multi-species transport to the global chemical kinetics of hydrogen combustion. A transport equation for the subgrid scale turbulent kinetic energy density is solved to produce the time and space dependent turbulent transport coefficients. The heat transfer coefficient governing the exchange of heat between fluid computational cells adjacent to wall cells is calculated by a modified Reynolds analogy formulation. The analysis of a MARK-III containment indicates very complex flow patterns that greatly influence fluid and wall temperatures and heat fluxes. 18 refs., 24 figs

Measurements of He II Thermal Counterflow Using PIV Technique

International Nuclear Information System (INIS)

Zhang, T.; Van Sciver, S.W.

2004-01-01

Our previous experiments on the measurements of He II thermal counterflow using Particle Image Velocimetry (PIV) have shown that there exists a substantial discrepancy between the measured and theoretical values of normal fluid velocity. It was assumed that this is due to the slip velocity between tracer particles and liquid helium. In the present work, tracer particles with a much smaller mean diameter and a more uniform size distribution were selected in order to reduce the effect of slip velocity, and an improved two phase fluidized bed technique was used to introduce the particles into liquid helium. The normal fluid velocity of thermal counterflow was then measured using the PIV technique at various heat fluxes and bath temperatures. The experimental results, however, still show the existence of discrepancy between PIV measured particle velocities and the theoretical normal fluid velocity. A preliminary explanation of these results is given based on an interaction of tracer particles with the superfluid component in the He II

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.

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.

Dynamics of premixed flames in a narrow channel with a step-wise wall temperature

Energy Technology Data Exchange (ETDEWEB)

Kurdyumov, Vadim N. [Department of Energy, CIEMAT, Avda. Complutense 22, 28040 Madrid (Spain); Pizza, Gianmarco [Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, Zurich CH-8092 (Switzerland); Combustion Research, Paul Scherrer Institute, Villigen CH-5232 (Switzerland); Frouzakis, Christos E. [Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, Zurich CH-8092 (Switzerland); Mantzaras, John [Combustion Research, Paul Scherrer Institute, Villigen CH-5232 (Switzerland)

2009-11-15

The effect of channel height, inflow velocity and wall temperature on the dynamics and stability of unity Lewis number premixed flames in channels with specified wall temperature is investigated with steady and transient numerical simulations using a two-dimensional thermo-diffusive model. The simplified model is capable of capturing many of the transitions and the combustion modes observed experimentally and in direct numerical simulations in micro- and meso-scale channels, and indicates that the thermal flame/wall interaction is the mechanism leading to the observed flame instabilities. Finally, an ad-hoc one-dimensional model based on the flame-sheet approximation is tested in its capacity to reproduce some of the flame dynamics of the two-dimensional thermo-diffusive model. (author)

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

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

Soot modeling of counterflow diffusion flames of ethylene-based binary mixture fuels

KAUST Repository

Wang, Yu; Raj, Abhijeet Dhayal; Chung, Suk-Ho

2015-01-01

of ethylene and its binary mixtures with methane, ethane and propane based on the method of moments. The soot model has 36 soot nucleation reactions from 8 PAH molecules including pyrene and larger PAHs. Soot surface growth reactions were based on a modified

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.

Reynolds number and geometry effects in laminar axisymmetric isothermal counterflows

KAUST Repository

Scribano, Gianfranco

2016-12-29

The counterflow configuration is a canonical stagnation flow, featuring two opposed impinging round jets and a mixing layer across the stagnation plane. Although counterflows are used extensively in the study of reactive mixtures and other applications where mixing of two streams is required, quantitative data on the scaling properties of the flow field are lacking. The aim of this work is to characterize the velocity and mixing fields in isothermal counterflows over a wide range of conditions. The study features both experimental data from particle image velocimetry and results from detailed axisymmetric simulations. The scaling laws for the nondimensional velocity and mixture fraction are obtained as a function of an appropriate Reynolds number and the ratio of the separation distance of the nozzles to their diameter. In the range of flow configurations investigated, the nondimensional fields are found to depend primarily on the separation ratio and, to a lesser extent, the Reynolds number. The marked dependence of the velocity field with respect to the separation ratio is linked to a high pressure region at the stagnation point. On the other hand, Reynolds number effects highlight the role played by the wall boundary layer on the interior of the nozzles, which becomes less important as the separation ratio decreases. The normalized strain rate and scalar dissipation rate at the stagnation plane are found to attain limiting values only for high values of the Reynolds number. These asymptotic values depend markedly on the separation ratio and differ significantly from the values produced by analytical models. The scaling of the mixing field does not show a limiting behavior as the separation ratio decreases to the smallest practical value considered.

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)

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.

Ethanol flame synthesis of carbon nanotubes in deficient oxygen environments

Science.gov (United States)

Hu, Wei-Chieh; Lin, Ta-Hui

2016-04-01

In this study, carbon nanotubes (CNTs) were synthesized using ethanol diffusion flames in a stagnation-flow system composed of an upper oxidizer duct and a lower liquid pool. In the experiments, a gaseous mixture of oxygen and nitrogen flowed from the upper oxidizer duct, and then impinged onto the vertically aligned ethanol pool to generate a planar and steady diffusion flame in a deficient oxygen environment. A nascent nickel mesh was used as the catalytic metal substrate to collect deposited materials. The effect of low oxygen concentration on the formation of CNTs was explored. The oxygen concentration significantly influenced the flame environment and thus the synthesized carbon products. Lowering the oxygen concentration increased the yield, diameter, and uniformity of CNTs. The optimal operating conditions for CNT synthesis were an oxygen concentration in the range of 15%-19%, a flame temperature in the range of 460 °C-870 °C, and a sampling position of 0.5-1 mm below the upper edge of the blue flame front. It is noteworthy that the concentration gradient of C2 species and CO governed the CNT growth directly. CNTs were successfully fabricated in regions with uniform C2 species and CO distributions.

Numerical Modelling of Soot Formation in Laminar Axisymmetric Ethylene-Air Coflow Flames at Atmospheric and Elevated Pressures

KAUST Repository

Rakha, Ihsan Allah

2015-01-01

The steady coflow diffusion flame is a widely used configuration for studying combustion kinetics, flame dynamics, and pollutant formation. In the current work, a set of diluted ethylene-air coflow flames are simulated to study the formation, growth

Effect of pressure on the lean limit flames of H2-CH4-air mixture in tubes

KAUST Repository

Zhou, Zhen; Shoshin, Yuriy; Hernandez Perez, Francisco; van Oijen, Jeroen A.; de Goey, Laurentius P.H.

2017-01-01

The lean limit flames of H2-CH4-air mixtures stabilized inside tubes in a downward flow are experimentally and numerically investigated at elevated pressures ranging from 2 to 5 bar. For the shapes of lean limit flames, a change from ball-like flame to cap-like flame is experimentally observed with the increase of pressure. This experimentally observed phenomenon is qualitatively predicted by numerical simulations. The structure of ball-like and cap-like lean limit flames at all tested pressures is analysed in detail based on the numerical predictions. The results show that the lean limit flames are located inside a recirculation zone at all tested pressures. For the leading edges of the lean limit flames at all tested pressures, the fuel transport is controlled by both convection and diffusion. For the trailing edge of the ball-like lean limit flame at 2 bar, the fuel transport is dominated by diffusion. However, with increasing pressure, the transport contribution caused by convection in the trailing edges of the lean limit flames increases. Finally, the influence of transport and chemistry on the predicted ultra lean flames and lean flammability limit is analysed at elevated pressures.

Effect of pressure on the lean limit flames of H2-CH4-air mixture in tubes

KAUST Repository

Zhou, Zhen

2017-05-25

The lean limit flames of H2-CH4-air mixtures stabilized inside tubes in a downward flow are experimentally and numerically investigated at elevated pressures ranging from 2 to 5 bar. For the shapes of lean limit flames, a change from ball-like flame to cap-like flame is experimentally observed with the increase of pressure. This experimentally observed phenomenon is qualitatively predicted by numerical simulations. The structure of ball-like and cap-like lean limit flames at all tested pressures is analysed in detail based on the numerical predictions. The results show that the lean limit flames are located inside a recirculation zone at all tested pressures. For the leading edges of the lean limit flames at all tested pressures, the fuel transport is controlled by both convection and diffusion. For the trailing edge of the ball-like lean limit flame at 2 bar, the fuel transport is dominated by diffusion. However, with increasing pressure, the transport contribution caused by convection in the trailing edges of the lean limit flames increases. Finally, the influence of transport and chemistry on the predicted ultra lean flames and lean flammability limit is analysed at elevated pressures.

Aerothermodynamic properties of stretched flames in enclosures

Science.gov (United States)

Rotman, D. A.; Oppenheim, A. K.

Flames are stretched by being pulled along their frontal surface by the flow field in which they reside. Their trajectories tend to approach particle paths, acquiring eventually the role of contact boundaries, -interfaces between the burnt and unburnt medium that may broaden solely as a consequence of diffusion. Fundamental properties of flow fields governing such flames are determined here on the basis of the zero Mach number model, providng a rational method of approach to the computational analysis of combustion fields in enclosures where, besides the aerodynamic properties flow, the thermodynamic process of compression must be taken into account. To illustrate its application, the method is used to reveal the mechanism of formation of a tulip-shape flame in a rectangular enclosure under nonturbulent flow conditions.

Characteristics of non-premixed oxygen-enhanced combustion: II. Flame structure effects on soot precursor kinetics resulting in soot-free flames

Energy Technology Data Exchange (ETDEWEB)

Skeen, S.A.; Axelbaum, R.L. [Department of Energy, Environmental, Chemical Engineering, Washington University in St. Louis, St. Louis, MO (United States); Yablonsky, G. [Department of Energy, Environmental, Chemical Engineering, Washington University in St. Louis, St. Louis, MO (United States); Parks College, Saint Louis University, St. Louis, MO (United States)

2010-09-15

A detailed computational study was performed to understand the effects of the flame structure on the formation and destruction of soot precursors during ethylene combustion. Using the USC Mech Version II mechanism the contributions of different pathways to the formation of benzene and phenyl were determined in a wide domain of Z{sub st} values via a reverse-pathway analysis. It was shown that for conventional ethylene-air flames two sequential reversible reactions play primary roles in the propargyl (C{sub 3}H{sub 3}) chemistry, namely (1) C{sub 2}H{sub 2}+CH{sub 3}= pC{sub 3} H{sub 4}+H, (2) pC{sub 3} H{sub 4}= C{sub 3} H{sub 3}+ H with the corresponding overall endothermic reaction of propargyl formation (3) C{sub 2} H{sub 2}+CH{sub 3}= C{sub 3} H{sub 3}+2H. The contributions of these reactions to propyne (pC{sub 3}H{sub 4}) and propargyl formation and propargyl self-combination leading to benzene and phenyl were studied as a function of physical position, temperature, Z{sub st}, and H concentration. In particular, the role of H radicals on soot precursor destruction was studied in detail. At low Z{sub st}, Reactions 1 and 2 contribute significantly to propyne and propargyl formation on the fuel side of the radical pool at temperatures greater than approx. 1600 K. At higher local temperatures near the radical pool where the concentration of H is significant, the reverse reactions begin to dominate resulting in soot precursor destruction. As Z{sub st} is increased, these regions merge and only net propargyl consumption is observed. Based on the equilibrium constant of Reaction 3, a Z{sub st} value was estimated above which the rate of propargyl formation as a soot precursor is greatly reduced (Z{sub st} = 0.3). This condition compares well with the experimental results for permanently blue counterflow flames in the literature. (author)

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 ...

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.

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.

Characteristics of autoignited laminar lifted flames in heated coflow jets of carbon monoxide/hydrogen mixtures

KAUST Repository

Choi, Byungchul

2012-06-01

The characteristics of autoignited lifted flames in laminar jets of carbon monoxide/hydrogen fuels have been investigated experimentally in heated coflow air. In result, as the jet velocity increased, the blowoff was directly occurred from the nozzle-attached flame without experiencing a stabilized lifted flame, in the non-autoignited regime. In the autoignited regime, the autoignited lifted flame of carbon monoxide diluted by nitrogen was affected by the water vapor content in the compressed air oxidizer, as evidenced by the variation of the ignition delay time estimated by numerical calculation. In particular, in the autoignition regime at low temperatures with added hydrogen, the liftoff height of the autoignited lifted flames decreased and then increased as the jet velocity increased. Based on the mechanism in which the autoignited laminar lifted flame is stabilized by ignition delay time, the liftoff height can be influenced not only by the heat loss, but also by the preferential diffusion between momentum and mass diffusion in fuel jets during the autoignition process. © 2012 The Korean Society of Mechanical Engineers.

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.

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

mainly between the fuel nozzle and the lifted flame edge. On the other hand, they were formed just prior to the flame edge for the non-autoignited lifted flames. The effect of fuel pyrolysis and partial oxidation were found to be important in explaining autoignited liftoff heights, especially in the Mild combustion regime. Flame structures of autoignited flames were investigated numerically for syngas (CO/H2) and methane fuels. The simulations of syngas fuel accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Autoignition characteristics of laminar nonpremixed methane jet flames in high- 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.

Effect of Lewis number on ball-like lean limit flames

KAUST Repository

Zhou, Zhen

2017-10-13

The lean limit flames for three different fuel compositions premixed with air, representing three different mixture Lewis numbers, stabilized inside a tube in a downward flow are examined by experiments and numerical simulations. The CH* chemiluminescence distribution in CH4–air and CH4–H2–air flames and the OH* chemiluminescence distribution in H2–air flames are recorded in the experiments. Cell-like flames are observed for the CH4–air mixture for all tested equivalence ratios. However, for CH4–H2–air and H2–air flames, ball-like lean limit flames are observed. Flame temperature fields are measured using Rayleigh scattering. The experimentally observed lean limit flames are predicted qualitatively by numerical simulation with the mixture-averaged transport model and skeletal mechanism of CH4. The results of the simulations show that the entire lean limit flames of CH4–H2–air and H2–air mixtures are located inside a recirculation zone. However, for the lean limit CH4–air flame, only the leading edge is located inside the recirculation zone. A flame structure with negative flame displacement speed is observed for the leading edges of the predicted lean limit flames with all three different fuel compositions. As compared with 1D planar flames, the fuel transport caused by convection is less significant in the present 2D lean limit flames for the three different fuel compositions. For the trailing edges of the three predicted lean limit flames, a diffusion dominated flame structure is observed.

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.)

Response to acoustic forcing of laminar coflow jet diffusion flames

KAUST Repository

Chrystie, Robin; Chung, Suk-Ho

2014-01-01

Toward the goal of understanding and controlling instability in combustion systems, we present a fundamental characterization of the interaction of the buoyancy-induced instability in flickering flames with forced excitation of fuel supply. Laminar

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

KAUST Repository

Selvaraj, Prabhu; Im, Hong G.

2017-01-01

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

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.

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.

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.

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.

Numerical investigation of drag and heat flux reduction mechanism of the pulsed counterflowing jet on a blunt body in supersonic flows

Science.gov (United States)

Zhang, Rui-rui; Huang, Wei; Yan, Li; Li, Lang-quan; Li, Shi-bin; Moradi, R.

2018-05-01

To design a kind of aerospace vehicle, the drag and heat flux reduction are the most important factors. In the current study, the counterflowing jet, one of the effective drag and heat flux reduction concepts, is investigated numerically by the two-dimensional axisymmetric Reynolds-averaged Navier-Stokes equations coupled with the SST k-ω turbulence model. An axisymmetric numerical simulation mode of the counterflowing jet on the supersonic vehicle nose-tip is established, and the numerical method employed is validated by the experimental schlieren images and experimental data in the open literature. A pulsed counterflowing jet scheme is proposed, and it uses a sinusoidal function to control the total and static pressures of the counterflowing jet. The obtained results show that the long penetration mode does not exist in the whole turnaround, even in a relatively small range of the jet total and static pressures, and this is different from the phenomenon obtained under the steady condition in the open literature. At the same time, it is observed that the variation of the physical parameters, such as the Stanton number induced by the pulsed jet, has an obvious periodicity and hysteresis phenomenon.

Analysis of cloud condensation nuclei composition and growth kinetics using a pumped counterflow virtual impactor and aerosol mass spectrometer

Directory of Open Access Journals (Sweden)

J. G. Slowik

2011-08-01

Full Text Available We present a new method of determining the size and composition of CCN-active aerosol particles. Method utility is illustrated through a series of ambient measurements. A continuous-flow thermal-gradient diffusion chamber (TGDC, pumped counterflow virtual impactor (PCVI, and Aerodyne time-of-flight mass spectrometer (AMS are operated in series. Ambient particles are sampled into the TGDC, where a constant supersaturation is maintained, and CCN-active particles grow to ~2.5 ± 0.5 μm. The output flow from the TGDC is directed into the PCVI, where a counterflow of dry N₂ gas opposes the particle-laden flow, creating a region of zero axial velocity. This stagnation plane can only be traversed by particles with sufficient momentum, which depends on their size. Particles that have activated in the TGDC cross the stagnation plane and are entrained in the PCVI output flow, while the unactivated particles are diverted to a pump. Because the input gas is replaced by the counterflow gas with better than 99 % efficiency at the stagnation plane, the output flow consists almost entirely of dry N₂ and water evaporates from the activated particles. In this way, the system yields an ensemble of CCN-active particles whose chemical composition and size are analyzed using the AMS. Measurements of urban aerosol in downtown Toronto identified an external mixture of CCN-active particles consisting almost entirely of ammonium nitrate and ammonium sulfate, with CCN-inactive particles of the same size consisting of a mixture of ammonium nitrate, ammonium sulfate, and organics. We also discuss results from the first field deployment of the TGDC-PCVI-AMS system, conducted from mid-May to mid-June 2007 in Egbert, Ontario, a semirural site ~80 km north of Toronto influenced both by clean air masses from the north and emissions from the city. Organic-dominated particles sampled during a major biogenic event exhibited higher CCN activity and/or faster

A simple one-step chemistry model for partially premixed hydrocarbon combustion

Energy Technology Data Exchange (ETDEWEB)

Fernandez-Tarrazo, Eduardo [Instituto Nacional de Tecnica Aeroespacial, Madrid (Spain); Sanchez, Antonio L. [Area de Mecanica de Fluidos, Universidad Carlos III de Madrid, Leganes 28911 (Spain); Linan, Amable [ETSI Aeronauticos, Pl. Cardenal Cisneros 3, Madrid 28040 (Spain); Williams, Forman A. [Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, CA 92093-0411 (United States)

2006-10-15

This work explores the applicability of one-step irreversible Arrhenius kinetics with unity reaction order to the numerical description of partially premixed hydrocarbon combustion. Computations of planar premixed flames are used in the selection of the three model parameters: the heat of reaction q, the activation temperature T{sub a}, and the preexponential factor B. It is seen that changes in q with equivalence ratio f need to be introduced in fuel-rich combustion to describe the effect of partial fuel oxidation on the amount of heat released, leading to a universal linear variation q(f) for f>1 for all hydrocarbons. The model also employs a variable activation temperature T{sub a}(f) to mimic changes in the underlying chemistry in rich and very lean flames. The resulting chemistry description is able to reproduce propagation velocities of diluted and undiluted flames accurately over the whole flammability limit. Furthermore, computations of methane-air counterflow diffusion flames are used to test the proposed chemistry under nonpremixed conditions. The model not only predicts the critical strain rate at extinction accurately but also gives near-extinction flames with oxygen leakage, thereby overcoming known predictive limitations of one-step Arrhenius kinetics. (author)

Mixed phase Pt-Ru catalyst for direct methanol fuel cell anode by flame aerosol synthesis

DEFF Research Database (Denmark)

Chakraborty, Debasish; Bischoff, H.; Chorkendorff, Ib

2005-01-01

A spray-flame aerosol catalyzation technique was studied for producing Pt-Ru anode electrodes for the direct methanol fuel cell. Catalysts were produced as aerosol nanoparticles in a spray-flame reactor and deposited directly as a thin layer on the gas diffusion layer. The as-prepared catalyst wa......Ru1/Vulcan carbon. The kinetics of methanol oxidation on the mixed phase catalyst was also explored by electrochemical impedance spectroscopy. (c) 2005 The Electrochemical Society.......A spray-flame aerosol catalyzation technique was studied for producing Pt-Ru anode electrodes for the direct methanol fuel cell. Catalysts were produced as aerosol nanoparticles in a spray-flame reactor and deposited directly as a thin layer on the gas diffusion layer. The as-prepared catalyst...... was found to be a mixture of nanocrystalline, mostly unalloyed Pt and an amorphous phase mostly of Ru and to a lesser extent of Pt oxides on top of the crystalline phase. The flame-produced Pt1Ru1 demonstrated similar onset potential but similar to 60% higher activity compared to commercially available Pt1...

Hydrogen-enriched non-premixed jet flames : analysis of the flame surface, flame normal, flame index and Wobbe index

NARCIS (Netherlands)

Ranga Dinesh, K.K.J.; Jiang, X.; Oijen, van J.A.

2014-01-01

A non-premixed impinging jet flame is studied using three-dimensional direct numerical simulation with detailed chemical kinetics in order to investigate the influence of fuel variability on flame surface, flame normal, flame index and Wobbe index for hydrogen-enriched combustion. Analyses indicate

Flame synthesis of carbon nano onions using liquefied petroleum gas without catalyst

International Nuclear Information System (INIS)

Dhand, Vivek; Prasad, J. Sarada; Rao, M. Venkateswara; Bharadwaj, S.; Anjaneyulu, Y.; Jain, Pawan Kumar

2013-01-01

Densely agglomerated, high specific surface area carbon nano onions with diameter of 30–40 nm have been synthesized. Liquefied petroleum gas and air mixtures produced carbon nano onions in diffusion flames without catalyst. The optimized oxidant to fuel ratio which produces carbon nano onions has been found to be 0.1 slpm/slpm. The experiment yielded 70% pure carbon nano onions with a rate of 5 g/h. X-ray diffraction, high-resolution electron microscopy and Raman spectrum reveal the densely packed sp 2 hybridized carbon with (002) semi-crystalline hexagonal graphite reflection. The carbon nano onions are thermally stable up to 600 °C. - Highlights: ►Flame synthesized carbon nano onions with 30–40 nm diameters. ►LPG/air, diffusion type flame used in absence of catalyst to produce nano onions. ►Carbon nano onion production rate is 5 g/hr and with 70% purity.

A two-dimensional analytical model of laminar flame in lycopodium dust particles

Energy Technology Data Exchange (ETDEWEB)

Rahbari, Alireza [Shahid Rajaee Teacher Training University, Tehran (Iran, Islamic Republic of); Shakibi, Ashkan [Iran University of Science and Technology, Tehran (Iran, Islamic Republic of); Bidabadi, Mehdi [Combustion Research Laboratory, Narmak, Tehran (Iran, Islamic Republic of)

2015-09-15

A two-dimensional analytical model is presented to determine the flame speed and temperature distribution of micro-sized lycopodium dust particles. This model is based on the assumptions that the particle burning rate in the flame front is controlled by the process of oxygen diffusion and the flame structure consists of preheat, reaction and post flame zones. In the first step, the energy conservation equations for fuel-lean condition are expressed in two dimensions, and then these differential equations are solved using the required boundary condition and matching the temperature and heat flux at the interfacial boundaries. Consequently, the obtained flame temperature and flame speed distributions in terms of different particle diameters and equivalence ratio for lean mixture are compared with the corresponding experimental data for lycopodium dust particles. Consequently, it is shown that this two-dimensional model demonstrates better agreement with the experimental results compared to the previous models.

A two-dimensional analytical model of laminar flame in lycopodium dust particles

International Nuclear Information System (INIS)

Rahbari, Alireza; Shakibi, Ashkan; Bidabadi, Mehdi

2015-01-01

A two-dimensional analytical model is presented to determine the flame speed and temperature distribution of micro-sized lycopodium dust particles. This model is based on the assumptions that the particle burning rate in the flame front is controlled by the process of oxygen diffusion and the flame structure consists of preheat, reaction and post flame zones. In the first step, the energy conservation equations for fuel-lean condition are expressed in two dimensions, and then these differential equations are solved using the required boundary condition and matching the temperature and heat flux at the interfacial boundaries. Consequently, the obtained flame temperature and flame speed distributions in terms of different particle diameters and equivalence ratio for lean mixture are compared with the corresponding experimental data for lycopodium dust particles. Consequently, it is shown that this two-dimensional model demonstrates better agreement with the experimental results compared to the previous models.

Instabilities and soot formation in spherically expanding, high pressure, rich, iso-octane-air flames

International Nuclear Information System (INIS)

Lockett, R D

2006-01-01

Flame instabilities, cellular structures and soot formed in high pressure, rich, spherically expanding iso-octane-air flames have been studied experimentally using high speed Schlieren cinematography, OH fluorescence, Mie scattering and laser induced incandescence. Cellular structures with two wavelength ranges developed on the flame surface. The larger wavelength cellular structure was produced by the Landau-Darrieus hydrodynamic instability, while the short wavelength cellular structure was produced by the thermal-diffusive instability. Large negative curvature in the short wavelength cusps caused local flame quenching and fracture of the flame surface. In rich flames with equivalence ratio φ > 1.8, soot was formed in a honeycomb-like structure behind flame cracks associated with the large wavelength cellular structure induced by the hydrodynamic instability. The formation of soot precursors through low temperature pyrolysis was suggested as a suitable mechanism for the initiation of soot formation behind the large wavelength flame cracks

Instabilities and soot formation in spherically expanding, high pressure, rich, iso-octane-air flames

Energy Technology Data Exchange (ETDEWEB)

Lockett, R D [School of Engineering and Mathematical Sciences, City University, Northampton Square, London EC1V OHB (United Kingdom)

2006-07-15

Flame instabilities, cellular structures and soot formed in high pressure, rich, spherically expanding iso-octane-air flames have been studied experimentally using high speed Schlieren cinematography, OH fluorescence, Mie scattering and laser induced incandescence. Cellular structures with two wavelength ranges developed on the flame surface. The larger wavelength cellular structure was produced by the Landau-Darrieus hydrodynamic instability, while the short wavelength cellular structure was produced by the thermal-diffusive instability. Large negative curvature in the short wavelength cusps caused local flame quenching and fracture of the flame surface. In rich flames with equivalence ratio {phi} > 1.8, soot was formed in a honeycomb-like structure behind flame cracks associated with the large wavelength cellular structure induced by the hydrodynamic instability. The formation of soot precursors through low temperature pyrolysis was suggested as a suitable mechanism for the initiation of soot formation behind the large wavelength flame cracks.

Investigations of Sooting Laminar Coflow Diffusion Flames at Elevated Pressures

KAUST Repository

Steinmetz, Scott

2016-01-01

diameters are successfully measured in nitrogen-diluted ethylene-air laminar coflow flames at pressures of 4, 8, 12, and 16 atm. An increase in particle size with pressure is found up to 12 atm, where particle sizes plateau. Particle size in the annulus

Design and construction of gas-fed burners for laboratory studies of flame structure

Science.gov (United States)

Dan Jimenez; Mark A. Finney; Jack Cohen

2010-01-01

The study of buoyant convection for diffusion flames in wildland fires is critical to understanding heating and cooling dynamics related to particle ignition. Studies based on solid biomass fuels are made difficult by short flame residence time associated with fine fuels. An alternative is to use artificial fuel gas rather than relying on pyrolysis of solid fuels to...

A computational study of ethylene–air sooting flames: Effects of large polycyclic aromatic hydrocarbons

KAUST Repository

Selvaraj, Prabhu

2015-11-05

An updated reduced gas-phase kinetic mechanism was developed and integrated with aerosol models to predict soot formation characteristics in ethylene nonpremixed and premixed flames. A primary objective is to investigate the sensitivity of the soot formation to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). The gas-phase chemical mechanism adopted the KAUST-Aramco PAH Mech 1.0, which utilized the AramcoMech 1.3 for gas-phase reactions validated for up to C2 fuels. In addition, PAH species up to coronene (C24H12 or A7) were included to describe the detailed formation pathways of soot precursors. In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph with expert knowledge (DRG-X) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames at low strain rate sooting conditions were considered, for which the sensitivity of soot formation characteristics to different nucleation pathways were investigated. Premixed flame experiment data at different equivalence ratios were also used for validation. The findings show that higher PAH concentrations result in a higher soot nucleation rate, and that the total soot volume and average size of the particles are predicted in good agreement with experimental results. Subsequently, the effects of different pathways, with respect to pyrene- or coronene-based nucleation models, on the net soot formation rate were analyzed. It was found that the nucleation processes (i.e., soot inception) are sensitive to the choice of PAH precursors, and consideration of higher PAH species beyond pyrene is critical for accurate prediction of the overall soot formation.

Autoignited lifted flames of dimethyl ether in heated coflow air

KAUST Repository

Al-Noman, Saeed M.

2018-05-16

Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone.For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Linear response of stretch-affected premixed flames to flow oscillations

Energy Technology Data Exchange (ETDEWEB)

Wang, H.Y.; Law, C.K. [Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544 (United States); Lieuwen, T. [School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332 (United States)

2009-04-15

The linear response of 2D wedge-shaped premixed flames to harmonic velocity disturbances was studied, allowing for the influence of flame stretch manifested as variations in the local flame speed along the wrinkled flame front. Results obtained from analyzing the G-equation show that the flame response is mainly characterized by a Markstein number {sigma}{sub C}, which measures the curvature effect of the wrinkles, and a Strouhal number, St{sub f}, defined as the angular frequency of the disturbance normalized by the time taken for the disturbance to propagate the flame length. Flame stretch is found to become important when the disturbance frequency satisfies {sigma}{sub C}St{sub f}{sup 2}{proportional_to} O(1), i.e. St{sub f}{proportional_to} O({sigma}{sub C}{sup -1/2}). Specifically, for disturbance frequencies below this order, stretch effects are small and the flame responds as an unstretched one. When the disturbance frequencies are of this order, the transfer function, defined as the ratio of the normalized fluctuation of the heat release rate to that of the velocity, is contributed mostly from fluctuations of the flame surface area, which is now affected by stretch. Finally, as the disturbance frequency increases to St{sub f}{proportional_to} O({sigma}{sub C}{sup -1}), i.e. {sigma}{sub C}St{sub f}{proportional_to} O(1), the direct contribution from the stretch-affected flame speed fluctuation to the transfer function becomes comparable to that of the flame surface area. The present study phenomenologically explains the experimentally observed filtering effect in which the flame wrinkles developed at the flame base decay along the flame surface for large frequency disturbances as well as for thermal-diffusively stable and weakly unstable mixtures. (author)

Autoignited lifted flames of dimethyl ether in heated coflow air

KAUST Repository

Al-Noman, Saeed M.; Choi, Byung Chul; Chung, Suk-Ho

2018-01-01

Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone.For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Flame synthesis of carbon nano onions using liquefied petroleum gas without catalyst

Energy Technology Data Exchange (ETDEWEB)

Dhand, Vivek, E-mail: vivekdhand2012@gmail.com [Centre for Knowledge Management of Nanoscience and Technology, 12-5-32/8, Vijayapuri Colony, Tarnaka, Secunderabad-500 017, A.P (India); Prasad, J. Sarada; Rao, M. Venkateswara [Centre for Environment, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad 500 085 (India); Bharadwaj, S. [Department of Physics, CVR College of Engineering and Osmania University, Hyderabad 501510, A.P (India); Anjaneyulu, Y. [TLGVRC, Jackson State University, JSU Box 18739, Jackson, MS 39217-0939 (United States); Jain, Pawan Kumar [International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur PO, Hyderabad 500005, Andhra Pradesh (India)

2013-03-01

Densely agglomerated, high specific surface area carbon nano onions with diameter of 30-40 nm have been synthesized. Liquefied petroleum gas and air mixtures produced carbon nano onions in diffusion flames without catalyst. The optimized oxidant to fuel ratio which produces carbon nano onions has been found to be 0.1 slpm/slpm. The experiment yielded 70% pure carbon nano onions with a rate of 5 g/h. X-ray diffraction, high-resolution electron microscopy and Raman spectrum reveal the densely packed sp{sup 2} hybridized carbon with (002) semi-crystalline hexagonal graphite reflection. The carbon nano onions are thermally stable up to 600 Degree-Sign C. - Highlights: Black-Right-Pointing-Pointer Flame synthesized carbon nano onions with 30-40 nm diameters. Black-Right-Pointing-Pointer LPG/air, diffusion type flame used in absence of catalyst to produce nano onions. Black-Right-Pointing-Pointer Carbon nano onion production rate is 5 g/hr and with 70% purity.

Acoustic excitation of diffusion flames with coherent structure in a plane shear layer.; Effects of acoustic excitation on combustion properties; Soshiki kozo wo tomonau sendan kakusan kaen no onkyo reiki.; Onkyo reiki ni yoru nensho tokusei no henka

Energy Technology Data Exchange (ETDEWEB)

Ishino, Y.; Kojima, T.; Oiwa, N.; Yamaguchi, S. [Nagoya Institute of Technology, Nagoya (Japan)

1993-10-25

This paper reports on experiments for acoustic excitation of plane shear structured flame. Flows of air separated into the higher velocity side and the lower velocity side by a partition on the center of a flow path merge at the measuring point to form a mixed layer with coherent structure. Fuel is supplied to this mixed layer with the flows so adjusted that the generated flame will attach to the partition on the lower velocity side. Acoustic excitation (at a sound pressure level of 100 dB to 120 dB) is performed in a speaker fitted on a wall on the higher velocity side. The paper mentions the results of the experiments as follows: the acoustic excitation produces such changes to diffusion flame in the plane shear layer as shorter flame and blue flame combustion and clarification of flame structures; as seen from spectral characteristics of temperature change in the flames, a flame acoustically excited strongly presents remarkable improvements in periodicity of the structure; as seen from sound pressure distribution in the flow direction at the measuring point, the flame zone of the flame acoustically excited strongly is positioned at the middle of the node and loop of a standing wave. 6 refs., 9 figs., 1 tab.

Linear analysis of the Richtmyer-Meshkov instability in shock-flame interactions

Science.gov (United States)

Massa, L.; Jha, P.

2012-05-01

Shock-flame interactions enhance supersonic mixing and detonation formation. Therefore, their analysis is important to explosion safety, internal combustion engine performance, and supersonic combustor design. The fundamental process at the basis of the interaction is the Richtmyer-Meshkov instability supported by the density difference between burnt and fresh mixtures. In the present study we analyze the effect of reactivity on the Richtmyer-Meshkov instability with particular emphasis on combustion lengths that typify the scaling between perturbation growth and induction. The results of the present linear analysis study show that reactivity changes the perturbation growth rate by developing a pressure gradient at the flame surface. The baroclinic torque based on the density gradient across the flame acts to slow down the instability growth of high wave-number perturbations. A gasdynamic flame representation leads to the definition of a Peclet number representing the scaling between perturbation and thermal diffusion lengths within the flame. Peclet number effects on perturbation growth are observed to be marginal. The gasdynamic model also considers a finite flame Mach number that supports a separation between flame and contact discontinuity. Such a separation destabilizes the interface growth by augmenting the tangential shear.

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.

Gravitational Influences on Flame Propagation through Non-Uniform, Premixed Gas Systems

Science.gov (United States)

Miller, Fletcher J.; Easton, John; Ross, Howard D.; Marchese, Anthony; Perry, David; Kulis, Michael

2001-01-01

Flame propagation through non-uniformly premixed (or layered) gases has importance both in useful combustion systems and in unintentional fires. As summarized previously, non-uniform premixed gas combustion receives scant attention compared to the more usual limiting cases of diffusion or uniformly premixed flames, especially regarding the role gravity plays. This paper summarizes our progress on furthering the knowledge of layered combustion, in which a fuel concentration gradient exists normal to the direction of flame spread. We present experimental and numerical results for flame spread through propanol-air layers formed near the flash point temperature (25 C) or near the stoichiometric temperature (33 C). Both the model and experimental results show that the removal of gravity results in a faster spreading flame, by as much as 80% depending on conditions. This is exactly the opposite effect as that predicted by an earlier model reported. We also found that having a gallery lid results in faster flame spread, an effect more pronounced at normal gravity, demonstrating the importance of enclosure geometry. Also reported here is the beginning of our spectroscopic measurements of fuel vapor.

On the Flame Height Definition for Upward Flame Spread

OpenAIRE

Consalvi, Jean L; Pizzo, Yannick; Porterie, Bernard; Torero, Jose L

2007-01-01

Flame height is defined by the experimentalists as the average position of the luminous flame and, consequently is not directly linked with a quantitative value of a physical parameter. To determine flame heights from both numerical and theoretical results, a more quantifiable criterion is needed to define flame heights and must be in agreement with the experiments to allow comparisons. For wall flames, steady wall flame experiments revealed that flame height may be define...

Stability of diffusion flame formed in a laminar flat plate boundary layer. Effect of fuel dilution; Soryu heiban kyokai sonai ni keiseisareru kakusan kaen no anteisei. Nenryo kishaku no eikyo

Energy Technology Data Exchange (ETDEWEB)

Takeuchi, M [National Institute for Resources and Environment, Tsukuba (Japan); Ueda, T; Mizumoto, M [Keio University, Tokyo (Japan). Faculty of Science and Technology; Amari, T [Keio University, Tokyo (Japan)

1998-10-25

A stability limit of the diffusion flame with fuel injection from a porous wall in a laminar flat plate boundary layer is measured as functions of fuel (CH4) concentration of CH4/N2 injectant mixture ({chi}) and its injection velocity (v). The free stream velocity (U{infinity}) is set as 0.6 m/s. The thermal condition at the wall is controlled by setting temperature at the upstream end of the porous wall as a reference temperature. When v >20 mm/s, the flame becomes unstable with the separation of leading flame edge with decreasing {chi}. The value of {chi} at the stability limit is constant without regard to v as long as the wall temperature is kept constant. As the wall temperature is decreased the value of {chi} increases. The separation is supposed to take place as a result of the limit of the reaction rate. When v <20 mm/s, the flame becomes unstable with the oscillation. The value of {chi} at the stability limit increases drastically with decreasing v. The oscillation takes place mainly due to the repeat of the extinction due to heat loss to the wall and the flame propagation in the combustible layer. 10 refs., 8 figs., 1 tab.

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

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

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

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

Study on dew point evaporative cooling system with counter-flow configuration

KAUST Repository

Lin, J.

2015-12-18

Dew point evaporative cooling has great potential as a disruptive process for sensible cooling of air below its entering wet bulb temperature. This paper presents an improved mathematical model for a single-stage dew point evaporative cooler in a counter-flow configuration. Longitudinal heat conduction and mass diffusion of the air streams, channel plate and water film, as well as the temperature difference between the plate and water film, are accounted for in the model. Predictions of the product air temperature are validated using three sets of experimental data within a discrepancy of 4%. The cooler’s heat and mass transfer process is analyzed in terms of its cooling capacity intensity, water evaporation intensity, and overall heat transfer coefficient along the channel. Parametric studies are conducted at different geometric and operating conditions. For the conditions evaluated, the study reveals that (1) the saturation point of the working air occurs at a fixed point regardless of the inlet air conditions, and it is mainly influenced by the working air ratio and channel height; (2) the intensity of the water evaporation approaches a minimum at 0.2 to 0.3m from the entrance; (3) the wet channel can be separated into two zones, and the overall heat transfer coefficient is above 100W/(m2·K) after the temperature of water film becomes higher than the working air temperature.

The CO/NOx emissions of swirled, strongly pulsed jet diffusion flames

KAUST Repository

Liao, Ying-Hao; Hermanson, James C.

2014-01-01

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

Methane Formation by Flame-Generated Hydrogen Atoms in the Flame Ionization Detector

DEFF Research Database (Denmark)

Holm, Torkil; Madsen, Jørgen Øgaard

1996-01-01

, and conceivably all hydrocarbons are quantitatively converted into methane at temperatures below 600 C, that is, before the proper combustion has started. The splitting of the C-C bonds is preceded by hydrogenation of double and triple bonds and aromatic rings. The reactions, no doubt, are caused by hydrogen...... atoms, which are formed in the burning hydrogen and which diffuse into the inner core of the flame. The quantitative formation of methane appears to explain the "equal per carbon" rule for the detector response of hydrocarbons, since all carbons are "exchanged" for methane molecules....

Effects of AC Electric Field on Small Laminar Nonpremixed Flames

KAUST Repository

Xiong, Yuan

2015-04-01

Electric field can be a viable method in controlling various combustion properties. Comparing to traditional actuators, an application of electric field requires very small power consumption. Especially, alternating current (AC) has received attention recently, since it could modulate flames appreciably even for the cases when direct current (DC) has minimal effects. In this study, the effect of AC electric fields on small coflow diffusion flames is focused with applications of various laser diagnostic techniques. Flow characteristics of baseline diffusion flames, which corresponds to stationary small coflow diffusion flames when electric field is not applied, were firstly investigated with a particular focus on the flow field in near-nozzle region with the buoyancy force exerted on fuels due to density differences among fuel, ambient air, and burnt gas. The result showed that the buoyancy force exerted on the fuel as well as on burnt gas significantly distorted the near-nozzle flow-fields. In the fuels with densities heavier than air, recirculation zones were formed very close to the nozzle exit. Nozzle heating effect influenced this near-nozzle flow-field particularly among lighter fuels. Numerical simulations were also conducted and the results showed that a fuel inlet boundary condition with a fully developed velocity profile for cases with long fuel tubes should be specified inside the fuel tube to obtain satisfactory agreement in both the flow and temperature fields with those from experiment. With sub-critical AC applied to the baseline flames, particle image velocimetry (PIV), light scattering, laser-induced incandescence (LII), and laser-induced fluores- cence (LIF) techniques were adopted to identify the flow field and the structures of OH, polycyclic aromatic hydrocarbons (PAHs), soot zone. Under certain AC condi- tions of applied voltage and frequency, the distribution of PAHs and the flow field near the nozzle exit were drastically altered from the

Investigation into periodic process of hydrogen isotope separation by counterflow method in the hydrogen-palladium system

International Nuclear Information System (INIS)

Andreev, B.M.; Selivanenko, I.L.; Vedeneev, A.I.; Golubkov, A.N.; Tenyaev, B.N.

1999-01-01

The key diagram and results of the investigation into working conditions of the pilot plant for hydrogen isotope separation embodying the concept of continuous counterflow separation in the hydrogen-palladium system are shown. The counterflow of phases in the plant is attained under the motion of palladium solid hydride phase relative to stationary blocks of flow rotation. The column separator is defined as section type one. The plant performs in periodic regime with accumulating vessels for light and heavy components of the separated mixture. Maximum concentration of the separated tritium ranged up to ∼ 96 % in the experiments of the deuterium-tritium separation. Minimum concentration of the residual tritium in the mixture ranged up to ∼ 0.1 %. The plant provides to reprocessing 4.5 moles of the gas a day [ru

An improved model for the analysis of evaporative counterflow cooling towers

International Nuclear Information System (INIS)

Nahavandi, A.N.; Oellinger, J.

1977-01-01

A rigorous approach is applied to the thermal design of counterflow cooling towers, by obviating the six simplifying assumptions in the classical Merkel method. It is indicated that: (1) neglecting evaporation losses is the main cause of inaccuracy in the Merkel results; (2) the error in the Merkel method may reach 12%; and (3) the present solution provides a more accurate and more ecologically favorable prediction for the cooling water tower. (Auth.)

Simulations of Flame Acceleration and DDT in Mixture Composition Gradients

Science.gov (United States)

Zheng, Weilin; Kaplan, Carolyn; Houim, Ryan; Oran, Elaine

2017-11-01

Unsteady, multidimensional, fully compressible numerical simulations of methane-air in an obstructed channel with spatial gradients in equivalence ratios have been carried to determine the effects of the gradients on flame acceleration and transition to detonation. Results for gradients perpendicular to the propagation direction were considered here. A calibrated, optimized chemical-diffusive model that reproduces correct flame and detonation properties for methane-air over a range of equivalence ratios was derived from a combination of a genetic algorithm with a Nelder-Mead optimization scheme. Inhomogeneous mixtures of methane-air resulted in slower flame acceleration and longer distance to DDT. Detonations were more likely to decouple into a flame and a shock under sharper concentration gradients. Detailed analyses of temperature and equivalence ratio illustrated that vertical gradients can greatly affect the formation of hot spots that initiate detonation by changing the strength of leading shock wave and local equivalence ratio near the base of obstacles. This work is supported by the Alpha Foundation (Grant No. AFC215-20).

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.

Studies of propane flame soot acting as heterogeneous ice nuclei in conjunction with single particle soot photometer measurements

Directory of Open Access Journals (Sweden)

I. Crawford

2011-09-01

Full Text Available The ice nucleation efficiency of propane flame soot particles with and without a sulphuric acid coating was investigated using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere. The test soot for cloud formation simulations was produced using a propane flame Combustion Aerosol Standard generator (CAST, Jing-CAST Technologies. The organic carbon content (OC of the test soot was altered in a reproducible fashion by changing the fuel/air mixture of the generator. The soot content of ice nuclei was subsequently investigated using a combination of a pumped counterflow virtual impactor (PCVI to separate and evaporate the ice crystals, and a DMT single particle soot photometer (SP2 to examine the mixing state of the BC containing ice residuals.

Ice nucleation was found to be most efficient for uncoated soot of low organic carbon content (~5 % organic carbon content where deposition freezing occurred at an ice saturation ratio S_ice ~ 1.22 at a temperature T = 226.6 K with 25 % of the test soot becoming active as ice nuclei. Propane flame soot of higher organic carbon content (~30 % and ~70 % organic carbon content showed significantly lower ice nucleation efficiency (an activated fraction of the order of a few percent in the experiments than the low organic carbon content soot, with water saturation being required for freezing to occur. Ice nucleation occurred over the range S_ice = 1.22–1.70, and T = 223.2–226.6 K. Analysis of the SP2 data showed that the 5 % organic carbon content soot had an undetectable OC coating whereas the 30 % organic carbon content soot had a thicker or less volatile OC coating.

The application of a sulphuric acid coating to the flame soot shifted the threshold of the onset of freezing towards that of the homogeneous freezing of sulphuric acid; for the minimum OC flame soot this inhibited nucleation since the

Numerical modelling of ion transport in flames

KAUST Repository

Han, Jie

2015-10-20

This paper presents a modelling framework to compute the diffusivity and mobility of ions in flames. The (n, 6, 4) interaction potential is adopted to model collisions between neutral and charged species. All required parameters in the potential are related to the polarizability of the species pair via semi-empirical formulas, which are derived using the most recently published data or best estimates. The resulting framework permits computation of the transport coefficients of any ion found in a hydrocarbon flame. The accuracy of the proposed method is evaluated by comparing its predictions with experimental data on the mobility of selected ions in single-component neutral gases. Based on this analysis, the value of a model constant available in the literature is modified in order to improve the model\\'s predictions. The newly determined ion transport coefficients are used as part of a previously developed numerical approach to compute the distribution of charged species in a freely propagating premixed lean CH4/O2 flame. Since a significant scatter of polarizability data exists in the literature, the effects of changes in polarizability on ion transport properties and the spatial distribution of ions in flames are explored. Our analysis shows that changes in polarizability propagate with decreasing effect from binary transport coefficients to species number densities. We conclude that the chosen polarizability value has a limited effect on the ion distribution in freely propagating flames. We expect that the modelling framework proposed here will benefit future efforts in modelling the effect of external voltages on flames. Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/13647830.2015.1090018. © 2015 Taylor & Francis.

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.

A computational study of radiation and gravity effect on temperature and soot formation in a methane air co-flow diffusion flame

Energy Technology Data Exchange (ETDEWEB)

Bhowal, Arup Jyoti, E-mail: arupjyoti.bhowal@heritageit.edu [Department of Mechanical Engineering, Heritage Institute of Technology, Chowbaga Road, Anandapur, Kolkata-700 107, West Bengal (India); Mandal, Bijan Kumar, E-mail: bkm375@yahoo.co.in [Department of Mechanical Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah – 711103, West Bengal (India)

2016-07-12

An effort has been made for a quantitative assessment of the soot formed under steady state in a methane air co flow diffusion flame by a numerical simulation at normal gravity and at lower gravity levels of 0.5 G, 0.1 G and 0.0001 G (microgravity). The peak temperature at microgravity is reduced by about 50 K than that at normal gravity level. There is an augmentation of soot formation at lower gravity levels. Peak value at microgravity multiplies by a factor of ∼7 of that at normal gravity. However, if radiation is not considered, soot formation is found to be much more.

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)

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)

Effect of burner geometry on swirl stabilized methane/air flames: A joint LES/OH-PLIF/PIV study

KAUST Repository

Liu, X.

2017-07-04

Large eddy simulation (LES) using a transported PDF model and OH-PLIF/PIV experiments were carried out to investigate the quarl effects on the structures of swirl stabilized methane/air flames. Two different quarls were investigated, one straight cylindrical quarl and one diverging conical quarl. The experiments show that the flames are significantly different with the two quarls. With the straight cylindrical quarl a compact blue flame is observed while with the diverging conical quarl the flame appears to be long and yellow indicating a sooty flame structure. The PIV results show the formation of a stronger flow recirculation inside the diverging conical quarl than that in the straight quarl. LES results reveal further details of the flow and mixing process inside the quarl. The results show that with the diverging quarl vortex breakdown occurs much earlier towards the upstream of the quarl. As a result the fuel is convected into the air flow tube and a diffusion flame is stabilized inside the air flow tube upstream the quarl. With the straight quarl, vortex breakdown occurs at a downstream location in the quarl. The scalar dissipation rate in the shear layer of the fuel jet is high, which prevents the stabilization of a diffusion flame in the proximity of the fuel nozzle; instead, a compact partially premixed flame with two distinct heat release layers is stablized in a downstream region in the quarl, which allows for the fuel and air to mix in the quarl before combustion and a lower formation rate of soot. The results showed that the Eulerian Stochastic Fields transported PDF method can well predict the details of the swirl flame dynamics.

Effect of burner geometry on swirl stabilized methane/air flames: A joint LES/OH-PLIF/PIV study

KAUST Repository

Liu, X.; Elbaz, Ayman M.; Gong, C.; Bai, X.S.; Zheng, H.T.; Roberts, William L.

2017-01-01

Large eddy simulation (LES) using a transported PDF model and OH-PLIF/PIV experiments were carried out to investigate the quarl effects on the structures of swirl stabilized methane/air flames. Two different quarls were investigated, one straight cylindrical quarl and one diverging conical quarl. The experiments show that the flames are significantly different with the two quarls. With the straight cylindrical quarl a compact blue flame is observed while with the diverging conical quarl the flame appears to be long and yellow indicating a sooty flame structure. The PIV results show the formation of a stronger flow recirculation inside the diverging conical quarl than that in the straight quarl. LES results reveal further details of the flow and mixing process inside the quarl. The results show that with the diverging quarl vortex breakdown occurs much earlier towards the upstream of the quarl. As a result the fuel is convected into the air flow tube and a diffusion flame is stabilized inside the air flow tube upstream the quarl. With the straight quarl, vortex breakdown occurs at a downstream location in the quarl. The scalar dissipation rate in the shear layer of the fuel jet is high, which prevents the stabilization of a diffusion flame in the proximity of the fuel nozzle; instead, a compact partially premixed flame with two distinct heat release layers is stablized in a downstream region in the quarl, which allows for the fuel and air to mix in the quarl before combustion and a lower formation rate of soot. The results showed that the Eulerian Stochastic Fields transported PDF method can well predict the details of the swirl flame dynamics.

An experimental and numerical study of nitrogen oxide formation mechanisms in ammonia-hydrogen-air flames

Science.gov (United States)

Kumar, Praveen

The demand for sustainable alternative fuels is ever-increasing in the power generation, transportation, and energy sectors due to the inherent non-sustainable characteristics and political constraints of current energy resources. A number of alternative fuels derived from cellulosic biomass, algae, or waste are being considered, along with the conversion of electricity to non-carbon fuels such as hydrogen or ammonia (NH3). The latter is receiving attention recently because it is a non-carbon fuel that is readily produced in large quantities, stored and transported with current infrastructure, and is often a byproduct of biomass or waste conversion processes. However, pure or anhydrous ammonia combustion is severely challenging due to its high auto-ignition temperature (650 °C), low reactivity, and tendency to promote NOx formation. As such, the present study focuses on two major aspects of the ammonia combustion. The first is an applied investigation of the potential to achieve pure NH3 combustion with low levels of emissions in flames of practical interest. In this study, a swirl-stabilized flame typically used in fuel-oil home-heating systems is optimized for NH3 combustion, and measurements of NO and NH3 are collected for a wide range of operating conditions. The second major focus of this work is on fundamental investigation of NO x formation mechanisms in flames with high levels of NH3 in H2. For laminar premixed and diffusion jet flames, experimental measurements of flame speeds, exhaust-gas sampling, and in-situ NO measurements (NO PLIF) are compared with numerically predicted flames using complex chemical kinetics within CHEMKIN and reacting CFD codes i.e., UNICORN. From the preliminary testing of the NOx formation mechanisms, (1) Tian (2) Konnov and (3) GRI-Mech3.0 in laminar premixed H2/NH 3 flames, the Tian and Konnov mechanisms are found to capture the reduction in measured flame speeds with increasing NH3 in the fuel mixture, both qualitatively and

Near wall combustion modeling in spark ignition engines. Part B: Post-flame reactions

International Nuclear Information System (INIS)

Demesoukas, Sokratis; Caillol, Christian; Higelin, Pascal; Boiarciuc, Andrei; Floch, Alain

2015-01-01

Highlights: • Models for the post flame reactions (CO and hydrocarbons) and heat release rate are proposed. • ‘Freezing’ effect of CO kinetics is captured but equilibrium CO concentrations are low. • Reactive–diffusive processes are modeled for hydrocarbons and the last stage of combustion is captured. - Abstract: Reduced fuel consumption, low pollutant emissions and adequate output performance are key features in the contemporary design of spark ignition engines. Zero-dimensional numerical simulation is an attractive alternative to engine experiments for the evaluation of various engine configurations. Both flame front reaction and post-flame processes contribute to the heat release rate. The contribution of this work is to highlight and model the role of post-flame reactions (CO and hydrocarbons) in the heat release rate. The modeling approach to CO kinetics used two reactions considered to be dominant and thus more suitable for the description of CO chemical mechanism. Equilibrium concentrations of all the species involved were calculated by a two-zone thermodynamic model. The computed characteristic time of CO kinetics was found to be of a similar order to the results of complex chemistry simulations. The proposed model captured the ‘freezing’ effect (reaction rate is almost zero) for temperatures lower than 1800 K and followed the trends of the measured values at exhaust. However, a consistent underestimation of CO levels at the exhaust was observed. The impact of the remaining CO on the combustion efficiency is considerable especially for rich mixtures. For a remaining 0.4% CO mass fraction, the impact on combustion inefficiency is 0.1%. Unburnt hydrocarbon, which have not reacted within the flame front before quenching, diffuse in the burnt gas and react. In this work, a global reaction rate models the kinetic behavior of hydrocarbon. The diffusion process was modeled by a relaxation equation applied on the calculated kinetic concentration

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

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)

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.

The anchoring mechanism of a bluff-body stabilized laminar premixed flame

KAUST Repository

Kedia, Kushal S.

2014-09-01

The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting bluff-body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane-air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. A steel bluff-body resulted in a slightly larger recirculation zone than a ceramic bluff-body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the bluff-body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting bluff-body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects. © 2014 The Combustion Institute.

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

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)

Experimental and chemical kinetic modeling study of small methyl esters oxidation: Methyl (E)-2-butenoate and methyl butanoate

Energy Technology Data Exchange (ETDEWEB)

Gail, S.; Sarathy, S.M.; Thomson, M.J. [Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON M5S 3G8 (Canada); Dievart, P.; Dagaut, P. [CNRS, 1C, Ave de la Recherche Scientifique, 45071 Orleans Cedex 2 (France)

2008-12-15

This study examines the effect of unsaturation on the combustion of fatty acid methyl esters (FAME). New experimental results were obtained for the oxidation of methyl (E)-2-butenoate (MC, unsaturated C{sub 4} FAME) and methyl butanoate (MB, saturated C{sub 4} FAME) in a jet-stirred reactor (JSR) at atmospheric pressure under dilute conditions over the temperature range 850-1400 K, and two equivalence ratios ({phi}=0.375,0.75) with a residence time of 0.07 s. The results consist of concentration profiles of the reactants, stable intermediates, and final products, measured by probe sampling followed by on-line and off-line gas chromatography analyses. The oxidation of MC and MB in the JSR and under counterflow diffusion flame conditions was modeled using a new detailed chemical kinetic reaction mechanism (301 species and 1516 reactions) derived from previous schemes proposed in the literature. The laminar counterflow flame and JSR (for {phi}=1.13) experimental results used were from a previous study on the comparison of the combustion of both compounds. Sensitivity analyses and reaction path analyses, based on rates of reaction, were used to interpret the results. The data and the model show that MC has reaction pathways analogous to that of MB under the present conditions. The model of MC oxidation provides a better understanding of the effect of the ester function on combustion, and the effect of unsaturation on the combustion of fatty acid methyl ester compounds typically found in biodiesel. (author)

Bounds for the propagation speed of combustion flames

International Nuclear Information System (INIS)

Fort, Joaquim; Campos, Daniel; Gonzalez, Josep R; Velayos, Joaquim

2004-01-01

We focus on a combustion model for premixed flames based on two coupled equations determining the spatial dynamics of temperature and fuel density. We rewrite these equations as a classical reaction-diffusion model, so that we can apply some known methods for the prediction of lower and upper bounds to the front speed. The predictions are compared to simulations, which show that our new bounds substantially improve those following from the linearization method, used in the previous work of Fort et al (2000 J. Phys. A: Math. Gen. 33 6953). Radiative losses lead to pulses rather than fronts. We find a bound for their speed which (in contrast to the linearization one) correctly predicts the order of magnitude of the flame speed

Bounds for the propagation speed of combustion flames

Energy Technology Data Exchange (ETDEWEB)

Fort, Joaquim [Departament de FIsica, Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia (Spain); Campos, Daniel [Grup de FIsica EstadIstica, Departament de FIsica, Universitat Autonoma de Barcelona, E-08193 Bellaterra (Spain); Gonzalez, Josep R [Grup de Mecanica de Fluids, Departament d' Enginyeria Mecanica, Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia (Spain); Velayos, Joaquim [Grup de Mecanica de Fluids, Departament d' Enginyeria Mecanica, Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia (Spain)

2004-07-23

We focus on a combustion model for premixed flames based on two coupled equations determining the spatial dynamics of temperature and fuel density. We rewrite these equations as a classical reaction-diffusion model, so that we can apply some known methods for the prediction of lower and upper bounds to the front speed. The predictions are compared to simulations, which show that our new bounds substantially improve those following from the linearization method, used in the previous work of Fort et al (2000 J. Phys. A: Math. Gen. 33 6953). Radiative losses lead to pulses rather than fronts. We find a bound for their speed which (in contrast to the linearization one) correctly predicts the order of magnitude of the flame speed.

UV Raman spectroscopy of H2-air flames excited with a narrowband KrF laser

Science.gov (United States)

Shirley, John A.

1990-01-01

Raman spectra of H2 and H2O in flames excited by a narrowband KrF excimer laser are reported. Observations are made over a porous-plug, flat-flame burner reacting H2 in air, fuel-rich with nitrogen dilution to control the temperature, and with an H2 diffusion flame. Measurements made from UV Raman spectra show good agreement with measurements made by other means, both for gas temperature and relative major species concentrations. Laser-induced fluorescence interferences arising from OH and O2 are observed in emission near the Raman spectra. These interferences do not preclude Raman measurements, however.

Perspectives of advanced thermal management in solar thermochemical syngas production using a counter-flow solid-solid heat exchanger

Science.gov (United States)

Falter, Christoph; Sizmann, Andreas; Pitz-Paal, Robert

2017-06-01

A modular reactor model is presented for the description of solar thermochemical syngas production involving counter-flow heat exchangers that recuperate heat from the solid phase. The development of the model is described including heat diffusion within the reactive material as it travels through the heat exchanger, which was previously identified to be a possibly limiting factor in heat exchanger design. Heat transfer within the reactive medium is described by conduction and radiation, where the former is modeled with the three-resistor model and the latter with the Rosseland diffusion approximation. The applicability of the model is shown by the analysis of heat exchanger efficiency for different material thicknesses and porosities in a system with 8 chambers and oxidation and reduction temperatures of 1000 K and 1800 K, respectively. Heat exchanger efficiency is found to rise strongly for a reduction of material thickness, as the element mass is reduced and a larger part of the elements takes part in the heat exchange process. An increase of porosity enhances radiation heat exchange but deteriorates conduction. The overall heat exchange in the material is improved for high temperatures in the heat exchanger, as radiation dominates the energy transfer. The model is shown to be a valuable tool for the development and analysis of solar thermochemical reactor concepts involving heat exchange from the solid phase.

Disproportionate entrance length in superfluid flows and the puzzle of counterflow instabilities

Science.gov (United States)

Bertolaccini, J.; Lévêque, E.; Roche, P.-E.

2017-12-01

Systematic simulations of the two-fluid model of superfluid helium (He-II) encompassing the Hall-Vinen-Bekharevich-Khalatnikov (HVBK) mutual coupling have been performed in two-dimensional pipe counterflows between 1.3 and 1.96 K. The numerical scheme relies on the lattice Boltzmann method. A Boussinesq-like hypothesis is introduced to omit temperature variations along the pipe. In return, the thermomechanical forcings of the normal and superfuid components are fueled by a pressure term related to their mass-density variations under an approximation of weak compressibility. This modeling framework reproduces the essential features of a thermally driven counterflow. A generalized definition of the entrance length is introduced to suitably compare entry effects (of different nature) at opposite ends of the pipe. This definition is related to the excess of pressure loss with respect to the developed Poiseuille-flow solution. At the heated end of the pipe, it is found that the entrance length for the normal fluid follows a classical law and increases linearly with the Reynolds number. At the cooled end, the entrance length for the superfluid is enhanced as compared to the normal fluid by up to one order of magnitude. At this end, the normal fluid flows into the cooling bath of He-II and produces large-scale superfluid vortical motions in the bath that partly re-enter the pipe along its sidewalls before being damped by mutual friction. In the superfluid entry region, the resulting frictional coupling in the superfluid boundary layer distorts the velocity profiles toward tail flattening for the normal fluid and tail raising for the superfluid. Eventually, a simple analytical model of entry effects allows us to re-examine the long-debated thresholds of T 1 and T 2 instabilities in superfluid counterflows. Inconsistencies in the T 1 thresholds reported since the 1960s disappear if an aspect-ratio criterion based on our modeling is used to discard data sets with the

Properties of flames propagating in rich propane-air mixtures at microgravity

Science.gov (United States)

Wang, S. F.; Pu, Y. K.; Jia, F.; Jarosinski, J.

Under normal gravity conditions it was found that the rich flammability limits for propane-air mixture are 9 2 C 3 H 8 equivalence ratio phi 2 42 for upward and 6 3 C 3 H 8 phi 1 60 for downward propagating flames An extremely large concentration gap exists between these two limits which is attributed to the influence of buoyancy and preferential diffusion in the mixture The present study enables a better understanding of flame behaviors in rich propane-air mixtures through microgravity experiments in which flame propagation can be examined in the absence of buoyancy The experiments were carried out in a cubic closed vessel of 80 mm inner length made of quartz glass A high-speed camera recorded flame propagation in the combustion vessel while the pressure history was measured by a transducer to indicate corresponding changes in heat release rate and the temperature development was measured by a thermocouple During the microgravity experiments the vessel was located inside a drop tower assembly The experimental data were compared with similar experiments conducted under normal gravity The flame characteristics were investigated for mixture concentrations between 6 5 C 3 H 8 and 9 2 C 3 H 8 Reliable data related to laminar burning velocity and flame thickness were obtained Some new details of the flame propagation near rich flammability limits were deduced Comparative experiments revealed the influence of gravity on combustion processes of rich propane-air

Migration of Organophosphate Flame Retardants from Closed Cell Foam to Settled Dust

Science.gov (United States)

Many industrial and consumer products, such as electrical and electronic products, furniture, plastics, textile, and building materials are manufactured with organophosphorus flame retardants (OPFRs). OPFRs can leach or diffuse out of the products and are released to the surround...

Migration of Organophorus Flame Retardants From Closed cell form to Settled Dust

Science.gov (United States)

Many industrial and consumer products, such as electrical and electronic products, furniture, plastics, textile, and building materials are manufactured with organophosphorus flame retardants (OPFRs). OPFRs can leach or diffuse out of the products and are released to the surround...

Computational Fluid-Particle Dynamics for the Flame Synthesis of Alumina Particles

DEFF Research Database (Denmark)

Johannessen, Tue; Pratsinis, Sotirie E.; Livbjerg, Hans

2000-01-01

A mathematical model for the dynamics of particle growth during synthesis of ultra fine particles in diffusion flames is presented. The model includes the kinetics of particle coalescence and coagulation, and when combined with a calculation of the temperature, velocity and gas composition distri...

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.

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.

Thermal-diffusional Instability in White Dwarf Flames: Regimes of Flame Pulsation

Energy Technology Data Exchange (ETDEWEB)

Xing, Guangzheng; Zhao, Yibo; Zhou, Cheng; Gao, Yang; Law, Chung K. [Center for Combustion Energy, Tsinghua University, Beijing 100084 (China); Modestov, Mikhail, E-mail: gaoyang-00@mails.tsinghua.edu.cn [Nordita, KTH Royal Institute of Technology and Stockholm University, SE-10691, Stockholm (Sweden)

2017-05-20

Thermal-diffusional pulsation behaviors in planar as well as outwardly and inwardly propagating white dwarf (WD) carbon flames are systematically studied. In the 1D numerical simulation, the asymptotic degenerate equation of state and simplified one-step reaction rates for nuclear reactions are used to study the flame propagation and pulsation in WDs. The numerical critical Zel’dovich numbers of planar flames at different densities ( ρ = 2, 3, and 4 × 10{sup 7} g cm{sup −3}) and of spherical flames (with curvature c = −0.01, 0, 0.01, and 0.05) at a particular density ( ρ = 2 × 10{sup 7} g cm{sup −3}) are presented. Flame front pulsation in different environmental densities and temperatures are obtained to form the regime diagram of pulsation, showing that carbon flames pulsate in the typical density of 2 × 10{sup 7} g cm{sup −3} and temperature of 0.6 × 10{sup 9} K. While being stable at higher temperatures, at relatively lower temperatures, the amplitude of the flame pulsation becomes larger. In outwardly propagating spherical flames the pulsation instability is enhanced and flames are also easier to quench due to pulsation at small radius, while the inwardly propagating flames are more stable.

A comprehensive experimental and modeling study of iso-pentanol combustion

KAUST Repository

Sarathy, Mani

2013-12-01

Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252. K, and at nominal pressures near 40 and 60. bar. Jet stirred reactor experiments are reported at 5. atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40. bar, for three equivalence ratios, and temperatures below 800. K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1-60. atm, temperatures of 650-1500. K, equivalence ratios of 0.25-4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions. © 2013 The Combustion Institute.

A comprehensive experimental and modeling study of iso-pentanol combustion

KAUST Repository

Sarathy, Mani; Park, Sungwoo; Weber, Bryan W.; Wang, Weijing; Veloo, Peter S.; Davis, Alexander C.; Togbé , Casimir; Westbrook, Charles K.; Park, Okjoo; Dayma, Guillaume; Luo, Zhaoyu; Oehlschlaeger, Matthew A.; Egolfopoulos, Fokion N.; Lu, Tianfeng; Pitz, William J.; Sung, Chihjen; Dagaut, P.

2013-01-01

Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252. K, and at nominal pressures near 40 and 60. bar. Jet stirred reactor experiments are reported at 5. atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40. bar, for three equivalence ratios, and temperatures below 800. K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1-60. atm, temperatures of 650-1500. K, equivalence ratios of 0.25-4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions. © 2013 The Combustion Institute.

REDUCTION OF NO FORMATION BY THERMAL EFFECT OF A TURBULNENT DIFFUSION FLAME H2/AIR MODELED BY THE CONCEPT OF LAMINAR FLAMELET

Directory of Open Access Journals (Sweden)

HADEF AMAR

2016-03-01

Full Text Available Highly exothermic reactions are responsible for the formation of harmful polluting chemical species to humans and the biosphere. In this context, nitrogen oxides (NOx are pollutants that are the subject of special attention on the part of regulators. In this work we studied the impact of a co-flow swirl on the internal structure of a turbulent diffusion flame H2-N2/Air and its role in reducing the formation of NO, which is modeled by the concept of laminar flamelet, while the flow field is modeled by the standard model k-ε, with a correction term for round jets.The results show good agreement with data from the experimental data.

Acoustic excitation of diffusion flames with coherent structure in a plane shear layer. ; Application of active combustion control to two-dimensional phase-locked arranging measurements. Soshiki kozo wo tomonau heimen sendai kakusan kaen no onkyo reiki. ; Nijigen iso heikin bunpu sokutei eno active nensho seigyo no oyo

Energy Technology Data Exchange (ETDEWEB)

Ishino, Y.; Kojima, T.; Oiwa, N.; Yamaguchi, S. (Nagoya Institute of Technology, Nagoya (Japan))

1993-11-25

The acoustic excitation of a plane diffusion flame enhances the periodicity of organized eddy controlled combustion. In this study, to clarify an effectiveness of application of active combustion control, phase characteristics of the excited eddy flames with high periodicity have been examined. A computer-aided phase-locked averaging method was employed to obtain graphical two-dimensional contour maps of the instantaneous profiles of temperature and CH emission. Both maps consisting of eight consecutive phases indicated clearly not only the periodic behavior of the organized eddy flame, but also the gas dynamic properties peculiar to those flames with coherent structure. In addition, the profiles of local contribution of the sound field to the combustion process were examined by calculating the two-dimensional distribution of the local Rayleigh index. Calculation results of the two-dimensional distribution of the local Rayleigh index indicated that the organized eddy flames have high sensitivity to sound, and play an important role in an interaction of sound and flame. 6 refs., 9 figs.

Speed ot travelling waves in reaction-diffusion equations

International Nuclear Information System (INIS)

Benguria, R.D.; Depassier, M.C.; Mendez, V.

2002-01-01

Reaction diffusion equations arise in several problems of population dynamics, flame propagation and others. In one dimensional cases the systems may evolve into travelling fronts. Here we concentrate on a reaction diffusion equation which arises as a simple model for chemotaxis and present results for the speed of the travelling fronts. (Author)

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.

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.

Characterisation and airborne deployment of a new counterflow virtual impactor inlet

Directory of Open Access Journals (Sweden)

T. Shingler

2012-06-01

Full Text Available A new counterflow virtual impactor (CVI inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE. The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i negligible organic contamination; (ii a significant sample flow rate to downstream instruments (∼15 l min⁻¹ that reduces the need for dilution; and (iii a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

OLYMPEX Counterflow Spectrometer and Impactor Field Campaign Report

Energy Technology Data Exchange (ETDEWEB)

Poellot, Michael [Univ. of North Dakota, Grand Forks, ND (United States)

2016-07-01

The U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility’s ARM Aerial Facility (AAF) Counterflow Spectrometer and Impactor (CSI) probe was flown on the University of North Dakota Cessna Citation research aircraft during the Olympic Mountain Experiment (OLYMPEX). The field campaign took place from November 12 through December 19, 2015, over the Olympic Mountains and coastal waters of Washington State as part of a National Aeronautics and Space Administration (NASA) Global Precipitation Measurement (GPM) validation campaign. The CSI was added to the Citation instrument suite to support the NASA Aerosol-Cloud Ecosystem (ACE) satellite program and flights of the NASA Lockheed Earth Resources (ER-2) aircraft. ACE funded extra ER-2 flights to focus on clouds that are weakly precipitating, which are also of interest to the DOE Atmospheric System Research (ASR) program.

Effect of hydrogen addition on autoignited methane lifted flames

KAUST Repository

Choin, Byung Chul

2012-01-01

Autoignited lifted flames in laminar jets with hydrogen-enriched methane fuels have been investigated experimentally in heated coflow air. The results showed that the autoignited lifted flame of the methane/hydrogen mixture, which had an initial temperature over 920 K, the threshold temperature for autoignition in methane jets, exhibited features typical of either a tribrachial edge or mild combustion depending on fuel mole fraction and the liftoff height increased with jet velocity. The liftoff height in the hydrogen-assisted autoignition regime was dependent on the square of the adiabatic ignition delay time for the addition of small amounts of hydrogen, as was the case for pure methane jets. When the initial temperature was below 920 K, where the methane fuel did not show autoignition behavior, the flame was autoignited by the addition of hydrogen, which is an ignition improver. The liftoff height demonstrated a unique feature in that it decreased nonlinearly as the jet velocity increased. The differential diffusion of hydrogen is expected to play a crucial role in the decrease in the liftoff height with increasing jet velocity.

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.

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.

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.

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.

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.

Radiation energy devaluation in diffusion combusting flows of natural gas

International Nuclear Information System (INIS)

Makhanlall, Deodat; Munda, Josiah L.; Jiang, Peixue

2013-01-01

Abstract: CFD (Computational fluid dynamics) is used to evaluate the thermodynamic second-law effects of thermal radiation in turbulent diffusion natural gas flames. Radiative heat transfer processes in gas and at solid walls are identified as important causes of energy devaluation in the combusting flows. The thermodynamic role of thermal radiation cannot be neglected when compared to that of heat conduction and convection, mass diffusion, chemical reactions, and viscous dissipation. An energy devaluation number is also defined, with which the optimum fuel–air equivalence for combusting flows can be determined. The optimum fuel–air equivalence ratio for a natural gas flame is determined to be 0.7. The CFD model is validated against experimental measurements. - Highlights: • Thermodynamic effects of thermal radiation in combusting flows analyzed. • General equation for second-law analyses of combusting flows extended. • Optimum fuel–air equivalence ratio determined for natural gas flame

Numerical Modelling of Soot Formation in Laminar Axisymmetric Ethylene-Air Coflow Flames at Atmospheric and Elevated Pressures

KAUST Repository

Abdelgadir, Ahmed

2015-03-30

A set of coflow diffusion flames are simulated to study the formation, growth, and oxidation of soot in flames of diluted hydrocarbon fuels, with focus on the effects of pressure. Firstly, we assess the ability of a high performance CFD solver, coupled with detailed transport and kinetic models, to reproduce experimental measurements of a series of ethylene-air coflow flames. Detailed finite rate chemistry describing the formation of Polycyclic Aromatic Hydro-carbons is used. Soot is modeled with a moment method and the resulting moment transport equations are solved with a Lagrangian numerical scheme. Numerical and experimental results are compared for various pressures. Finally, a sensitivity study is performed assessing the effect of the boundary conditions and kinetic mechanisms on the flame structure and stabilization properties.

The superflow state of 3He-B at a diffusive wall. Quasiclassical calculations

International Nuclear Information System (INIS)

Kopnin, N.B.; Soininen, P.I.

1992-01-01

The authors report first computations considering effects of a rough wall on the counterflow state in superfluid 3 He-B for high flow velocities. Using the quasiclassical Green's-function formalism supplemented by the boundary conditions for a diffusive wall, they calculate the order-parameter field and the supercurrent near a container wall for various pressures and temperatures. One of the results is that the current density at the wall as a function of the flow has a maximum at the velocity which is about half of the pair breaking velocity

Numerical Investigation of the Interaction of Counterflowing Jets and Supersonic Capsule Flows

Science.gov (United States)

Venkatachari, Balaji Shankar; Ito, Yasushi; Cheng, Gary; Chang, Chau-Lyan

2011-01-01

Use of counterflowing jets ejected into supersonic freestreams as a flow control concept to modify the external flowfield has gained renewed interest with regards to potential retropropulsion applications pertinent to entry, descent, and landing investigations. This study describes numerical computations of such a concept for a scaled wind-tunnel capsule model by employing the space-time conservation element solution element viscous flow solver with unstructured meshes. Both steady-state and time-accurate computations are performed for several configurations with different counterflowing jet Mach numbers. Axisymmetric computations exploring the effect of the jet flow rate and jet Mach number on the flow stability, jet interaction with the bow shock and its subsequent impact on the aerodynamic and aerothermal loads on the capsule body are carried out. Similar to previous experimental findings, both long and short penetration modes exist at a windtunnel Mach number of 3.48. It was found that both modes exhibit non-stationary behavior and the former is much more unstable than the latter. It was also found that the unstable long penetration mode only exists in a relatively small range of the jet mass flow rate. Solution-based mesh refinement procedures are used to improve solution accuracy and provide guidelines for a more effective mesh generation procedure for parametric studies. Details of the computed flowfields also serve as a means to broaden the knowledge base for future retropropulsion design studies.

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

Numerical and Experimental Study on Negative Buoyance Induced Vortices in N-Butane Jet Flames

KAUST Repository

Xiong, Yuan; Cha, Min; Chung, Suk-Ho

2015-01-01

Near nozzle flow field in flickering n-butane diffusion jet flames was investigated with a special focus on transient flow patterns of negative buoyance induced vortices. The flow structures were obtained through Mie scattering imaging with seed

Dynamics of the density of quantized vortex lines in counterflow turbulence: Experimental investigation

Science.gov (United States)

Varga, E.; Skrbek, L.

2018-02-01

Recently the interest in thermal counterflow of superfluid 4He, the most extensively studied form of quantum turbulence, has been renewed. Particularly, an intense theoretical debate has arisen about what form, if any, of the so-called Vinen equation accurately captures the dynamics of vortex line density, L . We address this problem experimentally, in a 21 cm long channel of square 7 ×7 mm2 cross section. Based on large statistics of second-sound data measured in nonequilibrium square-wave modulated thermally induced counterflow we investigate the phase portrait of the general form of the governing dynamical equation and conclude that for sparse tangles (L ≲105cm-2) all proposed forms of this equation based on the concept of a homogeneous random tangle of quantized vortices provide equally adequate descriptions of the growth of L , while for dense tangles (L >105cm-2) none of them is satisfactory or able to account for the significant slow-down in tangle growth rate as the steady state is approached. We claim, however, that agreement with theory is recovered if the geometrical parameter c2 introduced in numerical studies by K. W. Schwarz [Phys. Rev. B 38, 2398 (1988), 10.1103/PhysRevB.38.2398] is allowed to vary with vortex line density which also greatly improves the prediction of the observed early decay rate.

Breakdown of Counterflow Superfluidity in a Disordered Quantum Hall Bilayer

International Nuclear Information System (INIS)

Lee, D.K.K.; Eastham, P.R.; Cooper, N.R.

2011-01-01

We present a theory for the regime of coherent interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. We find that the system consists of domains of polarized superfluid phase. Injected currents introduce phase slips between the polarized domains which are pinned by disorder. We present a model of saturated tunneling domains that predicts a critical current for the breakdown of coherent tunneling that is extensive in the system size. This theory is supported by numerical results from a disordered phase model in two dimensions. We also discuss how our picture might be used to interpret experiments in the counterflow geometry and in two-terminal measurements

Quantized acoustoelectric current in the presence of large tunneling counterflow

DEFF Research Database (Denmark)

Gloos, K.; Utko, P.; Hansen, Jørn Bindslev

2004-01-01

A surface acoustic wave drives an electrical current through a short quantum wire. A second tunneling current is injected by biasing one side of the quantum wire. These two contributions to the total current, which flow in opposite directions, are controlled almost independently by the gate...... and the bias voltage, respectively. We have observed the quantization of the acoustoelectric current at up to ten times larger counterflowing tunneling currents. At large tunneling currents the acoustoelectric current can be strongly suppressed. However, this does not seem to be due to an electrostatic...... interaction between the two currents, but is probably caused by the complex potential landscape in the narrow channel of the quantum wire....

Quantized acoustoelectric current in the presence of large tunneling counterflow

International Nuclear Information System (INIS)

Gloos, K.; Utko, P.; Lindelof, P.E.; Hansen, J. Bindslev

2004-01-01

A surface acoustic wave drives an electrical current through a short quantum wire. A second tunneling current is injected by biasing one side of the quantum wire. These two contributions to the total current, which flow in opposite directions, are controlled almost independently by the gate and the bias voltage, respectively. We have observed the quantization of the acoustoelectric current at up to ten times larger counterflowing tunneling currents. At large tunneling currents the acoustoelectric current can be strongly suppressed. However, this does not seem to be due to an electrostatic interaction between the two currents, but is probably caused by the complex potential landscape in the narrow channel of the quantum wire

Soot Formation in Laminar Premixed Methane/Oxygen Flames at Atmospheric Pressure

Science.gov (United States)

Xu, F.; Lin, K.-C.; Faeth, G. M.

1998-01-01

Flame structure and soot formation were studied within soot-containing laminar premixed mc1hane/oxygen flames at atmospheric pressure. The following measurements were made: soot volume fractions by laser extinction, soot temperatures by multiline emission, gas temperatures (where soot was absent) by corrected fine-wire thermocouples, soot structure by thermophoretic sampling and transmission electron microscope (TEM), major gas species concentrations by sampling and gas chromatography, and gas velocities by laser velocimetry. Present measurements of gas species concentrations were in reasonably good agreement with earlier measurements due to Ramer et al. as well as predictions based on the detailed mechanisms of Frenklach and co-workers and Leung and Lindstedt: the predictions also suggest that H atom concentrations are in local thermodynamic equilibrium throughout the soot formation region. Using this information, it was found that measured soot surface growth rates could be correlated successfully by predictions based on the hydrogen-abstraction/carbon-addition (HACA) mechanisms of both Frenklach and co-workers and Colket and Hall, extending an earlier assessment of these mechanisms for premixed ethylene/air flames to conditions having larger H/C ratios and acetylene concentrations. Measured primary soot particle nucleation rates were somewhat lower than the earlier observations for laminar premixed ethylene/air flames and were significantly lower than corresponding rates in laminar diffusion flames. for reasons that still must be explained.

Asymptotic analysis to the effect of temperature gradient on the propagation of triple flames

Science.gov (United States)

Al-Malki, Faisal

2018-05-01

We study asymptotically in this paper the influence of the temperature gradient across the mixing layer on the propagation triple flames formed inside a porous wall channel. The study begins by formulating the problem mathematically using the thermo-diffusive model and then presents a thorough asymptotic analysis of the problem in the limit of large activation energy and thin flames. Analytical formulae for the local burning speed, the flame shape and the propagation speed in terms of the temperature gradient parameter have been derived. It was shown that varying the feed temperatures can significantly enhance the burning of the reactants up to a critical threshold, beyond which no solutions can be obtained. In addition, the study showed that increasing the temperature at the boundaries will modify the usual triple structure of the flame by inverting the upper premixed branch and extending it to the boundary, which may have great implications on the safety of the adopted combustion chambers.

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.

A Macroscopic Approach to the Lane Formation Phenomenon in Pedestrian Counterflow

Science.gov (United States)

Xiong, Tao; Zhang, Peng; Wong, S. C.; Shu, Chi-Wang; Zhang, Meng-Ping

2011-10-01

We simulate pedestrian counterflow by adopting an optimal path-choice strategy and a recently observed speed-density relationship. Although the whole system is symmetric, the simulation demonstrates the segregation and formation of many walking lanes for two groups of pedestrians. The symmetry breaking is most likely triggered by a small numerical viscosity or “noise", and the segregation is associated with the minimization of travel time. The underlying physics can be compared with the “optimal self-organization" mechanism in Helbing's social force model, by which driven entities in an open system tend to minimize their interaction to enable them to reach some ordering state.

Impact of flame-wall interaction on premixed flame dynamics and transfer function characteristics

KAUST Repository

Kedia, K.S.

2011-01-01

In this paper, we numerically investigate the response of a perforated-plate stabilized laminar methane-air premixed flame to imposed inlet velocity perturbations. A flame model using detailed chemical kinetics mechanism is applied and heat exchange between the burner plate and the gas mixture is incorporated. Linear transfer functions, for low mean inlet velocity oscillations, are analyzed for different equivalence ratio, mean inlet velocity, plate thermal conductivity and distance between adjacent holes. The oscillations of the heat exchange rate at the top of the burner surface plays a critical role in driving the growth of the perturbations over a wide range of conditions, including resonance. The flame response to the perturbations at its base takes the form of consumption speed oscillations in this region. Flame stand-off distance increases/decreases when the flame-wall interaction strengthens/weakens, impacting the overall dynamics of the heat release. The convective lag between the perturbations and the flame base response govern the phase of heat release rate oscillations. There is an additional convective lag between the perturbations at the flame base and the flame tip which has a weaker impact on the heat release rate oscillations. At higher frequencies, the flame-wall interaction is weaker and the heat release oscillations are driven by the flame area oscillations. The response of the flame to higher amplitude oscillations are used to gain further insight into the mechanisms. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.

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.

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.

Tulip flames: changes in shape of premixed flames propagating in closed tubes

Science.gov (United States)

Dunn-Rankin, D.; Sawyer, R. F.

The experimental results that are the subject of this communication provide high-speed schlieren images of the closed-tube flame shape that has come to be known as the tulip flame. The schlieren images, along with in-chamber pressure records, help demonstrate the effects of chamber length, equivalence ratio, and igniter geometry on formation of the tulip flame. The pressure/time records show distinct features which correlate with flame shape changes during the transition to tulip. The measurements indicate that the basic tulip flame formation is a robust phenomenon that depends on little except the overall geometry of the combustion vessel.

Theoretical analysis of the conical premixed flame response to upstream velocity disturbances considering flame speed development effects

Directory of Open Access Journals (Sweden)

Ghazaleh Esmaeelzade

2017-03-01

Full Text Available The effect of upstream velocity perturbations on the response of a premixed flame was investigated in terms of the flame transfer function dependency on excitation frequency. In this study, the assumption of constant flame speed was extended and the effect of flame speed development was considered; i.e., the flame speed would grow with the time after ignition or with the distance from a flame-holder. In the present study, the kinematics of a conical flame was investigated by linearization of the front tracking equation of flame to uniform and convected fluctuations of the flow velocity and the response was compared with that of a V-shaped flame and the experimental data in the previous studies. The results show that the effect of flame speed development could influence a decreasing gain and increase the phase of the flame response to the uniform velocity oscillations in low and moderate frequencies. Comparing the variations in the gain of flame response upon normalized frequency, show that a conical flame has lower values than the V-flame. In other words, these flames might be less susceptible to combustion instabilities than the V-flames. Furthermore, the variations in phase of the V-flames responses, which show a quasi-linear behavior with normalized frequency, have higher values than the saturated behavior in phase of the conical flame responses. Also, considering that the flame speed development induces an increase in the gain and phase of the conical flame response to the convected velocity oscillations in certain frequencies; because the developed flame front has longer length in comparison to the flame front in constant flame speed model. Therefore, the flame length may be longer than convective wavelength and the heat release would be generated in different points of the flame; consequently the flow oscillations might exert a stronger impact on the unsteady heat release fluctuations.

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)

Improvement of flame resistance of non-flame retardant cables by applying fire protection measures

International Nuclear Information System (INIS)

Takemura, Yujiro; Segoshi, Yoshinori; Jinno, Susumu; Mii, Kazuki

2017-01-01

The new regulatory requirements, which were put in force after the Fukushima Daiichi accident, impose the use of flame retardant cables on the plant components having safety functions for the purpose of fire protection. However, some Japanese nuclear power plants built in the early days use non-flame retardant cables that do not pass the demonstration test to check for the flame resistance. To cope with the new regulatory requirements, a fire protection measure for non-flame retardant cables was introduced to assure flame resistance of non-flame retardant cables equivalent to or higher than that of flame retardant cables. To illustrate the fire protection measure, both non-flame retardant cables and its cable tray are covered with fire protection sheet fabricated from incombustible material to form an assembly. Considering the demonstration test results, it can be concluded that flame resistance performance of non-flame retardant cables equivalent to or higher than that of flame retardant cables can be assured by forming the assembly even if an external fire outside the assembly and internal cable fire inside the assembly are assumed. This paper introduces the design of the assembly consisting of a bundle of cables and a cable tray and summarizes the results of demonstration tests. (author)

Self Induced Buoyant Blow Off in Upward Flame Spread on Thin Solid Fuels

Science.gov (United States)

Johnston, Michael C.; T'ien, James S.; Muff, Derek E.; Olson, Sandra L.; Ferkul, Paul V.

2013-01-01

) is as follows: The observed one-sided extinction is a blow- off induced by buoyant entrainment. It is known that the flammable diffusion flame regime is bounded by quenching and blow ]off limits when varying incoming air velocity. The narrowest samples tested (between 2 and 5 cm) begin within the flammable range, but as the flame grows, the buoyancy driven air velocity increases at the neighborhood of the flame base. The initially stable flame crosses the extinguishment boundary resulting in a flame blow-off. When one-side of the flame extinguishes, the remaining side shrinks due to the reduced heat transfer to the solid. This reduces the induced velocity and the flame becomes stable. It is proposed that this may have implications to upward flame growth beyond this experiment.

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.

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.

Examination of flame length for burning pulverized coal in laminar flow reactor

Energy Technology Data Exchange (ETDEWEB)

Kim, Jae Dong; Kim, Gyu Bo; Chang, Young June; Song, Ju Hun; Jeon, Chung Hwan [Pusan National University, Busan (Korea, Republic of)

2010-12-15

Because there has been a recent increase in the use of low calorific coal compared to standard coal, it is crucial to control the char flame length governing the burning life-time of coal in a coal-fired utility boiler. The main objective of this study is to develop a simplified model that can theoretically predict the flame length for burning coal in a laboratory-scale entrained laminar flow reactor (LFR) system. The char burning behavior was experimentally observed when sub-bituminous pulverized coal was fed into the LFR under burning conditions similar to those in a real boiler: a heating rate of 1000 K/s, an oxygen molar fraction of 7.7 %, and reacting flue gas temperatures ranging from 1500 to 2000 K. By using the theoretical model developed in this study, the effect of particle size on the coal flame length was exclusively addressed. In this model, the effect of particle mass was eliminated to compare with the experimental result performed under a constant mass feeding of coal. Overall, the computed results for the coal flame length were in good agreement with the experimental data, particularly when the external oxygen diffusion effect was considered in the model

Examination of flame length for burning pulverized coal in laminar flow reactor

International Nuclear Information System (INIS)

Kim, Jae Dong; Kim, Gyu Bo; Chang, Young June; Song, Ju Hun; Jeon, Chung Hwan

2010-01-01

Because there has been a recent increase in the use of low calorific coal compared to standard coal, it is crucial to control the char flame length governing the burning life-time of coal in a coal-fired utility boiler. The main objective of this study is to develop a simplified model that can theoretically predict the flame length for burning coal in a laboratory-scale entrained laminar flow reactor (LFR) system. The char burning behavior was experimentally observed when sub-bituminous pulverized coal was fed into the LFR under burning conditions similar to those in a real boiler: a heating rate of 1000 K/s, an oxygen molar fraction of 7.7 %, and reacting flue gas temperatures ranging from 1500 to 2000 K. By using the theoretical model developed in this study, the effect of particle size on the coal flame length was exclusively addressed. In this model, the effect of particle mass was eliminated to compare with the experimental result performed under a constant mass feeding of coal. Overall, the computed results for the coal flame length were in good agreement with the experimental data, particularly when the external oxygen diffusion effect was considered in the model

Breakdown of Counterflow Superfluidity in a Disordered Quantum Hall Bilayer

Directory of Open Access Journals (Sweden)

D. K. K. Lee

2011-01-01

Full Text Available We present a theory for the regime of coherent interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. We find that the system consists of domains of polarized superfluid phase. Injected currents introduce phase slips between the polarized domains which are pinned by disorder. We present a model of saturated tunneling domains that predicts a critical current for the breakdown of coherent tunneling that is extensive in the system size. This theory is supported by numerical results from a disordered phase model in two dimensions. We also discuss how our picture might be used to interpret experiments in the counterflow geometry and in two-terminal measurements.

The effect of CO{sub 2} dissolved in a diesel fuel on the jet flame characteristics

Energy Technology Data Exchange (ETDEWEB)

Xiao Jin; Huang Zhen; Qiao Xinqi; Hou Yuchun [Shanghai Jiao Tong University, Shanghai (China). Research Institute of Internal Combustion Engine

2008-03-15

This paper is concerned with an experimental study of the jet diffusion flame characteristics of fuel containing CO{sub 2}. Using diesel fuel containing dissolved CO{sub 2} gas, experiments were performed under atmospheric conditions with a diesel hole-type nozzle of 0.19 mm orifice diameter at constant injection pressure. In this study, four different CO{sub 2} mass fraction in diesel fuel such as 3.13%, 7.18%, 12.33% and 17.82% were used to study the effect of CO{sub 2} concentration on the jet flame characteristics. Jet flame characteristics were measured by direct photography, meanwhile the image colorimetry is used to assess the qualitative features of jet flame temperature. Experimental results show that the CO{sub 2} gas dilution effect and the atomization effect have a great influence on the flame structure and average temperature. When the injection pressure of diesel fuel increased from 4 MPa to 6 MPa, the low temperature flame length increased from 18.4 cm to 21.7 cm and the full temperature flame length decreased from 147.6 cm to 134.7 cm. With the increase of CO{sub 2} gas dissolved in the diesel fuel, the jet flame full length decreased for the jet atomization being improved greatly meanwhile the low temperature flame length increased for the CO{sub 2} gas dilution effect; with the increase of CO{sub 2} gas dissolved in the diesel fuel, the average temperature of flame increases firstly and then falls. Experimental results validate that higher injection pressure will improve jet atomization and then increased the flame average temperature. 27 refs., 13 figs.

Effect of cylindrical confinement on the determination of laminar flame speeds using outwardly propagating flames

Energy Technology Data Exchange (ETDEWEB)

Burke, Michael P.; Chen, Zheng; Ju, Yiguang; Dryer, Frederick L. [Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544 (United States)

2009-04-15

The effect of nonspherical (i.e. cylindrical) bomb geometry on the evolution of outwardly propagating flames and the determination of laminar flame speeds using the conventional constant-pressure technique is investigated experimentally and theoretically. The cylindrical chamber boundary modifies the propagation rate through the interaction of the wall with the flow induced by thermal expansion across the flame (even with constant pressure), which leads to significant distortion of the flame surface for large flame radii. These departures from the unconfined case, especially the resulting nonzero burned gas velocities, can lead to significant errors in flame speeds calculated using the conventional assumptions, especially for large flame radii. For example, at a flame radius of 0.5 times the wall radius, the flame speed calculated neglecting confinement effects can be low by {proportional_to}15% (even with constant pressure). A methodology to estimate the effect of nonzero burned gas velocities on the measured flame speed in cylindrical chambers is presented. Modeling and experiments indicate that the effect of confinement can be neglected for flame radii less than 0.3 times the wall radius while still achieving acceptable accuracy (within 3%). The methodology is applied to correct the flame speed for nonzero burned gas speeds, in order to extend the range of flame radii useful for flame speed measurements. Under the proposed scaling, the burned gas speed can be well approximated as a function of only flame radius for a given chamber geometry - i.e. the correction function need only be determined once for an apparatus and then it can be used for any mixture. Results indicate that the flow correction can be used to extract flame speeds for flame radii up to 0.5 times the wall radius with somewhat larger, yet still acceptable uncertainties for the cases studied. Flow-corrected burning velocities are measured for hydrogen and syngas mixtures at atmospheric and

Analysis of density wave instability in counter-flow steam generators using STEAMFREQ-X

International Nuclear Information System (INIS)

Chan, K.C.; Yadigaroglu, G.

1986-01-01

The STEAMFREQ-X computer model was developed to provide a more comprehensive modeling of the different phenomena that are important to stability analysis of counter-flow steam generators. It uses a frequency-domain analysis and considers heat-flux/flow coupling between the primary and secondary fluids in space and time. Predictions by STEAMFREQ-X were compared with data from both a multi-channel liquid-sodium heated steam generator and a set of single pipe test data. Predicted outlet steam qualities at instability thresholds were within 15% of experimental data for all test points. (orig.)

Reynolds averaged modelling of low momentum propane jet diffusion flames in cross flow

Energy Technology Data Exchange (ETDEWEB)

Majeski, A.J.; Chui, E.H. [Natural Resources Canada, Ottawa, ON (Canada). CANMET Energy Technology Centre; Kostiuk, L.W. [Alberta Univ., Edmonton, AB (Canada). Dept. of Mechanical Engineering

2003-07-01

It is common practice to use continuous low flow rate flares to dispose of unwanted or by-product combustible gases resulting from the manufacturing process or oil recovery operations. This study evaluates the usefulness of computational fluid dynamics (CFD) modelling in the context of low momentum flux reacting jets. The experimental data was gathered at the University of Alberta's Combustion Wind Tunnel. This data was used to compare data obtained from the CFD simulations. Only a small subset of the experimental conditions was used for the computational model. No attempt was made to fine tune any of the individual models. They were all part of the commercial CFD software package CFX-TASC flow, by ANSYS Inc. Flame length and angle results compared favourably with experiments. The shape of the plume changed significantly in the far field. This could be explained by distortion caused by the turbulence model used. A flame front model was incorporated in an effort to estimate combustion efficiency. The results obtained were not conclusive. 20 refs., 4 figs.

Numerical and Experimental Study on Negative Buoyance Induced Vortices in N-Butane Jet Flames

KAUST Repository

Xiong, Yuan

2015-07-26

Near nozzle flow field in flickering n-butane diffusion jet flames was investigated with a special focus on transient flow patterns of negative buoyance induced vortices. The flow structures were obtained through Mie scattering imaging with seed particles in a fuel stream using continuous-wave (CW) Argon-ion laser. Velocity fields were also quantified with particle mage velocimetry (PIV) system having kHz repetition rate. The results showed that the dynamic motion of negative buoyance induced vortices near the nozzle exit was coupled strongly with a flame flickering instability. Typically during the flame flickering, the negative buoyant vortices oscillated at the flickering frequency. The vortices were distorted by the flickering motion and exhibited complicated transient vortical patterns, such as tilting and stretching. Numerical simulations were also implemented based on an open source C++ package, LaminarSMOKE, for further validations.

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.

Flame analysis using image processing techniques

Science.gov (United States)

Her Jie, Albert Chang; Zamli, Ahmad Faizal Ahmad; Zulazlan Shah Zulkifli, Ahmad; Yee, Joanne Lim Mun; Lim, Mooktzeng

2018-04-01

This paper presents image processing techniques with the use of fuzzy logic and neural network approach to perform flame analysis. Flame diagnostic is important in the industry to extract relevant information from flame images. Experiment test is carried out in a model industrial burner with different flow rates. Flame features such as luminous and spectral parameters are extracted using image processing and Fast Fourier Transform (FFT). Flame images are acquired using FLIR infrared camera. Non-linearities such as thermal acoustic oscillations and background noise affect the stability of flame. Flame velocity is one of the important characteristics that determines stability of flame. In this paper, an image processing method is proposed to determine flame velocity. Power spectral density (PSD) graph is a good tool for vibration analysis where flame stability can be approximated. However, a more intelligent diagnostic system is needed to automatically determine flame stability. In this paper, flame features of different flow rates are compared and analyzed. The selected flame features are used as inputs to the proposed fuzzy inference system to determine flame stability. Neural network is used to test the performance of the fuzzy inference system.

A priori tests of combustion models based on a CH{sub 4}/H{sub 2} Triple Flame

Energy Technology Data Exchange (ETDEWEB)

Dombard, J.; Naud, B.; Jimenez Sanchez, C.

2008-07-01

This document reproduces the final project of Jerome Dombard, presented on June 25, 2008, for the obtention of the Master degree MIMSE (Master Ingenierie Mathematique, Statistique et Economique) of Bordeaux University (Universite Bordeaux 1). We make an a priori study of FPI/FGM-type turbulent combustion models using a 2D DNS of a triple flame. A reduced chemical scheme of 16 species and 12 reactions is used (ARM1, proposed by J.-Y. Chen at Berkeley University). The fuel (CH4/H2 mixture) and oxidizer (air) correspond to the inlet composition of the Sydney bluff-body stabilised flame experiments (flames HM1-3). First, we compute 1D laminar premixed flames. The purpose of those calculations is twofold: 1. check the differences between different computer programs and different treatments of molecular diffusion, and 2. calibrate the 2D-DNS of the laminar triple flame (mainly decide on the grid resolution). Then, the solution of the 2D laminar triple flame is used to test a priori FPI/FGM tables. Finally, preliminary considerations on sub-grid scale modelling in the context of Large Eddy Simulation are made. (Author) 14 refs.

Flame Motion In Gas Turbine Burner From Averages Of Single-Pulse Flame Fronts

Energy Technology Data Exchange (ETDEWEB)

Tylli, N.; Hubschmid, W.; Inauen, A.; Bombach, R.; Schenker, S.; Guethe, F. [Alstom (Switzerland); Haffner, K. [Alstom (Switzerland)

2005-03-01

Thermo acoustic instabilities of a gas turbine burner were investigated by flame front localization from measured OH laser-induced fluorescence single pulse signals. The average position of the flame was obtained from the superposition of the single pulse flame fronts at constant phase of the dominant acoustic oscillation. One observes that the flame position varies periodically with the phase angle of the dominant acoustic oscillation. (author)

Numerical simulation of growth of flames formed in two-dimensional mixing layer. 2nd Report. Effect of dilution of fuel; Nijigen kongo sonai ni keiseisareta kaen no seicho ni kansuru suchi simulation. 2. Nenryo no kishaku ni yoru eikyo

Energy Technology Data Exchange (ETDEWEB)

Noda, S [Toyohashi University of Technology, Aichi (Japan); Hashimoto, K [Sumitomo Metal Industries, Ltd., Osaka (Japan); Nakajima, T [Kobe University, Kobe (Japan). Faculty of Engineering

1994-07-25

The effect of fuel dilution on growth of flames formed in 2-D mixing layers was studied by numerical simulation. The methane mass fraction of fuel was adjusted to 1.0, 0.3 and 0.2 through dilution by nitrogen, while the oxygen mass fraction of an oxidizer was fixed at 0.27. Flame structure was complicated due to the flows separated by flame at the leading edge of flames, and three peaks of the second Damkohler number were observed. Fuel dilution by nitrogen caused blow-off of flames, and the mixing ratio of the fuel and oxidizer at the leading edge of flames was essential to blow-off of diffused flames. In the case where vortices were observed in a flow field, the first Damkohler number was important which was determined by the hydrodynamic characteristic time of coherent vortices and the chemical characteristic time of flame propagation based on the mixing ratio of the fuel and oxidizer at the leading edge of flames. The diffused flames were elongated by shearing force, and an exothermic reaction was suppressed and a flame stabilization decreased with a decrease in second Damkohler number. 10 refs., 9 figs., 1 tab.

Flame visualization in power stations

Energy Technology Data Exchange (ETDEWEB)

Hulshof, H J.M.; Thus, A W; Verhage, A J.L. [KEMA - Fossil Power Plants, Arnhem (Netherlands)

1993-01-01

The shapes and temperature of flames in power stations, fired with powder coal and gas, have been measured optically. Spectral information in the visible and near infrared is used. Coal flames are visualized in the blue part of the spectrum, natural gas flames are viewed in the light of CH-emission. Temperatures of flames are derived from the best fit of the Planck-curve to the thermal radiation spectrum of coal and char, or to that of soot in the case of gas flames. A measuring method for the velocity distribution inside a gas flame is presented, employing pulsed alkali salt injection. It has been tested on a 100 kW natural gas flame. 3 refs., 9 figs.

Combustion Synthesis of Nanomaterials Using Various Flame Configurations

KAUST Repository

Ismail, Mohamed Anwar

2016-02-01

interaction jet region, followed by a merged-jet region. The modified CWJ burner revealed appreciable mixing characteristics between the precursor and combustion gases within these regions, with a slight increase in the axial velocity due to the precursor injection. This led to more uniformity in particle size distribution of the synthesized nanoparticles with the poppet valve (first modification). The double-slit modification improved the uniformity of generated nanoparticles at a very wide range of stable experimental conditions. Images of OH fluorescence showed that flames are tightly attached to the burner tip and TTIP has no influence on these flames structures. The particle size was slightly affected by the operating conditions. The phase of TiO2 nanoparticles was mainly dependent on the equivalence ratio and fuel type, which impact flame height, heat release rate and high temperature residence time of the precursor vapor. For ethylene and methane flames, the anatase content is proportional to the equivalence ratio, whereas it is inversely proportional in the case of propane flames. The anatase content reduced by 8% as we changed Re between 8,000 and 19,000, implying that the Re has a slight effect on the anatase content. The synthesized TiO2 nanoparticles exhibited high crystallinity and the anatase phase was dominant at high equivalence ratios (φ >1.6) for C2H4, and at low equivalence ratios (φ <1.3) for the C3H8 flame. Concerning advanced nanoparticle synthesis, a multiple diffusion burner and flame spray pyrolysis (FSP) were adopted in this study to investigate the effect of doping/coating on TiO2 nanoparticles. The nanoparticles were characterized by the previously mentioned techniques in addition to thermogravimetric analysis (TGA) for carbon content, X-ray photoelectron spectroscopy (XPS) for surface chemistry, ultraviolet-visible spectroscopy (UV-vis) for light absorbance, inductively coupled plasma (ICP) for metal traces, and superconducting quantum

Study on dew point evaporative cooling system with counter-flow configuration

International Nuclear Information System (INIS)

Lin, J.; Thu, K.; Bui, T.D.; Wang, R.Z.; Ng, K.C.; Chua, K.J.

2016-01-01

Highlights: • Numerical model for a dew point evaporative cooler verified with experiments. • Saturation point of the working air is independent of the inlet air conditions. • The intensity of cooling capacity and water evaporation are studied. • The overall heat transfer coefficient for the working air is analyzed. • The conditions to achieve sub-wet bulb cooling are examined. - Abstract: Dew point evaporative cooling has great potential as a disruptive process for sensible cooling of air below its entering wet bulb temperature. This paper presents an improved mathematical model for a single-stage dew point evaporative cooler in a counter-flow configuration. Longitudinal heat conduction and mass diffusion of the air streams, channel plate and water film, as well as the temperature difference between the plate and water film, are accounted for in the model. Predictions of the product air temperature are validated using three sets of experimental data within a discrepancy of 4%. The cooler’s heat and mass transfer process is analyzed in terms of its cooling capacity intensity, water evaporation intensity, and overall heat transfer coefficient along the channel. Parametric studies are conducted at different geometric and operating conditions. For the conditions evaluated, the study reveals that (1) the saturation point of the working air occurs at a fixed point regardless of the inlet air conditions, and it is mainly influenced by the working air ratio and channel height; (2) the intensity of the water evaporation approaches a minimum at 0.2 to 0.3 m from the entrance; (3) the wet channel can be separated into two zones, and the overall heat transfer coefficient is above 100 W/(m"2·K) after the temperature of water film becomes higher than the working air temperature.

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)

Characteristics of autoignited laminar lifted flames in heated coflow jets of carbon monoxide/hydrogen mixtures

KAUST Repository

Choi, Byungchul; Chung, Suk-Ho

2012-01-01

and then increased as the jet velocity increased. Based on the mechanism in which the autoignited laminar lifted flame is stabilized by ignition delay time, the liftoff height can be influenced not only by the heat loss, but also by the preferential diffusion between

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

Science.gov (United States)