Forced Convection Heat Transfer in Circular Pipes
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
Forced heat convection in annular spaces
International Nuclear Information System (INIS)
This report deals with the experimental study of forced heat convection in annular spaces through which flow of air is passing when a uniform heat flux is dissipated across the inner wall. These observations took place chiefly in the region where thermal equilibrium are not yet established. Amongst other things it became apparent that, both in the region where thermal equilibrium conditions are on the way to establishment and where they are already established, the following relationship held good: the longitudinal temperature gradient, either on the wall or in the fluid stream, is proportional to the heat flux dissipated q, and inversely proportional to the average flow rate V: dT/dx = B (q/V). From this result the next step is to express the variations of the local convection coefficient ? (or of the Margoulis number M) in a relationship of the form: 1/M = ?(V) + F(x). If this relationship is compared with the classical empirical relationship ? = KVn (where n is close to 0.8), the relationship: 1/M = ?V1-n + F(x) is obtained (? is a constant for a given annular space); from this it was possible to coordinate the whole set of experimental results. (author)
Theory of heat transfer with forced convection film flows
Shang, Deyi
2010-01-01
Developing a new treatment of ""Free Convection Film Flows and Heat Transfer"" began in Shang's first monograph and is continued in this monograph. The current book displays the recent developments of laminar forced convection and forced film condensation. It is aimed at revealing the true features of heat and mass transfer with forced convection film flows to model the deposition of thin layers. The novel mathematical similarity theory model is developed to simulate temperature - and concentration - dependent physical processes. The following topics are covered in this book: Mathematical meth
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
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W.H.Azmi
2013-06-01
Full Text Available Nano?uids offer a significant advantage over conventional heat transfer ?uids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispersed in water. Regression equations are developed for the determination of the thermal conductivity and viscosity of nanofluids. The parameters influencing the decrease in convection heat transfer, observed by certain investigators, is explained.
Subcooled forced convection film boiling heat transfer
International Nuclear Information System (INIS)
In this article the theoretical calculation of film boiling heat transfer to subcooled liquid flowing upwards vertically is presented. Four heat transfer models have been proposed, analysis and comparison have been made in these models. The author takes into account both laminar and turbulent flow patterns for vapor film, different thermal boundary conditions and hydraulic conditions at the liquid-vapor interface, etc. The prediction is compared with reflooding experimental data, the steady state film boiling data. The agreement is satisfactory. At the same time, the heat transfer in self-modelling region is discussed
Plates of the dinosaur stegosaurus: forced convection heat loss fins?
Farlow, J O; Thompson, C V; Rosner, D E
1976-06-11
It is suggested that the plates along the arched back and tail of Stegosaurus served an important thermoregulatory function as forced convection "fins." Wind tunnel experiments on finned models, internal heat conduction calculations, and direct observations of the morphology and internal structure of stegosaur plates support this hypothesis, demonstrating the comparative effectiveness of the plates as heat dissipaters, controllable through input blood flow rate, temperature, and body orientation (with respect to wind). PMID:17748675
Forced convective post CHF heat transfer and quenching
International Nuclear Information System (INIS)
This paper discusses mechanisms in the post-CHF region which provide understanding and qualitative prediction capability for several current forced convective heat transfer problems. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced convective quenching are presented. Data reduction of quenching experiments is discussed, and the way in which the quenching transient may affect the results of different types of quenching experiments is investigated. This investigation provides an explanation of how minimum wall superheats greater than the homogeneous nucleation temperature result, as well as how these may appear to be either hydrodynamically or thermodynamically controlled. Finally, the results of a parametric study of the effects of the mechanisms upon the LOFT L2-3 hotpin calculation are presented
Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer
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Raf M. Podowski
2008-12-01
Full Text Available Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.
Comparative analysis of heat transfer correlations for forced convection boiling
International Nuclear Information System (INIS)
A critical survey was conducted of the most relevant correlations of boiling heat transfer in forced convection flow. Most of the investigations carried out on partial nucleate boiling and fully developed nucleate boiling have led to the formulation of correlations that are not able to cover a wide range of operating conditions, due to the empirical approach of the problem. A comparative analysis is therefore required in order to delineate the relative accuracy of the proposed correlations, on the basis of the experimental data presently available. The survey performed allows the evaluation of the accuracy of the different calculating procedure; the results obtained, moreover, indicate the most reliable heat transfer correlations for the different operating conditions investigated. This survey was developed for five pressure range (up to 180bar) and for both saturation and subcooled boiling condition
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Ghadimi, Mohammad; Ghadamian, Hossein [Islamic Azad Univ., Tehran (Iran, Islamic Republic of). Dept. of Energy Engineering, Science and Research Branch; Hamidi, Aliasghar A. [Tehran Univ. (Iran, Islamic Republic of). Dept. of Chemical Engineering; Fazelpour, Farivar [Islamic Azad Univ. of South Tehran Branch, Tehran (Iran, Islamic Republic of). Dept. of Energy System Engineering; Behghadam, Mehdi [Islamic Azad Univ. of Roudehen Branch, Tehran (Iran, Islamic Republic of). Dept. of Mechanical Engineering
2012-11-01
The present paper describes a two-dimensional finite volume numerical simulation of flow and heat transfer in airflow windows by free and forced convection techniques. The governing equations are the fully elliptic, Reynolds-averaged Navier-Stokes equations. The simple algorithm is employed to correct the pressure term. The second-order upwind scheme is used to discretize the convection terms. The (k-{epsilon}/RNG) turbulence model is applied for the flow simulation. The mesh used is the body-fitted, multi-plane grid system. Results on the variations of velocity and temperature profiles with geometrical parameters, at different temperature and velocity, for heat transfer by free and forced convection techniques are presented. Comparisons of the present results on temperature distribution for forced convection and for free convection with the available experimental forced convection data indicate that the airflow-influenced forced convection methods are considerably enhanced. (orig.)
Directory of Open Access Journals (Sweden)
S. I. ANWAR
2012-10-01
Full Text Available In this paper, convective heat transfer coefficient of Indian gooseberry (Emblica officinalis, in three different forms (shreds, slices and pieces, under forced convection mode has been determined. These forms were dried in laboratory drier. Values of constants C and n have been determined using experimental data and regression analysis for calculating values of convective heat transfer coefficient. It was found that the convective heat transfer coefficient varies with form of commodity being dried and decreases as the drying progresses. The value of convective heat transfer coefficient was highest for shredded form (30.39 W/m2oC followed by slices (25.88 W/m2oC and pieces (18.67 W/m2oC when compared at certain final moisture content. The data were also analyzed for per cent uncertainty.
Effect of Buoyancy on Forced Convection Heat Transfer in Vertical Channels - a Literature Survey
International Nuclear Information System (INIS)
This report contains a short resume of the available information from various sources on the effect of free convection flow on forced convection heat transfer in vertical channels. Both theoretical and experimental investigations are included. Nearly all of the theoretical investigations are concerned with laminar flow with or without internal heat generation. More consistent data are available for upward flow than for downward flow. Curves are presented to determine whether free convection or forced convection mode of heat transfer is predominant for a particular Reynolds number and Rayleigh number. At Reb > 105 free convection effects are negligible. Downward flow through a heated channel at low Reynolds number is unstable. Under similar conditions the overall heat transfer coefficient for downward flow tends to be higher than that for upward flow
International Nuclear Information System (INIS)
An experimental study on critical heat flux of forced convection boiling in uniformly heated vertical tube was carried out, using Freon-12 as the working fluid with critical heat flux qc measured in a 16 mm diameter circular tube. The effect of steam quality and mass velocity on CHF has been investigated. The results obtained were found to be essentially consistent with the CHF data measured by other authors. However, the influence of pressure on CHF was complex and coupled with mass velocity and steam quality. In the lower steam quality region, CHF decreased with increasing pressure, while in the higher steam quality region, CHF increased with increasing pressure
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
Dr. K. E. REBY ROY.; Rohit N Shenoy; Bibin Prasad
2012-01-01
Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dyna...
Analysis of Rectangular Microchannel under Forced convection heat transfer condition
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Dr. B.S.Gawali,
2011-03-01
Full Text Available Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturized high-performance pplications. Surface area is one of the important concepts for high flux heat transfer in Microchannel performance. Microchannel with hydraulic diameters 440?m, 476?m, 500?m and 550?m are analyzed for optimize microchannel hydraulic diameter. The microchannel having height of 400?m, 450?m, 500?m, 600?m with width of 500?m is analyzed numerically. Spacing between microchannel is also varied in range of 250?m, 300?m, 350?mand 400?m are considered for the analysis. Cu material microchannel having length of 30mm which carries 20 microchannels on top surface of the cu piece is to be considered. Flow rate also varied from 5lpm to 30 lpm for optimization with water as a medium. From numerical study it is observed that as hydraulic diameter increases from 444?m to 545 ?m the flow rate pressure drop also increases with decreases in diameter. Also heat in put to icrochannel as increases from 5 watt to 80 watt temperature drop is high at flow rate of 17lpm to 20 lpm. From analysis it is observed that as hydraulic diameter of microchannel is major concept of microchannel heat transfer which is dependent on flow rate of waterin microchannel. The microchannels with hydraulic diameter of 440?m to 600?m will follow temperature drop up-to 6 degree Kelvin with heat input of 5 watt to 80watt with flow rate of 5lpm to 25lpm.
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
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Dr. K. E. Reby Roy
2012-07-01
Full Text Available Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dynamics (CFD simulations using Fluent 6.3.26 are carried out for helical rectangular ducts wound over a cylindrical passage. The cylindrical passage is oriented horizontallyand acts as a counterflow heat exchanger. The analysis is done by changing the flow rates of four different fluids like Ethylene Glycol, Kerosene, Nano Fluid and Water. The fluid flow and heat transfer characteristics of the fluids are studied and Nusselt Number correlations with Dean Number are developed.
Boiling heat transfer with forced convection of LiBr-water solution in horizontal tube
International Nuclear Information System (INIS)
This paper examines the phenomena of boiling heat transfer and calculates the local heat transfer coefficient of a one-through boiler types regenerator for an absorption refrigerating machine. Boiling heat transfer with forced convection of Br-water solution in a horizontal tube is experimentally investigated. The results are summarized as follows. The flow patterns in the tube change from single phase flow to bubble flow, to stratified flow, to slug flow and finally, to annular flow. Local heat flux paries in proportion to the temperature difference between the tube wall and the solution. In the section where solution and vapor are supposed to flow as annular flow, the boiling heat transfer coefficient can be explained by the Dengler-Addoms expression. An the forced convection boiling heat transfer coefficients are higher than the pool boiling heat transfer coefficient
Forced convection heat transfer correlation for finned plates in a duct
International Nuclear Information System (INIS)
Forced convection heat transfer experiments were conducted for plate-fin in a duct using various fin spacing, fin height, duct width, Reynolds number for Prandtl numbers 2,014. Based upon analogy concept, mass transfer rate were measured instead of heat transfer rates. The heat transfer rates were enhanced with the increase of fin height and decrease of fin spacing as they increase the heat transfer area. Meanwhile, heat transfer rates were impaired with the increase of the duct width as the bypass flows increased to tip clearance region. Forced convection heat transfer correlations were developed for laminar and turbulent flow conditions and for narrow and wide ducts. The work draws attention to the tip clearance on the heat transfer of the finned plate in a duct. (author)
Prediction of forced convection heat transfer to Lead-Bismuth-Eutectic
Thiele, Roman
2013-01-01
The goal of this work is to investigate the capabilities of two different commercial codes, OpenFOAM and ANSYS CFX, to predict forced convection heat transfer in low Prandtl number fluids and investigate the sensitivity of these predictions to the type of code and to several input parameters.The goal of the work is accomplished by predicting forced convection heat transfer in two different experimental setups with the codes OpenFOAM and ANSYS CFX using three different turbulence models and va...
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V<0.3 m/s, due to the natural convection were found. The results may be useful during design of air distribution in rooms, e.g. low impulse ventilation, diffuse ventilation, etc.
International Nuclear Information System (INIS)
In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code 'STAR-CCM+'. We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 – 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.
International Nuclear Information System (INIS)
Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32 m, and 10 mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, hTP/hLo, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio hTP/hLo was expressed in terms of Martinelli parameter, Xtt, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large. (author)
Experiment of forced convection heat transfer using microencapsulated phase-change-material slurries
International Nuclear Information System (INIS)
The present study describes an experiment on forced convective heat transfer using a water slurry of Microencapsulated Phase-change-material. A normal paraffin hydrocarbon is microencapsulated by melamine resin, melting point of 28.1degC. The heat transfer coefficient and pressure drop in a circular tube were evaluated. The heat transfer coefficient using the slurry in case with and without phase change were compared to in case of using pure water. (author)
Nanofluid flow and forced convection heat transfer over a stretching surface considering heat source
Mohammadpour, M.; Valipour, P.; Shambooli, M.; Ayani, M.; Mirparizi, M.
2015-07-01
In this paper, magnetic field effects on the forced convection flow of a nanofluid over a stretching surface in the presence of heat generation/absorption are studied. The equations of continuity, momentum and energy are transformed into ordinary differential equations and solved numerically using the fourth-order Runge-Kutta integration scheme featuring the shooting technique. Different types of nanoparticles as copper (Cu), silver (Ag), alumina (Al2O3) and titania (TiO2) with water as their base fluid has been considered. The influence of significant parameters, such as magnetic parameter, volume fraction of the nanoparticles, heat generation/absorption parameter, velocity ratio parameter and temperature index parameter on the flow and heat transfer characteristics are discussed. The results show that the values of temperature profiles increase with increasing heat generation/absorption and volume fraction of the nanoparticles but they decrease with increasing velocity ratio parameter and temperature index parameter. Also, it can be found that selecting silver as nanoparticle leads to the highest heat transfer enhancement.
Forced convection heat exchange inside porous sintered metals
International Nuclear Information System (INIS)
Methods and results of investigating heat exchange in the process of liquid flow inside porous sintered metals have been analyzed. It has been shown that experimental data available include extremely conflicting correlations between heat transfer coefficient and Reynolds number, porosity, and relative wall thickness. Scattering of the data can attain one order of magnitude. The volume coefficient of heat transfer inside pores determined in papers does not correspond to its real value in the initial equations of the inner problem of porous cooling. Calculating and experimental method of determining the heat transfer coefficient has been developed and realized on the unit of radiation heating. More accurate experimental data on intraporous heat exchange have been obtained. It has been established that relative wall thickness does not affect the intensity of heat transfer inside pores
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris; Melikov, Arsen Krikor
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikinâ€™s body was as the skin...... temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ÂºC for cooling and at 20ÂºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant...... temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination of the...
An Experimental Study of Heat Transfer During Forced Air Convection
Xiao, Bowang; Wang, Gang; Wang, Qigui; Maniruzzaman, Mohammed; Sisson, Richard D.; Rong, Yiming
2011-10-01
Cast aluminum alloys are usually subject to solution treatment, quenching, and aging hardening for improved mechanical properties. Cooling rate during quenching plays an important role in residual stress, distortion, and mechanical property distributions in the resultant cast aluminum components. As the cooling rates of work pieces heavily depend on the interfacial heat transfer coefficient (HTC) between work pieces and quenchants, it is important to understand how HTC varies with different quenching conditions so that optimal quenching process can be achieved. In this study, a quenching system and an experimental procedure of obtaining HTC are presented. A series of experiments have been conducted to study the variations of HTC with respect to air temperature, air humidity, air velocity, and part orientation.
International Nuclear Information System (INIS)
In a high-level waste (HLW) repository, heat is generated by the radioactive decay of the waste. This can affect the safety of the repository because the surrounding environment can be changed by the heat transfer through the rock. Thus, it is important to determine the heat transfer coefficient of the atmosphere in the underground repository. In this study, the heat transfer coefficient was estimated by measuring the indoor environmental factors in the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) under forced convection. For the experiment, a heater of 5 kw capacity, 2 meters long, was inserted through the tunnel wall in the heating section of KURT in order to heat up the inside of the rock to 90 .deg. C, and fresh air was provided by an air supply fan connected to the outside of the tunnel. The results showed that the average air velocity in the heating section after the provision of the air from outside of the tunnel was 0.81 m/s with the Reynolds number of 310,000 ? 340,000. The seasonal heat transfer coefficient in the heating section under forced convection was 7.68 W/m2 K in the summer and 7.24 W/mm2 K in the winter
Energy Technology Data Exchange (ETDEWEB)
Yoon, Chan Hoon; Hwang, In Phil; Kim, Jin [Inha University, Incheon (Korea, Republic of); Kwon, Sang Ki [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2010-09-15
In a high-level waste (HLW) repository, heat is generated by the radioactive decay of the waste. This can affect the safety of the repository because the surrounding environment can be changed by the heat transfer through the rock. Thus, it is important to determine the heat transfer coefficient of the atmosphere in the underground repository. In this study, the heat transfer coefficient was estimated by measuring the indoor environmental factors in the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) under forced convection. For the experiment, a heater of 5 kw capacity, 2 meters long, was inserted through the tunnel wall in the heating section of KURT in order to heat up the inside of the rock to 90 .deg. C, and fresh air was provided by an air supply fan connected to the outside of the tunnel. The results showed that the average air velocity in the heating section after the provision of the air from outside of the tunnel was 0.81 m/s with the Reynolds number of 310,000 {approx} 340,000. The seasonal heat transfer coefficient in the heating section under forced convection was 7.68 W/m{sup 2} K in the summer and 7.24 W/mm{sup 2} K in the winter.
Experimental and theoretical study on forced convection film boiling heat transfer
International Nuclear Information System (INIS)
Theoretical solutions of forced convection film boiling heat transfer from horizontal cylinders in saturated liquids were obtained based on a two-phase laminar boundary layer film boiling model. It was clarified that author's experimental data for the cylinders with the nondimensional diameters, D, of around 1.3 in water and in Freon-113 agreed with the values of theoretical numerical solutions based on the two-phase laminar boundary layer model with the smooth vapor-liquid interface except those for low flow velocities. A forced convection film boiling heat transfer correlation including the radiation contribution from the cylinders with various diameters in saturated and subcooled liquids was developed based on the two-phase laminar boundary layer film boiling model and the experimental data for water and Freon-113 at wide ranges of flow velocities, surface superheats, system pressures and cylinder diameters. (author)
The effect of internal ribbing on forced convective heat transfer in circular-sectioned tubes
International Nuclear Information System (INIS)
This paper presents the results of an experimental examination of the effect of internal circumferential ribs on forced convection in circular-sectioned tubes. The work is relevant to the internal cooling of gas turbine rotor blades. The influence of rib geometry is investigated for three different rib configurations and simple design-type, empirical equations are developed for estimating heat transfer at rib and mid-rib locations. It is demonstrated that heat transfer may be improved by up to three fold in relation to fully developed forced convection in smooth-walled tubes. The geometric parameters which have been used for the experiments are typical of those currently applied to gas turbine blade cooling designs
Burnout in boiling heat transfer. Part III. High-quality forced-convection systems
International Nuclear Information System (INIS)
This is the final part of a review of burnout during boiling heat transfer. The status of burnout in high-quality forced-convection systems is reviewed, and recent developments are summarized in detail. A general guide to the considerable literature is given. Parametric effects and correlations for water in circular and noncircular ducts are presented. Other topics discussed include transients, steam-generator applications, correlations for other fluids, fouling, and augmentation
International Nuclear Information System (INIS)
For development of new reactor, supercritical water is expected to be used as coolant to improve thermal efficiency. However, the thermal characteristics of supercritical fluid is not revealed completely because its difficulty for experiment. Specific phenomena tend to occur near the pseudo-boiling point which is characterised by temperature corresponding to the saturation point in ordinary fluid. Around this point, the physic properties such as density, specific heat and thermal conductivity are drastically varying. Although there is no difference between gas and liquid phases in supercritical fluids, phenomena similar to boiling (with heat transfer deterioration) can be observed round the pseudo-boiling point. Experiments of heat transfer have been done for supercritical fluid in forced convective condition. However, these experiments were mainly realised inside stainless steel cylinder pipes, for which flow visualisation is difficult. Consequently, this work has been devoted to the development of method allowing the visualisation of supercritical flows. The experiment setup is composed of main loop and test section for the visualisation. Carbon dioxide is used as test fluid. Supercritical carbon dioxide flows upward in rectangular channel and heated by one-side wall to generate forced convection heat transfer. Through window at mid-height of the test section, shadowgraphy was applied to visualize density gradient distribution. The behavior of the density wave in the channel is visualized and examined through the variation of the heat transfer coefficient. (author)
Wall heat transfer coefficient for condensation and boiling in forced convection of sodium
International Nuclear Information System (INIS)
The wall heat transfer coefficient for sodium boiling and condensation in forced convective flow is theoretically derived by using the momentum-heat transfer analogy and the logarithmic law for velocity distribution in the liquid film. Only one constant in the logarithmic form is empirically determined. The results are insensitive to some of the approximations used, namely in the evaluation of E and of /tau/ /SUB i/ //tau/ /SUB w/. The results from the suggested correlation are in good agreement with the Zeigarnick and Litvinov data over the range of parameters that were specified in their experiments, while the Chen and NATOF correlations predict lower heat transfer coefficients. The suggested correlation predicts well the data in the high-heat transfer coefficient region. It can be argued that the data in the low heat transfer coefficient region are affected by unstable flow conditions and the uncertainties in the saturation-temperature measurements
Forced Convection Heat Transfer Experiments of the Finned Plate in a Duct
Energy Technology Data Exchange (ETDEWEB)
Chae, Myeongseon; Moon, Jeyoung; Chung, Bumjin [Kyung Hee Univ., Yongin (Korea, Republic of)
2014-05-15
The studies have been focused on the optimization of fin geometries to maximize the heat transfer rate. The forced convection heat transfer rates were affected largely by the fin spacing, fin height, and tip clearance. As the fin spacing decreases and fin height increases, heat transfers from the fins to the ambient are enhanced as they are directly proportional to the surface area. For a large tip clearance, the fluid tends to escape from the inner fin region to the outer wall region resulting in the decrease of the overall heat removal capability. Thus, the parametric influences of these variables are to be investigated to develop a generalized heat transfer correlation for the geometry. This study is a preliminary experimental study for plate-fin geometries such as fin spacing, fin height and duct width. Mass transfer experiments were carried out based on the analogy concept, using a copper sulfate electroplating system. The work has the relevance with the Reactor Cavity Cooling System performance enhancement study in the VHTR. Forced convection heat transfer experiments were performed for the vertical plate-fins in a duct. Based on the analogy between heat and mass transfer systems, mass transfer rates were measured using the cupric acid copper sulfate electroplating system. The fin spacings were varied from 0.002m to 0.007m, fin heights 0.01m and 0.015m, Re{sub Dh} from 10 to 6,500, and duct widths from 0.010m to 0.02m. The test results showed that the heat transfer rates enhanced with the increase of fin height and the decrease of fin spacing as they enlarge the heat transfer area. And the heat transfer rates were impaired with the increase of the duct width as the bypass flows increased to tip clearance region.
Forced Convection Heat Transfer Experiments of the Finned Plate in a Duct
International Nuclear Information System (INIS)
The studies have been focused on the optimization of fin geometries to maximize the heat transfer rate. The forced convection heat transfer rates were affected largely by the fin spacing, fin height, and tip clearance. As the fin spacing decreases and fin height increases, heat transfers from the fins to the ambient are enhanced as they are directly proportional to the surface area. For a large tip clearance, the fluid tends to escape from the inner fin region to the outer wall region resulting in the decrease of the overall heat removal capability. Thus, the parametric influences of these variables are to be investigated to develop a generalized heat transfer correlation for the geometry. This study is a preliminary experimental study for plate-fin geometries such as fin spacing, fin height and duct width. Mass transfer experiments were carried out based on the analogy concept, using a copper sulfate electroplating system. The work has the relevance with the Reactor Cavity Cooling System performance enhancement study in the VHTR. Forced convection heat transfer experiments were performed for the vertical plate-fins in a duct. Based on the analogy between heat and mass transfer systems, mass transfer rates were measured using the cupric acid copper sulfate electroplating system. The fin spacings were varied from 0.002m to 0.007m, fin heights 0.01m and 0.015m, ReDh from 10 to 6,500, and duct widths from 0.010m to 0.02m. The test results showed that the heat transfer rates enhanced with the increase of fin height and the decrease of fin spacing as they enlarge the heat transfer area. And the heat transfer rates were impaired with the increase of the duct width as the bypass flows increased to tip clearance region
Convective heat transfer on Mars
International Nuclear Information System (INIS)
An examination was made into the feasibility of using convective heat transfer on Mars to reject the waste heat from a Closed Brayton Cycle. Forced and natural convection were compared to thermal radiation. For the three radiator configurations studied, it was concluded that thermal radiation will yield the minimum mass and forced convection will result in the minimum area radiator. Other issues such as reliability of a fan motor were not addressed. Convective heat transfer on Mars warrants further investigation. However, the low density of the Martian atmosphere makes it difficult to utilize convective heat transfer without incurring a weight penalty
Analysis of Forced Convection Heat Transfer for Axial Annular Flow of Giesekus Viscoelastic Fluid
Energy Technology Data Exchange (ETDEWEB)
Mohseni, Mehdi Moayed; Rashidi, Fariborz; Movagar, Mohammad Reza Khorsand [Amirkabir University of Technology, Tehran (Iran, Islamic Republic of)
2015-02-15
Analytical solutions for the forced convection heat transfer of viscoelastic fluids obeying the Giesekus model are obtained in a concentric annulus under laminar flow for both thermal and hydrodynamic fully developed conditions. Boundary conditions are assumed to be (a) constant fluxes at the walls and (b) constant temperature at the walls. Temperature profiles and Nusselt numbers are derived from dimensionless energy equation. Subsequently, effects of elasticity, mobility parameter and viscous dissipation are discussed. Results show that by increasing elasticity, Nusselt number increases. However, this trend is reversed for constant wall temperature when viscous dissipation is weak. By increasing viscous dissipation, the Nusselt number decreases for the constant flux and increases for the constant wall temperature. For the wall cooling case, when the viscous dissipation exceeds a critical value, the generated heat overcomes the heat which is removed at the walls, and fluid heats up longitudinally.
Performance of a forced convection solar drier integrated with gravel as heat storage material
Energy Technology Data Exchange (ETDEWEB)
Mohanraj, M. [Dr Mahalingam College of Engineering and Technology, Pollachi (India). Dept. of Mechanical Engineering; Chandrasekar, P. [Swinburne Univ. of Technology, Sarawak (Malaysia). School of Engineering Sciences
2009-07-01
Sun drying is the most common method used in India to dry agricultural products such as grains, fruits and vegetables. The rate of drying depends on solar radiation, ambient temperature, wind velocity, relative humidity, initial moisture content, type of crops, crop absorptivity and mass product per unit exposed area. However, this method of spreading the crop in a thin layer on the ground has several disadvantages. This paper reported on a study that focused on developing a forced convection solar drier integrated with heat storage materials for drying various agricultural crops. The indirect forced convection solar drier, integrated with gravel as a sensible heat material, was used to dry pineapple slices under conditions similar to those found in Pollachi, India. The performance of the system was discussed along with the drying characteristics, drying rate, and specific moisture extraction rate. The results showed that the moisture content (wet basis) of pineapple was reduced from about 87.5 to 14.5 per cent (equilibrium moisture content) in about 29 hours in the bottom tray and 32 hours in the top tray. The thermal efficiency of the solar air heater was also reviewed. 9 refs., 5 figs.
International Nuclear Information System (INIS)
In the first part, free-convection and nucleate pool boiling heat transfer (up to burn-out heat flux) between a platinum wire of 0.15 mm in diameter in neon, deuterium and hydrogen has been studied at atmospheric pressure. These measurements were continued in liquid neon up to 23 bars (Pc ? 26.8 b). Film boiling heat transfer coefficients have been measured in pool boiling liquid neon at atmospheric pressure with three heating wires (diameters 0.2, 0.5, 2 mm). All the results have been compared with existing correlations. The second part is devoted to measurements of the critical heat flux limiting heat transfer with small temperature differences between the wall and the liquid neon flowing inside a tube (diameters 3 x 3.5 mm) heated by joule effect on 30 cm of length. Influences of flow stability, nature of electrical current, pressure, mass flow rate and subcooling are shown. In conclusion, the similarity of the heat transfer characteristics in pool boiling as well as in forced convection of liquid neon and hydrogen is emphasized. (author)
Yarin, Alexander; Freystein, Martin; Kolberg, Felix; Sinha-Ray, Sumit; Sahu, Rakesh; Spiegel, Lucas; Gambaryan-Roisman, Tatiana; Stephan, Peter
2015-03-01
To enhance heat transfer in forced convective boiling the microchannel bottom was amended by a nano-texture - periodic rectangular mats of electrospun polymer nanofibers. The fibers were ~ 300-500 nm in diameter and the mat thicknesses were about 6-15 ?m. The test fluid was FC-72 and the flow in microchannels contained trains of Taylor bubbles. The role of the nanofibers was to retain the warm microchannel bottom wet, to prevent dry-out and thus to enhance the heat removal rate. In the present experiments the time-average heat flux and heat transfer coefficient at the nanofiber-coated domains were found to be 1.5-2 times higher than those at the uncoated ones. Accordingly, a significant decrease (by 5-8 K) in the superheat was observed at the same Re of 387 and power supply of 36.1 kW/m2. At a higher Re of 432 and lower power supply of 28.1 kW/m2 similar trends in the heat removal rate and surface superheat were found. The significant enhancement of the heat transfer results from the fact that nanofiber mats facilitate wetting of surface under passing Taylor bubbles, thus delaying formation of vapor flow at the channel bottom. The interstices of the nanofiber mat act as the nucleation sites facilitating formation of tiny bubbles, which eventually results in a higher heat removal rate from the surface at a reduced superheat.
Forced convection heat transfer in structure packed beds: packing form and particle shape
Energy Technology Data Exchange (ETDEWEB)
Jian, Yang; Min, Zeng; Qiuwang, Wang [Xi' an Jiaotong University, Xi' an (China); Xiao, Yan [Nuclear Power Institute of China, Chengdu (China)
2009-07-01
A forced convection heat transfer inside micro pores of structure packed beds with spherical or ellipsoidal particles are numerically studied in this paper. Three-dimensional Navier Stokes equations and RNG k-{epsilon} turbulent model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are investigated in detail. The macroscopic hydrodynamic and heat transfer results are obtained from micro pore cells by using integrating method. The results show that, with the same physical parameters, the pressure drops in structure packed beds are much lower than those in randomly packed beds while the overall heat transfer efficiencies (except SC packing) are much higher. The traditional correlations of flow and heat transfer extracted from randomly packings are unavailable for structured packings, and some modified correlations are obtained. Furthermore, it finds that, with the same particle shape (sphere), the overall heat transfer performance of SC packing is the best and with the same packing form (FCC), the overall heat transfer performance of packing with long ellipsoidal particles is the best.
Reliability comparison of forced and natural convection residual heat removal in the GCFR
International Nuclear Information System (INIS)
Loss of cooling consequences for all breeder cores require an increased reliability of the engineered Residual Heat Removal (RHR) systems provided to assure abundant cooling of the core at decay heat levels. An upflow GCFR core design offers the capability for pressurized decay heat removal by natural convection, thus enhancing core cooling reliability and diversity. A quantitative assessment is presented for the Residual Heat Removal reliability achievable with and without natural convection. The reliability gains due to natural convection are limited by the demand frequency for PCRV depressurization and by the equipment which has to change state in order to establish natural convection. The coolant circulation diversity accomplished with natural convection is a major advantage
LAMINAR FORCED CONVECTION HEAT TRANSFER OVER STAGGERED CIRCULAR TUBE BANKS: A CFD APPROACH
Directory of Open Access Journals (Sweden)
Tahseen A. Tahseen
2013-06-01
Full Text Available This paper presents the numerical study of two-dimensional forced convection heat transfer for staggered tube banks in cross flow under incompressible, steady-state conditions. This system is solved for body-fitted coordinates using the finite volume method for flow over a bundle of cylindrical tubes. A constant heat flux is imposed on the surface of the tubes as the thermal boundary condition. The type of arrangement considered is a set of staggered tubes. Ratios of longitudinal pitch to tube diameter (ST/D of 1.25, 1.5, and 2 are considered. Reynolds numbers are varied from 25 to 250 and the Prandtl number is taken as 0.71. Velocity field vectors, temperature contours, and the local and average Nusselt numbers are analyzed in this paper. It can be seen that the predicted results are in good agreement with experimental and numerical results obtained previously. The obtained results show that the heat transfer rate increases with a reduction in the step of the longitudinal tube diameter. The local heat transfer depends strongly on the Reynolds number. The highest values are obtained at the surface opposite to the direction of flow. The heat transfer rate is insignificant in the areas of recycling.
An assessment of correlations of forced convection heat transfer to water at supercritical pressure
International Nuclear Information System (INIS)
Highlights: • Evaluates 26 correlations of heat transfer to supercritical water. • Compiles 3220 experimental data points for the evaluation. • Evaluates based on both entire database and three heat transfer regimes. • Provides a guide to choosing the best correlation for engineering design. - Abstract: The heat transfer of supercritical water is essential for supercritical water-cooled nuclear reactors. Many empirical correlations for heat transfer to supercritical water were proposed over the past few decades. Some evaluations of the correlations were conducted, and inconsistent conclusions appeared owing to limited correlations or experimental data. This work presents an extensive survey of the literature of correlations and experiments of forced convection heat transfer to water flowing upward in vertical tubes at supercritical pressure. There are 26 correlations found, and an experimental database containing 3220 data points from vertical tubes are compiled from nine independent laboratories. All available correlations are assessed against the experimental database. The results show that the best correlation has a mean absolute deviation of 12.8%, predicting 82.3% of the database within ±20%. The entire database is divided into three categories, and the correlations which can give the most accurate predictions of the experimental data from different categories are also identified. The results provide a guide to choosing a proper correlation for engineering practice. Some topics worthy of attention for future studies are indicated
Conjugate forced convection heat transfer in a plane channel: Longitudinally periodic regime
International Nuclear Information System (INIS)
The present paper studies the conjugate heat transfer problem in a parallel-plane channel. Laminar and stationary forced convection is studied, with a boundary condition given by a temperature distribution on the external side of the channel wall, which undergoes a sinusoidal longitudinal change. The local energy balance equation is written with reference to the fully developed region, where the temperature distribution can be expressed as a periodic function of the longitudinal coordinate. The temperature field in the solid wall and in the fluid, as well as the local and average Nusselt number, are determined analytically and numerically. A comparison between the values obtained analytically, by employing a complex temperature method, and those evaluated numerically, by employing a Bubnov-Galerkin finite element method, reveals an excellent agreement. (authors)
Improved modelling of turbulent forced convective heat transfer in straight ducts
International Nuclear Information System (INIS)
Ducts with non-circular cross sections are frequently encountered in industrial heat transfer equipment, e.g., compact heat exchangers, cooling channels in gas turbine blades, nuclear reactors, etc. This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in their fully developed state at low Reynolds number. The authors have developed a low Reynolds number version of the non-linear ?-? model combined with the heat flux models of SED, low Reynolds number version of GGDH and WET in general three dimensional geometries. The wall function approach is abandoned and the friction factor is predicted in agreement with experiments. The Nusselt numbers are also predicted very well. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with the linear and the nonlinear ?-? model, combined with the Lam-Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns are also of major concern
Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop
Energy Technology Data Exchange (ETDEWEB)
Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover
2010-09-01
This report develops a proposal to design and construct a forced convection test loop. A detailed test plan will then be conducted to obtain data on heat transfer, thermodynamic, and corrosion characteristics of the molten salts and fluid-solid interaction. In particular, this report outlines an experimental research and development test plan. The most important initial requirement for heat transfer test of molten salt systems is the establishment of reference coolant materials to use in the experiments. An earlier report produced within the same project highlighted how thermophysical properties of the materials that directly impact the heat transfer behavior are strongly correlated to the composition and impurities concentration of the melt. It is therefore essential to establish laboratory techniques that can measure the melt composition, and to develop purification methods that would allow the production of large quantities of coolant with the desired purity. A companion report describes the options available to reach such objectives. In particular, that report outlines an experimental research and development test plan that would include following steps: •Molten Salts: The candidate molten salts for investigation will be selected. •Materials of Construction: Materials of construction for the test loop, heat exchangers, and fluid-solid corrosion tests in the test loop will also be selected. •Scaling Analysis: Scaling analysis to design the test loop will be performed. •Test Plan: A comprehensive test plan to include all the tests that are being planned in the short and long term time frame will be developed. •Design the Test Loop: The forced convection test loop will be designed including extensive mechanical design, instrument selection, data acquisition system, safety requirements, and related precautionary measures. •Fabricate the Test Loop. •Perform the Tests. •Uncertainty Analysis: As a part of the data collection, uncertainty analysis will be performed to develop probability of confidence in what is measured in the test loop. Overall, the testing loop will allow development of needed heat transfer related thermophysical parameters for all the salts, validate existing correlations, validate measuring instruments under harsh environment, and have extensive corrosion testing of materials of construction.
Directory of Open Access Journals (Sweden)
P. MOHAJERI KHAMENEH,
2010-11-01
Full Text Available Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using a finite volume-based computational fluid dynamics technique. In this article, at each Z-location variation of dimensionless local temperature, nondimensional local heat flux variation and dimensionless local Nusselt number distribution along the tube length were compared between round tube and straight microchannel heat exchangers. Consequently, averaged computational Nusselt number was obtained for those indicated models and then validation study was performed for round tube counter flow type heat exchanger model. Finally, all of these numerical results for both kind of geometries in counter flow heat exchangers were discussed in details.
Energy Technology Data Exchange (ETDEWEB)
Francisco Valentin; Narbeh Artoun; Masahiro Kawaji; Donald M. McEligot
2015-08-01
Fundamental high pressure/high temperature forced convection experiments have been conducted in support of the development of a Very High Temperature Reactor (VHTR) with a prismatic core. The experiments utilize a high temperature/high pressure gas flow test facility constructed for forced convection and natural circulation experiments. The test section has a single 16.8 mm ID flow channel in a 2.7 m long, 108 mm OD graphite column with four 2.3kW electric heater rods placed symmetrically around the flow channel. This experimental study presents the role of buoyancy forces in enhancing or reducing convection heat transfer for helium at high pressures up to 70 bar and high temperatures up to 873 degrees K. Wall temperatures have been compared among 10 cases covering the inlet Re numbers ranging from 500 to 3,000. Downward flows display higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases due to the influence of buoyancy forces. In the entrance region, convection heat transfer is reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancyinduced mixing causes higher convection heat transfer and lower wall temperatures. However, their influences are reduced as the Reynolds number increases. This experimental study is of specific interest to VHTR design and validation of safety analysis codes.
Forced convection heat transfer with phase-change-material slurries: Turbulent flow in circular tube
Choi, Eunsoo; Cho, Young I.; Lorsch, Harold G.
1994-01-01
The present study investigates the increase in the convective heat transfer coefficient as well as the increase in the thermal capacity of a working fluid by using the latent heat from a solid-liquid phase change of particles. A long heating test section (627 diameters) with a uniform heat flux boundary condition is constructed in order to study the effects of the phase-change phenomenon produced by a phase-change-material (PCM) -- water slurry on the convective heat transfer coefficient in a turbulent flow. The study introduces a method to generate very fine PCM particles inside a flow loop using an emulsifier. With such fine PCM particles, the flow loop did not clog. Local pressure drops and local heat transfer coefficients are measured along the test section. The pressure drop significantly decreased at the point where the PCM particles in the slurry melted. The local convective heat transfer coefficient was found to vary significantly when the particles melted. This made it difficult to apply the log-mean-temperature-difference (LMTD) method to the analysis of the PCM slurry flow heat transfer. The study proposes a new three-region melting model, and provides an explanation of the physical mechanism of the convective heat transfer enhancement due to the PCM particles.
Laminar forced convection heat transfer to a single layer of ordered and disordered spheres
International Nuclear Information System (INIS)
We study laminar forced convection heat transfer to single layer arrays of equidistantly and non-equidistantly spaced spheres. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on sphere diameter and the free stream velocity) between 0.1 and 100. For equidistantly spaced arrays of spheres we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant arrays exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single spheres in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant arrays is lower than in equidistant arrays for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.
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Three-dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in microchannels with small rectangular sections having specific hydraulic diameters and distinct geometric configurations were investigated numerically. The numerical results indicated that the laminar heat transfer was to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center to center distance of the microchannels. The geometries and operating conditions of that indicated microchannel were created using a finite volume-based computational fluid dynamics technique. The aim of this paper is to obtain computational Nusselt number in laminar flow using Fluent CFD Solver and to validate it with available experimental studies. Acquired numerical results have an admissible compatibility with available experimental studies. In addition, conceivable temperature profiles and pressure drops have been obtained at each Z-location in this model. Then, pressure drop values in the present model were explored for each Reynolds number. Finally, the effects of geometric parameters on the average Nusselt number in the laminar flow were illustrated numerically.
International Nuclear Information System (INIS)
In the present study, spray cooling heat transfer was experimentally investigated for the case in which water is sprayed onto the surfaces of micro-fins in forced convection and nucleate boiling regions. The experimental results show that an increase in the droplet flow rate improves heat transfer due to forced convection and nucleate boiling in the both case of smooth surface and surfaces of micro-fins. However, the effect of subcooling for fixed droplet flow rate is very weak. Micro-fins surfaces enhance the spray cooling heat transfer significantly. In the dilute spray region, the micro-fin structure has a significant effect on the spray cooling heat transfer. However, this effect is weak in the dense spray region. A previously determined correlation between the Nusselt number and Reynolds number shows good agreement with the present experimental data for a smooth surface
Laminar forced convective/conductive heat transfer by finite element method
International Nuclear Information System (INIS)
The present study is directed at developing a finite element computer program for solution of decoupled convective/conductive heat transfer problems. Penalty function formulation has been used to solve momentum equations and subsequently transient energy equation is solved using modified Crank-Nicolson method. The optimal upwinding scheme has been employed in energy equation to remove oscillations at high Peclet number. (author)
International Nuclear Information System (INIS)
In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described
Energy Technology Data Exchange (ETDEWEB)
Wu, Shuang Ying; Chen, Su Jun; Xiao, Lan; Li, You Rong [Chongqing University, Chongqing (China)
2011-06-15
In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described.
Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said
2015-09-01
Forced convection is a phenomenon associated with the heat transfer fluid flows. The presence of convection affects simultaneously the thermal and hydrodynamic fields, the problem is thus coupled. This form of heat transfer inside ducts occurs in many practical applications such as solar collectors, heat exchangers, cooling of electronic components as well as chemical and nuclear. In this work, we are interested primarily for a numerical study of thermo-hydraulic performances of an incompressible turbulent flow of air through a heat sink composed of several rows of bars of square section. Profiles and the axial velocity fields, as well as profiles and the distribution of the Nusselt number are plotted for all the geometry considered and chosen for different sections. The effects of geometrical parameters of the model and the operating parameters on the dynamic and thermal behavior of the air are analyzed.
Forced Convection Boiling and Critical Heat Flux of Ethanol in Electrically Heated Tube Tests
Meyer, Michael L.; Linne, Diane L.; Rousar, Donald C.
1998-01-01
Electrically heated tube tests were conducted to characterize the critical heat flux (transition from nucleate to film boiling) of subcritical ethanol flowing at conditions relevant to the design of a regeneratively cooled rocket engine thrust chamber. The coolant was SDA-3C alcohol (95% ethyl alcohol, 5% isopropyl alcohol by weight), and tests were conducted over the following ranges of conditions: pressure from 144 to 703 psia, flow velocities from 9.7 to 77 ft/s, coolant subcooling from 33 to 362 F, and critical heat fluxes up to 8.7 BTU/in(exp 2)/sec. For the data taken near 200 psia, critical heat flux was correlated as a function of the product of velocity and fluid subcooling to within +/- 20%. For data taken at higher pressures, an additional pressure term is needed to correlate the critical heat flux. It was also shown that at the higher test pressures and/or flow rates, exceeding the critical heat flux did not result in wall burnout. This result may significantly increase the engine heat flux design envelope for higher pressure conditions.
Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State
Balouch, Masih N.
Worldwide research is being conducted to improve the efficiency of nuclear power plants by using supercritical water (SCW) as the working fluid. One such SCW reactor considered for future development is the CANDU-Supercritical Water Reactor (CANDU-SCWR). For safe and accurate design of the CANDU-SCWR, a detailed knowledge of forced-convection heat transfer in SCW is required. For this purpose, two supercritical fluid loops, i.e. a SCW loop and an R-134a loop are developed at Carleton University. The SCW loop is designed to operate at pressures as high as 28 MPa, temperatures up to 600 °C and mass fluxes of up to 3000 kg/m2s. The R-134a loop is designed to operate at pressures as high as 6 MPa, temperatures up to 140 °C and mass fluxes in the range of 500-6000 kg/m2s. The test loops designs allow for up to 300 kW of heating power to be imparted to the fluid. Both test loops are of the closed-loop design, where flow circulation is achieved by a centrifugal pump in the SCW loop and three parallel-connected gear pumps in the R-134a loop, respectively. The test loops are pressurized using a high-pressure nitrogen cylinder and accumulator assembly, which allows independent control of the pressure, while simultaneously dampening pump induced pressure fluctuations. Heat exchangers located upstream of the pumps control the fluid temperature in the test loops. Strategically located measuring instrumentation provides information on the flow rate, pressure and temperature in the test loops. The test loops have been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a seven-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test-section geometry. The design of both test loops allows for easy reconfiguration of the test-section orientation relative to the gravitational direction. All the test sections are of the directly-heated design, where electric current passing through the pressure retaining walls of the test sections provides the Joule heating required to heat up the fluid to supercritical conditions. A high-temperature dielectric gasket isolates the current carrying parts of the test section from the rest of the assembly. Temperature and pressure drop data are collected at the inlet and outlet, and along the heated length of the test section. The test loops and test sections are designed according to American Society of Mechanical Engineers (ASME) Pressure Piping B31.1, and Boiler and Pressure Vessel Code, Section VIII-Division 1 rules. The final test loops and test sections assemblies are certified by Technical Standards and Safety Authority (TSSA). Every attempt is made to use off-the-shelf components where possible in order to streamline the design process and reduce costs. Following a rigorous selection process, stainless steel Types 316 and 316H are selected as the construction materials for the test loops, and Inconel 625 is selected as the construction material for the test sections. This thesis describes the design of the SCW and R-134a loops along with the three test-section geometries (i.e., tubular, annular and bundle designs).
Burnout in boiling heat transfer. Part II: subcooled and low quality forced-convection systems
International Nuclear Information System (INIS)
Recent experimental and analytical developments regrading burnout in subcooled and low quality forced-convection systems are reviewed. Much data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries and power transients
Transient convective heat transfer
Scientific Electronic Library Online (English)
J., Padet.
2005-03-01
Full Text Available In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calcul [...] ation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions) and those that are due to internal causes (sources of variable power, instabilities, turbulence), and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature) are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.
Kakac, Sadik; Pramuanjaroenkij, Anchasa
2014-01-01
Intended for readers who have taken a basic heat transfer course and have a basic knowledge of thermodynamics, heat transfer, fluid mechanics, and differential equations, Convective Heat Transfer, Third Edition provides an overview of phenomenological convective heat transfer. This book combines applications of engineering with the basic concepts of convection. It offers a clear and balanced presentation of essential topics using both traditional and numerical methods. The text addresses emerging science and technology matters, and highlights biomedical applications and energy technologies. What’s New in the Third Edition: Includes updated chapters and two new chapters on heat transfer in microchannels and heat transfer with nanofluids Expands problem sets and introduces new correlations and solved examples Provides more coverage of numerical/computer methods The third edition details the new research areas of heat transfer in microchannels and the enhancement of convective heat transfer with nanofluids....
International Nuclear Information System (INIS)
A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a wafer–bond-and-etch-back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re < 1, while the top and bottom Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number. (paper)
Lee, Man
2012-02-22
A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a waferbond-and-etch- back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re < 1, while the top and bottom Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number. Â© 2012 IOP Publishing Ltd.
Burnout in boiling heat transfer. II. Subcooled and low-quality forced-convection systems
International Nuclear Information System (INIS)
Recent experimental and analytical developments regarding burnout in subcooled and low-quality forced-convection systems are reviewed. Many data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries. Other topics discussed include burnout with power transients and techniques to augment burnout. 86 references
The influence of molten pool geometry on forced convective heat transfer
Wei, Cheng-hua; Fang, Bo-lang; Liu, Wei-ping; Wang, Li-jun; Ma, Zhi-liang
2015-05-01
An investigation was conducted to determine the relationship between heat transfer coefficient and molten pool's geometry. It was accomplished by performing an experimental and numerical investigation using a cylinder dimple with two different serials of geometry: (1) cylinder dimples with fixed print diameter D=50mm and different depth, and (2) cylinder dimples with fixed depth d=10mm and different print diameter. The airflow speed varies from 50m/s to 250m/s in the turbulent regime. The results consist of flow characteristics, mainly velocity profile and heat transfer characteristics, including heat transfer coefficient and Nusselt number along flow direction, were obtained. The comparison was held against the smooth surface. Results showed that a centrally-located vortex was formed due to the flow separation. For heat transfer coefficient, such augmentations are present near the downstream edges and diminutions are present near the upstream edges of dimple rims, both slightly within each depression. It was found that the convection heat transfer coefficients with different geometry parameters have similar distribution along flow direction. A uniform piecewise linear function was built to describe the heat transfer characterizes for different molten pool print diameter.
International Nuclear Information System (INIS)
An experimental work is conducted on combined (free and forced) convection to study the local and average heat transfer for hydrodynamically fully developed and thermally developing laminar air flow in a horizontal circular cylinder. The experimental setup uses an aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30) subjected to a constant wall heat flux boundary condition. The investigation covers the Reynolds number range from 400 to 1600 and the heat flux range from 60 W/m2 to 400 W/m2. The fully developed condition is achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as the test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 180 cm (L/D = 60), and the other with length of 240 cm (L/D = 80), and two short calming sections with lengths of 60 cm (L/D = 20) and of 120 cm (L/D = 40). The surface temperature distribution along the cylinder surface and the local and average Nusselt number distributions with dimensionless axial distance Z+ were presented. For all entrance sections, an increase in the Nusselt number values was found as the heat flux increases. It was concluded that the free convection effects tended to decrease the heat transfer results at low Re number and to increase the heat transfer results for high Re number. The combined convection regime could be bounded by a suitable selection of Re number ranges and heat flux ranges. The obtained Richardson numbers (Ri) ranged approximately from 0.1171 to 7.125. The average Nusselt numbers were correlated with the Rayleigh numbers/Reynolds numbers
Forced convective boiling heat transfer of water in vertical rectangular narrow channel
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Highlights: â€¢ Chen correlation cannot well predict the coefficient of rectangular channel. â€¢ Kim and Mudawar correlation is the best one among the Chen type correlations. â€¢ Lazarek and Black correlation predicted 7.0% of data within the Â±30% error band. â€¢ The new correlation can well predict the coefficient with a small MAE of 14.4%. - Abstract: In order to research the characteristics of boiling flows in a vertical rectangular narrow channel, a series of convective boiling heat transfer experiments are performed. The test section is made of stainless steel with an inner diameter of 2 Ã— 40 mm and heated length of 1100 mm. The 3194 experimental data points are obtained for a heat flux range of 10â€“700 kW/m2, a mass flux range of 200â€“2400 kg/m2 s, a system pressure range of 0.1â€“2.5 MPa, and a quality range of 0â€“0.8. Eighteen prediction models are used to predict the flow boiling heat transfer coefficient of the rectangular narrow channel and the predicted value is compared against the database including 3194 data points, the results show that Chen type correlations and Lazarek and Black type correlations are not suitable for the rectangular channel very much. The Kim and Mudawar correlation is the best one among the 18 models. A new correlation is developed based on the superposition concept of nucleate boiling and convective boiling. the new correlation is shown to provide a good prediction against the database, evidenced by an overall MAE of 14.4%, with 95.2% and 98.6% of the data falling within Â±30% and Â±35% error bands, respectively
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The heat transfer resistance of a porous deposit can be expressed as the sum of two components: one associated with conduction through the fluid-filled deposit and a second associated with surface roughness. This simple relationship appears to hold under both single-phase forced-convection and boiling heat-transfer conditions. The conductive component of the total deposit resistance is always positive, whereas the roughness component is negative. Values for Îº and Rroughness measured in this investigation are as follows. Under single-phase forced-convection heat-transfer conditions, Îº = 1.3 Â± 0.2 W/mK and Rroughness = -4 x 10-6 m2K/W for magnetite deposits. Under flow-boiling heat-transfer conditions, Îº = 0.2 to 0.9 W/mK and Rroughness = -36 x 10-6 m2K/W for magnetite deposits, whereas Îº = 2.0 W/mK and Rroughness = -43 x 10-6 m2K/W for deposits composed of approximately equal proportions of copper and magnetite. (author)
Directory of Open Access Journals (Sweden)
M.M. Rahman
2012-12-01
Full Text Available This paper presents the numerical study on two-dimensional forced convection heat transfer across three in-line flat tubes confined in a channel under incompressible, steady-state conditions. This system is solved in body-fitted coordinates (BFC using the finite volume method (FVM. The constant heat flux is imposed on the surface of the tubes as the thermal boundary conditions. The range of the longitudinal pitch-to-diameter ratio (SL/Ds of 2.0–4.0 is considered, the Reynolds number varies within the range 25–300, and the Prandtl number is taken as 0.7. The temperature contours, local Nusselt number distributions at the tube surface and mean Nusselt number were analyzed. The strength of the heat transfer between the surface of the tubes and the air flow increases with an increase in Reynolds number and pitch-to-diameter ratio.
Kozlova, Sofya V; Ryzhkov, Ilya I
2014-09-01
In this paper, laminar convective heat transfer of water-alumina nanofluid in a circular tube with uniform heat flux at the tube wall is investigated. The investigation is performed numerically on the basis of two-component model, which takes into account nanoparticle transport by diffusion and thermophoresis. Two thermal regimes at the tube wall, heating and cooling, are considered and the influence of nanoparticle migration on the heat transfer is analyzed comparatively. The intensity of thermophoresis is characterized by a new empirical model for thermophoretic mobility. It is shown that the nanoparticle volume fraction decreases (increases) in the boundary layer near the wall under heating (cooling) due to thermophoresis. The corresponding variations of nanofluid properties and flow characteristics are presented and discussed. The intensity of heat transfer for the model with thermophoresis in comparison to the model without thermophoresis is studied by plotting the dependence of the heat transfer coefficient on the Peclet number. The effectiveness of water-alumina nanofluid is analyzed by plotting the average heat transfer coefficient against the required pumping power. The analysis of the results reveals that the water-alumina nanofluid shows better performance in the heating regime than in the cooling regime due to thermophoretic effect. PMID:25260328
Umer, Asim; Naveed, Shahid; Ramzan, Naveed
2015-11-01
Nanofluids, having 1-100 nm size particles in any base fluid are promising fluid for heat transfer intensification due to their enhanced thermal conductivity as compared with the base fluid. The forced convection of nanofluids is the major practical application in heat transfer equipments. In this study, heat transfer enhancements at constant wall heat flux under laminar flow conditions were investigated. Nanofluids of different volume fractions (1, 2 and 4 %) of copper (I) oxide nanoparticles in deionized water were prepared using two step technique under mechanical mixing and ultrasonication. The results were investigated by increasing the Reynolds number of the nanofluids at constant heat flux. The trends of Nusselt number variation with dimensionless length (X/D) and Reynolds numbers were studied. It was observed that heat transfer coefficient increases with increases particles volume concentration and Reynolds number. The maximum enhancement in heat transfer coefficient of 61 % was observed with 4 % particle volume concentration at Reynolds number (Re ~ 605).
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Highlights: ? Three different eddy viscosity turbulence models are validated. ? Two data sets and an extensive sensitivity study are employed for validation. ? Prediction errors for the velocity and temperature fields are analyzed. ? Turbulence Prandtl number has to be chosen in dependence of the turbulence model. - Abstract: This paper provides temperature and velocity distribution computations in heated annuli using RANS approach and employing three different turbulent viscosity models. In addition to comparison calculations an extensive sensitivity study was performed. The results show that the RANS approach and the turbulent viscosity models can be used for prediction of forced convection heat transfer to lead–bismuth-eutectic. However, the turbulent Prandtl number has to be carefully selected depending on the respective turbulence model.
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Turbulent mixed and forced convection were investigated experimentally in a horizontal duct which modeled both thermally and hydraulically a nuclear waste repository. Reynolds numbers ranged from 6,000 to 180,000. The maximum Grashof number achieved was 1.5 109. The experimental apparatus consisted of a horizontal duct, 30.5 cm wide by 45.2 cm high (12 inches by 18 inches), and 4.42 m (14.5 feet) in length. The duct had 29 heater sections in the axial direction, each 15.2 cm (6 inches) long. Heat flux profiles were imposed on the four walls of the duct by a network of resistance heater pads. The opposing sidewall heaters were connected by pairs in parallel, giving five independently controlled sets of heaters; floor, lower side, middle side, upper side, and ceiling. Since the power inputs to these five heater sets were controlled separately, a wide range of heat flux profiles could be investigated
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
Energy Technology Data Exchange (ETDEWEB)
Wenji, Song [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Graduate School of Chinese Academy of Sciences, Beijing 100039 (China); Rui, Xiao; Chong, Huang; Shihui, He; Kaijun, Dong; Ziping, Feng [Guangzhou Institute of Energy Conversion, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China); Key Laboratory of Renewable Energy and Gas Hydrate, CAS, No. 2 Nengyuan Road, Tianhe District, Guangzhou 510640 (China)
2009-11-15
Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid-liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within {+-}20%. (author)
Influence of Tip Clearance on Forced Convection Heat Transfer of a Finned Plate in a Duct
International Nuclear Information System (INIS)
Optimizations are required for a proper enhancement of cooling capability. An important phenomenological consideration is to be reveals for a finned plate in a duct. Due to the high friction near the fin region and low friction near the wall region, the forced flow tends to bypass from fin region to wall region. The bypass flow increases the net flow and enhances the heat transfer for a moderate tip clearance which is defined by the distance from the tip of the fin and the wall. Meanwhile for a large tip clearance, most of the flow bypasses and does not contribute the heat transfer and impairs the heat transfer. This study is a preliminary numerical study on the influence of the tip clearance on the heat transfer of the finned plate in a duct. The study aimed at supporting an experimental research exploring the phenomena for a very small tip clearance. Thus material properties and test conditions were chosen to meet the experimental conditions. It investigated the phenomena at Pr of 2,014 and ReS of 58.3. In order to investigate the small tip clearance phenomena, a simple numerical scheme was developed using a commercial CFD code. A case with the same experimental condition was tested using the numerical scheme and the error was about 12%. The results show the clear evidence of the flow bypass from the fin region to wall region, which impair the heat removal capacity of the finned plate in a duct. The study has the relevance with the reactor cavity cooling system performance enhancement activities in the VHTR. The numerical scheme will be tested for narrower and wider tip clearances and find an optimal tip clearance
Directory of Open Access Journals (Sweden)
Thamir K. Jassem
2013-05-01
Full Text Available An experimental forced laminar study was presented in this research for an air flowing through a circular channel for different angles ( ,30o,45o,60o, the channel was heated at constant heat flux , the channel also was packed with steel and glass spheres respectively . The tests were done for three values of Peclets number (2111.71,3945.42,4575.47 with changing the heat flux for each case and five times for each number.The results showed that the dimensionless temperature distribution Â will decrease with increasing the dimensionless channel length for all cases with changing Peclet number, heat flux and inclination angles, and its lowest value will be for glass spheres at highest flux, while at lower flux for , and the decreasing in dimensionless temperature was closed for both types of packed at other inclination angles.The study declared that the local Nusselt number decreases with increasing the dimensionless length of the channel for both packeds and for different applied heat flux, also through this study it was declared that the average Nusselt increases as Peclet number increases for both packed. Its value for the glass spheres is greater than the steel spheres with percentage (98.3% at small Peclet, and percentage (97.2% at large Peclet number for the horizontal tube, and (98.3% at small Peclet number and (97.8% at large Peclet number atÂ .Through this study its was found that average Nusselt number increases along the channel as the heat flux increases, because the bulk temperature will increase as the flow proceeds toward the end of the channel , so the heat transfer coefficient will increase.Â It was declared from this study that in the case of the steel packed the heat transfer will occur mainly by conduction, while in the case of glass packed the heat transfer will occur mainly by laminar forced convection, where the lowest Nusselt number (Nu=3.8 was found when the pipe is horizontal and lowest heat flux and lowest Peclet number.Â Â
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In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are presented for comparison as well. The channel cross-section is 120 mm wide and 12 mm high; the channel is operated with the lower and upper walls kept at fixed temperature whereas the sides are adiabatic. The ribs have a square cross section and are mounted 60° parallel-tilted (the angle is respect to main stream) in astaggered arrangement. The tested configurations differ each other for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls. Air flow rates have been varied in order to have Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number, calculated as the ratio Nu/Nu0, for the all tested ribbed channels ranging between 1.0 and 5.0.
Cucchi, M.; Fustinoni, D.; Gramazio, P.; Colombo, L. P. M.; Niro, A.
2014-04-01
In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are presented for comparison as well. The channel cross-section is 120 mm wide and 12 mm high; the channel is operated with the lower and upper walls kept at fixed temperature whereas the sides are adiabatic. The ribs have a square cross section and are mounted 60° parallel-tilted (the angle is respect to main stream) in astaggered arrangement. The tested configurations differ each other for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls. Air flow rates have been varied in order to have Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number, calculated as the ratio Nu/Nu0, for the all tested ribbed channels ranging between 1.0 and 5.0.
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Variation of forced convective heat transfer in a rectangular duct flow of a magnetic fluid under a magnetic field was investigated experimentally. Experiments were performed changing the magnetic field intensity, and this magnetic field could be varied from 0 mT to 600 mT. The Reynolds number based on the hydraulic diameter was set to 960, 1900 (laminar flow), and 2830 (turbulent flow). The results of the experiments show that in the case of laminar flow of the magnetic fluid, when a magnetic field is applied to a magnetic fluid flow, heat transfer locally increases in the region where the magnetic field exists. In contrast, in the turbulent flow of the magnetic fluid, heat transfer is not enhanced but reduced. In order to better understand this heat transfer phenomenon, we measured the velocity distribution of magnetic fluid flow by the Ultrasonic Velocity Profile (UVP) method. In the case of laminar flow, the result shows that the flow velocity at the center of the rectangular duct decreases and the velocity gradient in the near-wall region increases. Moreover, we calculated the flow resistance under a magnetic field by measurement of the pressure gradient, and the relationship between heat transfer and flow resistance was discussed.
Qasim, S. M.; Sahar, A. F. A.; Firas, A. A.
2015-11-01
A numerical study has been carried out to investigate the heat transfer by laminar forced convection of nanofluid taking Titania (TiO2) and Alumina (Al2O3) as nanoparticles and the water as based fluid in a three dimensional plain and U-longitudinal finned tube heat exchanger. A Solid WORKS PREMIUM 2012 is used to draw the geometries of plain tube heat exchanger or U-longitudinal copper finned tube heat exchanger. Four U-longitudinal copper fins have 100 cm long, 3.8cm height and 1mm thickness are attached to a straight copper tube of 100 cm length, 2.2 cm inner diameter and 2.39 cm outer diameter. The governing equations which used as continuity, momentum and energy equations under assumptions are utilized to predict the flow field, temperature distribution, and heat transfer of the heat exchanger. The finite volume approach is used to obtain all the computational results using commercial ANSYS Fluent copy package 14.0 with assist of solid works and Gambit software program. The effect of various parameters on the performance of heat exchanger are investigated numerically such as Reynolds' number (ranging from 270 to 1900), volume consternation of nanoparticles (0.2%, 0.4%, 0.6%, 0.8%), type of nanoparticles, and mass flow rate of nanofluid in the hot region of heat exchanger. For 0.8% consternation of nanoparticles, heat transfer has significant enhancement in both nanofluids. It can be found about 7.3% for TiO2 and about 7.5% for Al2O3 compared with the water only as a working fluid.
Theoretical and Computational Study of Forced-Convection Heat Transfer at Supercritical Pressures
Zhong, Jianguo
In the simulation of turbulent fluid flow and heat transfer at supercritical pressures, substantial difficulties have been encountered in the modeling of turbulence and bounda-ry layer. This is due to significant fluid property variations with respect to the local temperature and pressure, especially in the near-wall region of a heated wall, where large temperature differences occur. The classical turbulence models available in literature were typically developed for constant-property fluids, where an empirical wall function in the high-Re k-epsilon model, and a damping function in the low-Re k-epsilon model were derived based on the constant-property data to solve the boundary layer. As it can be found in the existing literature, large differences have been observed between the experimental and numerical simulation results of the heat transfer coefficient predictions in the en-hanced and deteriorated heat transfer situations for supercritical fluids. In this thesis, a novel near-wall treatment method is proposed to treat large property variations in the thermal and velocity sub-layers. In the near-wall region, the supercritical fluids can be considered thermal-conductive and viscous forces dominated. The thick-ness of the viscous sub-layer (VSL) and the conduction sub-layer (CSL) can be related to the wall shear stress and local Prandtl number information by using computational CFD models, such as that implemented in the NPHASE-CMFD code. The fluids' bulk and wall temperature information has been obtained from the literature review of experi-mental measurements. The wall temperature and heat transfer coefficient calculated from the k-epsilon model with the proposed wall treatment method have been found to be in good agreement with experimental data for both heat transfer enhancement and deterioration cases for two most widely used fluids: CO2 and water. The proposed model has been applied in the reactor-scale thermal-hydraulic analysis of different flow path designs in Gen-IV supercritical water nuclear reactors (SCWR). The main objective of this study has been to validate the performance of the current approach as a tool for the analysis of large-scale systems.
Thornton, E. A.
1979-01-01
Three practical problems in conduction/forced convection heat transfer are analyzed using a simplified engineering formulation of convective finite elements. Upwind and conventional finite element solutions are compared for steady-state and transient applications.
Aris, M.S.; McGlen, R.; Owen, I.; Sutcliffe, C.J.
2011-01-01
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs...
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Highlights: • Thermal conductivity is assumed a linear function of temperature. • It occurs due to diffusion-like radiation or a high temperature gradient. • Nusselt number and temperature profiles are obtained based on perturbation method. • Variable conductivity reveals enhancement obtained by porous media more clearly. • Nusselt number changes linearly with a linear change in conductivity. - Abstract: Effects of variation of the thermal conductivity on forced convection in a parallel-plates channel heat exchanger occupied by a fluid saturated porous medium are investigated analytically based on the perturbation methods. Walls of the channel are kept at a constant heat flux. Thermal conductivity of the medium is assumed to be a linear function of temperature (due to moderate radiation heat transfer in cellular foams or temperature dependent conductivity of the material). The Brinkman–Forchheimer–extended Darcy model for the flow field is used. Relations representing the temperature profile and Nusselt number as functions of porous medium shape parameter and thermal conductivity variation parameter are derived. Obtained Nusselt number and temperature profile are studied parametrically. No analytical investigation based on a variable conductivity approach for Brinkman–Forchheimer–extended Darcy model has been previously performed. Results show that a linear increase in the thermal conductivity of the medium results in a semi-linear increase in the Nusselt number
Simulation of MHD CuO-water nanofluid flow and convective heat transfer considering Lorentz forces
Sheikholeslami, Mohsen; Bandpy, Mofid Gorji; Ellahi, R.; Zeeshan, A.
2014-11-01
Magnetic field effect on CuO-water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo-Kleinstreuer-Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number.
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An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loop
Sivakumar, A.; Alagumurthi, N.; Senthilvelan, T.
2015-08-01
The microchannels are device used to remove high heat fluxes from smaller area. In this experimental research work the heat transfer performance of nanofluids of Al2O3/water and CuO/water were compared. The important character of such fluids is the enhanced thermal conductivity, in comparison with base fluid without considerable alteration in physical and chemical properties. The effect of forced convective heat transfer coefficient was calculated using serpentine shaped microchannel heat exchanger. Furthermore we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. The heat transfer coefficient for different particle concentration and temperature were analysed using forced convection heat transfer using nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the basefluid. The results also shows that CuO/water nanofluid has increased heat transfer coefficient compared with Al2O3/water and base fluids. Moreover the experimental results indicate there is increased forced convective heat transfer coefficient with the increase in nano particle concentration.
Study of the equivalent diameter concept for heat transfer by forced convection in annular channels
International Nuclear Information System (INIS)
This work describes a comparative analysis between experimental values of heat transfer coefficients in fully developed turbulent flow for a concentric annular channel, and those calculated with the empirical correlations obtained for tubes by Dittus-Boelter, Sieder and Tate, a modified Colburn equation, and that proposed by Gnielinski which applies the analogy between friction and heat transfer. The coefficients were calculated by means of two different equivalent diameters: 1) The hydraulic equivalent diameter; and 2) The heated equivalent diameter. It was concluded that the hydraulic equivalent diameter gives much better results than the heated equivalent diameter. (Author)
Simulation of MHD CuO–water nanofluid flow and convective heat transfer considering Lorentz forces
Energy Technology Data Exchange (ETDEWEB)
Sheikholeslami, Mohsen; Bandpy, Mofid Gorji [Department of Mechanical Engineering, Babol University of Technology, Babol (Iran, Islamic Republic of); Ellahi, R., E-mail: rellahi@engr.ucr.edu [Department of Mechanical Engineering, University of California Riverside (United States); Department of Mathematics and Statistics, FBAS, IIUI, H-10 Sector, Islamabad (Pakistan); Zeeshan, A. [Department of Mathematics and Statistics, FBAS, IIUI, H-10 Sector, Islamabad (Pakistan)
2014-11-15
Magnetic field effect on CuO–water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number. - Highlights: • This paper analyses the magnetic effect on CuO–water nanofluid. • Koo–Kleinstreuer–Li correlation and Lattice Boltzmann method are used. • Effects of pertinent parameters are presented through tables and graphs.
Experiments on a forced convection heat transfer at supercritical pressures - 6.32 mm ID tube
International Nuclear Information System (INIS)
The size of a sub-channel of the conceptual SCWR core design studied at KAERI is 6.5 mm. In order to provide heat transfer information in such a narrow sub-channel at supercritical pressure, an experiment was performed with a test section made of Inconel 625 tube of 6.32 mm ID. The test pressures were 7.75 and 8.12 MPa corresponding to 1.05 and 1.1 times the critical pressure of CO2, respectively. The mass flux and heat flux, which were in the range of 285 ? 1200 kg/m2s and 30 ? 170 kW/m2, were changed at a given system pressure. The corresponding Reynolds numbers are 1.8 x 104 ? 7.5 x 104. The effect of mass flux and heat flux was dominant factor in the supercritical pressure heat transfer while the effect of pressure was negligible. The Bishop's correlation predicted the test result most closely and Bae and Kim's recent correlation was the next. The heat transfer deterioration occurred when GR)b/Reb2.7 > 2.0 x 10-5. As soon as the heat transfer was deteriorated, it entered a new regime and did not recover the normal heat transfer nevertheless Grb/Reb2.7 reduced below 2.0 x 10-5. It may mean that the correlation must be developed for the normal and deterioration regime separately
Simulation of MHD CuO–water nanofluid flow and convective heat transfer considering Lorentz forces
International Nuclear Information System (INIS)
Magnetic field effect on CuO–water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number. - Highlights: • This paper analyses the magnetic effect on CuO–water nanofluid. • Koo–Kleinstreuer–Li correlation and Lattice Boltzmann method are used. • Effects of pertinent parameters are presented through tables and graphs
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (? = 0°, where ? is the inclination angle), inclined (? = 30° and 60°), and bottom-heated (? = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with ? = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number. PMID:26183389
Indian Academy of Sciences (India)
V Vasu; K Rama Krishna; A C S Kumar
2007-09-01
Nano?uids are a new class of heat transfer ?uids developed by suspending nanosized solid particles in liquids. Larger thermal conductivity of solid particles compared to the base ?uid such as water, ethylene glycol, engine oil etc. signi?cantly enhances their thermal properties. Several phenomenological models have been proposed to explain the anomalous heat transfer enhancement in nano?uids. This paper presents a systematic literature survey to exploit the characteristics of nano?uids, viz., thermal conductivity, speci?c heat and other thermal properties. An empirical correlation for the thermal conductivity of Al2O3 + water and Cu + water nano?uids, considering the effects of temperature, volume fraction and size of the nanoparticle is developed and presented. A correlation for the evaluation of Nusselt number is also developed and presented and compared in graphical form. This enhanced thermophysical and heat transfer characteristics make ?uids embedded with nanomaterials as excellent candidates for future applications.
An analytical approach for optimal design of heat sinks under forced convection
Miguel, Antonio
2013-06-01
Saving energy is just as important as generating energy. In this paper, we seek an optimized structure that achieves a certain level of heat transfer rate under a minimum pumping power to drive the fluid stream. Constraints are specified by the flow regime (laminar and turbulent), admissible boundary conditions on the walls (prescribed temperature and constant heat flux), and design standards. The study will help designers with more effective basic tools for the conceptual design of system and in establishing proper operating procedures.
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Karabacak, Rasim [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey); Yakar, Guelay, E-mail: gyakar@pau.edu.tr [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey)
2011-08-15
Highlights: {yields} The effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. {yields} Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. {yields} These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. {yields} The holes created turbulence in a region near the heating tube surface on the bottom of the fin. - Abstract: In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.
Experimental investigation of forced convective heat transfer in rectangular micro-channels
Directory of Open Access Journals (Sweden)
R. Kalaivanan
2010-05-01
Full Text Available This paper investigates the experimental program on the study of heat transfer characteristics in micro-channels. The two test sections used are of 47 and 50 micro-channels in rectangular cross-section of equivalent diameters 387 and 327 µm respectively. Each channel of length 192 mm is fabricated on a 304 stainless steel substrate (230 mm x 160 mm x 1.6 mm by photo chemical etching process. Covering the top with another plate of 0.5 mm thickness forms the channels by vacuum brazing. Experiments cover laminar region using the fluids ethanol, methanol and an ethanol-methanol mixture. The heat transfer coefficient is evaluated based on the heat carried away by the coolant and an average wall to mean fluid temperature difference. The Nusselt number is correlated through empirical correlations involving Reynolds number and Prandtl number with length parameter, the hydraulic diameter.
Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow
Ferrantelli, Andrea; Viljanen, Martti
2013-01-01
We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.
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Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% dp to 20% dp. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% dp to 20% dp
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Bu, S.S.; Yang, J.; Zhou, M.; Li, S.Y. [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Wang, Q.W., E-mail: wangqw@mail.xjtu.edu.cn [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Guo, Z.X. [Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854 (United States)
2014-04-01
Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% d{sub p} to 20% d{sub p}. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% d{sub p} to 20% d{sub p}.
A new mechanistic model of critical heat flux in forced-convection subcooled boiling
International Nuclear Information System (INIS)
Because of its practical importance and various industrial applications, the process of subcooled flow boiling has attracted a lot of attention in the research community in the past. However, the existing models are primarily phenomenological and are based on correlating experimental data rather than on a first-principle analysis of the governing physical phenomena. Even though the mechanisms leading to critical heat flux (CHF) are very complex, the recent progress in the understanding of local phenomena of multiphase flow and heat transfer, combined with the development of mathematical models and advanced Computational Fluid Dynamics (CFD) methods, makes analytical predictions of CHF quite feasible. Various mechanisms leading to CHF in subcooled boiling have been investigated. A new model for the predictions of the onset of CHF has been developed. This new model has been coupled with the overall boiling channel model, numerically implemented in the CFX 4 computer code, tested and validated against the experimental data of Hino and Ueda. The predicted critical heat flux for various channel operating conditions shows good agreement with the measurements using the aforementioned closure laws for the various local phenomena governing nucleation and bubble departure from the wall. The observed differences are consistent with typical uncertainties associated with CHF data
Directory of Open Access Journals (Sweden)
Sundus Hussein Abd
2012-01-01
Full Text Available In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while maintaining the surface area of all shapes to be the same. Each cylinder was heated to a certain temperature and put inside the duct at certain locations. The temperature of the cylinder was then monitored. The heat transfer coefficient were then calculated for forced convection for several Reynolds number (4555-18222.The study covered free convection impact for values of Rayleigh number ranging between (1069-3321. Imperical relationships were obtained for all cases of forced and free convection and compared with equations of circular cylindrical shapes found in literature. These imperical equations were found to be in good comparison with that of other sources.
Heterogeneous nanofluids: natural convection heat transfer enhancement
Bennacer Rachid; Oueslati Fakhreddine
2011-01-01
Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account th...
Effect of confinement on forced convection from a heated sphere in Bingham plastic fluids
Das, Pradipta K.; Gupta, Anoop K.; Nirmalkar, Neelkanth; Chhabra, Raj P.
2015-05-01
In this work, the momentum and heat transfer characteristics of a heated sphere in tubes filled with Bingham plastic fluids have been studied. The governing differential equations (continuity, momentum and thermal energy) have been solved numerically over wide ranges of conditions as: Reynolds number, 1 ? Re ? 100; Prandtl number, 1 ? Pr ? 100; Bingham number, 0 ? Bn ? 100 and blockage ratio,0 ? ? ? 0.5 where ? is defined as the ratio of the sphere to tube diameter. Over this range of conditions, the flow is expected to be axisymmetric and steady. The detailed flow and temperature fields in the vicinity of the surface of the sphere are examined in terms of the streamline and isotherm contours respectively. Further insights are developed in terms of the distribution of the local Nusselt number along the surface of the sphere together with their average values in terms of mean Nusselt number. Finally, the wall effects on drag are present only when the fluid-like region intersects with the boundary wall. However, heat transfer is always influenced by the wall effects. Also, the flow domain is mapped in terms of the yielded- (fluid-like) and unyielded (solid-like) sub-regions. The fluid inertia tends to promote yielding whereas the yield stress counters it. Furthermore, the introduction of even a small degree of yield stress imparts stability to the flow and therefore, the flow remains attached to the surface of the sphere up to much higher values of the Reynolds number than that in Newtonian fluids. The paper is concluded by developing predictive correlations for drag and Nusselt number.
Experimental study of micro-particle fouling under forced convective heat transfer
Scientific Electronic Library Online (English)
S. M., Peyghambarzadeh; A., Vatani; M., Jamialahmadi.
2012-12-01
Full Text Available Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, f [...] luid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formulate the asymptotic fouling resistance in terms of the mass transfer coefficient, thermophoresis velocity, and fluid shear rate. The results demonstrate that the prediction of the new model is in good agreement with the experimental observations.
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Lazarus Godson Asirvatham
2009-03-01
Full Text Available The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume of copper oxide (CuO nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% enhancement of the convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement was increased considerably as the Reynolds number increased. Possible reasons for the enhancement are discussed. Nanofluid thermo-physical properties and chaotic movement of ultrafine particles which accelerate the energy exchange process are proposed to be the main reasons for the observed heat transfer enhancement. A correlation for convective heat transfer coefficient of nanofluids, based on transport property and D/x for 8 mm tube has been evolved. The correlation predicts variation in the local Nusselt number along the flow direction of the nanofluid. A good agreement (±10% is seen between the experimental and predicted results.
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Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 ? Re ? 5000), the Rayleigh number (104 ? Ra ? 106) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra
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A model describing the thermal behaviour of a slurry of phase change material flow in a circular duct is presented. Reactors connected in series are considered for the representation of the circular duct with constant wall temperature. A phenomenological equation is formulated to take account of the heat generation due to phase change in the particles. Results of the simulation present a plateau of temperature along the longitudinal direction, characteristic of the phase change. The effect of different parameters such as the Reynolds number, the weight fraction and the temperature of the cold spring on the length of the plateau is analysed. A correlation resulting from numerical results is proposed for use in the determination of the characteristics of the exchanger for a phase change material slurry
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Degradation of the thermal performance of steam generators(SGs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SGs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors(PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure. Degradation mechanisms that may contribute to the loss of SG thermal performance include: fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SG or the integral preheater and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al. (1990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner and outer surfaces of the boiler tubes. Thus a complete understanding of the reasons fro the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary and secondary side heat transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively. The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer
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Budzynski, R.; Sobanski, R. [Politechnika Szczecinska, Szczecin (Poland)
1993-12-31
Results of an experimental investigation of mixed convection in laminar air flow across a tungsten wire are presented. Results are correlated by a 3-D surface equation of heat transfer, and compared with traditional equations for mixed convection that have been suggested by several investigators. Experimental results are given for Reynolds numbers in the range of 0.8 {<=} Re {<=} 17.5, Grashof numbers in the range of 1.5*10{sup -3} {<=} Gr {<=} 25 and buoyancy in the range of 6.10{sup -5} {<=} Gr/Re{sup 2} {<=} 11.5. (Authors). 7 refs., 5 figs.
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This paper explores the bearing that a non-uniform distribution of heat flux used as a wall boundary condition exerts on the heat transfer improvement in a round pipe. Because the overall heat load is considered fixed, the heat transfer improvement is viewed through a reduction in the maximum temperature (‘hot spot’) by imposing optimal distribution of heat flux. Two cases are studied in detail 1) fully developed and 2) developing flow. Peak temperatures in the heated pipe wall are calculated via an analytical approach for the fully developed case, while a numerical simulation based on CFD is employed for the developing case. By relaxing the heat flux distribution on the pipe wall, the numerical results imply that the optimum distribution of heat flux, which minimizes the peak temperatures corresponds with the ‘descending’ distribution. Given that the foregoing approach is quite different from the ‘ascending’ heat flux distribution recommended in the literature by means of the entropy generation minimization (EGM) method, it is inferred that the optimization of heat transfer and fluid flow, in comparison with the thermodynamic optimization, may bring forth quite different guidelines for the designs of thermal systems under the same constraints and circumstances. -- Highlights: • Considered the bearing of non-uniform distribution of heat flux on the hot spots. • Determined the optimal distribution of heat flux that minimizes the hot spots. • Results are compared with those obtained by EGM method
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Some correlations of forced convection burn-out data are based on the approximate linearity of the relationship between burn-out heat flux and the channel-averaged quality at the burn-out point. These correlations perform satisfactorily on data obtained from uniformly heated configurations. Therefore the further inference is sometimes made that the burn-out heat flux is uniquely related to the quality, and that the burn-out in non-uniformly heated configurations can be calculated from measurements made with uniform heating. This report presents burn-out data for Freon 12 flowing vertically upwards through both uniformly and non-uniformly heated round tubes. This data shows that the quality at burn-out does depend on the heat flux profile, and that the inference mentioned above is not justified. (author)
International symposium on transient convective heat transfer: book of abstracts
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The international symposium on convective heat transfer was held on 19-23 August 1996, in Cesme, Izmir, Turkey. The spesialists discussed forced convection, heat exchangers, free convection and multiphase media and phase change at the meeting. Almost 53 papers were presented in the meeting
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In the heat transfer studies by forced convection, we have few data about behavior of the fluids in an annular channel heated by a concentric pipe, such date is necessary to know the heat transfer coefficient that establish the interchange of energy and the thermic properties of the fluid with the geometry of the flow. In this work the objective, was to compare some empirical correlations that we needed for determinate the heat transfer coefficient for annular channels, where we obtained similar at the theoretical results of an experiment made by Miller and Benforado. It is important to know such coefficients because we can determinate the heat quantity transmitted to a probe zone, in which we simulate a nuclear fuel element that developed huge heat quantity that must be dispersed in short time. We give theoretical data of the heat forced transfer convection and we analyzed the phenomena in annular channels given some empirical correlations employed by some investigators and we analyzed each one. (Author)
Asirvatham, Lazarus Godson; Vishal, Nandigana; Gangatharan, Senthil Kumar; Lal, Dhasan Mohan
2009-01-01
The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume) of copper oxide (CuO) nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% e...
Steady, three-dimensional, internally heated convection
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Numerical calculations have been carried out of steady, symmetric, three-dimensional modes of convection in internally heated, infinite Prandtl number, Boussinesq fluids at a Rayleigh number of 1.4x104 in a spherical shell with inner/outer radius of 0.55 and in a 3x3x1 rectangular box. Multiple patterns of convection occur in both geometries. In the Cartesian geometry the patterns are dominated by cylindrical cold downflows and a broad hot upwelling. In the spherical geometry the patterns consist of cylindrical cold downwellings centered either at the vertices of a tetrahedron or the centers of the faces of a cube. The cold downflow cylinders are immersed in a background of upwelling within which there are cylindrical hot concentrations (plumes) and hot halos around the downflows. The forced hot upflow return plumes of internally heated spherical convection are fundamentally different from the buoyancy-driven plumes of heated from below convection
S.P. Anjali Devi; J. Wilfred Samuel Raj
2014-01-01
A study has been carried out on MHD boundary layer forced convection flow along a shrinking surface with variable heat flux in the presence of heat source. The flow is generated due to linear shrinking of the sheet and is influenced by uniform transverse magnetic field. The basic boundary layer momentum and heat transfer equations, which are nonlinear partial differential equations, are converted into nonlinear ordinary differential equations by means of similarity transformation. Numerical s...
Heterogeneous nanofluids: natural convection heat transfer enhancement
2011-01-01
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755
Heterogeneous nanofluids: natural convection heat transfer enhancement
Directory of Open Access Journals (Sweden)
Bennacer Rachid
2011-01-01
Full Text Available Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.
Heterogeneous nanofluids: natural convection heat transfer enhancement.
Oueslati, Fakhreddine Segni; Bennacer, Rachid
2011-01-01
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755
Internally heated convection and Rayleigh-B\\'enard convection
Goluskin, David
2015-01-01
This work reviews basic features of both internally heated (IH) convection and Rayleigh-B\\'enard (RB) convection, along with findings on IH convection from laboratory experiments and numerical simulations. In the first chapter, six canonical models of convection are described: three configurations of IH convection driven by constant and uniform volumetric heating, and three configurations of RB convection driven by the boundary conditions. The IH models are distinguished by differing pairs of thermal boundary conditions: top and bottom boundaries of equal temperature, an insulating bottom with heat flux fixed at the top, and an insulating bottom with temperature fixed at the top. The RB models also are distinguished by whether temperatures or heat fluxes are fixed at the top and bottom boundaries. Integral quantities important to heat transport are discussed, including the mean fluid temperature, the mean temperature difference between the boundaries, and the mean convective heat transport. Integral relations...
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Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re5). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.
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Future fusion reactor devices such as ITER or JT-60SA will produce thermonuclear fusion reaction in plasmas at several millions of degrees. The confinement in the center of the chamber is achieved by very intense magnetic fields generated by superconducting magnets. These coils have to be cooled down to 4.4 K through a forced flow of supercritical helium. The cyclic behavior of the machines leads to pulsed thermal heat loads which will have to be handled by the refrigerator. The HELIOS experiment built in CEA Grenoble is a scaled down model of the helium distribution system of the tokamak JT-60SA composed of a saturated helium bath and a supercritical helium loop. The thesis work explores HELIOS capabilities for experimental and numerical investigations on three heat load smoothing strategies: the use of the saturated helium bath as an open thermal buffer, the rotation speed variation of the cold circulator and the bypassing of the heated section. The developed model describes well the physical evolutions of the helium loop (pressure, temperature, mass flow) submitted to heat loads observed during experiments. Advanced controls have been tested and validated to improve the stability of the refrigerator and to optimize the refrigeration power. (author)
Terminal project heat convection in thin cylinders
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Heat convection in thin cylinders and analysis about natural convection for straight vertical plates, and straight vertical cylinders submersed in a fluid are presented some works carry out by different authors in the field of heat transfer. In the part of conduction, deduction of the equation of heat conduction in cylindrical coordinates by means of energy balance in a control volume is presented. Enthalpy and internal energy are used for the outlining of the equation and finally the equation in its vectorial form is obtained. In the convection part development to calculate the Nusselt number for a straight vertical plate by a forces analysis, an energy balance and mass conservation over a control volume is outlined. Several empiric correlations to calculate the Nusselt number and its relations with other dimensionless numbers are presented. In the experimental part the way in which a prototype rode is assembled is presented measurements of temperatures attained in steady state and in free convection for working fluids as air and water are showed in tables. Also graphs of Nusselt numbers obtained in the experimental way through some empiric correlations are showed (Author)
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Highlights: ? ANN was trained to predict the CHF with a better accuracy than GA. ? CHF increases with jet velocity. ? CHF decreases with an increase in L/d and the number of jets. ? CHF increases at first and then decreases with an increase of pressure. - Abstract: In this paper, a three-layer Back Propagation (BP) algorithm artificial neural network (ANN) for predicting critical heat flux (CHF) in saturated forced convective boiling on a heated surface with impinging jets was trained successfully with a root mean square (RMS) error of 17.39%. The input parameters of the ANN are liquid-to-vapor density ratio, ?l/?v, the ratio of characteristic dimension of the heated surface to the diameter of the impinging jet, L/d, reciprocal of the Weber number, 2?/?lu2(L - d), and the number of impinging jets, Nj. The output is dimensionless heat flux, qco/?vHfgu. Based on the trained ANN, the influence of principal parameters on CHF has been analyzed as follows. CHF increases with an increase in jet velocity and decreases with an increase in L/d and Nj. CHF increases with an increase in pressure at first and then decreases. Besides, a new correlation was generalized using genetic algorithm (GA) as a comparison with ANN to confirm the advantage of ANN.
Energy Technology Data Exchange (ETDEWEB)
Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
The goal of this NERI project was to perform research on high temperature fluoride and chloride molten salts towards the long-term goal of using these salts for transferring process heat from high temperature nuclear reactor to operation of hydrogen production and chemical plants. Specifically, the research focuses on corrosion of materials in molten salts, which continues to be one of the most significant challenges in molten salts systems. Based on the earlier work performed at ORNL on salt properties for heat transfer applications, a eutectic fluoride salt FLiNaK (46.5% LiF-11.5%NaF-42.0%KF, mol.%) and a eutectic chloride salt (32%MgCl2-68%KCl, mole %) were selected for this study. Several high temperature candidate Fe-Ni-Cr and Ni-Cr alloys: Hastelloy-N, Hastelloy-X, Haynes-230, Inconel-617, and Incoloy-800H, were exposed to molten FLiNaK with the goal of understanding corrosion mechanisms and ranking these alloys for their suitability for molten fluoride salt heat exchanger and thermal storage applications. The tests were performed at 850ÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â‹ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚ÂšC for 500 h in sealed graphite crucibles under an argon cover gas. Corrosion was noted to occur predominantly from dealloying of Cr from the alloys, an effect that was particularly pronounced at the grain boundaries Alloy weight-loss due to molten fluoride salt exposure correlated with the initial Cr-content of the alloys, and was consistent with the Cr-content measured in the salts after corrosion tests. The alloysÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â¢ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â€ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â™ weight-loss was also found to correlate to the concentration of carbon present for the nominally 20% Cr containing alloys, due to the formation of chromium carbide phases at the grain boundaries. Experiments involving molten salt exposures of Incoloy-800H in Incoloy-800H crucibles under an argon cover gas showed a significantly lower corrosion for this alloy than when tested in a graphite crucible. Graphite significantly accelerated alloy corrosion due to the reduction of Cr from solution by graphite and formation on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083ÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â¢ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â€ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â and 0.370ÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â¢ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â€ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â). In the 0.083ÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â¢ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â€ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â channel, the experimental heat transfer coefficient was shown to agree with values obtained from heat transfer correlations used for water. In the 0.370ÃƒÂƒÃ‚ÂƒÃƒÂ‚Ã‚Â¢ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â€ÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â D channel, the experimental heat transfer coefficient data was predictable by either a correlation for mixed convection, or forced convection depending on the value of Gr*/Re. These experiments provided new insights into the construction and operation of molten salt flow systems. The selection of multi-component salts for molten salt flow systems requires knowledge of properties such as melting point, heat capacity, density, and viscosity of these salts. Theoretical models have been developed for the prediction of these properties of multi-component salts.
Experimental study of turbulent forced convection in vertical eccentric annulus
Energy Technology Data Exchange (ETDEWEB)
Hosseini, R.; Ramezani, M.; Mazaheri, M.R. [Mechanical Engineering Dept., Amirkabir Univ. of Technology, 424 Hafez Ave., P.O. Box 15875-4413, Tehran (Iran)
2009-09-15
Forced convection in an open-ended vertical eccentric annulus with different eccentricities has been experimentally studied with several heat fluxes and inlet air velocities. Constant heat flux outer tube and insulated inner tube boundary conditions were used. The wall temperature of the outer tube was measured along the length for different eccentricities. Results indicate that with the increase of eccentricity, the convection heat transfer coefficient increases. Also, as the Reynolds number is increased a crest is formed in the temperature variation along the length. An empirical relation is derived for the Nusselt number as a function of the Reynolds number and eccentricity. (author)
Experimental study of turbulent forced convection in vertical eccentric annulus
International Nuclear Information System (INIS)
Forced convection in an open-ended vertical eccentric annulus with different eccentricities has been experimentally studied with several heat fluxes and inlet air velocities. Constant heat flux outer tube and insulated inner tube boundary conditions were used. The wall temperature of the outer tube was measured along the length for different eccentricities. Results indicate that with the increase of eccentricity, the convection heat transfer coefficient increases. Also, as the Reynolds number is increased a crest is formed in the temperature variation along the length. An empirical relation is derived for the Nusselt number as a function of the Reynolds number and eccentricity.
Suppression of saturated nucleate boiling by forced convective flow
International Nuclear Information System (INIS)
Tube-side forced convective boiling nitrogen and oxygen and thin film shell-side forced convective boiling R-11 data demonstrate a reduction in the heat transfer coefficient associated with nucleate boiling as the two-phase friction pressure drop increases. Techniques proposed in the literature to account for nucleate boiling during forced convective boiling are discussed. The observed suppression of nucleate boiling for the tube-side data is compared against the Chen correlation. Although general agreement is exhibited, supporting the interactive heat transfer mechanism theory, better agreement is obtained by defining a bubble growth region within the thermal boundary layer. The data suggests that the size of the bubble growth region is independent of the friction drop, but is only a function of the physical properties of the boiling liquid. 15 refs
Directory of Open Access Journals (Sweden)
Tasawar Hayat
2011-09-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge–Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Scientific Electronic Library Online (English)
Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.
2011-10-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in [...] contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge-Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Thornton, E. A.; Wieting, A. R.
1979-01-01
Conventional versus upwind convective finite elements, and lumped versus consistent formulations for practical conduction/forced convection analysis are evaluated on the basis of numerical studies, with finite element and finite difference lumped-parameter temperatures compared to closed-form analytical solutions for convection problems. Attention is given to two practical combined conduction and forced convection applications, stressing that the finite element method, showing superior accuracy, is competitive with the finite difference lumped-parameter method. Also considered are the computational time savings offered by the zero capacitance nodes procedure and comparative finite element and finite difference lumped-parameter computer times. The present study has reference to the design of actively cooled engine and airframe structures for hypersonic flight.
International Nuclear Information System (INIS)
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs adhered to the fin surface, thin wings punched-out of the fin material and TiNi shape memory alloy delta wings which changed their angles of attack based on the fin surface temperature. The longitudinal vortices generated from the wing designs induce secondary mixing of the cooler free stream air entering the fin stack with the warmer fluid close to the fin surfaces. The change in angle of the attack of the active delta wings provide heat transfer enhancement while managing flow pressure losses across the fin stack. A heat transfer enhancement of 37% compared to a plain fin stack was obtained from the 3-D tabs in a staggered arrangement. The punched-out delta wings in the staggered and inline arrangements provided enhancements of 30% and 26% respectively. Enhancements from the active delta wings were lower at 16%. However, as these devices reduce the pressure drop through the fin stack by approximately 19% in the de-activate position, over the activated position, a reduction in fan operating cost may be achieved for systems operating with inlet air temperatures below the maximum inlet temperature specification for the device. CFD analysis was also carried out to provide additional detail of the local heat transfer enhancement effects. The CFD results corresponded well with previously published reports and were consistent with the experimental findings. - Highlights: ? Heat transfer enhancements of heat pipe fin stacks was successfully achieved using fixed and active delta wing vortex generators. ? The active vortex generators, made from Ti-Ni, protruded into the flow stream at high temperatures and resume a low profile position at a low temperature set point. ? By considering wing spacing and the distance from the leading edge of the fin stack, heat transfer enhancements of up to 37%, compared to plane fin stacks, were achieved. ? By replacing the fixed delta wings with the active vortex generators, heat transfer enhancements of up to 16% was achieved and the pressure loss associated with flow obstructions was effectively managed.
Approximate physical burnout model for forced convection of saturated fluid
International Nuclear Information System (INIS)
Approximate physical burnout model for forced convection of saturated fluid is considered. Relationships for determining critical heat flux are presented. They correspond satisfactorily with experimental data. Experimental data on burnout in two-phase flow for various fluids including water and helium are presented
Rajesh Khatri; Pankaj Agarwal
2012-01-01
In this paper heat transfer and fluid flow characteristics in a channel has been theoretically investigated. In this study, FEM is employed to analyze a fluid flow inside a channel and then solve for the heat flow transfer through the same channel. The fluid flow is expressed by partial differential equation (Poisson’s equation).While, heat transfer is analyzed using the energy equation. The Navier Stokes equations along with the energy equation have been solved by using simple technique. The...
International Nuclear Information System (INIS)
This paper provides information on heat transfer enhancement due to jet mixing inside a cylindrical enclosure. The work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. The current research presents an interesting possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could provide the basis for a change of containment design and reduce the construction cost. It is found that the ratio of forced- and free convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry. To use the small-scale tests for large containment design, scale-up methods and criteria are important for matching the key governing parameters and fluid properties. In the present experiment, a cylindrical enclosure was constructed with a vertical wall of 2.29-m diameter and 0.8-m height and a vertically adjustable ceiling. A horizontal copper plate was installed at the bottom to provide an isothermal heating surface. Cold air was injected at several positions with varying pipe diameters and injecting orientations and was removed from the top of the enclosure. The experiment was performed with an extensive set of tests to study the combined natural- and forced convection heat transfer in a cylindrical enclosure mixed by an injected jet. The goals are to evaluate the key parameters governing the heat transfer augmentation by a forced jet and to investigate the effect of geometric factors, including jet diameter, jet injection orientation, and enclosure geometry (aspect ratio). Flow velocity measurement further provides a better understanding of the flow patterns developed inside the enclosure, which will determine the effectiveness of the whole volume mixing process. An additional experiment with a vertically cooled wall, similar to the condition of a passive containment cooling surface, is under construction. It will be used for further investigation of similar phenomena for steam condensation in the presence of non-condensable gases. From the current study with the cylindrical enclosure, it was found that the augmentation of natural-convection heat transfer by a forced jet is primarily determined by the jet Reynolds number, while the injecting nozzle diameter has a relatively weak effect. The jet orientation also plays an important role in determining the augmentation ratio, and of the four different jet orientations studied here, vertical downward injection at the center of the enclosure gives the highest augmentation, while azimuthal injection gives the lowest. The enclosure geometry and aspect ratio are also important factors, depending on the jet orientation. A combining rule is employed with a weighted relation to balance the contributions from separate heat-transfer correlations representing natural and forced dominated convections. It was found that under natural convection without the jet, the mean Nusselt number inside the large enclosure can be correlated by the enclosure Rayleigh number, RaD1/3, and under forced convection with a strong jet, the data are well represented as a function of the jet Reynolds number, Rej2/3. According to these relationships, a correlation form predicting mixed convection inside the enclosure was developed by a function that uses the ratio of forced- and free-convection heat transfer coefficients. This form can then be further reduced to predict the augmentation ratio as a function of the Archimedes number and a correcting factor accounting for jet orientation and enclosure geometry. Figure 1 presents the heat transfer augmentation as the Nusselt number ratio of mixed convection to natural convection (NuD/Nunc) versus the Archimedes number. The data, including four injecting orientations and three different jet diameters, are well correlated for each injection mode. It is found that the injection or
International Nuclear Information System (INIS)
Within the project 'Convection in a Cylinder' (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.
Directory of Open Access Journals (Sweden)
Rajesh Khatri
2012-03-01
Full Text Available In this paper heat transfer and fluid flow characteristics in a channel has been theoretically investigated. In this study, FEM is employed to analyze a fluid flow inside a channel and then solve for the heat flow transfer through the same channel. The fluid flow is expressed by partial differential equation (Poisson’s equation.While, heat transfer is analyzed using the energy equation. The Navier Stokes equations along with the energy equation have been solved by using simple technique. The domain is discretized using 2626 elements and that corresponds to a total number of nodes 2842. The channel has a constant heat flux at the two walls and the threedimensional numerical simulations. Numerical solutions were obtained using commercial software Ansys Fluent. The working fluid was air (Pr=0.7. The local Nusselt numbers are obtained, which can be used inestimation of flow and heat transfer performance in a channel In addition, local Nusselt numbers, velocity magnitude and temperature profiles, and pressure profiles are analyzed. Results showed that both fluid flow and temperature flow are influenced significantly with changing entrance velocity. The overall objective of thispaper is to study the flow characteristics and heat transfer analysis inside a channel while increasing entrance velocity.
Indian Academy of Sciences (India)
Ankur Kumar; Jyeshtharaj B Joshi; Arun K Nayak; Pallippattu K Vijayan
2015-05-01
In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.
International Nuclear Information System (INIS)
Liquid nitrogen was used as working fluid in a tube heated in cosine distribution to study burn-out phenomena in the present experiment. Two types of burn-out were observed. One occurred when flow pattern changed from churn flow or slug flow to annular flow. Another one is DNB phenomena when the flow was unstable. (author)
Effects of rib size and arrangement on forced convective heat transfer in a solar air heater channel
Skullong, Sompol; Thianpong, Chinaruk; Promvonge, Pongjet
2015-10-01
The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed solar air heater channel fitted with rib turbulators. The experiment was conducted for the airflow rate in terms of Reynolds numbers based on the hydraulic diameter of the channel in a range of 5000-24,000. In the present work, a comparative study between square and thin ribs (90Â°-rib) with three rib arrangements, namely, one ribbed wall (or single rib), in-line and staggered ribs on two opposite walls was first introduced. The study shows a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The comparison made at a single rib pitch and height also revealed that the thin rib performs better than the corresponding square one. Among the three arrangements, the in-line rib array provides higher heat transfer and friction loss than the staggered and the single one. However, the staggered thin rib provides the highest thermal performance. With this reason, only the staggered thin ribs at four different relative heights (BR = 0.1, 0.2, 0.3 and 0.4) and three relative pitches (PR = 0.5, 0.75 and 1.33) are investigated further. It is found that the staggered rib at BR = 0.4 and PR = 0.5 yields the highest heat transfer and friction factor but the maximum thermal performance is at BR = 0.2 and PR = 0.75.
Internally heated convection and Rayleigh-Bénard convection
Goluskin, David
2016-01-01
This Brief describes six basic models of buoyancy-driven convection in a fluid layer: three configurations of internally heated convection and three configurations of Rayleigh-Bénard convection. The author discusses the main quantities that characterize heat transport in each model, along with the constraints on these quantities. This presentation is the first to place the various models in a unified framework, and similarities and differences between the cases are highlighted. Necessary and sufficient conditions for convective motion are given. For the internally heated cases only, parameter-dependent lower bounds on the mean fluid temperature are proven, and results of past simulations and laboratory experiments are summarized and reanalyzed. The author poses several open questions for future study.
Directory of Open Access Journals (Sweden)
Amnart Boonloi
2014-01-01
Full Text Available The influences of modified V-shaped baffle in a square channel for heat transfer and thermal performance enhancement are presented numerically in three Dimensional (3D. The V-shaped baffles are modified in order to comfortable to installation in the square channel. The plates are used for clamping on both the upper and lower V-shaped baffles resulting the modified V-shaped baffle like orifice plate called “V-shaped orifice tubulators, VOT”. The effects of Blockage Ratios (BR = 0.05-0.20, flow attack angles (? = 20°, 30° and 45° and flow directions (V-Downstream and V-Upstream with a single Pitch Ratio (PR = 1 are investigated for Reynolds number based on the hydraulic diameter of the square channel (Dh, Re = 100-2000. The fully developed periodic flow and heat transfer are applied for the computational domain. The SIMPLE algorithm and the finite volume method are used in the current study. The numerical results show that the use of VOT not only increasing heat transfer rate, but also rise up very enlarge pressure loss due to reducing the flow area of the cross sectional area. In addition, the maximum thermal enhancement factors are found around 2.4 and 2.5 for BR = 0.10, ? = 30° at the highest Reynolds number of V-Downstream and V-Upstream, respectively.
Scientific Electronic Library Online (English)
Néstor Enrique, Cerquera Peña; Yaneth Liliana, Ruiz Osorio; Eduardo, Pastrana Bonilla.
2010-04-01
Full Text Available Teniendo como base la infraestructura existente de un horno tradicional de curado de tabaco, se rediseño e implementó en él un sistema de intercambio de calor por convección forzada que funciona con cisco de café como combustible. Este horno de curado de tabaco por convección forzada USCO-MADR fue e [...] valuado durante el periodo de cosecha, lográndose un manejo controlado de las variables de temperatura y humedad relativa dentro de él durante las tres etapas del curado de la hoja de tabaco; el equipo utilizado tuvo un excelente desempeño al emplear cisco de café como combustible con los siguientes consumos durante el proceso de curado: en la fase de “amarillamiento”, 8,92 kilogramos por hora; en la de “secado de paño y fijación de color”, 17,75 kilogramos por hora; y en la de “secado de vena”, 19,29 kilogramos por hora; el análisis comparativo de los costos operativos del horno evaluado, con los ajustes propuestos a éste, permiten presentarlo a la cadena de tabaco como una alternativa promisoria. Abstract in english A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure); a forced-convection heat exchanger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven) was evaluated during the harvesting [...] season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment used performed excellently when using coffee hulls as fuel, having the following approximate consumption during curing: 8.92 kilograms per hour during the yellowing stage, 17.75 kilograms per hour during the leaf drying and color fixation phase and 19.29 kilograms per hour during the stem drying stage. Comparative analysis of the oven´s operating costs along with the proposed adjustments to be made to it would allow its implementation as a promising alternative in the existing tobacco chain.
International Nuclear Information System (INIS)
Full text of publication follows: The prediction of the Critical Heat Flux (CHF) in a heat flux controlled boiling heat exchanger is important to assess the maximal thermal capability of the system. In the case of a nuclear reactor, CHF margin gain (using improved mixing vane grid design, for instance) can allow power up-rate and enhanced operating flexibility. In general, current nuclear core design procedures use quasi-1D approach to model the coolant thermal-hydraulic conditions within the fuel bundles coupled with fully empirical CHF prediction methods. In addition, several CHF mechanistic models have been developed in the past and coupled with 1D and quasi-1D thermal-hydraulic codes. These mechanistic models have demonstrated reasonable CHF prediction characteristics and, more remarkably, correct parametric trends over wide range of fluid conditions. However, since the phenomena leading to CHF are localized near the heater, models are needed to relate local quantities of interest to area-averaged quantities. As a consequence, large CHF prediction uncertainties may be introduced and 3D fluid characteristics (such as swirling flow) cannot be accounted properly. Therefore, a fully mechanistic approach to CHF prediction is, in general, not possible using the current approach. The development of CHF-enhanced fuel assembly designs requires the use of more advanced 3D coolant properties computations coupled with a CHF mechanistic modeling. In the present work, the commercial CFD code CFX-5 is used to compute 3D coolant conditions in a vertical heated tube with upward flow. Several CHF mechanistic models at low quality available in the literature are coupled with the CFD code by developing adequate models between local coolant properties and local parameters of interest to predict CHF. The prediction performances of these models are assessed using CHF databases available in the open literature and the 1995 CHF look-up table. Since CFD can reasonably capture 3D fluid flow characteristics in fuel rod bundles, this will eventually allow for numerical assessment of CHF performance of newly developed fuel assembly designs for scoping purposes before actual CHF testing. (authors)
Mechanistic modeling of CHF in forced-convection subcooled boiling
International Nuclear Information System (INIS)
Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels
Cryogenic forced convection refrigerating system
International Nuclear Information System (INIS)
This patent describes the method of refrigerating products by contact with a refrigerating gas which comprises introducing product into a refrigeration zone, contacting the product with the refrigerating gas for a sufficient time to refrigerate it to the appropriate extent and removing the refrigerated product. The improvement for producing the refrigeration gas from a liquid cryogen such that essentially all of the liquid cryogen is fully vaporized before contacting the product comprises: (a) introducing the liquid cryogen, selected from the group consisting of liquid air and liquid nitrogen, at elevated pressure into an ejector as the motive fluid to accelerate a portion of a warm refrigerating gas through the ejector while mixing the cryogen and gas to effect complete vaporization of the liquid cryogen and substantial cooling of the portion of the refrigerating gas resulting in a cold discharge gas which is above the liquefaction temperature of the cryogen; (b) introducing the cold discharge gas into a forced circulation pathway of refrigerating gas and producing a cold refrigerating gas which contacts and refrigerates product and is then at least partially recirculated; (c) sensing the temperature of the refrigerating gas in the forced circulation pathway and controlling the introduction of liquid cryogen with regard to the sensed temperature to maintain the temperature of the discharge gas above the liquefacton temperature of the cryogen utilized
Free convection heat transfer to supercritical helium
International Nuclear Information System (INIS)
The study of cryogenic free convective heat transfer from a sphere to supercritical helium is reported. The free convective heat transfer coefficient has been measured within the region of 4.2 to 25 K and 3 to 35 atmospheres. Measurements were made for sphere to helium temperature difference of 0.1 to 7 K. (author)
Fustinoni, D.; Gramazio, P.; Colombo, L.; Niro, A.
2015-11-01
In this paper we present experimental results for a rectangular channel with the lower and upper walls configured with V-shaped broken ribs pointing both upward and downward the flow stream, as well as in a mixed configuration. Results for V-shaped ribs upward pointing are also reported for comparison. The duct cross-section is 120-mm wide and 12-mm height; the rib- roughened walls are operated at fixed temperature whereas the channel side walls are adiabatic. The ribs have square cross section of 2 mm in side, a V-apex angle of 60Â°, and a pitch-to-side ratios of 10 and 40. Reynolds number has been varied between 700 and 7500. The results show that the flow regime seems to be turbulent even at the lowest tested value of Re, and the friction factor is quite independent of Re, i.e., the typical trend for k-roughened surfaces. Broken and continued ribs seems to perform with no evident differences between them, except at the lowest values of Re while prevailing the former or the latter according to p/e. Anyway, adopting ribbed surface is always advantageous and allows a maximum heat transfer enhancement factor of 4.9.
Implications of some of moist convection's other paths to heating.
Stevens, Bjorn
2015-04-01
Atmospheric moist convection plays a fundamental role in governing atmospheric circulation system and the response of the atmosphere to forcing. Although the role of deep convective heating in balancing the atmospheric energy budget and driving circulations has long been appreciated, other ways in which convection can be diabatic have been less extensively studied. For instance, the coupling between convection and radiative heating in the atmosphere appears to play an important role in determining the position of the ITCZ, and the strength of tropical variability, both on inter-seasonal (MJO) and inter-annual (ENSO) timescales. Likewise precipitation from very shallow convection is found, under some circumstances to be very efficient and to regulate the structure of the marine boundary layer in the trades. This may have implications for convective aggregation and the large-scale organization of convection, as well as for the susceptibility of clouds to forcing. In addition to addressing the above ideas, new frameworks (observations and simulation) for exploring the interplay of convection with large-scale circulations will be presented.
Directory of Open Access Journals (Sweden)
S. P. Anjali Devi
2014-01-01
Full Text Available A study has been carried out on MHD boundary layer forced convection flow along a shrinking surface with variable heat flux in the presence of heat source. The flow is generated due to linear shrinking of the sheet and is influenced by uniform transverse magnetic field. The basic boundary layer momentum and heat transfer equations, which are nonlinear partial differential equations, are converted into nonlinear ordinary differential equations by means of similarity transformation. Numerical solution of the resulting boundary value problem is obtained using Nachtsheim Swigert shooting iteration scheme for the satisfaction of asymptotic boundary conditions along with the Fourth Order Runge Kutta method. The effects of suction parameter, magnetic parameter, Prandtl number, heat source parameter, stretching/shrinking parameter and heat flux parameter on velocity and temperature are shown in several plots. The results are in good agreement with the earlier published works under some limiting cases. Skin friction coefficient and wall temperature are also explored for typical values of the parameter involved in the study.
EXPERIMENTAL AND NUMERICAL STUDY OF FORCED CONVECTION IN ENGINE BLOCK
Directory of Open Access Journals (Sweden)
BELSARE S.N.
2014-04-01
Full Text Available Experiments were conducted to study forced convection in three different engine blocks. Current study uses three engine blocks machined from plain Aluminium. Plain Cylinder Block, Cylinder Block with fins & Square Block with fins is used. Inside the cylinder block fine heaters were fitted to mimic heating at different locations. Temperatures at various locations were measured. Few experimental data is compared with CFD results by Fluent.
Thermal interaction between free convection and forced convection along a vertical conducting wall
Shu, Jian-Jun
2015-01-01
A theoretical study is presented in this paper to investigate the conjugate heat transfer across a vertical finite wall separating two forced and free convection flows at different temperatures. The heat conduction in the wall is in the transversal direction and countercurrent boundary layers are formed on the both sides of the wall. The governing equations of this problem and their corresponding boundary conditions are all cast into a dimensionless form by using a non-similarity transformation. These resultant equations with multiple singular points are solved numerically using a very efficient singular perturbation method. The effects of the resistance parameters and Prandtl numbers on heat transfer characteristics are investigated.
Pattern formation in spatially forced thermal convection
Weiss, S.; Seiden, G.; Bodenschatz, E.
2012-05-01
In this paper, we present experimental results on the interplay between two different symmetry breaking mechanisms in a pattern forming system, namely inclined layer convection (ILC) with a spatially modulated heated plate. By varying the relative strength and relative orientation, we explored in detail the interplay of these symmetry breaking mechanisms. We found a stabilization of spatio-temporal chaos and resonant interactions that led to superlattice patterns. The fundamental mechanisms observed should be equally applicable to other pattern forming systems.
Pattern formation in spatially forced thermal convection
International Nuclear Information System (INIS)
In this paper, we present experimental results on the interplay between two different symmetry breaking mechanisms in a pattern forming system, namely inclined layer convection (ILC) with a spatially modulated heated plate. By varying the relative strength and relative orientation, we explored in detail the interplay of these symmetry breaking mechanisms. We found a stabilization of spatio-temporal chaos and resonant interactions that led to superlattice patterns. The fundamental mechanisms observed should be equally applicable to other pattern forming systems. (paper)
Comparison of Thermal Comfort by Radiant Heating and Convective Heating
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Shigeru Imai
2015-01-01
Full Text Available Currently, convective heating with a heat-pump system, which has high energy efficiency, is popular for room heating. However, it is possible that energy savings using convective heating can be further improved using heat pumps that service both occupied and unoccupied spaces. Moreover, convective heating increases vertical temperature gradients in a room; thus, it is hard to say whether occupants are being provided with sufficient thermal comfort. The purpose of this study is to compare the thermal comfort provided by both radiant and convective heating systems. In this study, a small office room was modeled, and then temperature and airflow distributions in the room were calculated by Computational Fluid Dynamics (CFD simulations using ESP-r (Environmental research simulation software. Furthermore, distributions of Standard Effective Temperatures (SET* were calculated using the air temperature distributions obtained from the CFD simulations, which allows us to compare the thermal comfort provided by convective heating with that provided by radiant heating. The results show that radiant heating can provide satisfactory thermal comfort, even when the room air temperature is low. However, thermal comfort also depends on the temperature of blowing air, and blowing air must reach occupied regions; thus, only radiant heating cannot circulate sufficient air. In contrast, convective heating increases vertical temperature gradients in a room. Therefore, rather than using only radiant or convective heating, it may be more effective to combine them efficiently.
International Nuclear Information System (INIS)
This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6MPa in the heat flux ranges of 5-50kW/m2, vapor quality 0-100 percent and mass velocity of 150-600 kg/m2s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture
Turbulent mixed convection in asymmetrically heated vertical channel
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Mokni Ameni
2012-01-01
Full Text Available In this paper an investigation of mixed convection from vertical heated channel is undertaken. The aim is to explore the heat transfer obtained by adding a forced flow, issued from a flat nozzle located in the entry section of a channel, to the up-going fluid along its walls. Forced and free convection are combined studied in order to increase the cooling requirements. The study deals with both symmetrically and asymmetrically heated channel. The Reynolds number based on the nozzle width and the jet velocity is assumed to be 3 103 and 2.104; whereas, the Rayleigh number based on the channel length and the wall temperature difference varies from 2.57 1010 to 5.15 1012. The heating asymmetry effect on the flow development including the mean velocity and temperature the local Nusselt number, the mass flow rate and heat transfer are examined.
Latent Heating Processes within Tropical Deep Convection
van den Heever, S. C.; Mcgee, C. J.
2013-12-01
It has been suggested that latent heating above the freezing level plays an important role in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with observational evidence of diluted tropical deep convective cores. In this study, recent modifications to the HTH have been evaluated through the use of Lagrangian trajectory analysis of deep convective cores simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated microphysical, surface and radiation parameterization schemes. Idealized, high-resolution simulations of a line of tropical convective cells have been conducted. A two-moment microphysical scheme was utilized, and the initial and lateral boundary grid conditions were obtained from a large-domain CRM simulation approaching radiative convective equilibrium. As the tropics are never too far from radiative convective equilibrium, such a framework is useful for investigating the relationships between radiation, thermodynamics and microphysics in tropical convection. Microphysical impacts on latent heating and equivalent potential temperature (?e) have been analyzed along trajectories ascending within convective regions. Changes in ?e along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. It is apparent from the CRM simulations that mixing with dry environmental air decreases ?e along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase ?e above the freezing level. The along-trajectory contributions to latent heating from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation have also been quantified. Finally, the source regions of trajectories reaching the upper troposphere have been identified. The analysis indicates that while much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, the strongest updrafts are composed of air from closer to the surface. Thus, both the boundary layer and mid-level inflow appears to play an important role in deep convection developing within moist environments.
Boiling water reactor with forced convection coolant
International Nuclear Information System (INIS)
The forced convection of the primary coolant of the BWR is done via the wheels of circulating pumps. The wheels are connected via pump waves with the motors arranged outside the pressure vessel. The pump waves are placed in three radial bearings to avoid bending oscillations. The lubrication of these hydrostatic bearings is done with lubricating water from the primary water cleaning. This contains the primary pump, a regeneration preheater, an after-cooler, a mechanical filter and a mixed-bed filter. (DG)
Energy Technology Data Exchange (ETDEWEB)
Kodama, Shigeo, E-mail: skodama@neltd.co.jp [Nuclear Engineering Ltd., 1-3-7, Tosabori, Nishi Ward, Osaka 550-0001 (Japan); Yoshida, Kenji; Kataoka, Isao [Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
2014-03-15
Thermal stratification and mixing under single- and two-phase flow natural convection are studied experimentally in relation to the safety of nuclear reactors. Flow structure and temperature distribution were measured for a rod bundle with axially distributed heat flux wherein the upper parts of the heaters are heated and the lower parts are unheated. In this scenario, under conditions of single-phase flow, thermal stratification is quite appreciable. A drastic temperature change was observed at the interface of thermal stratification. In the heated region, upward flow occurred in the rod bundle and downward flow occurred at the peripheral region of the rod bundle. As the heat flux increased, radial mixing was observed between subchannels in the rod bundle. At the interface of thermal stratification, however, almost no mixing was observed. Under conditions of boiling two-phase flow, on the other hand, thermal stratification also occurred but mixing at the interface of thermal stratification was promoted due to agitation of flow induced by bubbles. As the heat flux increased and the void fraction in the heated section increased, the interface of thermal stratification gradually advanced toward the unheated section.
An assessment on forced convection in metal foams
International Nuclear Information System (INIS)
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896–0.956; samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
An assessment on forced convection in metal foams
Mancin, S.; Rossetto, L.
2012-11-01
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896-0.956 samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
Directory of Open Access Journals (Sweden)
Kovalenko A. V.
2015-01-01
Full Text Available In the article, we have suggested a general mathematical model of non-stationary and non-isothermal process of a binary electrolyte transfer in dilute solutions in an electro-membrane system (EMS, taking into account the joint action of gravitational convection, forced convection and electro convection in potential dynamic mode. This model is a boundary problem for a system of two-dimensional quasi-linear Navier-Stokes equation and Nernst-Planck-Poisson in partial derivatives equation. We have developed a theory of similarity of the process of heat and mass transfer in electro-membrane systems, specifically, in a desalting channel of electro dialysis apparatus, taking into account joint actions of concentration polarization, space charge, gravity convection, forced convection and electro convection. It is shown that the criterion of electro convection does not directly depend on the initial concentration, and, therefore, electro convection occurs at any initial concentration. At the same time, the criterion of concentration convection linearly dependents on the initial concentration, and, therefore, at high concentrations, concentration convection prevails, while at lower concentrations, the role of gravitational convection begins to fall whereas the role of electro convection increases. The theory of similarity of the process of heat and mass transfer in the desalting channel of electro dialysis apparatus built in this work taking into account the joint action of concentration polarization, space charge, gravity convection, forced convection and electro convection is important for engineering calculations, for scaling the results of experiments in an electro-membrane cell for industrial electro dialysis water desalting apparatus
Natural Convective Heat Transfer from Narrow Plates
Oosthuizen, Patrick H
2013-01-01
Natural Convective Heat Transfer from Narrow Plates deals with a heat transfer situation that is of significant practical importance but which is not adequately dealt with in any existing textbooks or in any widely available review papers. The aim of the book is to introduce the reader to recent studies of natural convection from narrow plates including the effects of plate edge conditions, plate inclination, thermal conditions at the plate surface and interaction of the flows over adjacent plates. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed.
Convective intitiation over a heated mountain: mechanisms and predictability
Kirshbaum, D.
2010-09-01
In conditionally unstable flows over orography, the strong horizontal convergence generated by elevated heating locally weakens convective inhibition and increases the likelihood of convective initiation. This generally serves to enhance the predictability of deep convection, except when the associated uplift lies just at the margin of the forcing needed for convective initiation. In such marginal cases, airflows with very small initial differences may experience substantially different evolutions. To investigate the processes that govern cloud development in such cases, this study analyzes ensembles of idealized, high-resolution 2d simulations of the diurnal cycle in conditionally unstable flow over a mountain ridge. The case considered is based on a well-observed event from the Convective and Orographic Precipitation Study (COPS) that has proven highly difficult to predict in NWP models. This event was characterized by strong conditional instability but also large convective inhibition and a very dry mid-troposphere that presented a hostile environment for ascending clouds. Within each ensemble, the members differ only in their random seeds of low-amplitude, white-noise thermal perturbations added to the initial flow (0600 local time). The members of each ensemble experience similar mesoscale evolution, with convective inhibition (CIN) eroding completely and large CAPE developing over the high terrain by noon. Shallow orographic cumuli form predictably in response, but only in some cases do these transition to deep cumulonimbi. The dynamical and microphysical mechanisms that determine the cloud evolution in these simulations are examined through parcel trajectory analysis and an entraining thermal model.
Numerical Investigation of Turbulent Forced Convection Nanofluids Inside an Annulus
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Farhad Vahidinia
2015-09-01
Full Text Available Turbulent forced convection heat transfer of Al2O3-water nanofluid has been studied numerically under uniform heat flux on the inner and outer walls in an annulus with rough tubes. Solid nanoparticles diameter were considered to be 18, 32, and 67 nm. Two-dimensional elliptic governing equations were used and the second-order upstream difference scheme and finite volume method were used for the discretization of governing equations. SIMPLEC algorithm has been established the relationship between pressure and velocity. The results demonstrated that the surface friction coefficient increases both in the inner and outer wall of the heat exchanger by increasing the diameter of nanoparticles. On the other side, the Nusselt number of nanofluid is greater than the base fluid for a given Reynolds number and nanoparticle volume fraction. Also, the Nusselt number decreases in the inner and outer wall with increasing the diameter of nanoparticles.
Convective Instability in a Fluid Mixture Heated from Above
Energy Technology Data Exchange (ETDEWEB)
La Porta, A.; Surko, C.M. [Department of Physics, University of California, San Diego, La Jolla, California 92093 (United States)
1998-04-01
Convection patterns in ethanol-water mixtures with negative {psi} are studied when the fluid is heated from above. Although the linear analysis predicts that the instability occurs at zero wave number, a large wave number pattern is observed. The onset is supercritical with a threshold that is experimentally indistinguishable from zero. The convection amplitude exhibits damped oscillations for sudden change in the forcing parameter. At the constant Rayleigh number the patterns first coarsen, then exhibit growth of narrow plumes. The instability appears to be related to salt fingering. {copyright} {ital 1998} {ital The American Physical Society}
International Nuclear Information System (INIS)
The steady laminar flow and thermal characteristics of a continuously moving vertical sheet of extruded material are studied close to and far downstream from the extrusion slot. The velocity and temperature variations, obtained by a finite volume method, are used to map out the entire forced, mixed and natural convection regimes. The effects of the Prandtl number (Pr) and the buoyancy force parameter (B) on the friction and heat transfer coefficients are investigated. Comparisons with experimental measurements and solutions by others in the pure forced and pure natural convection regions are made. In the mixed convection region, the results are compared with available finite-difference solutions of the boundary layer equations showing excellent agreement. The region close to the extrusion slot is characterized as a non-similar forced-convection dominated region in which NuxRex-1/2 drops sharply with increasing Richardson number (Rix). This is followed by a self-similar forced-convection dominated region in which NuxRex-1/2 levels off with increasing Rix until the buoyancy effect sets in. The existence and extent of the latter region depend upon the value of B. A non-similar mixed convection region where increasing buoyancy effect enhances the heat transfer rate follows. Finally, this region is followed downstream by a self-similar natural-convection dominated region in which NuxRex-1/2 approaches the pure natural convection asymptote at large Rix. Critical values of Rix to distinguish the various convection regimes are determined for different Pr and B
Prandtl Number Dependent Natural Convection with Internal Heat Sources
International Nuclear Information System (INIS)
Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra'), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool
Natural convection in a horizontal fluid layer periodically heated from above and below
Hossain, M. Z.; Floryan, J. M.
2015-08-01
Natural convection in a horizontal slot heated from above and from below has been considered. Each heating has a certain spatial distribution. It has been demonstrated that a wide variety of convection patterns can be generated by changing the relative position of both heating patterns. A significant intensification of convection, compared to convection resulting from heating applied at one wall only, results if there is no phase shift between both patterns, while a significant reduction of convection results from the phase shift corresponding to half of the heating wavelength. The system generates a nonzero mean shear stress at each wall for all phase shifts except shifts corresponding to half of and one full heating wavelength. This effect, which is generated within one convection cell, gives rise to a global force which may lead to a thermally induced drift of the walls if such a drift was allowed.
Free convection film flows and heat transfer
Shang, Deyi
2010-01-01
Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.
Natural and mixed convection heat transfer from a horizontal heated pipe buried in a porous medium
International Nuclear Information System (INIS)
The natural and mixed convection heat transfer from horizontal heated pipe (diam. 0.646'') embedded in a water saturated a porous medium (MIT Coarse Sand, porosity 30%) for three different depth of burial to pipe diameter ratios from 11.11, 9.69 and 4.55, have been studied experimentally. A galvanized steel box, 2.5 ft long x 1.25 ft wide x 0.5 ft deep, containing the coarse sand bed measuring 1.875 ft x 1.25 ft, was used for the experimental model. The pipe was heated by an electric heating element. In the mixed convection studies, water was allowed to flow in the sand bed and past the heated cylinder in a crosswise direction. The experimental results were correlated by using the normalized parameters - Nusselt number, Rayleigh number and Reynolds number, and the aspect ratios. In the case of crossflow past the pipe, the average Reynolds number at which forced convection becomes dominant over the natural convection mode of heat transfer has been experimentally determined. In order to check the accuracy of the experimental set up and instrumentations, natural convection heat transfer from the heated cylinder to water without the porous medium was conducted and the experimental results compared favorably with those of the previous investigators
Heat flux sensors for infrared thermography in convective heat transfer.
Carlomagno, Giovanni Maria; de Luca, Luigi; Cardone, Gennaro; Astarita, Tommaso
2014-01-01
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors' research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described. PMID:25386758
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Directory of Open Access Journals (Sweden)
Giovanni Maria Carlomagno
2014-11-01
Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
Boiling inception in trichlorotrifluoroethane during forced convection at high pressures
Dougall, R. S.; Lippert, T. E.
1972-01-01
The inception of bubbles during forced convection was studied experimentally by using trichlorotrifluoroethane (R-113 or Freon-113). The experiments were performed in a rectangular channel, 12.7 x 9.5 mm in cross section. Heating was from a 3.2 mm wide strip embedded in the longer side of the channel. The pressures studied ranged from 3.6 to 20.7 bar, mass velocities from 700 to 600 kg/sq m/sec, and inlet subcoolings from 26 to 97 C. Photographs of the flow were used to determine when bubbles first appeared on the heated surface. These data were compared with wall temperature measurements and inception theories. A reasonable method for calculating the complete boiling curve was found to agree with these results.
Energy Technology Data Exchange (ETDEWEB)
Ramis, M.K.; Jilani, G.; Jahangeer, S. [Department of Mechanical Engineering, National Institute of Technology Calicut, Kerala 673 601 (India)
2008-02-15
The main objective of this paper is to present a comparative study of uniform and non-uniform volumetric energy generation in a rectangular nuclear fuel element washed by upward moving stream of liquid sodium. Employing finite difference schemes, the boundary layer equations governing the flow and thermal fields in the fluid domain are solved simultaneously with two-dimensional energy equation in the solid domain by satisfying the continuity of temperature and heat flux at the solid-fluid interface. Numerical results are presented for a wide range of aspect ratio, A{sub r}, conduction-convection parameter, N{sub cc}, total energy generation parameter, Q{sub t}, and flow Reynolds number, Re{sub H}. It is concluded that for the same total energy generation, a somewhat realistic non-uniform volumetric energy generation puts greater restriction on the thermal power generation as compared to the idealistic uniform volumetric energy generation. Further, it is found that despite the total energy generation being the same for two cases, the non-uniform volumetric energy generation within the fuel element results in considerably higher energy dissipation rate. (author)
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Giovanni Maria Carlomagno; Luigi de Luca; Gennaro Cardone; Tommaso Astarita
2014-01-01
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dy...
Determination of the convective heat transfer coefficient
Spierings, D.; Bosman, F.; Peters, T.; Plasschaert, F.
1987-01-01
The value of the convective heat transfer coefficient (htc) is determined under different loading conditions by using a computer aided method. The thermal load has been applied mathematically as well as experimentally to the coronal surface of an axisymmetric tooth model. To verify the assumptions made for the mathematical tooth model, the results predicted with this model were compared with those of an experiment using mercury as the tooth surrounding medium. For all the other thermal loadin...
Development of a mechanistic model for forced convection subcooled boiling
Shaver, Dillon R.
The focus of this work is on the formulation, implementation, and testing of a mechanistic model of subcooled boiling. Subcooled boiling is the process of vapor generation on a heated wall when the bulk liquid temperature is still below saturation. This is part of a larger effort by the US DoE's CASL project to apply advanced computational tools to the simulation of light water reactors. To support this effort, the formulation of the dispersed field model is described and a complete model of interfacial forces is formulated. The model has been implemented in the NPHASE-CMFD computer code with a K-epsilon model of turbulence. The interfacial force models are built on extensive work by other authors, and include novel formulations of the turbulent dispersion and lift forces. The complete model of interfacial forces is compared to experiments for adiabatic bubbly flows, including both steady-state and unsteady conditions. The same model is then applied to a transient gas/liquid flow in a complex geometry of fuel channels in a sodium fast reactor. Building on the foundation of the interfacial force model, a mechanistic model of forced-convection subcooled boiling is proposed. This model uses the heat flux partitioning concept and accounts for condensation of bubbles attached to the wall. This allows the model to capture the enhanced heat transfer associated with boiling before the point of net generation of vapor, a phenomenon consistent with existing experimental observations. The model is compared to four different experiments encompassing flows of light water, heavy water, and R12 at different pressures, in cylindrical channels, an internally heated annulus, and a rectangular channel. The experimental data includes axial and radial profiles of both liquid temperature and vapor volume fraction, and the agreement can be considered quite good. The complete model is then applied to simulations of subcooled boiling in nuclear reactor subchannels consistent with the operating conditions of the AP1000 pressurized water reactor. The effects of both axial and lateral nonuniform power distributions inside reactor fuel elements are accounted for. Boiling flows are simulated for three different computational domains of increasing complexity: a quarter-subchannel bordering a single fuel pin, two subchannels surround by an array of 2 by 3 fuel pins, and in four subchannels surrounded by an array of 3 by 3 fuel pins. The predicted behavior is consistent with expectations. In the 3 by 3 array, the two-phase coolant is predicted to flow from the hot channels to the cold channels, enhancing heat exchange between subchannels. This, in turn, demonstrates that the new model is capable of capturing the turbulence- and buoyancy-induced coolant mixing across the neighboring channels.
Observation of dendritic growth under the influence of forced convection
Roshchupkina, O.; Shevchenko, N.; Eckert, S.
2015-06-01
The directional solidification of Ga-25wt%In alloys within a Hele-Shaw cell was visualized by X-ray radioscopy. The investigations are focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected during crystallization. Forced convection was produced by a specific electromagnetic pump. The direction of forced melt flow is almost horizontal at the solidification front. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting, fragmentation or freckle formation depending on the dendrite orientation, the flow direction and intensity. Forced flow eliminates solutal plumes and damps local fluctuations of solute. A preferential growth of the secondary arms occurs at the upstream side of the dendrites, whereas high solute concentration at the downstream side inhibits the formation of secondary branches.
Penetrative internally heated convection in two and three dimensions
Goluskin, David
2015-01-01
Convection of an internally heated fluid, confined between top and bottom plates of equal temperature, is studied by direct numerical simulation in two and three dimensions. The unstably stratified upper region drives convection that penetrates into the stably stratified lower region. The fraction of produced heat escaping across the bottom plate, which is one half without convection, initially decreases as convection strengthens. Entering the turbulent regime, this decrease reverses in two dimensions but continues monotonically in three dimensions. The mean fluid temperature, which grows proportionally to the heating rate ($H$) without convection, grows like $H^{4/5}$ when convection is strong in both two and three dimensions. The ratio of the heating rate to the fluid temperature is likened to the Nusselt number of Rayleigh-B\\'enard convection. Simulations are reported for Prandtl numbers between 0.1 and 10 and for Rayleigh numbers (defined in terms of the heating rate) up to $5\\times10^{10}$.
Forced convective cooling of a fin in a channel
International Nuclear Information System (INIS)
A numerical study of forced convection in a horizontal parallel-plate channel with a transverse fin located at lower channel wall is investigated. Through the use of a stream function vorticity transformation, solution of the transformed governing equations for the system is obtained using the control-volume method with non-uniform grid. The effects of the Reynolds number, thermal conductivity ratio of fin to fluid and fin profile area on heat transfer rate of the fin are presented. The results indicate that the optimum aspect ratio of a fin corresponding to the fin with maximum heat transfer rate increases with increasing Re but decreases with K for a fixed fin profile area. In addition, the optimum aspect ratio of a fin obtained from this study for smaller fin profile area and Re tends to approach that of analytical solution assuming constant heat-transfer coefficient. Finally, good agreements are found between the numerical predictions of this study and other experimental data.
Evaluation of heat removal from vertical cylinder by natural convection
International Nuclear Information System (INIS)
We have studied a cooling system in a maintenance facility to store low level radioactive wastes by using natural draft without forced air ventilation. A fundamental study of natural convection around the vertical cylindrical heaters was carried out experimentally and numerically, and the ambient air was used as a cooling fluid. It was found that the velocity and temperature of air at the center of the flow channel surrounded by 4 heaters was high. And also it was shown that natural heat transfer formula for system design was conservative in the interior temperature of heater due to the increase of flow rate by chimney effect. (author)
Natural convection heat transfer from helicoidal pipes
Energy Technology Data Exchange (ETDEWEB)
Xin, R.C.; Ebadlan, M.A. [Florida International Univ., Miami, FL (United States). Dept. of Mechanical Engineering
1995-12-31
An experimental investigation is reported on natural convection heat transfer in air from outer surface of uniformly heated helicoidal pipes with vertical and horizontal orientations. The temperatures along the flow direction and peripheral direction of the tube wall were measured. The test Rayleigh numbers range from 4,000 to 100,000. The local and average Nusselt numbers are evaluated and correlated. For the vertical case, the results are compared with those of single horizontal cylinder and column of horizontal cylinders. It is found that the heat transfer from the first turn is almost the same as that of single horizontal cylinder. The heat transfer coefficient on the outer coil wall ({psi} = 3{pi}/2) is higher than that on the inner coil wall ({psi} = 3{pi}/2) in the middle turns of the coil, due to the tube curvature. For the horizontal orientation, the results are well correlated with the tube diameter as characteristic length. The local heat transfer characteristics is discussed as well. The overall average Nusselt number of the horizontal coil is higher than that of the vertical coil in the laminar region.
Measurement of the convective heat-transfer coefficient
Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio
2014-01-01
We propose an experiment for investigating how objects cool down toward the thermal equilibrium with its surrounding through convection. We describe the time dependence of the temperature difference of the cooling object and the environment with an exponential decay function. By measuring the thermal constant tau, we determine the convective heat-transfer coefficient, which is a characteristic constant of the convection system.
Mixed convection in a horizontal porous duct with a sudden expansion and local heating from below
International Nuclear Information System (INIS)
Results are reported for an experimental and numerical study of forced and mixed convective heat transfer in a liquid-saturated, horizontal porous duct. The cross section of the duct has a sudden expansion with a heated region on the lower surface downstream and adjacent to the expansion. Calculated and measured Nusselt numbers for 0.1 1.5 and Ra/Pe1.5. Calculated Nusselt numbers are very close to those for the bottom-heated flat duct, and this result has several important implications for convective heat and mass transfer in geophysical systems and porous matrix heat exchangers
International Nuclear Information System (INIS)
The vaporisation of an appreciable quantity of a liquid in a turbulent gas stream explains the increase in the heat capacity of the fluid and the improvement in the heat-transfer coefficient. The present study makes it clear that even with a very slight vaporisation, the transfer coefficient can be much increased, the pressure drop remaining nearly constant. (authors)
Modelling of convective heat and mass transfer in rotating flows
Shevchuk, Igor V
2016-01-01
Â This monograph presents results of the analytical and numerical modeling of convective heat and mass transfer in different rotating flows caused by (i) system rotation, (ii) swirl flows due to swirl generators, and (iii) surface curvature in turns and bends. Volume forces (i.e. centrifugal and Coriolis forces), which influence the flow pattern, emerge in all of these rotating flows. The main part of this work deals with rotating flows caused by system rotation, which includes several rotating-disk configurations and straight pipes rotating about a parallel axis. Swirl flows are studied in some of the configurations mentioned above. Curvilinear flows are investigated in different geometries of two-pass ribbed and smooth channels with 180Â° bends. The author demonstrates that the complex phenomena of fluid flow and convective heat transfer in rotating flows can be successfully simulated using not only the universal CFD methodology, but in certain cases by means of the integral methods, self-similar and analyt...
Convection zone origins of solar atmospheric heating
Schatten, Kenneth H.; Mayr, Hans G.
1986-10-01
Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first 'wicks' material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these 'geysers' (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry.
Endwall convective heat transfer for bluff bodies
DEFF Research Database (Denmark)
Wang, Lei; Salewski, Mirko; Sundén, Bengt; Borg, Andreas; Abrahamsson, Hans
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study......, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the...... heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the...
Boiling of subcooled water in forced convection
International Nuclear Information System (INIS)
As a part of a research about water cooled high magnetic field coils, an experimental study of heat transfer and pressure drop is made with the following conditions: local boiling in tubes of small diameters (2 and 4 mm), high heat fluxes (about 1000 W/cm2), high coolant velocities (up to 25 meters/s), low outlet absolute pressures (below a few atmospheres). Wall temperatures are determined with a good accuracy, because very thin tubes are used and heat losses are prevented. Two regimes of boiling are observed: the establishment regime and the established boiling regime and the inception of each regime is correlated. Important delays on boiling inception are also observed. The pressure drop is measured; provided the axial temperature distribution of the fluid and the axial distributions of the wall temperatures, in other words the axial distribution of the heat transfer coefficients under boiling and non boiling conditions, at the same heat flux or the same wall temperatures, are taken in account, then total pressure drop can be correlated, but probably under certain limits of void fraction only. Using the same parameters, it seems possible to correlate the experimental values on critical heat flux obtained previously, which show very important effect of length and hydraulic diameter of the test sections. (authors)
International Nuclear Information System (INIS)
Among the topics discussed are: prediction of turbulent heat transfer in flows past a cylindrical cavity; turbulent heat transfer in rotating rectangular ducts; and visualization of secondary flow patterns in an isothermally heated curved pipe. Consideration is also given to: laminar mixed convection in the entrance region of a horizontal annulus; two-dimensional mixed convection along a flat plate; mixed convection flow about slender bodies of revolution; and measurement of laminar mixed convection from an inclined surface. Additional topics discussed include: the effects of opposing buoyancy on the flow of free and wall jets; mixed convection heat transfer around a rotating heated cylinder; and combined buoyancy effects of thermal and mass diffusion on laminar forced convection heat transfer in a vertical tube
International Nuclear Information System (INIS)
The vertical upward flow of water in a heated tube at a supercritical pressure was numerically simulated by means of a commercially available computational fluid dynamics code, FLUENT. To examine the reliability of the embedded turbulence models at a supercritical pressure, a series of simulations was performed with several two-equation turbulence models: a representative low-Reynolds number k-? model, k-? model, RNG k-? model as well as a standard k-? model. The turbulence models were evaluated by comparing the results of the simulations with the experimental data published by Yamagata et al. (1972). In normal heat transfer enhancement regions, the RNG k-? model with an enhanced wall treatment option reproduces best the experiment. But, in heat transfer deterioration regions, the difference between the simulations and the experiment are remarkable. The heat transfer trend predicted by the calculation with the k-? model and low-Reynolds number k-? model is qualitatively similar to that of the experiment. Therefore, in the heat transfer deterioration regions, the k-? models could be a candidate for a reasonable solution if some modifications are made. (author)
Heat transfer by natural convection in an internally heated reactor materials melt. Rev. 0
International Nuclear Information System (INIS)
The report is structured as follows: Heat flux calculations and estimates for In-Vessel Retention (IVR); Rayleigh-Benard convection and the Rayleigh number (criterion); Free convection in the horizontal layer of a liquid which is heated by internal heating sources, and a modified Rayleigh criterion; and Turbulent convection at the reactor vessel bottom during IVR. (P.A.)
International Nuclear Information System (INIS)
To continue with the equipment of the thermal hydraulics laboratory, it was designed thermal and mechanically an heat exchanger, to satisfy the requirements to have circuit that allows to carry out heat transfer experiments. The heat exchanger was manufactured and proven in the workshops of the Prototypes and Models Management, and it is expected that to obtain the foreseen results once completely installed the circuit, in the laboratory of thermal hydraulics of the Management of Nuclear Systems. (Author)
International Nuclear Information System (INIS)
A numerical investigation was conducted on the transient behavior of a hydrodynamically, fully developed, laminar flow of power-law fluids in the thermally developing entrance region of circular ducts taking into account the effect of viscous dissipation but neglecting the effect of axial conduction. In this regard, the unsteady state thermal energy equation was solved by using a finite difference method, whereas the steady state thermal energy equation without wall heat flux was solved analytically as the initial condition of the former. The effects of the power-law index and wall heat flux on the local Nusselt number and thermal entrance length were investigated. Moreover, the local Nusselt number of steady state conditions was correlated in terms of the power-law index and wall heat flux and compared with literature data, which were obtained by an analytic solution for Newtonian fluids. Furthermore, a relationship was proposed for the thermal entrance length
Performance of a convective, infrared and combined infrared- convective heated conveyor-belt dryer.
El-Mesery, Hany S; Mwithiga, Gikuru
2015-05-01
A conveyor-belt dryer was developed using a combined infrared and hot air heating system that can be used in the drying of fruits and vegetables. The drying system having two chambers was fitted with infrared radiation heaters and through-flow hot air was provided from a convective heating system. The system was designed to operate under either infrared radiation and cold air (IR-CA) settings of 2000 W/m(2) with forced ambient air at 30 °C and air flow of 0.6 m/s or combined infrared and hot air convection (IR-HA) dryer setting with infrared intensity set at 2000 W/m(2) and hot at 60 °C being blown through the dryer at a velocity of 0.6 m/s or hot air convection (HA) at an air temperature of 60 °C and air flow velocity 0.6 m/s but without infrared heating. Apple slices dried under the different dryer settings were evaluated for quality and energy requirements. It was found that drying of apple (Golden Delicious) slices took place in the falling rate drying period and no constant rate period of drying was observed under any of the test conditions. The IR-HA setting was 57.5 and 39.1 % faster than IR-CA and HA setting, respectively. Specific energy consumption was lower and thermal efficiency was higher for the IR-HA setting when compared to both IR-CA and HA settings. The rehydration ratio, shrinkage and colour properties of apples dried under IR-HA conditions were better than for either IR-CA or HA. PMID:25892769
Energy Technology Data Exchange (ETDEWEB)
Bories, S.; Mojtabi, A.; Prat, M.; Quintard, M. [Institut de Mecanique des Fluides de Toulouse, 31 (France)
2008-10-15
Multiple physico-chemical and transport phenomena take place in porous media. The study of these phenomena requires the knowledge of fluid storage, transfer and mechanical properties of these media. Like all polyphasic heterogenous systems, these properties depend on the morphology of the matrix and of the phenomena interacting in the different phases. This makes the heat transfers in porous media a particularly huge field of researches. This article makes a synthesis of these researches. Content: 1 - classification and characterization of porous media; 2 - modeling of transfer phenomena; 3 - heat transfer by conduction: concept of equivalent thermal conductivity (ETC), modeling of conduction heat transfer, ETC determination; 4 - heat transfer by convection: modeling of convection heat transfer, natural convection (in confined media, along surfaces or impermeable bodies immersed in a saturated porous medium), forced and mixed convection; 5 - radiant heat transfer: energy status equation, approximate solutions of the radiant transfer equation, use of the approximate solutions: case of fibrous insulating materials; 6 - conclusion. (J.S.)
Natural convection in a heat-generating fluid
International Nuclear Information System (INIS)
Experimental and theoretical studies on convective heat transfer from a heat-generating fluid confined in a closed volume are reviewed. Theoretical results are obtained by means of analytical estimates based on the relevant conservation laws and the current understanding of convective heat transfer processes. Four basic and one asymptotic regime of heat transfer are identified depending on the heat generation rate. Heat transfer in a quasi-two-dimensional geometry is analyzed. Transient heat transfer from a cooling fluid without internal heat sources is studied separately. Experimental results and theoretical predictions are compared
Forced Convection and Sedimentation Past a Flat Plate
Pelekasis, Nikolaos A.; Acrivos, Andreas
1995-01-01
The steady laminar flow of a well-mixed suspension of monodisperse solid spheres, convected steadily past a horizontal flat plate and sedimenting under the action of gravity, is examined. It is shown that, in the limit as Re approaches infinity and epsilon approaches 0, where Re is the bulk Reynolds number and epsilon is the ratio of the particle radius a to the characteristic length scale L, the analysis for determining the particle concentration profile has several aspects in common with that of obtaining the temperature profile in forced-convection heat transfer from a wall to a fluid stream moving at high Reynolds and Prandtl numbers. Specifically, it is found that the particle concentration remains uniform throughout the O(Re(exp -1/2)) thick Blasius boundary layer except for two O(epsilon(exp 2/3)) thin regions on either side of the plate, where the concentration profile becomes non-uniform owing to the presence of shear-induced particle diffusion which balances the particle flux due to convection and sedimentation. The system of equations within this concentration boundary layer admits a similarity solution near the leading edge of the plate, according to which the particle concentration along the top surface of the plate increases from its value in the free stream by an amount proportional to X(exp 5/6), with X measuring the distance along the plate, and decreases in a similar fashion along the underside. But, unlike the case of gravity settling on an inclined plate in the absence of a bulk flow at infinity considered earlier, here the concentration profile remains continuous everywhere. For values of X beyond the region near the leading edge, the particle concentration profile is obtained through the numerical solution of the relevant equations. It is found that, as predicted from the similarity solution, there exists a value of X at which the particle concentration along the top side of the plate attains its maximum value phi(sub m) and that, beyond this point, a stagnant sediment layer will form that grows steadily in time. This critical value of X is computed as a function of phi(sub s), the particle volume fraction in the free stream. In contrast, but again in conformity with the similarity solution, for values of X sufficiently far removed from the leading edge along the underside of the plate, a particle-free region is predicted to form adjacent to the plate. This model, with minor modifications, can be used to describe particle migration in other shear flows, as, for example, in the case of crossflow microfiltration.
Dielectrophoretic force-driven thermal convection in annular geometry
Yoshikawa, Harunori; Crumeyrolle, Olivier; Mutabazi, Innocent
2013-01-01
The thermal convection driven by the dielectrophoretic force is investigated in an- nular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and...
Endwall convective heat transfer for bluff bodies
DEFF Research Database (Denmark)
Wang, Lei; Salewski, Mirko
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.
International Nuclear Information System (INIS)
The practical objective of research on 'burn-out' is a reliable method giving the maximum safe rating for any water cooled reactor. Experimental work, which began at numerous centres about 10 years ago, has been concerned principally with endeavouring to understand the phenomenon as it applies to simple geometries such as round and rectangular channels. Many millions of pounds have been spent on this work and several thousand separate experimental results obtained. This considerable effort has achieved little real success in providing an explanation of 'burn-out' however. Many conflicting views have arisen and correlations so far developed have been shown to give calculated 'burn-out' heat fluxes varying by a factor of the order of 5> when applied to a typical reactor situation. While some uncertainty may be due to experimental variations, inadequate analytical effort is considered to be the primary cause of the present confused situation. To overcome this various analytical studies are being initiated by the Reactor Development Division at Winfrith and a detailed plan is being evolved for bringing effort to bear on certain fundamental aspects of boiling which have been neglected and which in some oases will require the development of special experimental techniques. This report describes the result of some work already carried out. It concerns an initial examination made on a large group of 'burn-out' data and describes the development of a correlation which predicts 'burn-out' heat fluxes to within an R.M.S. error of less than 10% over a very wide range of operating conditions including pressure. (author)
A numerical study of Li-SF6 wick combustion - Forced and mixed convective burning
Damaso, R. C.; Chen, L.-D.
1992-01-01
A numerical study is conducted to study Li-SF6 wick diffusion flames under mixed convective burning conditions at a pressure of 0.01 MPa. Both planar and cylindrical wicks are considered. The model is based on a conserved scalar approach. The objective of this study is to assess the effects of particular parameters on the burning rate and heat transfer. The flat-plate solution yields a fuel mass burning rate per unit surface area following the x exp -1/2 dependence of the classical similarity solution, where x is the streamwise distance. Cylindrical wick geometries yield enhanced burning rates over planar wicks. For the case of mixed convective burning, the burning rate results approach either the forced or natural convective burning limits as ambient streamwise velocity is changed. Critical Richardson numbers specifying these burning limits are determined for a given condition. Reducing gravity results in a lower burning rate because the influence of natural convection is diminished. Under reduced gravity of 1/1000 of the sea-level value, mixed convective burning nearly resembles forced convection.
Solution of heat removal from nuclear reactors by natural convection
Zitek Pavel; Valenta Vaclav
2014-01-01
This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR).The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor) for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection...
Analysis of natural convection in volumetrically-heated melt pools
International Nuclear Information System (INIS)
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation
Analysis of natural convection in volumetrically-heated melt pools
Energy Technology Data Exchange (ETDEWEB)
Sehgal, B.R.; Dinh, T.N.; Nourgaliev, R.R. [Royal Inst. of Tech., Stockholm (Sweden). Div. of Nuclear Power Safety
1996-12-01
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation. Refs, figs, tabs.
Convection zone origins of solar atmospheric heating
International Nuclear Information System (INIS)
Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first wicks material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these geysers (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry. 39 references
Natural and forced convection film boiling over axisymmetric bodies at high temperatures
International Nuclear Information System (INIS)
Natural and forced convection film boiling over axisymmetric bodies is analysed in the case of high surface temperatures. In these conditions, the global heat transfer is no more driven by simple conduction through the vapour film, an assumption that is commonly made in film boiling modelling for simplicity reasons, but rather by convection through this film. Therefore a mathematical method was developed which enables a full description of the vapour flow by including the inertia and convection terms of the momentum and energy equations. It is based on classical two-phase boundary layer integral methods where polynomial functions of order 5 are used to describe the velocity and the temperature profiles in the vapour flow. Also, a simple scaling analysis is described in order to understand when inertial and convective effects in the vapour flow become important. It is shown for example that for any given fluid, these effects will become predominant even at a low surface superheat when the fluid pressure is increased near its critical pressure. The developed models are then compared with three simpler models: a similar model which uses order 3 polynomial functions, a model where the convective effects are modelled by just using an effective latent heat, and an even simpler model where convection and inertia in the vapour film are not considered at all. It is shown on some examples with very high surface temperatures that if convective and inertial effects are totally neglected, the global heat transfer is clearly underestimated compared to the ones calculated with the two other models, which are quite similar. However, if other important parameters such as vapour production or vapour film thickness at the front stagnation point are calculated, the results given by the model with the effective latent heat diverge from those given by the developed models. (author)
Heat flow control in thermo-magnetic convective systems using engineered magnetic fields
Lee, Jaewook; Nomura, Tsuyoshi; Dede, Ercan M.
2012-09-01
We present the design of a magnetically controlled convective heat transfer system. The underlying thermo-magnetic instability phenomenon is described, and enhanced convective fluid flow patterns are determined using non-linear programming techniques plus a design sensitivity analysis. Specifically, the magnetic fluid body force is computed by finding the optimal distribution and magnetization direction of a magnetic field source, where the objective is to minimize the maximum temperature of a closed loop heat transfer system. Sizeable fluid recirculation zones are induced by arranging magnetic field generation elements in configurations similar to Halbach arrays. Applications include improved heat flow control for electromechanical systems.
A Study of Nucleate Boiling with Forced Convection in Microgravity
Merte, Herman, Jr.
1999-01-01
The ultimate objective of basic studies of flow boiling in microgravity is to improve the understanding of the processes involved, as manifested by the ability to predict its behavior. This is not yet the case for boiling heat transfer even in earth gravity, despite the considerable research activity over the past 30 years. The elements that constitute the nucleate boiling process - nucleation, growth, motion, and collapse of the vapor bubbles (if the bulk liquid is subcooled) - are common to both pool and flow boiling. It is well known that the imposition of bulk liquid motion affects the vapor bubble behavior relative to pool boiling, but does not appear to significantly influence the heat transfer. Indeed, it has been recommended in the past that empirical correlations or experimental data of pool boiling be used for design purposes with forced convection nucleate boiling. It is anticipated that such will most certainly not be possible for boiling in microgravity, based on observations made with pool boiling in microgravity. In earth gravity buoyancy will act to remove the vapor bubbles from the vicinity of the heater surface regardless of how much the imposed bulk velocity is reduced, depending, of course, on the geometry of the system. Vapor bubbles have been observed to dramatically increase in size in pool boiling in microgravity, and the heat flux at which dryout took place was reduced considerably below what is generally termed the critical heat flux (CHF) in earth gravity, depending on the bulk liquid subcooling. However, at heat flux levels below dryout, the nucleate pool boiling process was enhanced considerably over that in earth gravity, in spite of the large vapor bubbles formed in microgravity and perhaps as a consequence. These large vapor bubbles tended to remain in the vicinity of the heater surface, and the enhanced heat transfer appeared to be associated with the presence of what variously has been referred to as a liquid microlayer between the bubble and the heater surface. The enhancement of the boiling process with low velocities in earth gravity for those orientations producing the formation of a liquid macrolayer described above, accompanied by "sliding" vapor bubbles, has been demonstrated. The enhancement was presented as a function of orientation, subcooling, and heated length, while a criterion for the heat transfer for mixed natural/forced convection nucleate boiling was given previously. A major unknown in the prediction and application of flow boiling heat transfer in microgravity is the upper limit of the heat flux for the onset of dryout (or critical heat flux - CHF), for given conditions of fluid-heater surfaces, including geometry, system pressure and bulk liquid subcooling. It is clearly understood that the behavior in microgravity will be no different than on earth with sufficiently high flow velocities, and would require no space experimentation. However, the boundary at which this takes place is still an unknown. Previous results of CHF measurements were presented for low velocity flow boiling at various orientations in earth gravity as a function of flow velocity and bulk liquid subcooling, along with preliminary measurements of bubble residence times on a flat heater surface. This showed promise as a parameter to be used in modeling the CHF, both in earth gravity and in microgravity. The objective of the work here is to draw attention to and show results of current modeling efforts for the CHF, with low velocities in earth gravity at different orientations and subcoolings. Many geometrical possibilities for a heater surface exist in flowing boiling, with boiling on the inner and outer surfaces of tubes perhaps being the most common. If the vapor bubble residence time on and departure size from the heater surface bear a relationship to the CHF, as results to be given indicate, it is important that visualization of and access to vapor bubble growth be conveniently available for research purposes. In addition, it is desirable to reduce the number of variables as much as p
Heat convection on cylinder at high Prandtl numbers
International Nuclear Information System (INIS)
Natural convection flow on a vertical cylinder is considered here when the Prandtl numbers is large. Little work has been done in this field apart form some experimental studies which are for lower Prandtl numbers. Here, the singular perturbation technique is used to solve this problem. The method adopted is to split the flow into a thin layer close to the surface of the cylinder, surrounded by a much thicker layer where the velocity is reduced to zero. It is shown that at high Prandtl numbers, the velocity boundary layer tends to be somewhat larger due to large kinematic viscosity relative to thermal diffusivity. The motion of the outer layer, however, seems to be caused by the drag force exerted by the inner layer, not due to the buoyancy itself. The basic properties of the flow are evaluated. The heat transfer coefficient is shown to give good prediction for all ranges of Prandtl numbers
DEFF Research Database (Denmark)
Taherian, Hessam; Yazdanshenas, Eshagh
Due to scarcity of literature on forced-convection heat transfer in a solar collector with rhombic cross-section absorbing tubes, a series of experiments was arranged and conducted to determine heat transfer coefficient. In this study, a typical rhombic cross-section finned tube of flat...
Time evolution simulation of heat removal in a small water tank by natural convection
International Nuclear Information System (INIS)
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity distribution over a flat plate.
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin; Ibsen, Lars Bo; Sfahani, M. G.
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity ...
Cao, Y.; Faghri, A.
1991-01-01
The performance of a thermal energy storage module is simulated numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid with low Prandtl numbers are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. The numerical results show that module geometry is crucial to the design of a space-based thermal energy storage system.
Predictions of laminar natural convection in heated cavities
International Nuclear Information System (INIS)
Several examples of laminar, natural convection in heated cavities are discussed with illustrative calculations. These include convection in a square cavity at high Rayleigh number; in a narrow cavity at moderate aspect ratio; in a rectangular cavity heated from below; in a trapezoidal cavity, and in a rectangular cavity containing a conducting obstruction. The steady equations for the velocity, pressure and temperature are solved in the Boussinesq approximation, using a standard Galerkin formulation of the finite-element method. (author)
Energy Technology Data Exchange (ETDEWEB)
Pakdee, W.; Rattanadecho, P. [Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, Rangsit Campus, Klong Luang, Pathumtani 12120 (Thailand)
2006-12-15
Numerical investigations of transient natural convection flow through a fluid-saturated porous medium in a rectangular cavity with a convection surface condition were conducted. Physical problem consists of a rectangular cavity filled with porous medium. The cavity is insulated except the top wall that is partially exposed to an outside ambient. The exposed surface allows convective transport through the porous medium, generating a thermal stratification and flow circulations. The formulation of differential equations is non-dimensionalized and then solved numerically under appropriate initial and boundary conditions using the finite difference method. The finite different equation handling the boundary condition of the open top surface is derived. The two-dimensional flow is characterized mainly by two symmetrical vortices driven by the effect of buoyancy. A lateral temperature gradient in the region close to the top wall induces the buoyancy force under an unstable condition. Unsteady effects of associated parameters were examined. It was found that the heat transfer coefficient, Rayleigh number and Darcy number considerably influenced characteristics of flow and heat transfer mechanisms. Furthermore, the flow pattern is found to have a local effect on the heat convection rate. (author)
Nie, Ji; Sobel, Adam H
2016-01-01
Extratropical extreme precipitation events are usually associated with large-scale flow disturbances, strong ascent and large latent heat release. The causal relationships between these factors are often not obvious, however, and the roles of different physical processes in producing the extreme precipitation event can be difficult to disentangle. Here, we examine the large-scale forcings and convective heating feedback in the precipitation events which caused the 2010 Pakistan flood within the Column Quasi-Geostrophic framework. A cloud-revolving model (CRM) is forced with the large-scale forcings (other than large-scale vertical motion) computed from the quasi-geostrophic omega equation with input data from a reanalysis data set, and the large-scale vertical motion is diagnosed interactively with the simulated convection. Numerical results show that the positive feedback of convective heating to large-scale dynamics is essential in amplifying the precipitation intensity to the observed values. Orographic li...
Details of Exact Low Prandtl Number Boundary-Layer Solutions for Forced and For Free Convection
Sparrow, E. M.; Gregg, J. L.
1959-01-01
A detailed report is given of exact (numerical) solutions of the laminar-boundary-layer equations for the Prandtl number range appropriate to liquid metals (0.003 to 0.03). Consideration is given to the following situations: (1) forced convection over a flat plate for the conditions of uniform wall temperature and uniform wall heat flux, and (2) free convection over an isothermal vertical plate. Tabulations of the new solutions are given in detail. Results are presented for the heat-transfer and shear-stress characteristics; temperature and velocity distributions are also shown. The heat-transfer results are correlated in terms of dimensionless parameters that vary only slightly over the entire liquid-metal range. Previous analytical and experimental work on low Prandtl number boundary layers is surveyed and compared with the new exact solutions.
Slow forced and free convection in inclined channels
International Nuclear Information System (INIS)
Mixed convection at low Reynolds numbers in tilted rectangular channels of intermediate aspect ratios is dealt with. A laminar flow enters into an inclined enclosure whose upper plate is adiabatic whereas a constant heat flux is supplied to the bottom plate. The analytic solution of the linearised problem is first obtained. Subsequently the numerical solution of the pertinent equations is carried on by a computer's time saving iterative procedure. (Author)
Nadia Potoceanu
2007-01-01
The paper presented the most aspects of convective circulate mode of heat transfer : heat transfer through the boundary layer formed at the surface of the heat generator; heat transfer in the heat carrier and heat transfer through the boundary layer formed at the heated surface
Solution of heat removal from nuclear reactors by natural convection
Directory of Open Access Journals (Sweden)
Zitek Pavel
2014-03-01
Full Text Available This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR.The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
Solution of heat removal from nuclear reactors by natural convection
Zitek, Pavel; Valenta, Vaclav
2014-03-01
This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR).The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor) for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
Turbulent forced convection in a helicoidal pipe with substantial pitch
Energy Technology Data Exchange (ETDEWEB)
Yang, G.; Ebadian, M.A. [Florida International Univ., Miami, FL (United States). Dept. of Mechanical Engineering
1995-12-31
Fully developed turbulent convective heat transfer in a circular cross section helicoidal pipe with finite pitch is numerically studied. The {kappa}-{epsilon} model is used to model the turbulent behavior. The time averaged momentum and energy equations are derived in the helicoidal coordinate system. The results indicate that the temperature distribution in the cross section will be asymmetric as the pitch of the coil increases. Unlike that in laminar flow, an increase in the Prandtl number will reduce the torsion effect on the heat transfer in a helicoidal pipe. The results also indicate that the pitch effect will be enhanced as the flow rate increases.
A Study on the Mixed Convection Heat Transfer in a Vertical Cylinder Using Electroplating System
International Nuclear Information System (INIS)
Hydrogen economy has drawn public attentions as a promising future energy source. Hydrogen is a non-petroleum-based, non-toxic, renewable and clean burning energy source. Hydrogen is the secondary energy, which means that it is produced by consuming the first energy such as coal, gas, petroleum etc. This again means that it is clean so long as it is produced by clean methods. One of the promising production methods of hydrogen is to use the heat from an HTGR(High Temperature Gas-cooled Reactor), a next generation nuclear reactor for a safe and reliable operation as well as for efficient and economic generation of energy. The knowledge of detailed heat transfer phenomena in gaseous phase emerges as an important factor for HTGR, where buoyancy effect plays a significant role. Large and expensive test facilities are to be constructed to assess the detailed mixed convection phenomena. However, using analogy concept, heat transfer system can be transformed to mass transfer system and vice versa. If a simple mass transfer system could be devised, and the experimental solution from that system could be obtained, then this could theoretically lead to a solution for a similar heat transfer system. In this study, a copper electroplating system was selected as the mass transfer system. A copper electroplating system with limiting current technique has a good advantage to simulate heat transfer system as mass transfer coefficient, analogous with heat transfer coefficient, can be directly obtained from the information of the bulk concentration and electric current between electrodes. This study simulated the mixed convective heat transfer phenomena in a vertical cylinder using copper electroplating system. The mixed convection phenomenon is observed when the forced and natural convections are of comparable magnitudes in one system. The mixed convection is classified as laminar and turbulent flows depending on the exchange mechanism and also as buoyancy aided and buoyancy opposed flows depending on the directions of forced flows with respect to the buoyancy forces. For a laminar flow, the heat transfer rate of buoyancy aided flow is larger than the corresponding forced convection heat transfer due to the increased flow velocity while the heat transfer rate of buoyancy opposed flow is smaller. However for a turbulent flow, the buoyancy opposed flow shows higher heat transfer rates than corresponding forced convective flow due to the increased turbulent production and the buoyancy aided flow shower lower heat transfer rates due to laminarization. Mixed convection heat transfer in a vertical cylinder with aiding flow and opposing flow studied experimentally for Reynolds numbers ranging from 4,000 to 10,000 with a constant Grashof number 6.2x109 and Prandtl number about 2000. The experimental results reproduced the trend of mixed convection heat transfer phenomena in a turbulent situation and agree well with the study performed by Y. Parlatan(1996). The analogy experimental method successfully simulates the mixed convection heat transfer system and seems to be a useful tool for heat transfer studies for HTGR as well as the systems with high buoyancy condition and high Prandtl number fluid, as the electroplating method not only provides useful information regarding heat transfer but also has a cost-effective advantage over any other comparable experimental method
SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD
Directory of Open Access Journals (Sweden)
A. R. M. Rosdzimin
2010-12-01
Full Text Available In this paper, mixed (forced–natural convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is used for the computation of the temperature field. We found that the combination of nine- and four-velocity models can be applied to the calculation without losing its accuracy. The results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number at the top surface of the heated square. The computational results demonstrate that the flow pattern, formation of vortex and also the Nusselt number are influence by the Reynolds number and Richardson number.
International Nuclear Information System (INIS)
Highlights: • Convective–radiative radial fins with base convective heating were analyzed. • Homogeneous material and functionally graded material fins were investigated. • Fin efficiency and the effects of dimensionless parameters in fins were analyzed. - Abstract: This paper studies a radial fin of uniform thickness with convective heating at the base and convective–radiative cooling at the tip. The fin is assumed to experience uniform internal heat generation. The exposed surfaces of the fin lose heat by simultaneous convection and radiation to the surroundings. Two types of fin materials are investigated: homogeneous material and functionally graded material (FGM). For the homogeneous material, the thermal conductivity is assumed to be a linear function of temperature, while for the FGM fin the thermal conductivity is modeled as a linear function of the dimensionless radial coordinate. The analysis is conducted using the differential transformation method (DTM). The accuracy of DTM is verified by comparing the results for the simplified versions of the present model with an exact analytical solution derived here. Once the accuracy of DTM is authenticated, the method is used to generate results for the general problem formulated here. These results illustrate the effects of various dimensionless parameters on the thermal performance of homogeneous material fins and FGM fins
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-WC thermal-hydraulic computer program
International Nuclear Information System (INIS)
Most of the previous convection experiments for nanofluids have been performed for internal tube flow with constant heat flux boundary condition. In contrast, a simple experimental apparatus measuring convective heat transfer coefficient from a heated wire to external nanofluids is proposed and its working principles are explained in detail. The convective heat transfer coefficient provided by the present system might be used as a useful indication justifying the adoption of prepared nanofluids as new efficient heat transfer fluids. Validation experiments by comparing convective heat transfer coefficients between the conventional correlation and measured values are carried out for base fluids. Also the effect of increased thermal conductivity of nano lubrication oil on the enhancement of convective heat transfer coefficient is investigated
Aerosol Radiative Effects on Deep Convective Clouds and Associated Radiative Forcing
Fan, J.; Zhang, R.; Tao, W.-K.; Mohr, I.
2007-01-01
The aerosol radiative effects (ARE) on the deep convective clouds are investigated by using a spectral-bin cloud-resolving model (CRM) coupled with a radiation scheme and an explicit land surface model. The sensitivity of cloud properties and the associated radiative forcing to aerosol single-scattering albedo (SSA) are examined. The ARE on cloud properties is pronounced for mid-visible SSA of 0.85. Relative to the case excluding the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. Cloud droplet and ice particle number concentrations, liquid water path (LWP), ice water path (IWP), and droplet size decrease significantly when the ARE is introduced. The ARE causes a surface cooling of about 0.35 K and significantly high heating rates in the lower troposphere (about 0.6K/day higher at 2 km), both of which lead to a more stable atmosphere and hence weaker convection. The weaker convection and the more desiccation of cloud layers explain the less cloudiness, lower cloud optical depth, LWP and IWP, smaller droplet size, and less precipitation. The daytime-mean direct forcing induced by black carbon is about 2.2 W/sq m at the top of atmosphere (TOA) and -17.4 W/sq m at the surface for SSA of 0.85. The semi-direct forcing is positive, about 10 and 11.2 W/sq m at the TOA and surface, respectively. Both the TOA and surface total radiative forcing values are strongly negative for the deep convective clouds, attributed mostly to aerosol indirect forcing. Aerosol direct and semi-direct effects are very sensitive to SSA. Because the positive semi-direct forcing compensates the negative direct forcing at the surface, the surface temperature and heat fluxes decrease less significantly with the increase of aerosol absorption (decreasing SSA). The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable and dryer atmosphere due to enhanced surface cooling and atmospheric heating.
Scientific Electronic Library Online (English)
M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.
2014-06-01
Full Text Available In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are [...] numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases.
Energy Technology Data Exchange (ETDEWEB)
Ghalambaz, M.; Noghrehabadi, A.; Ghanbarzadeh, A., E-mail: m.ghalambaz@gmail.com, E-mail: ghanbarzadeh.a@scu.ac.ir [Department of Mechanical Engineering, Shahid Chamran University of Ahvaz, Ahvaz (Iran, Islamic Republic of)
2014-04-15
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
International Nuclear Information System (INIS)
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
Convective heat transfer around vertical jet fires: An experimental study
Energy Technology Data Exchange (ETDEWEB)
Kozanoglu, Bulent, E-mail: bulentu.kozanoglu@udlap.mx [Universidad de las Americas, Puebla (Mexico); Zarate, Luis [Universidad Popular Autonoma del Estado de Puebla (Mexico); Gomez-Mares, Mercedes [Universita di Bologna (Italy); Casal, Joaquim [Universitat Politecnica de Catalunya (Spain)
2011-12-15
Highlights: Black-Right-Pointing-Pointer Experiments were carried out to analyze convection around a vertical jet fire. Black-Right-Pointing-Pointer Convection heat transfer is enhanced increasing the flame length. Black-Right-Pointing-Pointer Nusselt number grows with higher values of Rayleigh and Reynolds numbers. Black-Right-Pointing-Pointer In subsonic flames, Nusselt number increases with Froude number. Black-Right-Pointing-Pointer Convection and radiation are equally important in causing a domino effect. - Abstract: The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice.
The optimization of longitudinal convective fins with internal heat generation
International Nuclear Information System (INIS)
The solution of the optimization problem for longitudinal convective fins of constant thickness, triangular or parabolic profile, and uniform internal heat generation, is presented. The cases considered are those of a given heat generation density, total heat generation and heat generation per unit width of the fin, when either the heat dissipation or the width of the fin is prescribed. The results are set forth in a nondimensional form, which are presented graphically. The effect of the fin's thermal conductivity upon the optimum dimensions is discussed, and limiting values for the heat generation and the heat dissipation, which may be imposed on the fin for a feasible optimization, are also obtained. (Auth.)
Convective Behavior Of Low Prandtl Number Fluid Heated On Centrifuge
Inatomi, Yuko
2008-02-01
It has been shown by numerical simulations and by analysis of crystals that buoyancy convection in liquid might be suppressed at a particular rotation rate on a centrifuge. In the present study a damping effect that the Coriolis force has on buoyancy convection in low Prandtl number on a centrifuge was numerically analyzed under the condition of a free-swinging configuration. It is shown that the nondimensional angular velocity characterizes the minimum value of the flow velocity.
An experimental investigation of forced convection flat plate solar air heater with storage material
Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed
2012-01-01
Solar air heater (SAH) is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radi...
Thermal Performance of Convective-Radiative Heat Transfer in Porous Fins
Directory of Open Access Journals (Sweden)
Majid SHAHBABAEI
2014-01-01
Full Text Available Forced and natural convection in porous fins with convective coefficient at the tips under radiation and convection effects are investigated in this paper. Aluminum and copper as fin materials are investigated. In forced and natural convection, air and water are applied as working fluids, respectively. In order to solve this nonlinear equation, Homotopy Perturbation Method (HPM and Variational Iteration Method (VIM are used. To verify the accuracy of the methods, a comparison is made to the exact solution (BVP. In this work, the effects of porosity parameter (, Radiation parameter (? and Temperature-Ratio parameter (µ on non-dimensional temperature distribution for both of the flows are shown. The results show that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, it is shown that both VIM and HPM are capable of being used to solve this nonlinear heat transfer equation.doi:10.14456/WJST.2014.64
Reynolds stress and heat flux in spherical shell convection
Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P
2010-01-01
Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Aims. We compute turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow ...
Convective heat transfer in building simulation; Convectieve warmteoverdracht in gebouwsimulatie
Energy Technology Data Exchange (ETDEWEB)
Peeters, L.F.R. [Departement Mechanica, Vrije Universiteit Brussel, Brussels (Belgium)
2012-05-15
The sensitivity of the convective heat transfer correlations to air movements related to the normal use of a room is discussed. Also the parameters determining and the control variables directing the selection of the convection coefficient correlation during an energy calculation are discussed. The current approach for modelling convective heat transfer in common building simulation programs is discussed. As is the additional research required to further improve convection modelling. [Dutch] Er zijn drie vormen van warmteoverdracht: straling, geleiding en convectie. Terwijl de eerste twee accuraat en relatief eenvoudig analytisch kunnen worden uitgeschreven, is dat voor convectie minder evident. Een convectieve stroming is afhankelijk van een hele reeks parameters, die in min of meerdere mate relevant zijn voor de energieberekeningen in gebouwsimulatie. Het is dus van belang om te weten waar die sensitiviteiten liggen en hoe die worden gemodelleerd in gebouwsimulatie. Dit artikel geeft een analyse van de relevante parameters voor modellering van convectie in twee veel gebruikte simulatiecodes.
Validation of PARET for the modeling of heat transfer under natural convection core cooling
International Nuclear Information System (INIS)
The PARET code is a one-dimensional, coupled thermal-hydraulic and point-kinetics code, which was originally developed for the analysis of SPERT-I transients and later adapted for the analysis of transient behavior in research reactors. Due to its ease of transportability and relative simplicity of input preparation, it is widely used internationally and is particularly attractive for research reactors with limited computational facilities. The thermal-hydraulic modeling of the current version of PARET accounts for buoyancy forces in the core and external pressure gradients that may arise from density differences between the core inlet and outlet. This feature of PARET makes it a useful tool for the analysis of research reactors cooled by natural convection as well as those cooled by forced convection. Since PARET has been applied to the analysis of the International Atomic Energy Agency 10-MW benchmark cores for protected and unprotected transients and also for the analysis of SPERT-I transients, its forced convection heat-removal model is reliable. However, there has been little experience with the capability of PARET to model heat removal in cores cooled by natural convection. This paper reports the results of some experiments performed at the Malaysian PUSPATI reactor to compare PARET predictions for power increases under natural convection core cooling to measured data
D'Orazio, A.; Karimipour, A.; Nezhad, A. H.; Shirani, E.
2014-11-01
Laminar mixed convective heat transfer in two-dimensional rectangular inclined driven cavity is studied numerically by means of a double population thermal Lattice Boltzmann method. Through the top moving lid the heat flux enters the cavity whereas it leaves the system through the bottom wall; side walls are adiabatic. The counter-slip internal energy density boundary condition, able to simulate an imposed non zero heat flux at the wall, is applied, in order to demonstrate that it can be effectively used to simulate heat transfer phenomena also in case of moving walls. Results are analyzed over a range of the Richardson numbers and tilting angles of the enclosure, encompassing the dominating forced convection, mixed convection, and dominating natural convection flow regimes. As expected, heat transfer rate increases as increases the inclination angle, but this effect is significant for higher Richardson numbers, when buoyancy forces dominate the problem; for horizontal cavity, average Nusselt number decreases with the increase of Richardson number because of the stratified field configuration.
Numerical simulations on natural convective heat transfer and active cooling of IFMIF Test Cell
Energy Technology Data Exchange (ETDEWEB)
Chen, Yuming, E-mail: yuming.chen@kit.edu [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany); Arbeiter, Frederik; Heinzel, Volker; Kondo, Keitaro [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany); Mittwollen, Martin [Institute for Material Handling and Logistics, Karlsruhe Institute of Technology (Germany); Tian, Kuo [Institute for Neutron Physics and Reactor Technology, Karlsruhe Institute of Technology (Germany)
2014-10-15
Highlights: •Thermo-hydraulic simulations of the IFMIF Test-Cell were carried out with Ansys-CFX V.14 on a reference case. •The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). •A kind of CFX Beta feature was used; in which multiple fluid domains associated with individual turbulence (laminar) models were defined and solved in a single simulation. •The simulations provide the key information on the flow and heat transfer of the Test-Cell components. -- Abstract: The International Fusion Materials Irradiation Facility (IFMIF) is designated to generate a materials irradiation database for the future fusion reactors. The Test Cell (TC) accommodates the Test Modules and the lithium target assembly. Due to the nuclear heat generation, all the Test Modules inside the TC will be actively cooled. Other components like supporting structures, pipelines, cables etc., will be passively cooled by natural convection. The heat will be removed from the steel liners surrounding the TC by active water cooling. This paper concerns the thermo-hydraulic simulations of the Test Cell using Ansys-CFX. The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). The simulations provide the only means for validating the design before the construction and operation.
Extinction transition in bacterial colonies under forced convection
Neicu, T.; Pradhan, A.; Larochelle, D. A.; Kudrolli, A.
2000-01-01
We report the spatio-temporal response of {\\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the c...
International Nuclear Information System (INIS)
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. In the case of mixed and forced convection flows, the higher is the velocity, the more important is the storage capacity. - Highlights: ? We study effect of inside convection correlation on energy stored in PCM wall. ? We developed a 1D conduction model for multilayer walls, with phase change material. ? Correlations have been constructed for mixed convection in all flow regimes. ? Up to 200% variation of energy stored in PCM layer, depending on convection correlation. ? Ventilation can increase the energy stored in the PCM layer.
Convective Heat Transport in Compressible Fluids
Furukawa, Akira; Onuki, Akira
2002-01-01
We present hydrodynamic equations of compressible fluids in gravity as a generalization of those in the Boussinesq approximation used for nearly incompressible fluids. They account for adiabatic processes taking place throughout the cell (the piston effect) and those taking place within plumes (the adiabatic temperature gradient effect). Performing two-dimensional numerical analysis, we reveal some unique features of plume generation and convection in transient and steady states of compressib...
Convective Heat Transfer In Porous Ceramic Materials
BÜYÜKALACA, Orhan
1999-01-01
In this study heat transfer in porous ceramic materials, which offer a potential as an alternative heat transfer medium in a number of systems in which heat transfer takes place, is investigated experimentally. Experiments were performed for five different specimens at various air flow rates and specimen temperatures. The volumetric heat transfer coefficient was determined using the results of the experiments. A characteristic length obtained from the pressure drop data was used in ...
S. Venkatachalapathy; Udayakumar, M
2010-01-01
Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD) software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a...
Convective heat transfer around vertical jet fires: an experimental study.
Kozanoglu, Bulent; Zárate, Luis; Gómez-Mares, Mercedes; Casal, Joaquim
2011-12-15
The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice. PMID:21962859
International Nuclear Information System (INIS)
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented
Convective Heat Transfer in the Reusable Solid Rocket Motor of the Space Transportation System
Ahmad, Rashid A.; Cash, Stephen F. (Technical Monitor)
2002-01-01
This simulation involved a two-dimensional axisymmetric model of a full motor initial grain of the Reusable Solid Rocket Motor (RSRM) of the Space Transportation System (STS). It was conducted with CFD (computational fluid dynamics) commercial code FLUENT. This analysis was performed to: a) maintain continuity with most related previous analyses, b) serve as a non-vectored baseline for any three-dimensional vectored nozzles, c) provide a relatively simple application and checkout for various CFD solution schemes, grid sensitivity studies, turbulence modeling and heat transfer, and d) calculate nozzle convective heat transfer coefficients. The accuracy of the present results and the selection of the numerical schemes and turbulence models were based on matching the rocket ballistic predictions of mass flow rate, head end pressure, vacuum thrust and specific impulse, and measured chamber pressure drop. Matching these ballistic predictions was found to be good. This study was limited to convective heat transfer and the results compared favorably with existing theory. On the other hand, qualitative comparison with backed-out data of the ratio of the convective heat transfer coefficient to the specific heat at constant pressure was made in a relative manner. This backed-out data was devised to match nozzle erosion that was a result of heat transfer (convective, radiative and conductive), chemical (transpirating), and mechanical (shear and particle impingement forces) effects combined.
Khaled, M.; Garnier, B.; Harambat, F.; Peerhossaini, H.
2010-02-01
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented.
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
International Nuclear Information System (INIS)
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
Energy Technology Data Exchange (ETDEWEB)
Canaan, R.E.
1995-12-01
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
Leung, C. W.; Wong, T. T.; Kang, H. J.
The experimental investigations were consisting of two parts. The first part was carried out to study the effect of corner geometry on the steady-state forced convection inside horizontal isosceles triangular ducts with sharp corners. The electrically-heated triangular duct was used to simulate the triangular passage of a plate-fin compact heat exchanger. The isosceles triangular ducts were manufactured with duralumin, and fabricated with the same length of 2.4m and hydraulic diameter of 0.44m, but five different apex angles (i.e. ?a=15?,30?, 40?,60?, and 90?) respectively. The investigation was performed under turbulent flow condition covering a wide range of Reynolds number (i.e. 7000part was performed to investigate the effect of surface roughness on the forced convection of the same system. Horizontal equilateral triangular ducts with an apex angle of 60? were fabricated with the same length and hydraulic diameter, but different average surface roughnesses of 1.2 m,3.0 m and 11.5 m respectively. It was concluded that the duct with a higher surface roughness will have a better heat transfer performance. Non-dimensional expressions for the determination of the heat transfer coefficient of the triangular ducts with different apex angles and surface roughnesses were also developed.
A contribution to incipient boiling in the case of subcooled boiling with forced convection
International Nuclear Information System (INIS)
The literature gives contradictory statements about incipient subcooled boiling. To clear up these contradictions it seems important to study the effect of different thermo- and hydrodynamic parameters, like heating surface load, system pressure, local supercooling, and flowrate. Further influencing quantities investigated here are the concentration dissolved gases and the surface condition of the heat surface. To carry out the experimental investigations a measuring method which has already been used by Mayinger applied. With this method, incipient boiling can be determined as the first measurable heat transfer improvement in comparison with single-phase forced convection. Besides, photographs sould make it possible to give statements on the quantity and size of the bubbles on the heating surface. (orig./GL)
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Nagano, K.; Mochida, T.; Ochifuji, K. [Hokkaido University (Japan). Graduate School of Engineering
2002-08-01
Estimation of the heat recovery rate in high-temperature underground storage (> 50{sup o}C) is required before such a system can be built. However, if high-temperature water is injected into and stored in the aquifer, large-scale natural convection could occur that might reduce the heat recovery rate. This study aims to clarify the universal quantitative condition under which natural convection appears and exerts an observable influence for a system with forced horizontal flow in the saturated porous medium. The authors investigated this using both experiments and computer simulations. A test section simulating an aquifer was made. Warm water was injected into the test section, which was filled with glass beads. The temperature distribution and the flow rate profile at the outlet were measured. The authors found that the limit condition at which natural convection influences the forced horizontal flow can be determined from the velocity profile and modified Rayleigh number Ra*. In addition, the heat transfer coefficients of the upper and the lower side of walls were estimated. A computer simulation was made for calculation of the temperature field and the velocity vector in the porous medium under natural convection and under forced convection. As calculated temperature fields and flow rate distributions at the outlet were similar to the experimental results, it is thought that this program can be applied to evaluation of the temperature and the velocity of aquifer thermal energy storage. In addition, an index expressing the degree of influence of natural convection on forced convection was proposed. (author)
Heat transfer by natural convection into an horizontal cavity
International Nuclear Information System (INIS)
At this thesis it is studied the heat transfer by natural convection in an horizontal cavity, it is involved a boiling's part that is described the regimes and correlations differences for boiling's curve. It is designed a horizontal cavity for realize the experimental part and it's mention from equipment or instrumentation to succeed in a experimentation that permits to realize the analysis of heat transfer, handling as water fluid at atmospheric pressure and where it's present process from natural convection involving part boiling's subcooled. The system consists of heater zone submerged in a horizontal cavity with water. Once part finished experimental with information to obtained it's proceeded to obtain a correlation, realized starting from analysis dimensionless such as: Jakob, Bond and Grasoft (Boiling) besides of knows in natural convection: Prandtl and Nusselt. The mathematical model explains the behavior for natural convection continued part boiling's subcooled. It is realize analysis graphics too where it's show comparing with Globe Dropkin and Catton equations by natural convection with bottom heating. (Author)
Investigating Convective Heat Transfer with an Iron and a Hairdryer
Gonzalez, Manuel I.; Lucio, Jesus H.
2008-01-01
A simple experimental set-up to study free and forced convection in undergraduate physics laboratories is presented. The flat plate of a domestic iron has been chosen as the hot surface, and a hairdryer is used to generate an air stream around the plate. Several experiments are proposed and typical numerical results are reported. An analysis and…
Convective Heat Transport in Compressible Fluids
Furukawa, A; Furukawa, Akira; Onuki, Akira
2002-01-01
We present hydrodynamic equations of compressible fluids in gravity as a generalization of those in the Boussinesq approximation used for nearly incompressible fluids. They account for adiabatic processes taking place throughout the cell (the piston effect) and those taking place within plumes (the adiabatic temperature gradient effect). Performing two-dimensional numerical analysis, we reveal some unique features of plume generation and convection in transient and steady states of compressible fluids. As the critical point is approached, overall temperature changes induced by plume arrivals at the boundary walls are amplified, giving rise to overshoot behavior in transient states and significant noises of the temperature in steady states. The velocity field is suggested to assume a logarithmic profile within boundary layers. Random reversal of macroscopic shear flow is examined in a cell with unit aspect ratio. We also present a simple scaling theory for moderate Rayleigh numbers.
Calculational method for combined natural circulation and forced-convection flow in a channel
International Nuclear Information System (INIS)
This paper presents a finite element solution for combined natural circulation and forced convection flow in a channel. Because the buoyancy force plays an important role in a mixed convection flow, an iteration scheme was used in solving the coupled energy-momentum equations. The momentum equations and the pressure equation are solved to calculate velocity profiles instead of solving the momentum equations with the continuity equation. Though the pressure equation is obtained by using the continuity equation, the continuity principle is reinforced into the momentum equations and the pressure equation at each iteration. Calculations are performed for the combined natural circulation and forced convection case and the forced convection only case
Determination of drying kinetics and convective heat transfer coefficients of ginger slices
Akpinar, Ebru Kavak; Toraman, Seda
2015-12-01
In the present work, the effects of some parametric values on convective heat transfer coefficients and the thin layer drying process of ginger slices were investigated. Drying was done in the laboratory by using cyclone type convective dryer. The drying air temperature was varied as 40, 50, 60 and 70 °C and the air velocity is 0.8, 1.5 and 3 m/s. All drying experiments had only falling rate period. The drying data were fitted to the twelve mathematical models and performance of these models was investigated by comparing the determination of coefficient (R 2), reduced Chi-square (? 2) and root mean square error between the observed and predicted moisture ratios. The effective moisture diffusivity and activation energy were calculated using an infinite series solution of Fick's diffusion equation. The average effective moisture diffusivity values and activation energy values varied from 2.807 × 10-10 to 6.977 × 10-10 m2/s and 19.313-22.722 kJ/mol over the drying air temperature and velocity range, respectively. Experimental data was used to evaluate the values of constants in Nusselt number expression by using linear regression analysis and consequently, convective heat transfer coefficients were determined in forced convection mode. Convective heat transfer coefficient of ginger slices showed changes in ranges 0.33-2.11 W/m2 °C.
Electro-magneto convective heat transfer in magnetic liquids
International Nuclear Information System (INIS)
Magneto convective and electro-magneto convective heat transfer coefficients have been measured in a magnetic liquid (kerosene) and a super-paramagnetic colloid (hematite in distilled water) from a single platinum wire (diameter 0.025 mm) mounted along the axis of a copper cylinder. For this study, we used two copper cylinders (diameters = 20.5 mm and 53 mm). An inhibition in heat transfer coefficient was observed in the presence of magnetic field. A similar effect was also noticed when electric and magnetic fields were applied simultaneously. The heat transfer coefficient was found to be independent of the size of the cylinder over the range of the space ratio 820 to 2120 but an enhancement of heat transfer coefficient was observed when the inclination of the cylinder was increased from the horizontal to vertical positions
MHD forced and free convection boundary layer flow near the leading edge
International Nuclear Information System (INIS)
Magnetohydrodynamic forced and free convection flow of an electrically conducting viscous incompressible fluid past a vertical flat plate with uniform heat flux in the presence of a magnetic field acting normal to the plate that moves with the fluid has been studied near the leading edge of the plate. The coupled non-linear equations are solved by the method of superposition for the values of the Prandtl number ranges from 0.01 to 10.0. The velocity and the temperature profiles are presented graphically and the values of the wall shear-stress as well as the heat transfer rate are presented in tabular form showing the effect of the buoyancy force and the applied magnetic field. To show the accuracy of the present method some typical values are compared with the available one. (author). 17 refs, 3 figs, 2 tabs
Dendrite growth under forced convection: analysis methods and experimental tests
International Nuclear Information System (INIS)
An analysis is given of the nonisothermal growth of a dendrite crystal under forced fluid flow in a binary system. The theoretical model utilized employs a free moving crystalâ€“liquid interface and makes use of the Oseen approximation for the equations of motion of the liquid. A criterion for the stable growth of two-dimensional and three-dimensional parabolic dendrites is derived under the assumption of an anisotropic surface tension at the crystalâ€“liquid interface, which generalizes the previous known results for the stable growth of a dendrite with convection in a one-component fluid and for the growth of a dendrite in a two-component system at rest. The criterion obtained within the Oseen hydrodynamic approximation is extended to arbitrary Peclet numbers and dendrite growth with convection in a nonisothermal multicomponent system. Model predictions are compared with experimental data on crystal growth kinetics in droplets processed in electromagnetic and electrostatic levitation facilities. Theoretical and simulation methods currently being developed are applied to crystallization processes under earthly and reduced gravity conditions. (reviews of topical problems)
Natural convection heat transfer in moderate aspect ratio enclosures
International Nuclear Information System (INIS)
Local and average heat transfer coefficients for natural convection between parallel plates separated by slats to create enclosures of moderate aspect ratio have been experimentally determined using an interferometric technique. The effects of Rayleigh number, tilt and slat angle, and aspect ratio on the Nusselt number have been determined. The Rayleigh number range tested was up to 7 x 104, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 0.25 and 4. The angles of tilt of the enclosure with respect to the horizontal were 45, 60 and 90 deg. Slat angles of 45, 60 90 and 135 deg were studied. The results obtained in a previous investigation [1] for aspect ratios of 9 to 36 are included to show continuity. The results indicate that the convective heat transfer is a strong function of the aspect ratio for aspect ratios less than 4. For aspect ratios in the range of 0.5 to 4, spacers between the plates increase, rather than decrease, natural convection heat transfer compared to that for long enclosures. Slat angles less than 90 deg (i.e., oriented downward) reduce convective heat transfer
Measurement of the Convective Heat-Transfer Coefficient
Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio
2014-01-01
We propose an experiment for investigating how objects cool down toward the thermal equilibrium with their surroundings. We describe the time dependence of the temperature difference of the cooling objects and the environment with an exponential decay function. By measuring the thermal constant t, we determine the convective heat-transfer…
Heat convection in a set of three vertical cylinders
International Nuclear Information System (INIS)
Experimental results on temperature and heat flow in a set of three vertical cylinders with internal generation of heat, water submerged and in free convection are presented in this work . Temperature distribution, Nusselt number and convective coefficient (h) for each rod, developed for the distance between the axis of cylinders in vertical position, as a consequence of the application of power in its outside, are analyzed. Experimental information about heat transfer by free convection in vertical cylinders and surfaces is analyzed. Information of the several author who have carried out studies about the heat transfer on vertical cylinders was compiled, and the proposed equations with the experimental data obtained in the thermo fluids laboratory of National Institute of Nuclear Research (ININ) were tested. The way in which separation distance, s, distribution temperature array, Nusselt number, and convective coefficient calculated for the proposed channel with the Keyhani, Dutton and experimental equations are tabulated and they are plotted for each power value and for each separation between rods. The scheme of the used equipment and the experimentation description as well as the observations of tests and graphical results are included. (Author)
International Nuclear Information System (INIS)
In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.
Energy Technology Data Exchange (ETDEWEB)
Mokhtari, F [Physics Department, Faculty of Science, University of Mouloud Mammeri, Tizi Ouzou (Algeria); Bouabdallah, A; Zizi, M [LTSE Laboratory, University of Science and Technology USTHB. BP 32 Elalia, Babezzouar, Algiers (Algeria); Hanchi, S [UER Mecanique/ E.M.P B.P 17, Bordj El Bahri, Algiers (Algeria); Alemany, A, E-mail: abouab2002@yahoo.f [Laboratoire EPM, CNRS, Grenoble (France)
2010-03-01
In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.
Natural convection in porous media with heat generation
International Nuclear Information System (INIS)
Heat transfer characteristics of a fluid saturated porous media are investigated for the case of uniform internal heat generation with cooling from above. Analytical models of conduction and single phase cellular convection show good agreement with previous Rayleigh number correlations and with experimental data obtained by Joule heating of salt water in a sand bed. An approximate dryout criterion is also derived for two phase boiling heat transfer in a fixed bed which is neither channeled nor fluidized. Correlation of dryout data using this criterion is encouraging, especially considering the analytical rather than correlational basis of the criterion
Natural convection in porous media with heat generation
International Nuclear Information System (INIS)
The heat transfer characteristics of fluid-saturated porous media are investigated for the case of uniform internal heat generation with cooling from above. Analytical models of conduction and single-phase cellular convection show good agreement with previous Rayleigh number correlations and with experimental data obtained by Joule heating of saltwater in a sand bed. An approximate dryout criterion is also derived for two-phase boiling heat transfer in a fixed bed which is neither channeled nor fluidized. Correlation of dryout data using this criterion is encouraging, especially considering the analytical rather than correlational basis of the criterion
Natural convection in porous media with heat generation. [LMFBR
Energy Technology Data Exchange (ETDEWEB)
Hardee, H.C.; Nilson, R.H.
1977-06-01
The heat transfer characteristics of fluid-saturated porous media are investigated for the case of uniform internal heat generation with cooling from above. Analytical models of conduction and single-phase cellular convection show good agreement with previous Rayleigh number correlations and with experimental data obtained by Joule heating of saltwater in a sand bed. An approximate dryout criterion is also derived for two-phase boiling heat transfer in a fixed bed which is neither channeled nor fluidized. Correlation of dryout data using this criterion is encouraging, especially considering the analytical rather than correlational basis of the criterion.
Natural convection in porous media with heat generation
Energy Technology Data Exchange (ETDEWEB)
Hardee, H.C. Jr.; Nilson, R.H.
1976-12-01
Heat transfer characteristics of a fluid saturated porous media are investigated for the case of uniform internal heat generation with cooling from above. Analytical models of conduction and single phase cellular convection show good agreement with previous Rayleigh number correlations and with experimental data obtained by Joule heating of salt water in a sand bed. An approximate dryout criterion is also derived for two phase boiling heat transfer in a fixed bed which is neither channeled nor fluidized. Correlation of dryout data using this criterion is encouraging, especially considering the analytical rather than correlational basis of the criterion.
Optimisation of convective heat dissipation from ventilated brake discs
Galindo-Lopez, Carlos Hannover
2009-01-01
Fast heat dissipation from brake discs is sought in current vehicles, where high power braking duties demand harmonic combination of strength, (undamped) disc mass and cooling abilities for a wide speed range. This work analyses the convective heat dissipation from ventilated brake discs and proposes means for its optimisation. The focus of research is the ventilation geometry of a standard brake disc with an outer diameter of 434mm and radial channels of 101mm in length. After...
Scaling of heat transport near onset in rapidly rotating convection
Ecke, Robert E.
2015-10-01
We consider the scaling of heat transport in the geostrophic regime of rotating Rayleigh-Bénard convection near onset for small Ekman number Ek from the perspective of weakly nonlinear theory. We show that available heat transport data from numerical simulation [1] for Ek law slope depends sensitively on the magnitude of the coefficients a and b. Similar arguments apply to Pr = 7 numerical data although the weakly nonlinear expansion appears valid for a smaller range of ? than in the Pr = 1 case.
Relating Convective and Stratiform Rain to Latent Heating
Tao, Wei-Kuo; Lang, Stephen; Zeng, Xiping; Shige, Shoichi; Takayabu, Yukari
2010-01-01
The relationship among surface rainfall, its intensity, and its associated stratiform amount is established by examining observed precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The results show that for moderate-high stratiform fractions, rain probabilities are strongly skewed toward light rain intensities. For convective-type rain, the peak probability of occurrence shifts to higher intensities but is still significantly skewed toward weaker rain rates. The main differences between the distributions for oceanic and continental rain are for heavily convective rain. The peak occurrence, as well as the tail of the distribution containing the extreme events, is shifted to higher intensities for continental rain. For rainy areas sampled at 0.58 horizontal resolution, the occurrence of conditional rain rates over 100 mm/day is significantly higher over land. Distributions of rain intensity versus stratiform fraction for simulated precipitation data obtained from cloud-resolving model (CRM) simulations are quite similar to those from the satellite, providing a basis for mapping simulated cloud quantities to the satellite observations. An improved convective-stratiform heating (CSH) algorithm is developed based on two sources of information: gridded rainfall quantities (i.e., the conditional intensity and the stratiform fraction) observed from the TRMM PR and synthetic cloud process data (i.e., latent heating, eddy heat flux convergence, and radiative heating/cooling) obtained from CRM simulations of convective cloud systems. The new CSH algorithm-derived heating has a noticeably different heating structure over both ocean and land regions compared to the previous CSH algorithm. Major differences between the new and old algorithms include a significant increase in the amount of low- and midlevel heating, a downward emphasis in the level of maximum cloud heating by about 1 km, and a larger variance between land and ocean in the new CSH algorithm.
Non intrusive measurement of the convective heat transfer coefficient
Energy Technology Data Exchange (ETDEWEB)
Rebay, M.; Mebarki, G.; Padet, J. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Arfaoui, A. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM; Maad, B.R. [Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM
2010-07-01
The efficiency of cooling methods in thermal systems such as radiators and heat exchangers must be improved in order to enhance performance. The evaluation of the heat transfer coefficients between a solid and a fluid is necessary for the control and the dimensioning of thermal systems. In this study, the pulsed photothermal method was used to measure the convective heat transfer coefficient on a solid-fluid interface, notably between an air flow and a heated slab mounted on a PVC flat plate. This configuration simulated the electronic air-cooling inside enclosures and racks. The influence of the deflector's inclination angle on the enhancement of heat transfer was investigated using 2 newly developed identification models. The first model was based on a constant heat transfer coefficient during the pulsed experiment, while the second, improved model was based on a variable heat transfer coefficient. The heat transfer coefficient was deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab. Temperature evolutions were derived by infrared thermography, a camera for cartography and a detector for precise measurement in specific locations. The results show the improvement of measurement accuracies when using a model that considers the temporal evolution of the convective heat transfer coefficient. The deflection of air flow on the upper surface of the heated slab demonstrated better cooling of the slab by the deflection of air flow. 11 refs., 1 tab., 8 figs.
Evaporation of a binary liquid film by forced convection
Directory of Open Access Journals (Sweden)
Nasr Abdelaziz
2011-01-01
Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.
Extinction transition in bacterial colonies under forced convection
Neicu, T; Larochelle, D A; Kudrolli, A
2000-01-01
We report the spatio-temporal response of {\\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the colony growing in the absence of the hostile environment as predicted by the model of Dahmen, Nelson and Shnerb. A quantitative comparison is not possible without considering the slow dynamics and the time-dependent interaction of the population with the hostile environment.
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N. Nesenchuk
2014-09-01
Full Text Available Directions pertaining to intensification of convective heat transfer in a soft heating device have been experimentally investigated in the paper and the most efficient one has been selected that is creation of artificial roughness on the device surface. The considered heating device for a heat supply system of a mobile object has been made of soft polymer material (polyvinyl chloride. Following evaluation results of heat exchange intensification a criteria equation has been obtained for calculation of external heat transfer with due account of heat transfer intensification.
A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media
Liu, Qing; He, Ya-Ling; Li, Qing; Tao, Wen-Quan
2013-01-01
In this paper, a two-dimensional (2D) multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating convection heat transfer in porous media at the representative elementary volume scale. In the model, a MRT-LB equation is used to simulate the flow field, while another MRT-LB equation is employed to simulate the temperature field. The effect of the porous media is considered by introducing the porosity into the equilibrium moments, and adding a forc...
M. A. Mansour
2013-01-01
The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with t...
Free convection from a constant heat flux elliptic tube
International Nuclear Information System (INIS)
An experimental investigation is presented on free convection of air around the outer surface of a constant heat flux elliptic tube. The local and average Nusselt number distribution is reported for different values of Rayleigh number and different tube inclination angles. The test Rayleigh number, based on input heat flux, ranges from 1.1x107 to 8x107. Average Nusselt numbers are evaluated and correlated with Rayleigh number for the elliptic tube with vertical major axis. Comparison between the convection characteristics of isothermal and constant heat flux elliptic tubes has been presented. Also, the effect of elliptic tube orientation on the average Nusselt number is shown. It is found that a higher value of average Nusselt number is achieved when the major axis of the tube is vertical
Coupled Convective and Radiative Heat Transfer Simulation for Urban Environments
Gracik, Stefan; Sadeghipour, Mostapha; Pitchurov, George; Liu, Jiying; Heidarinejad, Mohammad; Srebric, Jelena; Building Science Group, Penn State Team
2013-11-01
A building's surroundings affect its energy use. An analysis of building energy use needs to include the effects of its urban environment, as over half of the world's population now lives in cities. To correctly model the energy flow around buildings, an energy simulation needs to account for both convective and radiative heat transfer. This study develops a new model by coupling OpenFOAM and Radiance, open source packages for simulating computational fluid dynamics (CFD) and solar radiation, respectively. The model currently provides themo-fluid parameters including convective heat transfer coefficients, pressure coefficients, and solar heat fluxes that will be used as inputs for building energy simulations in a follow up study. The model uses Penn State campus buildings immersed in the atmospheric boundary layer flow as a case study to determine the thermo-fluid parameters around buildings. The results of this case study show that shadows can reduce the solar heat flux of a building's surface by eighty percent during a sunny afternoon. Convective heat transfer coefficients can vary by around fifty percent during a windy day.
Modelling Marangoni convection in laser heat treatment
Drezet, Jean-Marie; Pellerin, S.; Bezençon, Cyrille; Mokadem, Sélim
2004-01-01
Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto the damaged component. This new technique aims to be used for the repair and reshape single crystal gas turbine components. Because of the very localised melting pool, the high temperature gradients produced during the process must be carefully controlled in order to avoid both the columnar-...
Directory of Open Access Journals (Sweden)
Sumon Saha
2006-10-01
Full Text Available Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical values of the Reynolds numbers, based on the enclosure height, are chosen as Re = 50, 100 and 200, and steady, laminar results are obtained in the range of Richardson number as 0 = Ri = 10 and a fixed Prandtl number of 0.71. The parametric studies for a wide range of governing parameters show consistent performance of the present numerical approach to obtain as stream functions and temperature profiles. Heat transfer rates at the heated walls are presented in terms of average Nusselt numbers. The computational results indicate that the heat transfer coefficient is strongly affected by Reynolds number and Richardson number. An empirical correlation is developed by using Nusselt number, Reynolds number and Richardson number.
Silk cocoon drying in forced convection type solar dryer
International Nuclear Information System (INIS)
The thin layer silk cocoon drying was studied in a forced convection type solar dryer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 oC. The cocoon was dried from the initial moisture content of about 60-12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.
Free and forced convective-diffusion solutions by finite element methods
International Nuclear Information System (INIS)
Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure
Theoretical Convective Heat Transfer Model Developement of Cold Storage Using Taguchi Analysis.
Directory of Open Access Journals (Sweden)
Dr.N.Mukhopadhyay
2015-01-01
Full Text Available Energy crisis is one of the most important problems the world is facing now-a-days. With the increase of cost of electrical energy operating cost of cold storage storing is increasing which forces the increased cost price of the commodities that are kept. In this situation if the maximum heat energy(Q is absorbed by the evaporator inside the cold room through convective heat transfer process in terms of –heat transfer due to convection and heat transfer due to condensation, more energy has to be wasted to maintain the evaporator space at the desired temperature range of 2- 8 degree centigrade. In this paper we have proposed a theoretical heat transfermodel of convective heat transfer incold storage using Taguchi L9 orthogonal array. Velocity of air(V, Temperature difference(dT, RelativeHumidity(RHare the basic variable and three ranges are taken each of them in the model development. Graphical interpretations from the model justifies the reality
Convective Heat Transfer in Nanofluids: A Computational Approach
Giraldo, Mauricio; Sanín, Daniel; Flórez, Whady F.
2010-09-01
Nanofluids are a novel strategy to increase heat transfer characteristics of fluids by the addition of solid particles with diameters below 100 nm. Experimental measurements have shown that this approach can greatly increase heat conductivity, even above that predicted by Maxwell's theory. This paper shows a direct numerical simulation of the flow and thermal behaviour of a nanofluid loaded with alumina nanoparticles. The Boundary Element Method is used given its capabilities to deal with moving boundary problems. Results showed strong convective currents caused by the presence of the nanoparticles, which in time increase total heat flow in the cavity.
Natural Convection Heat Transfer Experiments on an Inclined Helical Coil
International Nuclear Information System (INIS)
Research interests for the compact heat exchanger increase with growing needs on compact nuclear systems. Accordingly, the heat exchangers of helical coil types are adopted replacing the once-through type and the U-tube type ones. When the helical coil type heat exchangers are used for nuclear propulsions, the heat transfer of inclined helical coil becomes an important problem due to the shaking of the ship. This study measured the natural convection heat transfer from the outside surface of the helical coil in a circular duct varying the coil inclination and turn number. It is also conducted using a circular duct having same height of the helical coil. Based upon the analogy concept, a mass transfer system was used instead of a heat transfer system. A cupric acid-copper sulfate electroplating system was employed as the mass transfer systems
Convective heat transfer with buoyancy effects from thermal sources on a flat plate
Tewari, S. S.; Jaluria, Y.
1991-06-01
An experimental study is carried out on the thermal interaction between two finite-size heat sources, located on a flat plate that is well insulated on the back. Both the horizontal and the vertical orientations of the surface are studied by measuring the flow velocities, the temperature field, and the local heat flux. The investigation is directed at the pure natural convection circumstance (no forced flow velocity) and the buoyancy-dominated mixed-convection circumstance (presence of a relatively small forced flow velocity). Large temperature gradients occur in the vicinity of the heat sources, resulting in a substantial diffusion of heat along the plate length. However, the effect of conduction is found to be highly localized. The orientation of the surface has a very strong effect on the interaction of the wakes from the heat sources for the circumstances considered. An upstream source is found to have a very strong influence on the temperature of a downstream source in the vertical surface orientation but has a much weaker influence in the horizontal orientation. In the latter circumstance the presence of a small forced flow velocity may actually increase the temperature of a downstream source by tilting the wake from the upstream source toward the downstream source.
Stafford, J.; Walsh, E.; Egan, V.
2009-01-01
Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer dat...
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)
MHD mixed convective heat transfer flow about an inclined plate
Ayd?n, Orhan; Kaya, Ahmet
2009-11-01
Mixed convection heat transfer about a semi-infinite inclined plate in the presence of magneto and thermal radiation effects is studied. The fluid is assumed to be incompressible and dense. The nonlinear coupled parabolic partial differential equations governing the flow are transformed into the non-similar boundary layer equations, which are then solved numerically using the Keller box method. The effects of the mixed convection parameter R i, the angle of inclination ?, the magnetic parameter M and the radiation-conduction parameter R d on the velocity and temperature profiles as well as on the local skin friction and local heat transfer parameters. For some specific values of the governing parameters, the results are compared with those available in the literature and a fairly good agreement is obtained.
Natural convection in vertical heat-generating porous annuli
International Nuclear Information System (INIS)
In recent years, natural convection in porous media induced by internal heat generation has received considerable attention for its important applications in many geophysical and energy related engineering problems. These include, but are not limited to, heat removal from fuel debris in nuclear reactors, underground disposal of radioactive materials, and exothermic chemical reactions in packed-bed reactors. Recently, Beukema et al. (1984) studied natural convection in porous media with internal heat generation from the metabolism of the agricultural products confined in an isothermally cooled parallelepiped. Other existing reports on this topic focused mostly on a rectangular enclosure with two types of boundary conditions, i.e., cooled at the horizontal boundaries or at the side walls. However, a heat generating porous annulus frequently encountered in many engineering practices has not received much attention. Furthermore, there appears to be no studies on the non-Darcy effects for the case of a heat generating porous medium. The purpose of the paper is to numerically investigate the heat transfer process occurring in porous annuli by taking both inertial and viscous into account
Energy Technology Data Exchange (ETDEWEB)
Davidson, J.H.
1998-06-01
This progress report describes the thermodynamic testing and modeling of a thermosyphon heat exchanger used in solar water heating systems. Testing of a four tube-in-shell thermosyphon heat exchanger was performed in two parts. The first portion of the test increased the collector fluid while the storage tank remained isothermal. After the collector fluid temperature was raised to 95 C, the second part of the test allowed the storage tank to gain heat. The test was performed for two collector flow rates. Measured values included collector side forced flow rate, temperature differences across the heat exchanger, vertical temperature distribution in the storage tank, vertical water temperature profile in the heat exchanger, and pressure drop on the thermosyphon side of the heat exchanger. The overall heat transfer coefficient-area product (UA) values obtained confirmed that models which assume UA depends solely on thermosyphon flow rate do not adequately characterize thermosyphon heat exchangers. This is because heat transfer in thermosyphon exchangers occurs in the mixed convection, rather than forced flow, regime. A linear regression equation was developed to better predict UA using the Prandtl, Reynolds, and Grashof numbers and dimensionless parameters based on fluid properties calculated for the average hot and cold leg temperatures. 9 figs.
AERIAL MEASUREMENTS OF CONVECTION CELL ELEMENTS IN HEATED LAKES
Energy Technology Data Exchange (ETDEWEB)
Villa-Aleman, E; Saleem Salaymeh, S; Timothy Brown, T; Alfred Garrett, A; Malcolm Pendergast, M; Linda Nichols, L
2007-12-19
Power plant-heated lakes are characterized by a temperature gradient in the thermal plume originating at the discharge of the power plant and terminating at the water intake. The maximum water temperature discharged by the power plant into the lake depends on the power generated at the facility and environmental regulations on the temperature of the lake. Besides the observed thermal plume, cloud-like thermal cells (convection cell elements) are also observed on the water surface. The size, shape and temperature of the convection cell elements depends on several parameters such as the lake water temperature, wind speed, surfactants and the depth of the thermocline. The Savannah River National Laboratory (SRNL) and Clemson University are collaborating to determine the applicability of laboratory empirical correlations between surface heat flux and thermal convection intensity. Laboratory experiments at Clemson University have demonstrated a simple relationship between the surface heat flux and the standard deviation of temperature fluctuations. Similar results were observed in the aerial thermal imagery SRNL collected at different locations along the thermal plume and at different elevations. SRNL will present evidence that the results at Clemson University are applicable to cooling lakes.
Forced convection analysis for generalized Burgers nanofluid flow over a stretching sheet
Khan, Masood; Khan, Waqar Azeem
2015-10-01
This article reports the two-dimensional forced convective flow of a generalized Burgers fluid over a linearly stretched sheet under the impacts of nano-sized material particles. Utilizing appropriate similarity transformations the coupled nonlinear partial differential equations are converted into a set of coupled nonlinear ordinary differential equations. The analytic results are carried out through the homotopy analysis method (HAM) to investigate the impact of various pertinent parameters for the velocity, temperature and concentration fields. The obtained results are presented in tabular form as well as graphically and discussed in detail. The presented results show that the rate of heat transfer at the wall and rate of nanoparticle volume fraction diminish with each increment of the thermophoresis parameter. While incremented values of the Brownian motion parameter lead to a quite opposite effect on the rates of heat transfer and nanoparticle volume fraction at the wall.
Olson, Sandra
2011-01-01
To better evaluate the buoyant contributions to the convective cooling (or heating) inherent in normal-gravity material flammability test methods, we derive a convective heat transfer correlation that can be used to account for the forced convective stretch effects on the net radiant heat flux for both ignition delay time and burning rate. The Equivalent Low Stretch Apparatus (ELSA) uses an inverted cone heater to minimize buoyant effects while at the same time providing a forced stagnation flow on the sample, which ignites and burns as a ceiling fire. Ignition delay and burning rate data is correlated with incident heat flux and convective heat transfer and compared to results from other test methods and fuel geometries using similarity to determine the equivalent stretch rates and thus convective cooling (or heating) rates for those geometries. With this correlation methodology, buoyant effects inherent in normal gravity material flammability test methods can be estimated, to better apply the test results to low stretch environments relevant to spacecraft material selection.
A correlation for natural convection heat transfer from inclined plate-finned heat sinks
International Nuclear Information System (INIS)
Steady-state natural convection heat transfer from inclined plate-finned heat sinks to air is numerically investigated by using an experimentally validated model. The heat sinks with parallel arrangement of uniform rectangular cross section plate fins are inclined from the vertical in both forward and backward directions in order to investigate the effect of inclination on convection. Our previously validated numerical model for vertically oriented heat sinks is directly used without changing any model parameters, but only by varying the direction of the gravitational acceleration to create the effect of inclination. The flow and temperature fields are resolved using a finite volume computational fluid dynamics code. Performing a large number of simulations for the heat sink base inclination angles of ±4°, ±10°, ±20°, ±30°, ±45°, ±60°, ?65°, ?70°, ±75°, ±80°, ±85°, ±90° from the vertical, the dependence of the convective heat-transfer rate to the inclination angle and Rayleigh number is investigated. Scale analyses are performed in order to generalize estimates for the convection heat-transfer rates. A single correlation is suggested and shown to be valid for a very wide range of angles from ?60° (upward) to +80° (downward) in a wide range of Rayleigh numbers from 0 to 2 × 108. -- Highlights: ? Natural convection heat transfer from inclined plate-finned heat sinks is investigated. ? A correlation for estimating convection heat-transfer rates is suggested. ? The correlation is shown to be valid in a very wide range of angles, ?60° ? ? ? +80°. ? The correlation is verified with all available experimental data in literature. ? Flow separation and fin height play the most significant roles at high inclinations
Turbulent convective heat transfer around a horizontal rectangular cylinder
International Nuclear Information System (INIS)
A procedure for numerical analysis of turbulent convective heat transfer around a horizontal rectangular cylinder is presented. The ?-? turbulent model is used and the staggered grid system is introduced for the velocity components. Finite difference equations are derived from elliptic partial differential equations using hybrid scheme. TDMA and line-by-line SIMPLE algorithms are used to solve finite difference equations. The variation of mean velocity, turbulent kinetic energy, dissipation rate of turbulent kinetic energy and temperature behind the cylinder are presented. The heat transfer coefficient from a rectangular cylinder in cross flow is represented by the relation Nu=0.0282 Re0.766 Pr1/3 for 1035. (Author)
International Nuclear Information System (INIS)
The electro-thermo-convective motion in a plane horizontal dielectric liquid layer subjected to simultaneous action of electric field and thermal gradient is numerically investigated. We consider the case of a strong unipolar charge injection C = 10 from above or below. Therefore in this context, we only take into account the Coulomb force, disregarding the dielectric one. The effect of the electric field on the heat transfer is analyzed through the characterization of the time history of the Nusselt number as well as its evolution according to the characteristic dimensionless electric parameter T. It is demonstrated that the electric effects dominate the buoyancy ones resulting in an electrically induced convection which significantly enhance the heat transfer.
Liquid oil painting: Free and forced convection in an enclosure with mechanical and thermal forcing
Sheard, Gregory J; King, Martin P
2012-01-01
A fluid dynamics video is linked to this article, which have been submitted to the Gallery of Fluid Motion as part of the 65th American Physical Society meeting of the Division of Fluid Dynamics, held in San Diego, California, USA, over 17-20 November 2012. The video serves to visualize flows generated in a rectangular enclosure that are subjected to both mechanical and thermal forcing through a common horizontal boundary. This system exhibits features consistent with either horizontal convection or lid-driven cavity flows depending on the ratio between thermal and mechanical stirring, and three different cases are visualized in the linked videos.
Convective heat transfer analysis in aggregates rotary drum reactor
International Nuclear Information System (INIS)
Heat transport characterisation inside rotary drum dryer has a considerable importance linked to many industrial applications. The present paper deals with the heat transfer analysis from experimental apparatus installed in a large-scale rotary drum reactor applied to the asphalt materials production. The equipment including in-situ thermal probes and external visualization by mean of infrared thermography gives rise to the longitudinal evaluation of inner and external temperatures. The assessment of the heat transfer coefficients by an inverse methodology is resolved in order to accomplish a fin analysis of the convective mechanism inside baffled (or flights) rotary drum. The results are discussed and compared with major results of the literature. -- Highlights: ? A thermal and flow experimentation is performed on a large-scale rotary drum. ? Four working points is chosen in the frame of asphalt materials production. ? Evaluation of the convective transfer mechanisms is calculated by inverse method. ? The drying stage is performed in the combustion area. ? Wall/aggregates heat exchanges have a major contribution in the heating stage
Heat transport measurements in turbulent rotating Rayleigh-Benard convection
Energy Technology Data Exchange (ETDEWEB)
Ecke, Robert E [Los Alamos National Laboratory; Liu, Yuanming [Los Alamos National Laboratory
2008-01-01
We present experimental heat transport measurements of turbulent Rayleigh-Benard convection with rotation about a vertical axis. The fluid, water with Prandtl number ({sigma}) about 6, was confined in a cell which had a square cross section of 7.3 cm x 7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers 2 x 10{sup 5} < Ra < 5 x 10{sup 8} and Taylor numbers 0 < Ta < 5 x 10{sup 9}. We show the variation of normalized heat transport, the Nusselt number, at fixed dimensional rotation rate {Omega}{sub D}, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range 10{sup 7} to about 10{sup 9} is roughly 0.29 with a Ro dependent coefficient or equivalently is also well fit by a combination of power laws of the form a Ra{sup 1/5} + b Ra{sup 1/3} . The range of Ra is not sufficient to differentiate single power law or combined power law scaling. The overall impact of rotation on heat transport in turbulent convection is assessed.
Validating The Goddard Convective-Stratiform Heating Algorithm for PMM
Lang, S. E.; Tao, W. K.; Takayabu, Y. N.; Shige, S.; Johnson, R. H.; Ciesielski, P. E.
2014-12-01
RMS errors from 2007 through 2008 are computed between the equivalent rainfall obtained from integrating the cloud heating retrieved by the latest Goddard Convective-Stratiform Heating (CSH) algorithm (i.e., the TRMM 3G31 daily gridded product) and the observed surface rainfall obtained from the TRMM 2B31 Combined Algorithm at various time and space scales as well as for varying rain intensities both over land and ocean areas. RMS errors are significantly reduced by spatial and temporal averaging, noticeably lower over ocean than over land, and decrease with increasing rain intensity. There is a mean global positive heating bias of ~24%, which is much higher over land (~41%) than over ocean (~16%); these biases are reduced to (~5%) globally, ~23% over land and a negative bias of just ~2% over ocean when the residual background heating in far from rain regions is not included.
Optimization of a Radial Flow Heat Sink Under Natural Convection
Bhowmik, Himangshu
2014-01-01
A steady-state three-dimensional numerical model is developed to predict natural convection heat transfer from a radial flow heat sink. The considered medium is air. The effect of several design parameters, such as the fin length and height, number of fins, and the heat sink base radius, on heat transfer is investigated. The Taguchi method, known to be a very useful tool for selecting the best levels of control factors, is employed. Five factors and four levels for each factor are chosen. Sixteen kinds of models are analyzed, and the total heat transfer for each model is obtained. The results are used to estimate the optimum design values of the parameters affecting the heat sink performance. The reliability of these values is verified. The average heat transfer rate of the optimum model is shown to increase by 60% as compared to the reference model. Finally, the heat transfer data at different outer radii of the radial flow heat sink are correlated.
DEFF Research Database (Denmark)
Taherian, Hessam; Yazdanshenas, Eshagh
2006-01-01
Due to scarcity of literature on forced-convection heat transfer in a solar collector with rhombic cross-section absorbing tubes, a series of experiments was arranged and conducted to determine heat transfer coefficient. In this study, a typical rhombic cross-section finned tube of flat-plate collectors used as the test section. Two correlations were proposed for the Nusselt number as a function of the Reynolds number and the Prandtl number based on hydraulic diameter for various heat fluxes. The temperature distribution along the finned tube for the fluid and the wall were also illustrated.
Core flows and heat transfer induced by inhomogeneous cooling with sub- and supercritical convection
Dietrich, W.; Hori, K.; Wicht, J.
2016-02-01
The amount and spatial pattern of heat extracted from cores of terrestrial planets is ultimately controlled by the thermal structure of the lower rocky mantle. Using the most common model to tackle this problem, a rapidly rotating and differentially cooled spherical shell containing an incompressible and viscous liquid is numerically investigated. To gain the physical basics, we consider a simple, equatorial symmetric perturbation of the CMB heat flux shaped as a spherical harmonic Y11 . The thermodynamic properties of the induced flows mainly depend on the degree of nonlinearity parametrised by a horizontal Rayleigh number Rah =q? Ra , where q? is the relative CMB heat flux anomaly amplitude and Ra is the Rayleigh number which controls radial buoyancy-driven convection. Depending on Rah we identify and characterise three distinctive flow regimes through their spatial patterns, heat transport and flow speed scalings: in the linear conductive regime the radial inward flow is found to be phase shifted 90° eastwards from the maximal heat flux as predicted by a linear quasi-geostrophic model for rapidly rotating spherical systems. The advective regime is characterised by an increased Rah where nonlinearities become significant, but is still subcritical to radial convection. There the upwelling is dispersed and the downwelling is compressed by the thermal advection into a spiralling jet-like structure. As Rah becomes large enough for the radial convection to set in, the jet remains identifiable on time-average and significantly alters the global heat budget in the convective regime. Our results suggest, that the boundary forcing not only introduces a net horizontal heat transport but also suppresses the convection locally to such an extent, that the net Nusselt number is reduced by up to 50%, even though the mean CMB heat flux is conserved. This also implies that a planetary core will remain hotter under a non-homogeneous CMB heat flux and is less well mixed. A broad numerical parameter investigation regarding Rayleigh number and the relative heat flux anomaly further fosters these results.
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Directory of Open Access Journals (Sweden)
Theodor Mateescu
2007-01-01
Full Text Available The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
Neshat, E.; Hossainpour, S.; Bahiraee, F.
2014-06-01
Both of experimental and numerical investigations were performed to understand unsteady natural convection from outer surface of helical coils. Four helical coils with two different curvature ratios were used. Each coil was mounted in the shell both vertically and horizontally. The cold water was entered the coil and the hot water in the shell was cooling by unsteady natural convection. A CFD code was developed to simulate natural convection heat transfer. Equations of tube and shell are solved simultaneously. Statistical analyses have been done on data points of temperature and natural convection Nusselt number. It was revealed that shell-side fluid temperature and the Nusselt number of the outer surface of coils are functions of in-tube fluid mass flow rate, specific heat of fluids and geometrical parameters including length, inner diameter of the tube and the volume of the shell, and time.
Origin of Knudsen forces on heated microbeams
Zhu, Taishan
2010-09-09
The presented work probes the fundamentals of Knudsen forces. Using the direct simulation Monte Carlo (DSMC) method, the flows induced by temperature inhomogeneity within a representative configuration and the Knudsen force acting on a heated microbeam are captured as functions of Knudsen number in the entire flow regime. Both flow strength and Knudsen force peak in the transition regime and negative Knudsen force absent in experimental data is observed. The mechanisms of the thermally induced flows and Knudsen forces are studied. It has been found that thermal edge flow is the main driven source for the formation of the Knudsen force on microbeams and domain configuration plays an important role in the process.
Energy Technology Data Exchange (ETDEWEB)
Schumacher, Courtney
2012-12-13
Heating associated with tropical cloud systems drive the global circulation. The overall research objectives of this project were to i) further quantify and understand the importance of heating in tropical convective cloud systems with innovative observational techniques, and ii) use global models to determine the large-scale circulation response to variability in tropical heating profiles, including anvil and cirrus cloud radiative forcing. The innovative observational techniques used a diversity of radar systems to create a climatology of vertical velocities associated with the full tropical convective cloud spectrum along with a dissection of the of the total heating profile of tropical cloud systems into separate components (i.e., the latent, radiative, and eddy sensible heating). These properties were used to validate storm-scale and global climate models (GCMs) and were further used to force two different types of GCMs (one with and one without interactive physics). While radiative heating was shown to account for about 20% of the total heating and did not have a strong direct response on the global circulation, the indirect response was important via its impact on convection, esp. in how radiative heating impacts the tilt of heating associated with the Madden-Julian Oscillation (MJO), a phenomenon that accounts for most tropical intraseasonal variability. This work shows strong promise in determining the sensitivity of climate models and climate processes to heating variations associated with cloud systems.
Natural convection heat transfer below downward facing horizontal surfaces
International Nuclear Information System (INIS)
The laminar steady-state natural convection below an infinite strip and below a circular plate, heated at their bottom sides, has been calculated analytically for a uniform surface temperature as well as for a uniform surface heat flux. This convection is driven by the non-uniform temperature distribution near the edges of the plate. This particular feature makes the problem a basically elliptic one, a fact that was not taken into account in earlier studies. In contrast to the flow near inclined heated plates the horizontal situation cannot be described by a boundary layer theory alone. Similarity solutions have been obtained employing the method of matched asymptotic expansions. A potential flow or a Stokes' flow with an unknown upstream velocity is taken as the outer expansion and a boundary layer flow as the inner expansion. Both expansions are matched by an energy balance in the region near the stagnation point at the plate center. In this way the unknown upstream velocity is fixed. This iterative procedure is first restricted to the limiting cases of low and high Prandtl numbers. Then an interpolation of these asymptotes yields heat transfer correlations for arbitrary Prandtl numbers. In comparison to earlier approximations these results are independent of any empirical boundary layer profiles and are based on experimental data to a very limited extend. (orig.)
Development and performance evaluation of forced convection potato solar dryer
International Nuclear Information System (INIS)
This research paper deals with the design development and testing of a forced convection solar dryer, for drying and converting to flour of high moisture content vegetables like potatoes. The angle of solar collector was made adjustable for the absorption of maximum solar radiation by the absorber plate. The air flow rate was controlled by adjustable gate valve to find the optimum flow rate for dehydration of the product. The penetration of solar radiation raised the temperature of the absorber plate of the dryer to 110 deg. C during the operation under stagnation or no load conditions. The maximum air temperature attained in the solar air heater, under this condition was 80 deg. C. The dryer was loaded with 12 Kg of blanched potato chips having an initial moisture content of 89.75%, and the final desired moisture content of 6.95% was achieved within five hours without losing the color of potato chips, while the moisture contents reduction was from 89.75% to 33.75% for five hours in open sun drying under shade. The drying cost for 1 Kg of potatoes was calculated as Rs. 245 and it was Rs. 329 in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for blanched potato chips drying, and it turned out be Rs.163177.67/- which was much higher than the capital cost of the dryer (Rs. 25000). The payback period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)
Natural convection of ferrofluids in partially heated square enclosures
International Nuclear Information System (INIS)
In this study, natural convection of ferrofluid in a partially heated square cavity is numerically investigated. The heater is located to the left vertical wall and the right vertical wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure are assumed to be adiabatic. Finite element method is utilized to solve the governing equations. The influence of the Rayleigh number (104?Ra?5×105), heater location (0.25H?yh?0.75H), strength of the magnetic dipole (0???2), horizontal and vertical location of the magnetic dipole (?2H?a??0.5H, 0.2H?b?0.8H) on the fluid flow and heat transfer characteristics are investigated. It is observed that different velocity components within the square cavity are sensitive to the magnetic dipole source strength and its position. The length and size of the recirculation zones adjacent to the heater can be controlled with magnetic dipole strength. Averaged heat transfer increases with decreasing values of horizontal position of the magnetic dipole source. Averaged heat transfer value increases from middle towards both ends of the vertical wall when the vertical location of the dipole source is varied. When the heater location is changed, a symmetrical behavior in the averaged heat transfer plot is observed and the minimum value of the averaged heat transfer is attained when the heater is located at the mid of vertical wall. - Highlights: • Free convection of ferrofluids in a partially heated cavity is numerically studied. • Velocities are sensitive to the magnetic dipole source strength and its position. • Averaged Nu increases with decreasing x-location values of the magnetic dipole. • Averaged Nu increases from middle towards ends when y-location of dipole changes
Experimental study of an upward sub-cooled forced convection in a rectangular channel
Kouidri, A.; Madani, B.; Roubi, B.; Hamadouche, A.
2015-08-01
The upward sub-cooled forced convection in a rectangular channel is investigated experimentally. The aim of the present work is the studying of the local heat transfer phenomena. Concerning the experimentation: the n-pentane is used as a working fluid, the independent variables are: the velocity in the range from 0.04 to 0.086 m/s and heat flux density with values between 1.8 and 7.36 W/cm2. The results show that the local Nusselt number distribution is not uniform along the channel; however, uniformity is observed in the mean Nusselt number for Reynolds under 1600. On the other hand, a new correlation to predict the local fluid temperature is established as a function of local wall temperature. The wall's heat is dissipated under the common effect of the sub-cooled regime; therefore, the local heat transfer coefficient is increased. The study of the thermal equilibrium showed that for Reynolds less than 1500; almost all of the heat flux generated by the heater cartridges is absorbed by the fluid.
Development of convective heat transfer correlations for common designs of solar dryer
International Nuclear Information System (INIS)
Highlights: ? Separate experimental methods of hcpf evaluation are proposed for different dryers. ? Correlation for hcpf in terms of dimensionless numbers for each dryer is proposed. ? Single correlation for hcpf representing different dryer designs is also developed. ? Levenberg–Marquardt algorithm is used to develop temperature dependent correlation. ? Close agreement of experimental and predicted hcpf validates proposed correlations. - Abstract: The knowledge of convective heat transfer coefficient hcpf (absorber plate to flowing air) is necessary to predict or evaluate thermal performance of any solar dryer. In order to determine hcpf, laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform no-load steady state experiments for natural and forced air circulation. The dryers are operated under indoor simulation conditions for absorbed thermal energy and air flow rate for the range of 300–800 W/m2 and 1–3 m/s, respectively. Separate methods depending on mode of heat utilisation are proposed for determination of hcpf for different dryers. Correlations of hcpf in terms of dimensionless numbers are developed for each dryer operating under natural and forced convection. Levenberg–Marquardt algorithm is used to develop temperature dependent correlations. A close agreement between experimental and predicted hcpf values obtained from proposed correlations for natural convection dryers demonstrates their reliability. However, for forced convection dryers, there is a need to use temperature dependent Nu–Re correlation for more accurate results. The low uncertainty ranging from 0.3% to 0.8% in the determination of hcpf confirms the accuracy of experimental data obtained for various dryer designs operated under different conditions.
Convective heat transfer for viscoelastic fluid in a curved pipe
Energy Technology Data Exchange (ETDEWEB)
Norouzi, M.; Kayhani, M.H. [Shahrood University of Technology, Mechanical Engineering Department, Shahrood (Iran); Nobari, M.R.H. [Amirkabir University of Technology, Mechanical Engineering Department, Tehran (Iran); Joneidi, A.A. [Eindhoven University of Technology, Mechanical-Polymer Technology Group, Eindhoven (Netherlands)
2010-10-15
In this paper, fully developed convective heat transfer of viscoelastic flow in a curved pipe under the constant heat flux at the wall is investigated analytically using a perturbation method. Here, the curvature ratio is used as the perturbation parameter and the Oldroyd-B model is applied as the constitutive equation. In the previous studies, the Dirichlet boundary condition for the temperature at the wall has been used to simplify the solution, but here exactly the non-homogenous Neumann boundary condition is considered to solve the problem. Based on this solution, the non-axisymmetric temperature distribution of Dean flow is obtained analytically and the effect of flow parameters on the flow field is investigated in detail. The current analytical results indicate that increasing the Weissenberg number, viscosity ratio, curvature ratio, and Prandtl number lead to the increase of the heat transfer in the Oldroyd-B fluid flow. (orig.)
Analysis of heat and mass transfer
International Nuclear Information System (INIS)
The contents of this book are: Theory of Heat Conduction and Heat-conduction Equations; Thermal Conductivity; Steady Heat Conduction; Unsteady Heat Conduction; Forced Convection in Laminar Flow; Forced Convection in Turbulent Flow; Dimensional Analysis; Forced Convection in Separated Flow; Natural Convection; Radiation of Strongly Absorbing Media; and Radiation of Weakly Absorbing Media
Directory of Open Access Journals (Sweden)
M. A. Mansour
2013-09-01
Full Text Available The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with the corresponding boundary conditions. By using approximations of finite difference method, the non-dimensional governing equations are discritized. For natural convection the influential parameters are Rayleigh number Ra, the rotational angle of the enclosure? and the Hartmann number Ha, through which different thermo-fluid characteristics inside the enclosure are obtained. In the present study, the obtained results are presented in terms of streamlines, isotherms and average Nusselt number along the heat source. The result shows that with increasing Ha, the diffusive heat transfer become prominent even though Rayleigh number increases. Optimum heat transfer rate is obtained at higher values of Ra in the absence of magnetic force.
Directory of Open Access Journals (Sweden)
S. Venkatachalapathy
2010-01-01
Full Text Available Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a constant heat flux source at the bottom are of square cross-section and arranged in an in-line manner. Each heat source is attached with one thermocouple, which is connected to a data acquisition system and a computer. The steady state temperatures of heat sources, air inlet, outlet and enclosure walls are measured. The analysis is carried out by varying the heat fluxes and outlet areas. The heat transfer coefficient, Nusselt number and Grashof number are obtained. Results indicate that the heat sources inside the array are hotter and the heat transfer coefficient increases almost linearly with heat source surface temperatures. Grashof number and outlet opening areas strongly influence the Nusselt number. The heat transfer coefficient for the inner heat sources in a row is lower than those near the enclosure walls. The results of numerical analysis are compared with the experiments and there is a good agreement between the two.
Heat transfer reduction at the separation point on a spinning sphere in mixed convection
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Ozturk, Aysegul; Ece, Mehmet Cem [Trakya Univ., Dept. of Mechanical Engineering, Edirne (Turkey)
2002-07-01
The unsteady laminar thermal boundary-layer flow over an impulsively started translating and spinning isothermal body of revolution in the case of mixed convection is investigated. Velocity components and temperature are obtained as series of functions in powers of time. The general results are applied to a spinning sphere and the development of the surface heat flux evaluated at the separation point as it advances upstream is determined. The surface heat flux evaluated at the separation point as it moves forward decreases due to the increasing magnitude and influence of the centrifugal force and it is augmented by the opposing flow and reduced by the aiding flow. Reduction of the surface heat flux at the separation point is as low as 50 per cent as compared to the heat flux at the front stagnation point. (Author)
DORE, VALENTINA
2010-01-01
The motion of buoyancy driven plumes is, on all scales, the most common heat and momentum transfer mechanism in geophysical flows, well known as Free Convection. Similarly, density stratification due to heating inequalities is also an ordinary scenario in nature. Free Convection phenomenon coupled with a density stratified fluid setting leads to the so-called Penetrative Free Convection (PFC). When a fluid, in static equilibrium, is stably stratified a thermal forcing can produce an unstable ...
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Favre, E.
1997-09-26
coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas which drastically changes the heat and mass transfer across the liquid layer. Two experiments were considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow appears as petals or rays when the aspect ratio. The lateral confinement selects the azimuthal wavelength. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be `weak`, even for the largest values of the Marangoni number (Ma = 1.3 10{sup 5}). In the case of mercury, the thermo-capillary effect is reduced to zero to impurities at the surface which have special trajectories we describe and compare to a simpler experiment. Only the buoyancy forces induce a unstationary, weakly turbulent flow as soon as the heating power exceeds 4W (Ra = 4.5 10{sup 3}, calculated with h = 1 mm). The past part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number and the buoyancy force with the help of the literature. Results concerning heat transfer, in particular the exponent of the law Nusselt number vs. heating power, were compared with available data. (author) 115 refs.
Natural convective heat transfer of lithium under magnetic field
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Uda, N.; Hayase, M.; Chikaoka, T.; Inoue, S.; Horiike, H. E-mail: horiike@nucl.eng.osaka-u.ac.jp; Miyazaki, K
2000-11-01
The magnetohydrodynamic (MHD) effect on a natural convection heat transfer of liquid metal lithium was experimentally studied. The test section consisted of a lithium pool and, a heater pin, which was settled in the center of the pool. The results are summarized as follows, (1) with increasing B (Ly{>=}50), the Nusselt number decreased to 70 of the highest value at 0.05 T of the magnetic field; (2) large scale circulation flow profile and temperature profile were found to change drastically with the application of the weak field of 0.01 T.
International Nuclear Information System (INIS)
A penalty finite element analysis with bi-quadratic elements is performed to investigate the influence of uniform and non-uniform heating of bottom wall on mixed convection lid driven flows in a square cavity. In the present investigation, bottom wall is uniformly and non-uniformly heated while the two vertical walls are maintained at constant cold temperature and the top wall is well insulated and moving with uniform velocity. A complete study on the effect of Gr shows that the strength of circulation increases with the increase in the value of Gr irrespective of Re and Pr. As the value of Gr increases, there occurs a transition from conduction to convection dominated flow at Gr=5.103 and Re=1 for Pr=0.7. A detailed analysis of flow pattern shows that the natural or forced convection is based on both the parameters Ri (Gr/Re2) and Pr. As the value of Re increases from 1 to 102, there occurs a transition from natural convection to forced convection depending on the value of Gr irrespective of Pr. Particularly for higher value of Grashof number (Gr=105), the effect of natural convection is dominant up to Re=10 and thereafter the forced convection is dominant with further increase in Re. As Pr increases from 0.015 to 10 for a fixed Re and Gr (Gr=103), the inertial force gradually becomes stronger and the intensity of secondary circulation gradually weakens. The local Nusselt number (Nub) plot shows that the heat transfer rate is very high at the edges of the bottom wall and then decreases at the center of the bottom wall for the uniform heating and that contrasts lower heat transfer rate at the edges for the non-uniform heating of the bottom wall. It is also observed that Nul shows non-monotonic behavior with both uniform and non-uniform heating cases for Re=10 at higher value of Pr. The average Nusselt number plot for the left or right wall shows a kink or inflexion at Gr=104 for highest value of Pr. Thus the overall power law correlation for average Nusselt number may not be obtained for mixed convection effects at higher Pr. (authors)
Yano, Ryosuke
2014-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation usin...
Numerical simulation of combined natural and forced convection during thermal-hydraulic transients
International Nuclear Information System (INIS)
The single-phase COMMIX (COMponent MIXing) computer code performs fully three-dimensional, transient, thermal-hydraulic analyses of liquid-sodium LMFBR components. It solves the conservation equations of mass, momentum, and energy as a boundary-value problem in space and as an initial-value problem in time. The concepts of volume porosity, surface permeability and distributed resistance, and heat source have been employed in quasi-continuum (rod-bundle) applications. Results from three transient simulations involving forced and natural convection are presented: (1) a sodium-filled horizontal pipe initially of uniform temperature undergoing an inlet velocity rundown transient, as well as an inlet temperature transient; (2) a 19-pin LMFBR rod bundle undergoing a velocity transient; and, (3) a simulation of a water test of a 1/10-scale outlet plenum undergoing both velocity and temperature transients
Study of turbulent natural-circulation flow and low-Prandtl-number forced-convection flow
International Nuclear Information System (INIS)
Calculational methods and results are discussed for the coupled energy and momentum equations of turbulent natural circulation flow and low Prandtl number forced convection flow. The objective of this paper is to develop a calculational method for the study of the thermal-hydraulic behavior of coolant flowing in a liquid metal fast breeder reactor channel under natural circulation conditions. The two-equation turbulence model is used to evaluate the turbulent momentum transport property. Because the analogy between momentum transfer and heat transfer does not generally hold for low Prandtl number fluid and natural circulation flow conditions, the turbulent thermal conductivity is calculated independently using equations similar to the two-equation turbulence model. The numerical technique used in the calculation is the finite element method
A theoretical study of the spheroidal droplet evaporation in forced convection
Energy Technology Data Exchange (ETDEWEB)
Li, Jie, E-mail: leejay1986@163.com; Zhang, Jian
2014-11-07
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time. - Highlights: • Fully algebraic solutions for the spheroidal droplet evaporation rate is obtained. • We examine the effect of aspect ratio on the droplet evaporation. • We propose a calculation method of Nusselt number for spheroidal droplet.
Forced convection of ferro-fluids in a vented cavity with a rotating cylinder
International Nuclear Information System (INIS)
In this study, numerical investigation of the forced convection of ferro-fluid in a square cavity with ventilation ports in the presence of an adiabatic rotating cylinder is carried out. The governing equations are solved with a finite element based solver. The effects of Reynolds number (20 ? Re ? 400), angular rotational speed of the cylinder (-500 ? ? ? 500), strength and location of the magnetic dipole (0 ? ? ? 250), (0.2 ? a ? 0.8, -0.8 ? b ? -0.2) on the flow and thermal fields are numerically studied. It is observed that the length and size of the recirculation zones can be controlled with magnetic dipole strength and angular rotational speed of the cylinder. When the magnetic dipole is closer to the bottom wall of the cavity, flow is accelerated towards the bottom wall with larger influence area. The increasing values of the angular rotational speed of the cylinder in the clockwise direction enhance the heat transfer
Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia
2009-01-01
This paper aims to contribute to a better understanding of convective heat transfer. For this purpose, the reason why thermal diffusivity should be placed before the Laplacian operator of the heat flux, and the role of the velocity gradient in convective heat transfer are analysed. The background to these analyses is that, when the energy…
Convective Heat Transfer of Magnetic Nanofluids in a Microtube
Directory of Open Access Journals (Sweden)
Kuo Jung Lo
2015-05-01
Full Text Available This paper conducts an analysis of convective heat transfer of magnetic nanofluids in an isothermally heated microtube. The main purpose is to investigate the influences of particle volume fraction and external magnetic field strength on the fluid velocity, temperature, pressure, pressure drop, flow drag, and heat transfer rate. Firstly, a flow and heat transfer model is built. A water-based magnetite (Fe3O4 nanofluid is then pre-pared, and a thermal flow test system is further de-signed, so as to verify the theoretical model with experimental data. Finally, the thermal flow fields and the corresponding characteristics are numerically analyzed by using the marching implicit (MI procedure. The results reveal that when the particle volume fraction is increased, the average flow drag also increases while the average heat transfer rate goes down. Furthermore, as the external magnetic field strength is increased, the average flow drag rises and the average heat transfer rate also rises in the general case.
International Nuclear Information System (INIS)
The natural convection heat transfer from a heated vertical plate with wall temperature decreasing linearly from the lower end and embedded in a water saturated porous medium (coarse sand of 23% porosity) has been investigated experimentally. The steady state isothermal lines in the porous medium were developed with the aid of a computer program from the measured temperature data for several rates of heat input. The computed values of the local Nusselt number and modified Rayleigh number were used to obtain the empirical correlations. In order to check the accuracy of the measuring instruments and experimental procedures in simulating the thermal field, experimental investigations of heat transfer by natural convection from a vertical heated plate with constant wall temperature and embedded in the water saturated sand were conducted and the experimental results were compared with those previous investigations. The agreement was very good indicating that the instrumentation and experimental set-up used in the present study were reasonably satisfactory
International Nuclear Information System (INIS)
The paper deals with the investigation of thermo-magnetic convection in a paramagnetic liquid subjected to a non-uniform magnetic field in weightlessness conditions. Indeed, in zero-g space conditions such as realized in International Space Station (ISS), or in artificial satellite, or in free-flight space vessels, the classical thermo-gravitational convection in fluid disappears. In any cases, it may be useful to restore the convective thermal exchange inside fluids such as liquid oxygen. In this paper, the restoration of heat exchange by the way of creation of magnetic convection is numerically studied.
Crumeyrolle, Olivier; Egbers, Christoph; Mutabazi, Innocent; Dahley, M. Norman; Smieszek, Marlene
2012-07-01
We investigate numerically the thermal convection of an annular dielectric liquid sheet under the effect of the dielectrophoretic force, as observed when a dielectric liquid is permeated by an inhomogeneous electric field. This is of particular interest for space applications as natural convection cannot appear and forced convection from moving parts such as pumps is undesirable due to the expected wearing and lower reliability. Hence heat exchanger relying on the dielectrophoretic force to create convection could provide light, compact and reliable heat exchanger for aerospace cooling systems \\cite{crumeyrolleP}. We investigate the case of a radius ratio equal to 0.5 and Prandtl number of 65. This setup is under experimental investigation at LAS, BTU Cottbus, both on ground and during parabolic flight. The 3D linear stability analysis, that takes the finite size of the system into account, shows that the critical mode is non-axisymmetric and under the form of two counteroriented helices, rather than under the form of rolls as predicted in past investigations\\cite{crumeyrolleT}. Due to the short duration of microgravity during parabolic flight (22 seconds), 3D time-dependent DNS are required with realistic initial conditions. The simulations show that the helices are difficult to observe, as the flow pattern is dominated by convection plumes. We report that transient thermal transfer at the inner cylinder is strongly enhanced by those structures, while the thermal transfer close to the outer cylinder is weaker. J.S. Paschkewitz and {D.M.} Pratt, Exp. Therm. Fluid Sci., 21,, 187 (2000). M. Takashima, Q. J. Mech. appl. Math. 33,, 93 (1980).
Cherba?ski, Robert
2015-05-01
This paper presents a comparative study on heat transfer in a packed column. Two methods of heating are considered: microwave and convective. Transient one-dimensional mathematical models were proposed to describe the both alternatives. To account for significant differences in the temperatures between the gas and solid phase a heterogeneous model was applied in the modelling. The numerical simulations were carried out for different operating conditions. The effects of the gas inlet temperature and the microwave power, the bed porosity, the penetration depth of microwaves and the gas velocity were examined. The simulation results were compared on the basis of the time profiles of the average bed temperature and the outlet gas temperature. The same electric power utilized in the microwave heated packed column and the convective heated packed column was established as the key criterion for the comparisons. The compared profiles intersect indicating the time ranges in which the one or the other solution provides higher temperature of the bed. It was displayed that the microwave heated packed column should be preferred when longer heating times are required. In turn, the convective heated packed column is the better choice when shorter heating times are needed.
Convective heat transfer enhancement inside tubes using inserted helical coils
Ali, R. K.; Sharafeldeen, M. A.; Berbish, N. S.; Moawed, M. A.
2016-01-01
Convective heat transfer was experimentally investigated in tubes with helical coils inserts in turbulent flow regime within Reynolds number range of 14400 â‰¤ Re â‰¤ 42900. The present work aims to extend the experimental data available on wire coil inserts to cover wire diameter ratio from 0.044 to 0.133 and coil pitch ratio from 1 to 5. Uniform heat flux was applied to the external surface of the tube and air was selected as fluid. The effects of Reynolds number and wire diameter and coil pitch ratios on the Nusselt number and friction factor were studied. The enhancement efficiency and performance criteria ranges are of (46.9-82.6%) and (100.1-128%) within the investigated range of the different parameters, respectively. Correlations are obtained for the average Nusselt number and friction factor utilizing the present measurements within the investigated range of geometrical parameters and Re.
Convective heat and mass transfer in rotating disk systems
Energy Technology Data Exchange (ETDEWEB)
Shevchuk, Igor V. [MBtech Powertrain GmbH, Fellbach-Schmiden (Germany)
2009-07-01
The book describes results of investigations of a series of convective heat-and-mass transfer problems in rotating-disk systems, namely, over free rotating disks, under conditions of transient heat transfer, solid-body rotation of fluid, orthogonal flow impingement onto a disk, swirl radial flow between parallel co-rotating disks, in cone-disk systems and for Prandtl and Schmidt numbers larger than one. Methodology used included integral methods, self-similar and approximate analytical solutions, as well as CFD. The book is aimed at the professional audience of academic researchers, industrial R and D engineers, university lecturers and graduate/postgraduate students working in the area of rotating-disk systems. (orig.)
Natural convection heat transfer of fluid with temperature-dependent specific heat
International Nuclear Information System (INIS)
The present study investigates natural convection from a heated vertical plate of fluid with temperature-dependent specific heat, which is introduced as a model of microencapsulated phase change material slurries (MCPCM slurries). The temperature dependence of specific heat is represented by Gauss function with three physical parameters (peak temperature, width of phase change temperature and latent heat). Boundary layer equations are solved numerically, and the velocity and temperature fields of the flow are obtained. The relation between the heat transfer coefficients and the physical parameters of specific heat is discussed. The results show that the velocities and temperatures are smaller, and the heat transfer coefficients are larger comparing with those of the fluid with constant specific heat. (author)
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Chato, J.C.; Crowley, J.M.
1981-05-01
A multi-faceted research program has been performed to investigate in detail several aspects of free and forced convective cooling of underground electric cable systems. There were two main areas of investigation. The first one reported in this volume dealt with the fluid dynamic and thermal aspects of various components of the cable system. In particular, friction factors for laminar flow in the cable pipes with various configurations were determined using a finite element technique; the temperature distributions and heat transfer in splices were examined using a combined analytical numerical technique; the pressure drop and heat transfer characteristics of cable pipes in the transitional and turbulent flow regime were determined experimentally in a model study; and full-scale model experimental work was carried out to determine the fluid dynamic and thermal characteristics of entrance and exit chambers for the cooling oil. The second major area of activity, reported in volume 2, involved a feasibility study of an electrohydrodynamic pump concept utilizing a traveling electric field generated by a pumping cable. Experimental studies in two different configurations as well as theoretical calculations showed that an electrohydrodynamic pump for the moving of dielectric oil in a cable system is feasible.
Shang, De-Yi
2012-01-01
This book presents recent developments in our systematic studies of hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, as well as accelerating film flow of non-Newtonian power-law fluids (FFNF). These new developments provided in this book are (i) novel system of analysis models based on the developed New Similarity Analysis Method; (ii) a system of advanced methods for treatment of gas temperature- dependent physical properties, and liquid temperature- dependent physical properties; (iii) the organically combined models of the governing mathematical models with those on treatment model of variable physical properties; (iv) rigorous approach of overcoming a challenge on accurate solution of three-point boundary value problem related to two-phase film boiling and condensation; and (v) A pseudo-similarity method of dealing with thermal boundary layer of FFNF for greatly simplifies the heat-transfer analysis and numerical calculati...
International Nuclear Information System (INIS)
The helically coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This paper aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube, especially focusing on oscillation phenomena of dryout point. A glass-made helically coiled tube was used to observe the inside water boiling behavior flowing upward, which was heated by high temperature oil outside the tube. This oil was also circulated through a glass made tank to provide the heat source for water evaporation. The criterion for oscillation of dryout point was found to be a function of inlet liquid velocity and hot oil temperature. The observation results suggest the mechanism of dryout point oscillation mainly consists of intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the helical tube. In addition, the oscillation characteristics were experimentally confirmed. As inlet liquid velocity increases, oscillation amplitude also increases but oscillation cycle does not change so much. As hot oil temperature increases, oscillation amplitude and cycle gradually decreases. (author)
Direct numerical simulation of liquid sodium droplet combustion in forced convection air flow
International Nuclear Information System (INIS)
In case of sodium leakage in liquid metal fast breeder reactor, the liquid sodium comes out in droplet form from a pipe accompanied with ignition and combustion Combustion heat and reaction products might affect integrity of steel liners in piping rooms. A direct numerical simulation code, COMET, is developed to simulate the combustion of a liquid sodium droplet. The extended MAC method coupled with a higher-order upwind scheme is used to calculate reacting compressible flow. Multicomponent counter diffusion of chemical species, mass and energy transfer by sodium evaporation, and heat transfer by radiation and thermal conductivity are calculated coupling with the flow. Chemical reaction of sodium, oxygen and water vapor is calculated by using the equation-solving methods of equilibrium constants. Thermodynamic properties of the mixed gas are evaluated based on the molecular transport theories. By using COMET, the single droplet combustion of liquid sodium in forced convection air flow is numerically simulated. Spatial distributions such as combustion heat, temperature, pressure, and chemical species behaviors such as formation, decomposition and transport are analyzed and discussed. The change of the droplet diameter agrees closely with the d-square law that has been experimentally observed and theoretically derived. (author)
Convective Heating of the LIFE Engine Target During Injection
Energy Technology Data Exchange (ETDEWEB)
Holdener, D S; Tillack, M S; Wang, X R
2011-10-24
Target survival in the hostile, high temperature xenon environment of the proposed Laser Inertial Fusion Energy (LIFE) engine is critical. This work focuses on the flow properties and convective heat load imposed upon the surface of the indirect drive target while traveling through the xenon gas. While this rarefied flow is traditionally characterized as being within the continuum regime, it is approaching transition where conventional CFD codes reach their bounds of operation. Thus ANSYS, specifically the Navier-Stokes module CFX, will be used in parallel with direct simulation Monte Carlo code DS2V and analytically and empirically derived expressions for heat transfer to the hohlraum for validation. Comparison of the viscous and thermal boundary layers of ANSYS and DS2V were shown to be nearly identical, with the surface heat flux varying less than 8% on average. From the results herein, external baffles have been shown to reduce this heat transfer to the sensitive laser entrance hole (LEH) windows and optimize target survival independent of other reactor parameters.
Experimental study of natural convection heat transfer in a volumetrically heated semicircular pool
International Nuclear Information System (INIS)
Highlights: • Natural convection tests were performed to study high modified Rayleigh number. • The relations between the Nu and Ra? were determined for boundary conditions. • The upward and downward natural convection heat transfer rates were affected by Pr. • From the test, the Rayleigh numbers was obtained from 5.7 × 106 up to 7.0 × 1011. • The resultant correlations can be applied to determine the reactor vessel failure. - Abstract: SIGMA CP (Simulant Internal Gravitated Material Apparatus Circular Pool) tests were performed to study natural convection characterized by a high modified Rayleigh number (Ra?) in a semicircular pool. The test apparatus had a 500 mm diameter, 250 mm height and 100 mm width. Two thin cartridge heaters, which had a sheath diameter of 6 mm and a length of 2000 mm, were used to simulate internal heating in the pool. They were uniformly distributed in the semicircular pool to supply a maximum heating power of 3 kW to the pool. The Ra? was varied from 106 to 1011 based on the power input. The working fluid Prandtl number (Pr) ranged between 4 and 8 for water and was 0.7 for air. Particular attention was paid to the influence of Pr on the natural convection heat transfer in the pool. A number of relations between the Nusselt number (Nu) and Ra? were determined for the boundary conditions in the semicircular pool. The boundary condition is that the upper and lower walls are cooled isothermally. The experimental results indicated that both the upward and downward natural convection heat transfer rates were affected by Pr. The Rayleigh numbers were determined up to 7 × 1011 from this test. The resultant engineering correlations can be applied to simulations of a spectrum of loss-of-coolant accidents to determine the reactor vessel failure time
Directory of Open Access Journals (Sweden)
Zeinali Heris Saeed
2011-01-01
Full Text Available Abstract In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.
Zeinali Heris, Saeed; Noie, Seyyed Hossein; Talaii, Elham; Sargolzaei, Javad
2011-12-01
In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.
Grooms, Ian
2014-01-01
The non-hydrostatic, quasigeostrophic approximation for rapidly rotating Rayleigh-B\\'enard convection admits a class of exact `single mode' solutions. These solutions correspond to steady laminar convection with a separable structure consisting of a horizontal planform characterized by a single wavenumber multiplied by a vertical amplitude profile, with the latter given as the solution of a nonlinear boundary value problem. The heat transport associated with these solutions is studied in the regime of strong thermal forcing (large reduced Rayleigh number $\\widetilde{Ra}$). It is shown that the Nusselt number $Nu$, a nondimensional measure of the efficiency of heat transport by convection, for this class of solutions is bounded below by $Nu\\gtrsim \\widetilde{Ra}^{3/2}$, independent of the Prandtl number, in the limit of large reduced Rayleigh number. Matching upper bounds include only logarithmic corrections, showing the accuracy of the estimate. Numerical solutions of the nonlinear boundary value problem for ...
Transient natural convection heat and mass transfer in crystal growth
Han, Samuel S.
1988-01-01
A numerical analysis of transient combined heat and mass transfer across a rectangular cavity is performed by a numerical method based on the SIMPLE algorithm. The physical parameters are selected to represent a range of possible crystal growth in solutions. Numerical results are compared with available experimental data to confirm the accuracy of the results. Good qualitative agreements are obtained for the average mass transfer rate across the cavity. Also, qualitative agreements are observed for the global development of thermal and solute fields. It is found that the thermal and solute fields become highly oscillatory when the thermal and solute Grashof numbers are large. Oscillations are probably caused by a number of different instability mechanisms. By reducing the gravity some of these instabilities were made to disappear at the lower Grashof numbers. Transient temperature and solute distribution near the crystal growing surface are highly non-uniform at the higher Grashof numbers. These non-uniformities are less severe in the reduced gravity environments but still exist. The effects of convection on the rate of average mass transfer are more than one order of magnitude higher than those of conduction in the range of Grashof numbers studied. Dependency of mass transfer rate on the Grashof number indicates that the convection effects many not be negligible even in the microgravity environments for the range of parameters investigated.
Convective heat transfer in vee-trough linear concentrators
Meyer, B. A.; Mitchell, J. W.; El-Wakil, M. M.
1982-01-01
Natural convection heat-transfer coefficients have been experimentally determined for trough-type collectors. The effects of Rayleigh number, tilt angle, and ideal concentration ratio on the Nusselt number have been experimentally determined over ranges representative of collector operation. The Rayleigh number range tested was up to 10,000,000, the tilt angle was varied from 30-90 deg and ideal concentration ratios of 2, 3, 4 and 5 were tested. The experimental results are supported by finite element solutions to the governing equations. A method of extending the results to truncated trough collectors and CPC collectors is suggested. The convective losses are compared to those for conventional flat plate collectors. The critical Rayleigh number increases with concentration ratio. Nusselt number increases more rapidly at high concentration ratio than at low values. Correlation equations are developed over the range of parameters tested. These results are useful to designers of collectors of this type, and allow estimation of the thermal losses during operation.
Convection patterns in end-heated inclined enclosures.
Delgado-Buscalioni, R
2001-07-01
The natural convection in inclined side-heated rectangular boxes with adiabatic walls is theoretically and numerically investigated. The study is focused on the characterization of the convection patterns arising at the core of the basic steady unicellular flow and covers the whole range of Prandtl numbers (0number R identical with K Ra, defined in terms of the local streamwise temperature gradient, K Delta T/L. The critical value of R for transversal and longitudinal modes is determined by the linear stability analysis of the basic plane-parallel flow, which also provides the stability diagram in the (Pr-alpha) chart. Anyhow, the effect of confinement can decisively change the stability properties of the core: if the steady unicell reaches the boundary layer regime (BLR) the local temperature gradient vanishes at the core leaving a completely stable core region. A theoretical determination of the frontier of the BLR in the space of parameters (alpha, R, and cavity size) yields an extra criterion of stability that has been displayed in the stability diagram. As confirmed by numerical calculations, the core-flow instabilities can only develop for Prlow Pr (liquid metals). An analytical relationship for its frequency in terms of alpha, Ra, and Pr is derived. Throughout the paper, numerical calculations in two- and three-dimensional enclosures illustrate each type of multicellular flow and examples of instability interactions near the codimension-2 lines predicted by the theory. PMID:11461387
Experimental study of natural convective heat transfer in a vertical hexagonal sub channel
Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur
2012-06-01
The development of new practices in nuclear reactor safety aspects and optimization of recent nuclear reactors, including the APWR and the PHWR reactors, needs a knowledge on natural convective heat transfer within sub-channels formed among several nuclear fuel rods or heat exchanger tubes. Unfortunately, the currently available empirical correlation equations for such heat transfer modes are limited and researches on convective heat transfer within a bundle of vertical cylinders (especially within the natural convection modes) are scarcely done. Although boundary layers around the heat exchanger cylinders or fuel rods may be dominated by their entry regions, most of available convection correlation equations are for fully developed boundary layers. Recently, an experimental study on natural convective heat transfer in a subchannel formed by several heated parallel cylinders that arranged in a hexagonal configuration has been being done. The study seeks for a new convection correlation for the natural convective heat transfer in the sub-channel formed among the hexagonal vertical cylinders. A new convective heat transfer correlation equation has been obtained from the study and compared to several similar equations in literatures.
Evaluation of finite element formulations for transient conduction forced-convection analysis
Thornton, E. A.; Wieting, A. R.
1979-01-01
Numerical studies clarifying the advantages and disavantages of conventional versus upwind convective finite elements are presented along with lumped versus consistent formulations for practical conduction forced-convection analysis. A finite-element procedure for treatment of negligible capacitance fluid nodes is presented. The procedure is based on procedures used in finite-element structural dynamics to treat nodes with negligible structural mass. Two finite-element programs and a finite-difference lumped-parameter program used in the studies are discussed. Evaluation studies utilizing three convection and two combined conduction-convection problems are then presented and discussed. Additionally, the computational time saving offered by the finite element procedure is considered for a practical combined conduction-convection problem.
Energy Technology Data Exchange (ETDEWEB)
Triplett, C.E.
1996-12-01
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan`s investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra){sup n}, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan`s aligned array results and to other studies of natural convection in horizontal tube arrays.
International Nuclear Information System (INIS)
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan's investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra)n, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan's aligned array results and to other studies of natural convection in horizontal tube arrays
Fritts, David C.
2004-01-01
The specific objectives of this research effort included the following: 1) Quantification of gravity wave propagation throughout the lower and middle atmosphere in order to define the roles of topographic and convective sources and filtering by mean and low-frequency winds in defining the wave field and wave fluxes at greater altitudes; 2) The influences of wave instability processes in constraining wave amplitudes and fluxes and generating turbulence and transport; 3) Gravity wave forcing of the mean circulation and thermal structure in the presence of variable motion fields and wave-wave interactions, since the mean forcing may be a small residual when wave interactions, anisotropy, and momentum and heat fluxes are large; 4) The statistical forcing and variability imposed on the thermosphere at greater altitudes by the strong wave forcing and interactions occurring in the MLTI.
Modeling a forced to natural convection boiling test with the program LOOP-W
International Nuclear Information System (INIS)
Extensive testing has been conducted in the Simulant Boiling Flow Visualization (SBFV) loop in which water is boiled in a vertical transparent tube by circulating hot glycerine in an annulus surrounding the tube. Tests ranged from nonboiling forced convection to oscillatory boiling natural convection. The program LOOP-W has been developed to analyze these tests. This program is a multi-leg, one-dimensional, two-phase equilibrium model with slip between the phases. In this study, a specific test, performed at low power where non-boiling forced convection was changed to boiling natural convection and then to non-boiling again, has been modeled with the program LOOP-W
Scientific Electronic Library Online (English)
Cláudia R., Andrade; Edson L., Zaparoli.
Full Text Available This work studies the forced convection problem in internal flow between concentric annular ducts, with radial fins at the internal tube surface. The finned surface heat transfer is analyzed by two different approaches. In the first one, it is assumed one-dimensional heat conduction along the intern [...] al tube wall and fins, with the convection heat transfer coefficient being a known parameter, determined by an uncoupled solution. In the other way, named conjugated approach, the mathematical model (continuity, momentum, energy and K-epsilon equations) applied to tube annuli problem was numerically solved using finite element technique in a coupled formulation. At first time, a comparison was made between results obtained for the conjugated problem and experimental data, showing good agreement. Then, the temperature profiles under these two approaches were compared to each other to analyze the validity of the one-dimensional classical formulation that has been utilized in the heat exchanger design.
Fast Prediction Method for Steady-State Heat Convection
Wáng, Yì
2012-03-14
A reduced model by proper orthogonal decomposition (POD) and Galerkin projection methods for steady-state heat convection is established on a nonuniform grid. It was verified by thousands of examples that the results are in good agreement with the results obtained from the finite volume method. This model can also predict the cases where model parameters far exceed the sample scope. Moreover, the calculation time needed by the model is much shorter than that needed for the finite volume method. Thus, the nonuniform POD-Galerkin projection method exhibits high accuracy, good suitability, and fast computation. It has universal significance for accurate and fast prediction. Also, the methodology can be applied to more complex modeling in chemical engineering and technology, such as reaction and turbulence. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Maksimov Vyacheslav I.; Nagornova Tatiana A.; Shestakov Igor A.
2015-01-01
Results of mathematical modeling of convective heat transfer in air area surrounded on all sides enclosing structures, in the presence of heat source at the lower boundary of the media are presented. Solved the system of differential equations of unsteady Navier-Stokes equations with the appropriate initial and boundary conditions. The process of convective heat transfer is calculated using the models of turbulence Prandtl and Prandtl-Reichard. Takes into account the processes of heat exchang...
International Nuclear Information System (INIS)
Highlights: â†’ Transport processes in isothermal hexagonal sheath with 19 heat generating rods is studied. â†’ Correlation is given to predict the maximum temperature considering all transport processes. â†’ Effective thermal conductivity of rod bundle can be obtained using max temperature. â†’ Data on the critical Rayleigh numbers for p/d ratios of 1.1-2.0 is presented. â†’ Radiative heat transfer contributes to heat dissipation of 38-65% of total heat. - Abstract: A numerical study of conjugate natural convection and surface radiation in a horizontal hexagonal sheath housing 19 solid heat generating rods with cladding and argon as the fill gas, is performed. The natural convection in the sheath is driven by the volumetric heat generation in the solid rods. The problem is solved using the FLUENT CFD code. A correlation is obtained to predict the maximum temperature in the rod bundle for different pitch-to-diameter ratios and heat generating rates. The effective thermal conductivity is related to the heat generation rate, maximum temperature and the sheath temperature. Results are presented for the dimensionless maximum temperature, Rayleigh number and the contribution of radiation with changing emissivity, total wattage and the pitch-to-diameter ratio. In the simulation of a larger system that contains a rod bundle, the effective thermal conductivity facilitates simplified modelling of the rod bundle by treating it as a solid of effective thermal conductivity. The parametric studies revealed that the contribution of radiation can be 38-65% of the total heat generation, for the parameter ranges chosen. Data for critical Rayleigh number above which natural convection comes into effect is also presented.
Multiregional coupled conduction--convection model for heat transfer in an HTGR core
International Nuclear Information System (INIS)
HEXEREI is a three-dimensional, coupled conduction-convection heat transfer and multichannel fluid dynamic analysis computer code with both steady-state and transient capabilities. The program was developed to provide thermal-fluid dynamic analysis of a core following the general design for high-temperature gas-cooled reactors (HTGRs); its purpose was to provide licensing evaluations for the U.S. Nuclear Regulatory Commission. In order to efficiently model the HTGR core, the nodal geometry of HEXEREI was chosen as a regular hexagonal array perpendicular to the axis of and bounded by a right circular cylinder. The cylindrical nodal geometry surrounds the hexagonal center portion of the mesh; these two different types of nodal geometries must be connected by interface nodes to complete the accurate modeling of the HTGR core. HEXEREI will automatically generate a nodal geometry that will accurately model a complex assembly of hexagonal and irregular prisms. The accuracy of the model was proven by a comparison of computed values with analytical results for steady-state and transient heat transfer problems. HEXEREI incorporates convective heat transfer to the coolant in many parallel axial flow channels. Forced and natural convection (which permits different flow directions in parallel channels) is included in the heat transfer and fluid dynamic models. HEXEREI incorporates a variety of steady-state and transient solution techniques that can be matched with a particular problem to minimize the computational time. HEXEREI was compared with a code of similar capabilities that was based on a Cartesian mesh. This code modeled only one specific core design, and the mesh spacing was closer than that generated by HEXEREI. Good agreement was obtained with the detail provided by the representations
Optimal Heat Transport in Rayleigh-B\\'enard Convection
Sondak, David; Waleffe, Fabian
2015-01-01
Steady flows that optimize heat transport are obtained for two-dimensional Rayleigh-B\\'enard convection with no-slip horizontal walls for a variety of Prandtl numbers $Pr$ and Rayleigh number up to $Ra\\sim 10^9$. Power law scalings of $Nu\\sim Ra^{\\gamma}$ are observed with $\\gamma\\approx 0.31$, where the Nusselt number $Nu$ is a non-dimensional measure of the vertical heat transport. Any dependence of the scaling exponent on $Pr$ is found to be extremely weak. On the other hand, the presence of two local maxima of $Nu$ with different horizontal wavenumbers at the same $Ra$ leads to the emergence of two different flow structures as candidates for optimizing the heat transport. For $Pr \\lesssim 7$, optimal transport is achieved at the smaller maximal wavenumber. In these fluids, the optimal structure is a plume of warm rising fluid which spawns left/right horizontal arms near the top of the channel, leading to downdrafts adjacent to the central updraft. For $Pr > 7$ at high-enough Ra, the optimal structure is a...
Heat Transfer Characteristics on Toroidal Convection Loop with Nanofluids
International Nuclear Information System (INIS)
Experimental studies on single-phase toroidal circulation loop(thermosyphon) have been performed in the present study with Ag-nanofluids as a working fluids. The present paper deals with an experimental study on the heat transfer behavior of single-phase toroidal loop. Toroidal loop charged with nanofluid has been constructed and a number of tests have been carried out. Different geometric parameter, e.g., orientation has been investigated. The tests were conducted employing two fluids: distilled water and Ag-nanofluid of various volume concentrations. The experiments at Rayleigh number from 105 to 106 showed a systematic and slight deterioration in natural convective heat transfer. It was observed that the deterioration due to the particle concentration was in the range of 5-10%. At a given particle concentration of 0.05%, abrupt decrease in the Nusselt number and the Raleigh number was observed. The present study with toroidal loop shows that the application of nanofluids for heat transfer intensification should not be decided only by the effective thermal conductivity with increasing particle concentration
Energy Technology Data Exchange (ETDEWEB)
Moon, Hee Gon; Wang, Sem Yung; Shim, Ho kyoung [Kwangju Institute of Science and Technology, Kwangju (Korea, Republic of)
2004-07-01
This paper presents the adjoint variable Design Sensitivity Analysis (DSA) for thermal systems considering both conduction and convection heat transfer. Both nodal temperature and total heat flow are considered to be objective functions and design sensitivity formulas are derived for each case. For the case of convection heat transfer, the adjoint analysis is carefully proceeded to obtain a precise result. A topology optimization example is examined for a simple planar square plate in order to design a heat exchanger as verification.
Natural Convection Heat and Mass Transfer from Falling Films in Vertical Channels
Buck, Gregory Allen
1990-01-01
In the design of solar collector/regenerators for use in open cycle absorption refrigeration (OCAR) units, the problem of predicting evaporation rates and solution temperatures is of paramount importance in determining overall cycle performance. This transport of heat and mass is dominated by natural convection with buoyant forces primarily generated as a result of film heating by the solar flux, but aided by the evaporation of water (the lighter species) into the rising moist air stream. In order to better understand the mechanism of these combined buoyant interactions, the governing equations for natural convection flow in a vertical channel bounded by a heated falling film (simulating a glazed collector/regenerator) were solved using several different finite difference techniques. The numerical results were validated against existing experimental and numerical results for simplified boundary conditions. The appropriate nondimensionalization for the falling film boundary condition was established, ostensibly for the first time, and a parametric study for an air-water vapor mixture has been presented. Curve fits to the numerical results were determined for engineering design applications. To further confirm the validity of the numerical solutions, an experimental apparatus was constructed using electric resistance heat to simulate the constant heat flux of the solar source. Water was introduced at the top of this heated vertical surface at various flow rates and under various supplied heat fluxes, and a natural convection channel flow generated between the heated falling film and a parallel, plexiglass surface. Film temperatures and moist air velocity profiles were measured at various streamwise (vertical) locations for comparison with the numerical results. In general, measured film temperatures were 15 to 20 percent lower than the predicted values, but came to within 3 percent of the predictions when experimental uncertainty was incorporated into the numerical inputs. Photographic smoke-wire measurements of the induced moist air velocity were about 20 percent higher than the numerical predictions for small channel gap spacing, and about 50 percent higher for large gap spacing. These trends in the data indicate that a redistribution of the supplied heat flux from the film to the moist air is required to lower predicted film temperatures and raise predicted gas velocities. Physically plausible arguments to explain this redistribution and suggestions for improving the numerical predictions and the experimental measurements are offered.
International Nuclear Information System (INIS)
Graphical abstract: Photograph of the experimental set-up. - Highlights: • Thermal performance of an indirect-mode solar dryer is investigated. • Mathematical models are obtained for thin layer drying of thymus and mint. • Both thymus and mint show the constant and falling rate drying periods. - Abstract: An indirect-mode forced convection solar dryer was designed and fabricated. The thermal performance of the solar dryer under Tanta (latitude, 30° 47? N and longitude, 31° E) prevailing weather conditions was experimentally investigated. The system consists of a double pass v-corrugated plate solar air heater connected to a drying chamber. A blower was used to force the heated air to the drying chamber. Drying experiments were performed for thymus (initial moisture content 95% on wet basis) and mint (initial moisture content 85% on wet basis) at an initial temperature of 29 °C. The final moisture contents for thymus and mint were reached after 34 and 5 h, respectively. Fourteen mathematical models of thin layer drying were tested to specify the suitable model for describing the drying behavior of the studied products. It was found that, Midilli and Kucuk model is convenient to describe the thin layer solar drying of mint. However, the Page and modified Page models were found to be the best among others for describing the drying curves of thymus
Energy Technology Data Exchange (ETDEWEB)
Surducan, E.; Surducan, V.; Neamtu, C., E-mail: camelia.neamtu@itim-cj.ro [National Institute for Research and Development of Isotopic and Molecular Technologies (INCDTIM), 67-103 Donat St., 400293, Cluj?Napoca (Romania); Limare, A.; Di Giuseppe, E. [Institut de Physique du Globe de Paris (IPGP), Univ. Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, 75005, Paris (France)
2014-12-15
We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm{sup 3} convection tank is filled with a water?based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals.
Heat transfer enhancement in a convective field by applying ionic wind
International Nuclear Information System (INIS)
This paper reports that this study has been conducted to pursue the heat transfer enhancement in a convective field by applying electric field. Firstly, aimed at thinning boundary layer, swirl motions were caused by utilizing the ionic wind in a channel flow with parallel wire-electrode arrangement. Secondly, ionic wind was induced at right angle to the primary flow at regular intervals by using cross wire-electrode arrangement. Thirdly, to utilize the dynamical effect of adding particles under the Coulomb force, electric field was applied to gas-solid suspensions flow field. On the basis of these results, fundamental characteristics of the combined flow structure and the heat transfer in the EHD field were clarified, and the possibility of the practical application will be insighted
Effects of turbulence models on forced convection subcooled boiling in vertical pipe
International Nuclear Information System (INIS)
Highlights: • Subcooled boiling phenomena are simulated by FLUENT code. • Capacities of turbulence model, wall function and two-phase treatment are studied. • k–? models perform better than k–w models for predicting subcooled boiling. • Grid with Y+ near to 1 could not give good results though k–w models are used. - Abstract: In this study, a two-fluid model coupled with wall boiling model was adopted to investigate two-phase forced convection subcooled boiling process in a vertical heated pipe based on FLUENT 14.5. The influences of turbulence models (k–? models and k–w models) on flow and heat transfer in pipe were studied. The performances of different turbulent models, wall functions and two-phase turbulence treatments were analyzed based on four sets of grids by comparing the calculated results with Bartolemei experimental data. The results of k–? models showed better agreement with experimental data than these of k–w models. The Dispersed and Per Phase treatments for two-phase turbulence parameters performed no better than the Mixture treatment. The enhanced wall function could not cope the situation for grid with near-wall Y-plus near to 1. k–? models with appropriate mesh size performed excellent in predicting subcooled boiling in vertical channel. This work can be referred for choosing turbulence model when analyzing subcooled boiling by CFD methodology
Directory of Open Access Journals (Sweden)
M.A. Ahmed
2015-09-01
Full Text Available In this paper, turbulent forced convection of nanofluids flow in triangular-corrugated channels is numerically investigated over Reynolds number ranges of 1000–5000. Four different types of nanofluids which are Al2O3, CuO, SiO2 and ZnO–water with nanoparticles diameters in the range of 30–70 nm and the range of nanoparticles volume fraction from 0% to 4% have been considered. The governing equations of mass, momentum and energy are solved using finite volume method (FVM. The low Reynolds number k–? model of Launder and Sharma is adopted as well. It is found that the average Nusselt number, pressure drop, heat transfer enhancement, thermal–hydraulic performance increase with increasing in the volume fraction of nanoparticles and with decreasing in the diameter of nanoparticles. Furthermore, the SiO2–water nanofluid provides the highest thermal–hydraulic performance among other types of nanofluids followed by Al2O3, ZnO and CuO–water nanofluids. Moreover, the pure water has the lowest heat transfer enhancement as well as thermal–hydraulic performance.
Subcooled forced-convection film boiling in the forward stagnation region of a sphere or cylinder
International Nuclear Information System (INIS)
An analysis is made of forced-convection film boiling in stagnation flow of subcooled liquids. The role of liquid viscosity in film boiling is determined by postulating the existence of a hydrodynamic boundary layer superposed on potential flow and using a perturbation technique. The viscous boundary layer due to shear stress at the vapor-liquid interface is shown to perturb the velocity field only slightly at large liquid subcooling. While the inviscid solution cannot be used to describe liquid motion when the liquid temperature is near its saturation temperature, the vapor is found to move only under the influence of the potential flow pressure distribution, thereby eliminating the coupling between the liquid boundary layer and vapor film without any significant errors in the heat-transfer problem. A rational interpolation formula between these two limiting cases leads to a simple expression for the film boiling heat transfer incorporating the major effects of wall superheat and liquid subcooling. The applicability of this formula to subcooled film boiling from a sphere or a cylinder is demonstrated. (author)
An experimental investigation of forced convection flat plate solar air heater with storage material
Directory of Open Access Journals (Sweden)
Aissa Walid
2012-01-01
Full Text Available Solar air heater (SAH is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radiation and some meteorological data (temperature, pressure, relative humidities, etc. for measuring days are obtained from the Egyptian Meteorological Authority, Aswan station. Inlet and outlet temperatures of air from a SAH have been recorded. In this work, attempt has been made to present the temperature distribution in non dimensional form that makes it useable for any region and not restricted to local conditions. The variation of solar radiation, air heater efficiency, Nusselt number and temperature distribution along the air heater are discussed. Comparisons between the calculated values of outlet air temperatures, average air temperatures and storage material temperatures and the corresponding measured values showed good agreement. Comparison between current work and those in previous investigations showed fair agreement.
Second law analysis of forced convection in a circular duct for non-Newtonian fluids
Energy Technology Data Exchange (ETDEWEB)
Mahmud, Shohel; Fraser, Roydon Andrew [Department of Mechanical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ont. (Canada)
2006-09-15
The second law characteristics of fluid flow and heat transfer inside a circular duct under fully developed forced convection for non-Newtonian fluids are presented. Heat flux is kept constant at the duct wall. Analytical expressions for dimensionless entropy generation number (N{sub S}), irreversibility distribution ratio (F), and Bejan number (Be) are obtained as functions of dimensionless radius (R), Peclet number (Pe), modified Eckert number (Ec), Prandtl number (Pr), dimensionless temperature difference ({omega}), and fluid index (m or n). Spatial distributions of local and average entropy generation number, irreversibility ratio, and Bejan number are presented graphically. For a particular value of fluid index, n=1 (or m=2), the general entropy generation number expression for a non-Newtonian power-law fluid reduces to the expression for Newtonian fluid as expected. Furthermore, entropy generation minimization is applied to calculate an optimum fluid index (n{sub EGM}). A correlation is proposed that calculates n{sub EGM}as a function of group parameter (EcxPr/{omega}) and Peclet number (Pe) within +/-5% accuracy. Finally, for some selected fluid indices, the governing equations are solved numerically in order to obtain Nusselt number. It is observed that the numerically obtained asymptotic Nusselt number shows excellent agreement with the analytically obtained Nusselt number. (author)
Yano, Ryosuke
2015-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation using the direct simulation Monte Carlo method. Numerical results indicate that strongly nonequilibrium states are formed in vicinities of two walls, which do not allow us to discuss the transition of the thermal conduction to the thermal convection of thermally relativistic fluids under the gravitational force in the framework of the relativistic Navier-Stokes-Fourier equation, when the flow-field is under the transition regime between the rarefied and continuum regimes, whereas such strongly nonequilibrium states are not formed in vicinities...
Describing the Heat Transport of Turbulent Rayleigh--B\\'enard Convection by POD methods
Lülff, Johannes
2015-01-01
Rayleigh--B\\'enard convection, which is the buoyancy-induced motion of a fluid enclosed between two horizontal plates, is an idealised setup to study thermal convection. We analyse the modes that transport the most heat between the plates by computing the proper orthogonal decomposition (POD) of numerical data. Instead of the usual POD ansatz of finding modes that describe the energy best, we propose a method that is optimal in describing the heat transport. Thereby, we can determine the modes with the major influence on the heat transport and the coherent structures in the convection cell. We also show that in lower-dimensional projections of numerical convection data, the newly developed modes perform consistently better than the standard modes. We then use this method to analyse the main modes of three-dimensional convection in a cylindrical vessel as well as two-dimensional convection with varying Rayleigh number and varying aspect ratio.
Energy Technology Data Exchange (ETDEWEB)
Hayat, T. [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia); Iqbal, Z., E-mail: zahidiqbal_qau@yahoo.com [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Mustafa, M. [Research Centre for Modeling and Simulation, National University of Sciences and Technology, Sector H-12, Islamabad 44000 (Pakistan); Alsaedi, A. [Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia)
2012-11-15
Highlights: Black-Right-Pointing-Pointer Boundary layer flow of an upper-convected Maxwell (UCM) fluid over a moving surface. Black-Right-Pointing-Pointer Convective boundary conditions have been used. Black-Right-Pointing-Pointer Series solutions are obtained by homotopy analysis method (HAM). Black-Right-Pointing-Pointer Graphical results for various interesting parametric values. - Abstract: This study discusses the flow and heat transfer in an upper-convected Maxwell (UCM) fluid over a moving surface in the presence of a free stream velocity. The convective boundary conditions have been handled. Similarly transformations are invoked to convert the partial differential equations governing the steady flow of a Maxwell fluid into an ordinary differential system. This system is solved by a homotopic approach. The effects of influential parameters such as Deborah number ({beta}), Prandtl number (Pr), Eckert number (Ec), suction parameter (S) and ratio ({lambda}) have been thoroughly examined.
Free convection heat transfer across rectangular-celled diathermanous honeycombs
International Nuclear Information System (INIS)
Experimental obtained Nusselt number-Rayleigh number plots are presented for free convective heat transfer across inclined honeycomb panels filled with air. The honeycomb cells were rectangular in shape with very long cell dimensions across the slope and comparatively short dimensions up the slope. Elevation aspect ratios, A/sub E/, investigated were 3, 5 and 10; angles of inclination, theta, measured from the horizontal, were 0, 30, 60, 75 and 90 deg. The effect on the Nusselt number, of the emissivities of the plates bounding the honeycomb, and of the emissivity of honeycomb material, was also investigated. The measurements confirmed that the critical Rayleigh number and the post-critical heat transfer depend on the radiant properties of the honeycomb cells. The critical Rayleigh numbers at theta=0 were well predicted by the methods of Sun and Edwards. For 030 deg. The theta=90 deg data were found to be closely correlated by an equation of the form recently proposed by Bejan and Tien
Coupled heat and mass transfer in a convective tunnel dryer
International Nuclear Information System (INIS)
The mechanism of drying in a convective tunnel dryer with air heated in solar collectors was approached first experimentally with a pilot laboratory unit, then numerically taking into account the coupled heat and mass transfers. In the present study, several experimental essays were conducted followed by the adoption of a simulation tool describing the opening conditions of the tunnel dryer and a behavioural model that can be of great interest in the design and the automation of such industrial units. Indeed, behavioural models of thermodynamic system are characterised by the interactions of a large number of complex phenomenon, which call for various types of energy. This dynamic feature requires a modeling approach, using physical phenomenon such as energy storage. energy transformation and energy dissipation as data. The pseudo-bond graph methodology was used in modelling the drying system. This methodology was very suitable for thermo fluid process. It accepts the use of elements that do not exist in the traditional bond graph methods. An explicit pseudo-bond graph model who describes the process of water evaporation under the tray is studies in this paper and the governing equations are determined using bond graph properties.(Author)
A note on the heat transfer in convective fins
International Nuclear Information System (INIS)
In this paper a generalized approach to the problem of heat transfer through convective fins is given. The proper dimensionless variables, which specify the general problem are identified, and upper bounds of the values of the dimensionless number Nsub(r) defined as 'the ratio of the heat transferred by the fin to that of the corresponding bare surface' are derived. It was shown that these limiting values of the Nsub(r) are 1/?B1 and ?2/B1 for longitudinal fins and spines respectively, where B1 is the Biot number hb/k, while for annular fins of constant thickness and hyperbolic profile, Nsub(r) 1, where K(?) is a number determined by the profile of the fin and the ratio ? = x2/x1 of the outside to the inside radii. It was also shown that for longitudinal fins and spinces the possible adverse insulating effect by the use of the fin is avoided, if one selects the value of ?hA/kC < 1, which is a rather stricter criterion than the one reported in the literature, namely that of hA/kC < 1 [2-5]. An example is given to show how one may utilize the appropriate value of Nsub(r) and the fin effectiveness e, to obtain the dimensions of the fin. (orig.)
International Nuclear Information System (INIS)
A new version of the ENERGY series code, ENERGY-IV, was written for predicting coolant temperature distributions in wire-wrapped rod assemblies used in the Liquid Metal Fast Breeder Reactor. The ENERGY-IV Code is applicable to both steady-state forced and mixed convection operation for a single isolated assembly. (The SUPERENERGY Code, [Basehore (1980)] is applicable to core wide forced convection analysis.) ENERGY-IV is an empirical code designed to be fast running. Hence the core designer can use it as an inexpensive thermal hydraulic design or diagnosis tool
Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type
Terres, H.; Chávez, S.; Lizardi, A.; López, R.; Vaca, M.; Flores, J.; Salazar, A.
2015-01-01
In this work, experimental results to determine the convection heat transfer coefficient in the cooking process of apple using a solar cooker box-type are presented. Experimental data of temperatures for water, surface and central point of the apple were used. To determine the convection coefficient, the apple was modelled as a sphere. The temperatures evolution was defined using thermocouples located at water, surface and central point in the vegetables. Using heat transfer convection equations in transitory state and the temperatures measured, the Biot number and the convection coefficient were determined.
Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type
International Nuclear Information System (INIS)
In this work, experimental results to determine the convection heat transfer coefficient in the cooking process of apple using a solar cooker box-type are presented. Experimental data of temperatures for water, surface and central point of the apple were used. To determine the convection coefficient, the apple was modelled as a sphere. The temperatures evolution was defined using thermocouples located at water, surface and central point in the vegetables. Using heat transfer convection equations in transitory state and the temperatures measured, the Biot number and the convection coefficient were determined
Negative Knudsen force on heated microbeams
Zhu, Taishan
2011-11-18
Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was, however, neither observed in experiment [A. Passian et al., Phys. Rev. Lett. 90, 124503 (2003)], nor captured in the latest numerical study [J. Nabeth et al., Phys. Rev. E 83, 066306 (2011)]. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed. The discovered phenomenon could find its applications in novel mechanisms for pressure sensing and actuation.
International Nuclear Information System (INIS)
The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k .Îµ turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082âˆ¼1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%
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Ma, R.Y. [California State Polytechnic Univ., Pomoma, CA (United States). Dept. of Mechanical Engineering
1993-09-01
Tests were performed to determine the convective heat loss characteristics of a cavity receiver for a parabolid dish concentrating solar collector for various tilt angles and wind speeds of 0-24 mph. Natural (no wind) convective heat loss from the receiver is the highest for a horizontal receiver orientation and negligible with the reveler facing straight down. Convection from the receiver is substantially increased by the presence of side-on wind for all receiver tilt angles. For head-on wind, convective heat loss with the receiver facing straight down is approximately the same as that for side-on wind. Overall it was found that for wind speeds of 20--24 mph, convective heat loss from the receiver can be as much as three times that occurring without wind.
Large-scale tomographic PIV in forced and mixed convection using a parallel SMART version
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Kuehn, Matthias; Ehrenfried, Klaus; Bosbach, Johannes; Wagner, Claus [German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology, Goettingen (Germany)
2012-07-15
Large-scale tomographic particle image velocimetry (tomographic PIV) was used to study large-scale flow structures of turbulent convective air flow in an elongated rectangular convection cell. Three flow cases have been investigated, that is, pure forced convection and mixed convection at two different Archimedes numbers. The Reynolds number was constant at Re=1.04 x 10{sup 4} for all cases, while the Archimedes numbers were Ar=2.1 and 3.6 for the mixed convection cases, corresponding to Rayleigh numbers of Ra=1.6 x 10{sup 8} and 2.8 x 10{sup 8}, respectively. In these investigations, the size of the measurement volume was as large as 840 mm x 500 mm x 240 mm. To allow for statistical analysis of the measured instantaneous flow fields, a large number of samples needed to be evaluated. Therefore, an efficient parallel implementation of the tomographic PIV algorithm was developed, which is based on a version of the simultaneous multiplicative reconstruction technique (SMART). Our algorithm distinguishes itself amongst other features by the fact that it does not store any weighting coefficients. The measurement of forced convection reveals an almost two-dimensional roll structure, which is orientated in the longitudinal cell direction. Its mean velocity field exhibits a core line with a wavy shape and a wavelength, which corresponds to the height and depth of the cell. In the instantaneous fields, the core line oscillates around its mean position. Under the influence of thermal buoyancy forces, the global structure of the flow field changes significantly. At lower Archimedes numbers, the resulting roll-like structure is shifted and deformed as compared to pure forced convection. Additionally, the core line oscillates much more strongly around its mean position due to the interaction of the roll structure with the rising hot air. If the Archimedes number is further increased, the roll-like structure breaks up into four counter-rotating convection rolls as a result of the increased influence of buoyancy forces. Moreover, large-scale tomographic PIV reveals that the orientation of these rolls reflects a 'W'-like shape in the horizontal X-Z-plane of the convection cell. (orig.)
Effects of Control on the Onset of Marangoni-Benard Convection with Uniform Internal Heat Generation
Norfifah Bachok; Norihan Md. Arifin; Fadzilah Md Ali
2008-01-01
The effect of control on the onset of Marangoni-Benard convection in a horizontal layer of fluid with internal heat generation heated from below and cooled from above is investigated. The resulting eigenvalue problem is solved exactly. The effects of control are studied by examining the critical Marangoni numbers and wave numbers. It is found that the onset of Marangoni-Benard convection with internal heat generation can be delayed through the use of control.
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Ziyaddin RECEBL?
2008-01-01
Full Text Available In some studies, the effect of magnetic field on heat convection has been investigated given that physical properties are constant regardless of temperature. The effect of magnetic field on heat convection and fluids whose physical properties change by temperature has been investigated in this study as physical properties of fluids change by the effect of temperature. Momentum, continuity and energy equations including electromagnetic force affecting the fluid were used in the solution. Temperatures at axial and radial directions and Nusselt numbers were calculated depending on magnetic field intensity and other physical properties of fluid by solving the equation system written in cylindrical coordinates system by means of one of the numerical methods which is finite difference method. According to results, velocity and temperature of the cooled fluid decreased following an increase in the intensity of magnetic field placed vertically to flow direction. As determined in the previous one, this study also indicated that the increase in Reynolds number increases Nusselt number, and increasing the effect of magnetic field decreases Nusselt number. The theoretical results of the present study are in conformity with the results of our previous one.
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This paper is concerned with an unsteady, laminar, free convective flow over a heated sphere with the effect of internal heat generation/absorption. The dimensionless governing equations have been solved employing the finite difference method as well as a perturbation method for short time and an asymptotic method for long time. We examine the effects of the physical parameters, such as, the Prandtl number, Pr, and the heat generation/absorption parameter, ?, on the friction factor and heat transfer rate as well as the velocity and temperature profiles. It is observed that when the Prandtl number, Pr, is increased, the friction factor decreases while the heat transfer rate increases. In the presence of internal heat generation, the friction factor increases while the heat transfer rate reduces. The reverse pattern is found with the heat absorption parameter. The momentum and thermal boundary layers become thicker with an increase of the heat generation parameter. A comparison among the numerical solutions, the perturbation solutions for short time and the asymptotic solutions for long time has been presented which provides a good agreement among the solutions. (authors)
Explicit finite element analysis of convective-conductive heat transfer
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The present paper discusses an explicit finite element approach to problems in transient convective-conductive heat transfer in a fluid region. The governing equations are the incompressible Navier-Stokes equations coupled with the thermal energy equation. Plane and axisymmetric problems are considered in terms of the primitive variables: velocity, pressure and temperature. The space discretization is based on 4-node or 9-node quadrilateral finite elements, while a finite difference method is used for time integration. Due to the complexity of the governing equations, an explicit time discretization method is choosen in connection with a diagonal mass representation. To deal with the necessarily implicit incompressibility constraint and the associated pressure terms, a fractional-step method is developed for marching in time. In this way, the pressure field is fully decoupled from and solved alternatively with the momentum equations. A weak treatment of the prescribed tangential components of velocity is introduced in order to avoid the spurious phenomenon of chequerboard splitting of the discrete pressure field encountered in other studies. To illustrate the proposed fractional-step method, numerical examples are presented in plane and axisymmetric configurations using both bilinear and biquadratic local approximations. The solutions obtained are found in good agreement with previously published results. (orig.)
Convection initiation along a dryline under conditions of weak synoptic forcing
Trier, S.; Chen, F.; Manning, K.
2003-04-01
During the spring and early summer a dryline characterized by relatively weak temperature (density) gradients but large moisture contrasts is a common feature over the southern Great Plains (SGP) region of the United States. In the spring, when strong vertical wind shear is present, the dryline often serves as a focus for severe convective storms. In the current study we examine a quasi-stationary dryline present over the SGP for a five-day period from 19 to 23 June 1998. In contrast to the more widely reported spring cases, this dryline episode and its associated deep convection occurred in the absence of strong vertical wind shear and organized synoptic-scale ascent. The detailed mechanisms of convection initiation along drylines are not well understood. The current case allows a unique opportunity for the assessment of planetary boundary layer (PBL)-based deep convection initiation mechanisms, which are likely to be of general importance in dryline convection cases, without the complicating effects of strong large-scale forcing. Numerical simulations with the Pennsylvania State/NCAR Mesoscale Model Version 5 (MM5), at both deep convection resolving resolutions and at coarser resolution over regional scale domains, have accurately reproduced the initiation and short term (3-6 h) evolution of deep convection, as revealed by satellite, radar, and surface rainfall observations. Convection initiation was both observed and simulated at the eastern edge of the dryline, which later retrogressed westward during the lifecycle of the slowly eastward moving convection. During the five-day episode there was little severe weather reported, however, localized heavy rainfall amounts were common in the large-CAPE environment along and slightly east of the dryline. Trajectory analysis with model output elucidates the thermodynamic destabilization of the PBL air that feeds the updrafts of the developing convection. Model-based sensitivity studies are planned to examine the role of land-surface atmosphere interactions (particularly the impact of soil moisture and temperature), which are likely to be especially important for convection initiation in the weakly forced synoptic environment. Results from this analysis will be discussed at the conference.
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The model laws for the initial film boiling at forced convection are realized in vertical tubes. The local conditions in the investigated area were regarded to be most effective and sufficient for the description. The theory was confirmed by experimental data. (orig.)
Method of cleaning convective heating surfaces from deposits
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Keylin, I.I.; Dedikov, S.N.; Nikitin, A.I.
1982-01-01
According to the main USSR cert. of auth. 767500, sprayed water with temperature 70-100/sup 0/C is supplied to the heating surface to be cleaned having temperature of 150-350/sup 0/C. In this case the sprayed water turns into steam on the hot deposits, they are loosened and are washed away by technical water. However, in cleaning the surfaces with vertical pipes, only the front coils of the first 2-3 rows are exposed to the effect of the hot technical water. According to the new proposal, when the temperature in the gas line reaches 350/sup 0/C, exhaust fans are turned on, the slide valves are closed and sprayed water with temperature of 70-100/sup 0/C is supplied to the cleaning section, which coming into contact with the hot heating surfaces evaporates, forming a steam bath in the gas line. The deposits are separated and washed away by water from the frontal coils. Within 45 minutes after the beginning of cleaning, the slide valves are opened, and because of the thrust created by the smokestack, the boiler is ventilated for 5-15 minutes. Water passes through the entire packet of cleanable surface, sprinkling the inner coils. After this the slide valves are closed, and cleaning lasts for another 45 minutes. Then air is supplied into the convective shaft to ventilate the vertical coils in the opposite direction. Cleaning is done until the temperature in the gas line drops to 150/sup 0/C, i.e., 5-6 h. Then the surfaces are flushed with technical water from hoses. The proposed method excludes the need for deslagging and cleaning of the vertical packets by manual tool.
Fang, Pingping
1998-12-01
An extended numerical investigation of fully developed, forced convective laminar flows with heat transfer in eccentric annuli has been carried out. Both Newtonian and non-Newtonian (power-law or Ostwald-de Waele) fluids are studied, representing typical applications in petrochemical, bio-chemical, personal care products, polymer/plastic extrusion and food industries. For the heat transfer problem, with an insulated outer surface, two types of thermal boundary conditions have been considered: Constant wall temperature (T), and uniform axial heat flux with constant peripheral temperature (H1) on the inner surface of the annulus. The governing differential equations for momentum and energy conservation are solved by finite-difference methods. Velocity and temperature distributions in the flow cross section, the wall shear-stress distribution, and isothermal f Re, Nu i,T and Nu i,H1 values for different eccentric annuli (0/le?/*/le0.6,/ 0.2/le r/sp/*/le0.8) are presented. In Newtonian flows, the eccentricity is found to have a very strong influence on the flow and temperature fields. In an annulus with relatively large inner cylinder eccentricity, the flow tends to stagnate in the narrow section and has higher peak velocities in the wide section of the annulus. There is considerable flow maldistribution in the azimuthal direction, which in turn produces greater nonuniformity in the temperature field and a consequent degradation in the average heat transfer. Also, the H1 wall condition sustains higher heat transfer coefficients relative to the T boundary condition on the inner surface. For viscous, power-law type non-Newtonian flows, both shear thinning (n1) fluids are considered. Here, the non-linear shear behavior of the fluid is found to further aggravate the flow and temperature maldistribution, and once again the eccentricity is seen to exhibit a very strong influence on the friction and heat transfer behavior. Finally, the hydrodynamic characteristics of fully developed axial laminar flow of Newtonian fluids in eccentric annuli with a rotating inner cylinder are investigated. These are of significant importance to the design and operation of oil and gas drilling wells. Using finite-difference method to solve the governing flow equations in bipolar coordinates, computational results for a wide range of annulus geometry (0/le r/sp/*/le1,/ 0/le/varepsilon/sp/*/le0.8), and rotational Reynolds number (0/le Rer/le150) are presented, where the rotational speeds are restricted to the sub-critical Taylor number regime. The results delineate the effects of annuli r/sp/* and ?sp/*, and inner cylinder rotation speed on the flow structure and frictional losses.
Cebeci, Tuncer
1989-01-01
This book is designed to accompany Physical and Computational Aspects of Convective Heat Transfer by T Cebeci and P Bradshaw and contains solutions to the exercises and computer programs for the numerical methods contained in that book Physical and Computational Aspects of Convective Heat Transfer begins with a thorough discussion of the physical aspects of convective heat transfer and presents in some detail the partial differential equations governing the transport of thermal energy in various types of flows The book is intended for senior undergraduate and graduate students of aeronautical, chemical, civil and mechanical engineering It can also serve as a reference for the practitioner
Onset of nuclear boiling in forced convection (Method of detection)
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Local onset of boiling in any pressure water cooling systems, as a PWR for instance, can mean a possible dangerous mismatch between the produced heat and the cooling capabilities. Its consequences can lead to serious accidental conditions and a reliable technique to detect such a phenomenon is therefore of particular need. Most techniques used up to now rely basically on local measurements and assume therefore usually the previous knowledge of the actual hot or boiling spot. The method proposed here based on externally located accelerometers appears to be sensitive to the global behaviour of the mechanical structure and is therefore not particularly bound to any exact localization of the sensors. The vibrations produced in the mechanical structure of the heated assembly are measured by accelerometers placed on the external surfaces that are easily accessible. The onset of the boiling, the growth and condensation of the bubbles on the heated wall, induces a resonance in the structure and an excitation at its particular eigen frequencies. Distinctive peaks are clearly observed in the spectral density function calculated from the accelerometer signal as soon as bubbles are produced. The technique is shown to be very sensitive even at the earliest phase of boiling and quite independent on sensor position. A complete hydrodynamic analysis of the experimental channels have been performed in order to assess the validity of the method both in steady conditions and during rapid power transients
Tri–Dimensional Numerical Analysis for Forced Convection over a Forward Facing Step
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J.A. Jiménez–Berna
2010-01-01
Full Text Available A finite volume discretization technique is used to develop a numerical code to simulate the flow structures and forced convection in a forward facing step channel. The velocity field and pressure distribution inside the computacional domain are linked by the SIMPLE algorithm. The duct dimensions are defined in terms of the step height, such that the aspect and expansion ratios are four and two respectivelly. The total length in the streamflow direction is sixty times the step height, while the step edge is located twenty times the step height after the channel inlet. The boundary conditions at the channel inlet correspond to a fully developed flow at a constant temperature T0. The heating conditions are those of considereing the bottom wall at a high constant temperature (Tw>T0 and the other walls as adiabatic ones. Results for the location and size of the re–circulating zones, as well as the flow structures and temperature distributions at different planes inside the computational domain for three different Reynolds parameters are presented.
Experimental transient natural convection heat transfer from a vertical cylindrical tank
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In this paper heat transfer experimental data is presented and compared to general correlations proposed in the literature for transient laminar free convection from a vertical cylindrical tank. The experimental data has been obtained from heating and cooling experiments carried out with a cylindrical full-scale hot water storage tank working under real operating conditions. The experimental device and the data acquisition system are described. The calculation procedures established to obtain the experimental values of the heat transfer coefficients, as well as the data reduction process, are detailed. The local convection and radiation heat transfer coefficients are obtained from different heating power conditions for local Rayleigh numbers within the range of 1x105-3x108. The great quantity of available experimental data allows a detailed analysis with a reliable empirical base. The experimental local convection heat transfer coefficients are correlated and compared to correlations proposed in open literature for engineering calculations. - Highlights: ? Experimental data of transient local convection heat transfer coefficients from a cylindrical tank for heating and cooling processes is obtained. ? The transient behaviour of the convection coefficients is dependent on temperature difference evolutions between the surface and the air. ? The Nu.Ra-1/4 ratio decreases proportionally in (Ts-T?)-0.9. ? A new correlation based on the semi-infinite region theory for laminar transient free convection is proposed.
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Maksimov Vyacheslav I.
2015-01-01
Full Text Available Results of mathematical modeling of convective heat transfer in air area surrounded on all sides enclosing structures, in the presence of heat source at the lower boundary of the media are presented. Solved the system of differential equations of unsteady Navier-Stokes equations with the appropriate initial and boundary conditions. The process of convective heat transfer is calculated using the models of turbulence Prandtl and Prandtl-Reichard. Takes into account the processes of heat exchange region considered with the environment. Is carried out the analysis of the dimensionless heat transfer coefficient at interfaces “air – enclosures”. The distributions average along the gas temperature range are obtained.
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This report is the user's manual for the computer code CONDIF that has been developed for solving natural and forced convection problems. It describes the preparation of input data and the solution of a test problem
Natural versus forced convection in laminar starting plumes
Rogers, Michael C.; Morris, Stephen W.
2009-01-01
A starting plume or jet has a well-defined, evolving head that is driven through the surrounding quiescent fluid by a localized flux of either buoyancy or momentum, or both. We studied the scaling and morphology of starting plumes produced by a constant flux of buoyant fluid from a small, submerged outlet. The plumes were laminar and spanned a wide range of plume Richardson numbers Ri. Ri is the dimensionless ratio of the buoyancy forces to inertial effects, and is thus our measurements cross...
Short-wave convective turbulence and anomalous electron heat conduction of a plasma
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A turbulent mechanism of anomalous electron transfer in a magnetized plasma due to excitation in the plasma of short-wave convective oscillations is suggested. Stationary spectra of short-wave convective turbulence are found in the framework of a general static approach and anomalous electron heat conduction of the plasma is calculated
Heat transfer in the thermo-electro-hydrodynamic convection under microgravity conditions
Tadie Fogaing, Mireille; Yoshikawa, Harunori,; Crumeyrolle, Olivier; Mutabazi, Innocent
2014-01-01
This article deals with the thermal convection in a dielectric fluid confined in a finite-length plane capacitor with a temperature gradient under microgravity conditions. The dielectrophoretic force resulting from differential polarization of the fluid plays the role of buoyancy force associated with an electric effective gravity. It induces the convection when the Rayleigh number based on this electric gravity exceeds a critical value. Two-dimensional numerical simulation for a geometry wit...
Convection-radiation heat transfer to steam in rod bundle geometry/
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An engineering heat transfer model was developed to predict the total heat transfer coefficients above the froth region in a nuclear reactor core undergoing a slow core uncovering. The model consists of a new heat transfer correlation for convection to steam and a one-dimensional thermal radiation equation. Above the froth region, large wall-to-bulk temperature ratios can take place; therefore, variable property effects on flow and heat transfer were examined because they can affect the heat transfer conditions to a considerable extent. The convective heat transfer coefficients and rod surface temperatures were calculated by using various correlations. The comparison of the results showed that the new correlation accurately predicts the convective heat transfer coefficients and, when combined with the radiation equation, the wall temperatures. The use of this model should be of value in modeling small-break loss-of-coolant accidents and preliminary design work
The computer code CONDIF-01 (release 2) for transient convective-conductive heat transfer
International Nuclear Information System (INIS)
CONDIF-01 is a finite element computer code developed at J.R.C. Ispra to solve natural and forced convection problems, for use in Post Accident Heat Removal studies following a hypothetical fast-reactor core meltdown. The new version of the code is capable of analysing problems in which there exists initially a liquid (solid) region which may change phase to solid (liquid), as time proceeds. A variant of the enthalpy method is employed to model the phase change process. The presence of structures enclosing the liquid (solid) region is accounted for, but such structures are assumed to remain in the solid phase. Plane and axisymmetric situations may be analysed. The essential characteristics of the code are outlined here. This report gives instructions for preparing input data to CONDIF-01, release 2, and describes two test problems in order to illustrate both the input and the output of the code
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Conjugate mixed convection with buoyancy assisted laminar flow in the entrance region of a vertical channel is considered numerically. The problem is solved by a finite volume method for a thick walled, two-regional channel which has constant and uniform outside wall temperatures. The effects of wall thermal conduction as well as assisted buoyancy force on the flow and heat transfer are discussed in detail. Results are presented for a Prandtl number of 0.7, solid-to-fluid thermal conductivity ratios of 1? k* < ?, wall thickness-to-channel length ratios of 0? l* ?5, Reynolds numbers of 200? Re ?1000, and for various Grashof numbers. The critical buoyancy parameter (Gr/Re), above which the flow reversal occurs, increases linearly with the increasing l*/k*, while it is independent on the Reynolds number. (authors)
Heat Transfer in MHD Mixed Convection Flow of a Ferrofluid along a Vertical Channel
Gul, Aaiza; Khan, Ilyas; Shafie, Sharidan; Khalid, Asma; Khan, Arshad
2015-01-01
This study investigated heat transfer in magnetohydrodynamic (MHD) mixed convection flow of ferrofluid along a vertical channel. The channel with non-uniform wall temperatures was taken in a vertical direction with transverse magnetic field. Water with nanoparticles of magnetite (Fe3O4) was selected as a conventional base fluid. In addition, non-magnetic (Al2O3) aluminium oxide nanoparticles were also used. Comparison between magnetic and magnetite nanoparticles were also conducted. Fluid motion was originated due to buoyancy force together with applied pressure gradient. The problem was modelled in terms of partial differential equations with physical boundary conditions. Analytical solutions were obtained for velocity and temperature. Graphical results were plotted and discussed. It was found that temperature and velocity of ferrofluids depend strongly on viscosity and thermal conductivity together with magnetic field. The results of the present study when compared concurred with published work. PMID:26550837
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Numerical analysis of natural convection inside a heat generated fluid was performed for four different spherical geometries that match the experimental vessels used by Asfia et al. [5-7]. The transient calculations were performed with the CFX 5.7 fluid dynamic software. The simulations show that the highest heat flux is just below the rim of the cavity and it can be 50 times higher than at the bottom. Based on the numerical results, the local values of heat transfer coefficient and the distributions of global Nusselt number were calculated. The present, three-dimensional simulation results were compared with the numerical results of Mayinger et al. [3] and Reineke et al. [4], and with the experimental data of Asfia et al. [5-7]. The agreement between the results that is well inside the experimental scatter verifies the selected modeling approach. (author)
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Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat adiabatic surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air, i.e. where air-conditioning is being used, being considered. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Biuniqueness type approximation. Although the flow is in general three-dimensional, the flow over each slat is assumed to be the same and attention can therefore be restricted to flow over a single slat by using repeating boundary conditions. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless distance of the slat center point (the pivot point) from the surface (4) the dimensionless slat size (5) the dimensionless slat spacing (6) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined. (author)
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Two-phase flow heat transfer has been exhaustively studied over recent years. However, in this field several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling have been studied using different approaches, numerical, analytical and experimental. In this work, an experimental analysis, data representation and heat transfer coefficient prediction on two-phase heat transfer on nucleate and convective boiling are presented. An empirical correlation is obtained based on genetic algorithms search engine over a dimensional analysis of the two-phase flow heat transfer problem. (author)
Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger
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Taoufik Mnasri
2008-01-01
Full Text Available This study presents analytical models allowing to study a forced convection laminar flow in non-established dynamic and thermic regimes. We treated a flow in a bitubular exchanger in permanent thermal contact with a semi-infinite medium, such as the ground. The wall temperature as well as the wall heat flux evolve in the course of time until a quasi-steady mode. The theoretical method is original because it uses Green's functions method to determine the analytical solutions of the heat propagation equation on the wall during the heating phase. These analytical solutions allow to identify the temperature distribution versus time. The complexity of the system geometry as well as the infinity of the medium surrounding the exchanger make the traditional methods of numerical resolution unable to solve the problem. We used, to solve it, the finite volume method coupled with the finite element method at the boundary. We studied the effect of Reynolds number, the fluid entry temperature and the transfer duration on the axial evolution of the heat transfer coefficient. We illustrated also the profile of the temperature field in the fluid medium.
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Sourtiji Ehsan
2012-01-01
Full Text Available A numerical study of natural convection heat transfer through an alumina-water nanofluid inside L-shaped cavities in the presence of an external magnetic field is performed. The study has been carried out for a wide range of important parameÂters such as Rayleigh number, Hartmann number, aspect ratio of the cavity and solid volume fraction of the nanofluid. The influence of the nanoparticle, buoyancy force and the magnetic field on the flow and temperature fields have been plotted and discussed. The results show that after a critical Rayleigh number depending on the aspect ratio, the heat transfer in the cavity rises abruptly due to some significant changes in flow field. It is also found that the heat transfer enhances in the presence of the nanoparticles and increases with solid volume fraction of the nanofluid. In addition, the performance of the nanofluid utilization is more effective at high RayÂleigh numbers. The influence of the magnetic field has been also studied and deÂduced that it has a remarkable effect on the heat transfer and flow field in the cavity that as the Hartmann number increases the overall Nusselt number is significantly decreased specially at high Rayleigh numbers.
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Paper describes design and peculiar features of application of heat flux measuring sensor, operation of which is based on the Seebeck transverse effect. Sensor capabilities are shown using investigation into free-convection heat transfer as an example. Paper present the results of measuring of instantaneous value of heat flux on a heated vertical surface under various air flow regimes within free-convection boundary layer
Natural versus forced convection in laminar starting plumes
Rogers, Michael C
2009-01-01
A starting plume or jet has a well-defined, evolving head that is driven through the surrounding quiescent fluid by a localized flux of either buoyancy or momentum, or both. We studied the scaling and morphology of starting plumes produced by a constant flux of buoyant fluid from a small, submerged outlet. The plumes were laminar and spanned a wide range of plume Richardson numbers Ri. Ri is the dimensionless ratio of the buoyancy forces to inertial effects, and is thus our measurements crossed over the transition between buoyancy-driven plumes and momentum-driven jets. We found that the ascent velocity of the plume, nondimensionalized by Ri, exhibits a power law relationship with Re, the Reynolds number of the injected fluid in the outlet pipe. We also found that as the threshold between buoyancy-driven and momentum-driven flow was crossed, two distinct types of plume head mophologies existed: confined heads, produced in the Ri > 1 regime, and dispersed heads, which are found in the Ri < 1 regime. Head di...
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Prasad Kerehalli
2015-01-01
Full Text Available An analysis is carried out to study the effects of temperature-dependent transport properties on the fully developed free and forced MHD convection flow in a vertical channel. In this model, viscous and Ohmic dissipation terms are also included. The governing nonlinear equations (in non-dimensional form are solved numerically by a second order finite difference scheme. A parametric study is performed in order to illustrate the interactive influences of the model parameters; namely, the magnetic parameter, the variable viscosity parameter, the mixed convection parameter, the variable thermal conductivity parameter, the Brinkmann number and the Eckert number. The velocity field, the temperature field, the skin friction and the Nusselt number are evaluated for several sets of values of these parameters. For some special cases, the obtained numerical results are compared with the available results in the literature: Good agreement is found. Of all the parameters, the variable thermo-physical transport property has the strongest effect on the drag, heat transfer characteristics, the stream-wise velocity, and the temperature field.
Numerical simulation of turbulent forced convection in liquid metals
Vodret, S.; Vitale Di Maio, D.; Caruso, G.
2014-11-01
In the frame of the future generation of nuclear reactors, liquid metals are foreseen to be used as a primary coolant. Liquid metals are characterized by a very low Prandtl number due to their very high heat diffusivity. As such, they do not meet the so-called Reynolds analogy which assumes a complete similarity between the momentum and the thermal boundary layers via the use of the turbulent Prandtl number. Particularly, in the case of industrial fluid-dynamic calculations where a resolved computation near walls could be extremely time consuming and could need very large computational resources, the use of the classical wall function approach could lead to an inaccurate description of the temperature profile close to the wall. The first aim of the present study is to investigate the ability of a well- established commercial code (ANSYS FLUENT v.14) to deal with this issue, validating a suitable expression for the turbulent Prandtl number. Moreover, a thermal wall-function developed at Universite Catholique de Louvain has been implemented in FLUENT and validated, overcoming the limits of the solver to define it directly. Both the resolved and unresolved approaches have been carried out for a channel flow case and assessed against available direct numerical and large eddy simulations. A comparison between the numerically evaluated Nusselt number and the main correlations available in the literature has been also carried out. Finally, an application of the proposed methodology to a typical sub-channel case has been performed, comparing the results with literature correlations for tube banks.
Preliminary Numerical Analysis of Convective Heat Transfer Loop Using MARS Code
Energy Technology Data Exchange (ETDEWEB)
Lee, Yongjae; Seo, Gwang Hyeok; Jeun, Gyoodong; Kim, Sung Joong [Hanyang Univ., Seoul (Korea, Republic of)
2014-05-15
The MARS has been developed adopting two major modules: RELAP5/MOD3 (USA) for one-dimensional (1D) two-fluid model for two-phase flows and COBRA-TF code for a three-dimensional (3D), two-fluid, and three-field model. In addition to the MARS code, TRACE (USA) is a modernized thermal-hydraulics code designed to consolidate and extend the capabilities of NRC's 3 legacy safety code: TRAC-P, TRAC-B and RELAP. CATHARE (French) is also thermal-hydraulic system analysis code for Pressurized Water Reactor (PWR) safety. There are several researches on comparing experimental data with simulation results by the MARS code. Kang et al. conducted natural convection heat transfer experiments of liquid gallium loop, and the experimental data were compared to MARS simulations. Bang et al. examined the capability of the MARS code to predict condensation heat transfer experiments with a vertical tube containing a non-condensable gas. Moreover, Lee et al. adopted MELCOR, which is one of the severe accident analysis codes, to evaluate several strategies for the severe accident mitigation. The objective of this study is to conduct the preliminary numerical analysis for the experimental loop at HYU using the MARS code, especially in order to provide relevant information on upcoming experiments for the undergraduate students. In this study, the preliminary numerical analysis for the convective heat transfer loop was carried out using the MARS Code. The major findings from the numerical simulations can be summarized as follows. In the calculations of the outlet and surface temperatures, the several limitations were suggested for the upcoming single-phase flow experiments. The comparison work for the HTCs shows validity for the prepared input model. This input could give useful information on the experiments. Furthermore, the undergraduate students in department of nuclear engineering, who are going to be taken part in the experiments, could prepare the program with the input, and will be provided with expected results for the single-phase and forced convective phenomena. For the future study, different materials for the heating part are considered, such as other metals or silicon carbide (SiC) tube, which is a candidate material of fuel claddings for current and next-generation reactors.
International Nuclear Information System (INIS)
Highlights: â†’ We investigate laminar convective heat transfer in channels with periodic cavities. â†’ Heat transfer rates are lower than for the flat channel. â†’ This is ascribed to the steady circulating motion within the cavities. â†’ Diffusion in a low Prandtl number fluid can locally overcome the heat transfer decrease due to advection only for isothermal boundary conditions. - Abstract: Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry investigated is a channel with a periodic series of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number based on the hydraulic diameter between Rem = 24.9 and Rem = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers and wall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heat transfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transfer rates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature and very low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effect of different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometry is also considered.
Energy Technology Data Exchange (ETDEWEB)
Stalio, E., E-mail: enrico.stalio@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy); Angeli, D., E-mail: diego.angeli@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy); Barozzi, G.S., E-mail: giovanni.barozzi@unimore.it [Dipartimento di Ingegneria Meccanica e Civile, Universita degli Studi di Modena e Reggio Emilia, Via Vignolese 905/B, 41125 Modena (Italy)
2011-10-15
Highlights: > We investigate laminar convective heat transfer in channels with periodic cavities. > Heat transfer rates are lower than for the flat channel. > This is ascribed to the steady circulating motion within the cavities. > Diffusion in a low Prandtl number fluid can locally overcome the heat transfer decrease due to advection only for isothermal boundary conditions. - Abstract: Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry investigated is a channel with a periodic series of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number based on the hydraulic diameter between Re{sub m} = 24.9 and Re{sub m} = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers and wall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heat transfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transfer rates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature and very low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effect of different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometry is also considered.
Influence of Pr on Natural Convection Heat Transfer of an Open Channel Finned Plate
International Nuclear Information System (INIS)
The finned plate provides the extended heat transfer area and improves the heat transfer. However when the fin spacing becomes small, the pressure drop increases due to frictional loss, heat transfer is impaired. Thus there is an optimal fin spacing. For the natural convection heat transfer, the heated thermal boundary layer drives the flow and the influence of the Prandtl number on the heat transfer will be very important as the thickness of the thermal boundary layer depends on it. This study aims at investigating the influence of the Prandtl number on the natural convection heat transfer of the finned plate. Numerical analyses were performed by varying the Pr from 2 to 2,014. Numerical analysis was performed for the natural convection heat transfer of a finned plate in an open channel. In order to investigate the influence of the Prandtl number on the heat transfer, four different values of Prandtl numbers were simulated and compared. As expected, the velocity profiles were almost similar except for the fact that the boundary layer develops earlier for smaller Prandtl number fluid. However the temperature profiles varied drastically depending on the values of the Prandtl number. As the Prandtl number increases, the thermal boundary layer reduces. The comparisons of the results with Le Fevre natural convection heat transfer correlation for vertical plate shows that as the Pr increases, the NuL of the finned plate becomes similar to that of the flat plate of the same heat transfer area
A reassessment of the heat transport by variable viscosity convection with plates and lids
International Nuclear Information System (INIS)
The heat transport by a viscous fluid with temperature dependent viscosity has been studied numerically. As opposed to previous models, the top surface of the fluid clearly defines a tectonic plate with horizontally uniform velocity and subduction. Past studies failed to incorporate plates, the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity; there is a strong buffering between internal temperature and heat loss. Past studies of parameterized convection which incorporated parameters indicative of strong buffering between internal temperature and total heat output still provide the most physically plausible representation of the Earth's thermal evolution
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
Directory of Open Access Journals (Sweden)
Kaprawi S., Dyos Santoso
2013-01-01
Full Text Available This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretized thermal equation. The circular cylinder has the aspect ratio equal to unity while the elliptical cylinder has the aspect ratio less than unity by reducing the minor axis and increasing the major axis to obtain the same cross section as circular cylinder. The results of the calculations show that the skin friction change significantly, but in contrast with the elliptical cylinders have greater convection heat transfer than that of circular cylinder. Some results of calculations are compared to the analytical solutions given by the previous authors.
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
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Kaprawi, S.; Santoso, Dyos [Mechanical Department of Sriwijaya University, Jl. Raya Palembang-Prabumulih Km. 32 Inderalaya 50062 Ogan Ilir (Indonesia)
2013-07-01
This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretized thermal equation. The circular cylinder has the aspect ratio equal to unity while the elliptical cylinder has the aspect ratio less than unity by reducing the minor axis and increasing the major axis to obtain the same cross section as circular cylinder. The results of the calculations show that the skin friction change significantly, but in contrast with the elliptical cylinders have greater convection heat transfer than that of circular cylinder. Some results of calculations are compared to the analytical solutions given by the previous authors.
International Nuclear Information System (INIS)
Natural convection in low aspect ratio rectangular enclosures is considered along with three-dimensional convection within rectangular boxes, natural convection flow visualization in irradiated water cooled by air flow over the surface, free convection in vertical slots, the stratification in natural convection in vertical enclosures, the flow structure with natural convection in inclined air-filled enclosures, and natural convection across tilted, rectangular enclosures of small aspect ratio. Attention is given to the effect of wall conduction and radiation on natural convection in a vertical slot with uniform heat generation of the heated wall, a numerical study of thermal insulation enclosure, free convection in a piston-cylinder enclosure with sinusoidal piston motion, natural convection heat transfer between bodies and their spherical enclosure, an experimental study of the steady natural convection in a horizontal annulus with irregular boundaries, three-dimensional natural convection in a porous medium between concentric inclined cylinders, a numerical solution for natural convection in concentric spherical annuli, and heat transfer by natural convection in porous media between two concentric spheres
Natural convection heat transfer in vertical triangular subchannel in Zirconia-water nanofluid
Tandian, N. P.; Alkharboushi, A. A. K.; Kamajaya, K.
2015-09-01
Natural convection heat transfer in vertical triangular sub-channel has important role in cooling mechanism of the APWR and the PHWR nuclear reactors. Unfortunately, natural convection correlation equations for such geometry are scarcely available. Recent studies showed that ZrO2-water nanofluid has a good prospect to be used in the nuclear reactor technology due to its low neutron absorption cross section. Although several papers have reported transport properties of ZrO2-water nanofluids, practically there is no correlation equation for predicting natural convection heat transfer in a vertical triangular sub-channel in ZrO2-water nanofluid. Therefore, a study for finding such heat transfer correlation equation has been done by utilizing Computational Fluid Dynamics software and reported in this paper. In the study, natural convection heat transfer in a vertical triangular sub-channel has been simulated at several values of heat transfer flux within 9.1 to 30.9 kW/m2 range and ZrO2 concentrations of 0 (pure water), 0.27, and 3 volume-% of ZrO2. The study shows that the ZrO2 concentration has no significant influence to the natural convection heat transfer at those concentration levels. The obtained theoretical heat transfer correlation equations were verified through experiment, and they showed very similar results. The correlation equations are reported in this paper.
The role of a convective surface in models of the radiative heat transfer in nanofluids
Energy Technology Data Exchange (ETDEWEB)
Rahman, M.M., E-mail: mansurdu@yahoo.com; Al-Mazroui, W.A.; Al-Hatmi, F.S.; Al-Lawatia, M.A.; Eltayeb, I.A.
2014-08-15
Highlights: • The role of a convective surface in modelling with nanofluids is investigated over a wedge. • Surface convection significantly controls the rate of heat transfer in nanofluid. • Increased volume fraction of nanoparticles to the base-fluid may not always increase the rate of heat transfer. • Effect of nanoparticles solid volume fraction depends on the types of constitutive materials. • Higher heat transfer in nanofluids is found in a moving wedge rather than in a static wedge. - Abstract: Nanotechnology becomes the core of the 21st century. Nanofluids are important class of fluids which help advancing nanotechnology in various ways. Convection in nanofluids plays a key role in enhancing the rate of heat transfer either for heating or cooling nanodevices. In this paper, we investigate theoretically the role of a convective surface on the heat transfer characteristics of water-based nanofluids over a static or moving wedge in the presence of thermal radiation. Three different types of nanoparticles, namely copper Cu, alumina Al{sub 2}O{sub 3} and titanium dioxide TiO{sub 2} are considered in preparation of nanofluids. The governing nonlinear partial differential equations are made dimensionless with the similarity transformations. Numerical simulations are carried out through the very robust computer algebra software MAPLE 13 to investigate the effects of various pertinent parameters on the flow field. The obtained results presented graphically as well as in tabular form and discussed from physical and engineering points of view. The results show that the rate of heat transfer in a nanofluid in the presence of thermal radiation significantly depends on the surface convection parameter. If the hot fluid side surface convection resistance is lower than the cold fluid side surface convection resistance, then increased volume fraction of the nanoparticles to the base fluid may reduces the heat transfer rate rather than increases from the surface of the wedge to the nanofluid. This finding is new and has not been reported in any open literature.
The role of a convective surface in models of the radiative heat transfer in nanofluids
International Nuclear Information System (INIS)
Highlights: • The role of a convective surface in modelling with nanofluids is investigated over a wedge. • Surface convection significantly controls the rate of heat transfer in nanofluid. • Increased volume fraction of nanoparticles to the base-fluid may not always increase the rate of heat transfer. • Effect of nanoparticles solid volume fraction depends on the types of constitutive materials. • Higher heat transfer in nanofluids is found in a moving wedge rather than in a static wedge. - Abstract: Nanotechnology becomes the core of the 21st century. Nanofluids are important class of fluids which help advancing nanotechnology in various ways. Convection in nanofluids plays a key role in enhancing the rate of heat transfer either for heating or cooling nanodevices. In this paper, we investigate theoretically the role of a convective surface on the heat transfer characteristics of water-based nanofluids over a static or moving wedge in the presence of thermal radiation. Three different types of nanoparticles, namely copper Cu, alumina Al2O3 and titanium dioxide TiO2 are considered in preparation of nanofluids. The governing nonlinear partial differential equations are made dimensionless with the similarity transformations. Numerical simulations are carried out through the very robust computer algebra software MAPLE 13 to investigate the effects of various pertinent parameters on the flow field. The obtained results presented graphically as well as in tabular form and discussed from physical and engineering points of view. The results show that the rate of heat transfer in a nanofluid in the presence of thermal radiation significantly depends on the surface convection parameter. If the hot fluid side surface convection resistance is lower than the cold fluid side surface convection resistance, then increased volume fraction of the nanoparticles to the base fluid may reduces the heat transfer rate rather than increases from the surface of the wedge to the nanofluid. This finding is new and has not been reported in any open literature
Jiao, Anjun; Zhang, Yuwen; Ma, Hongbin; Critser, John
2009-01-01
Heat and mass transfer in a circular tube subject to the boundary condition of the third kind is investigated. The closed form of temperature and concentration distributions, the local Nusselt number based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Lewis number and Biot number on heat and mass transfer were investigated.
NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION AIR FLOW IN A RECTANGULAR VENTURI CHANNEL
Directory of Open Access Journals (Sweden)
IGO SERGE WENDSIDA
2014-01-01
Full Text Available In this work, a laminar forced convection air flow in a rectangular vertical venturi has been numerically simulated. Mathematical transformation has been used to transform the irregular profile of the venturi wall into straight line. Transfers equations are solved using finite volume method, Gauss and Thomas algorithms. A computing algorithm has been generated for the problem simulation. Hydrodynamics and thermals effects are investigated in detail. Results are presented as velocity and streamlines patterns, and temperature profiles.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
U.S. Muhammed; A.M.I. El-Okene; M. Isiaka
2012-01-01
The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with or...
International Nuclear Information System (INIS)
The analytical solution of the nonstationary problem on the convective heat exchange by the laminar liquid flow with an account of the axial heat conductivity upwards by the flow from the heated zone is obtained. The mathematical difficulties are related to the fact, that in contrast to the existing solutions the conditions of conjugation by temperature and heat flow at the boundary of the pre-entry and heated sections depend on the coordinates and time. The temperature fields in the pre-entry and heated sections as well as the local and averaged characteristics of the stationary and nonstationary heat exchange are plotted with application of the obtained solution
Open Channel Natural Convection Heat Transfer on a Vertical Finned Plate
Energy Technology Data Exchange (ETDEWEB)
Park, Joo Hyun; Heo, Jeong Hwan; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)
2013-10-15
The natural convection heat transfer of vertical plate fin was investigated experimentally. Heat transfer systems were replaced by mass-transfer systems, based on the analogy concept. The experimental results lie within the predictions of the existing heat transfer correlations of plate-fin for the natural convections. An overlapped thermal boundary layers caused increasing heat transfer, and an overlapped momentum boundary layers caused decreasing heat transfer. As the fin height increases, heat transfer was enhanced due to increased inflow from the open side of the fin spacing. When fin spacing and fin height are large, heat transfer was unaffected by the fin spacing and fin height. Passive cooling by natural convection becomes more and more important for the nuclear systems as the station black out really happened at the Fukushima NPPs. In the RCCS (Reactor Cavity Cooling System) of a VHTR (Very High Temperature Reactor), natural convection cooling through duct system is adopted. In response to the stack failure event, extra cooling capacity adopting the fin array has to be investigated. The finned plate increases the surface area and the heat transfer increases. However, the plate of fin arrays may increase the pressure drop and the heat transfer decreases. Therefore, in order to enhance the passive cooling with fin arrays, the parameters for the fin arrays should be optimized. According to Welling and Wooldridge, a natural convection on vertical plate fin is function of Gr, Pr, L, t, S, and H. The present work investigated the natural convection heat transfer of a vertical finned plate with varying the fin height and the fin spacing. In order achieve high Rayleigh numbers, an electroplating system was employed and the mass transfer rates were measured using a copper sulfate electroplating system based on the analogy concept.
Material transport in a convective surface mixed layer under weak wind forcing
Mensa, Jean A.; Özgökmen, Tamay M.; Poje, Andrew C.; Imberger, Jörg
2015-12-01
Flows in the upper ocean mixed layer are responsible for the transport and dispersion of biogeochemical tracers, phytoplankton and buoyant pollutants, such as hydrocarbons from an oil spill. Material dispersion in mixed layer flows subject to diurnal buoyancy forcing and weak winds (| u10 | = 5m s-1) are investigated using a non-hydrostatic model. Both purely buoyancy-forced and combined wind- and buoyancy-forced flows are sampled using passive tracers, as well as 2D and 3D particles to explore characteristics of horizontal and vertical dispersion. It is found that the surface tracer patterns are determined by the convergence zones created by convection cells within a time scale of just a few hours. For pure convection, the results displayed the classic signature of Rayleigh-Benard cells. When combined with a wind stress, the convective cells become anisotropic in that the along-wind length scale gets much larger than the cross-wind scale. Horizontal relative dispersion computed by sampling the flow fields using both 2D and 3D passive particles is found to be consistent with the Richardson regime. Relative dispersion is an order of magnitude higher and 2D surface releases transition to Richardson regime faster in the wind-forced case. We also show that the buoyancy-forced case results in significantly lower amplitudes of scale-dependent horizontal relative diffusivity, kD(?), than those reported by Okubo (1970), while the wind- and buoyancy-forced case shows a good agreement with Okubo's diffusivity amplitude, and the scaling is consistent with Richardson's 4/3rd law, kD ? ?4/3. These modeling results provide a framework for measuring material dispersion by mixed layer flows in future observational programs.
Energy Technology Data Exchange (ETDEWEB)
Nanjundappa, C.E., E-mail: cenanju@hotmail.com [Department of Mathematics, Dr. Ambedkar Institute of Technology, Bangalore-560 056 (India); Shivakumara, I.S., E-mail: shivakumarais@gmail.com [Department of Mathematics, Bangalore University, Bangalore-560 001 (India); Prakash, H.N., E-mail: prakashahn83@gmail.com [Government Pre-University College, B H Road, Tumkur-572 102 (India)
2014-12-15
We investigate the influence of Coriolis force on the onset of thermomagnetic convection in ferrofluid saturating a porous layer in the presence of a uniform vertical magnetic field using both linear and weakly non-linear analyses. The modified Brinkman–Forchheimer-extended Darcy equation with Coriolis term has been used to describe the fluid flow. The linear theory based on normal mode method is considered to find the criteria for the onset of stationary thermomagnetic Convection and weakly non-linear analysis based on minimal representation of truncated Fourier series analysis containing only two terms has been used to find the Nusselt number Nu as functions of time. The range of thermal Rayleigh number R beyond which the bifurcation becomes subcritical increases with increasing ?, Da{sup ?1} and Ta. The global quantity of the heat transfer rate decreases by increasing the Taylor number Ta. The results obtained, during the above analyses, have been presented graphically and the effects of various parameters on heat and mass transfer have been discussed. Finally, we have drawn the steady streamlines for various parameters.
Kumar, Varun; Kumar, Manoj; Shakher, Chandra
2014-09-20
In this paper, the local convective heat transfer coefficient (h) is measured along the surface of an electrically heated vertical wire using digital holographic interferometry (DHI). Experiments are conducted on wires of different diameters. The experimentally measured values are within the range as given in the literature. DHI is expected to provide a more accurate local convective heat transfer coefficient (h) as the value of the temperature gradient required for the calculation of "h" can be obtained more accurately than by other existing optical interferometric techniques without the use of a phase shifting technique. This is because in digital holography phase measurement accuracy is expected to be higher. PMID:25322139
Kamajaya, K.; Umar, E.; Sudjatmi
2015-09-01
Study on convection heat transfer using water-Al2O3 nanofluid as the working fluid in the vertical sub-channel has been conducted. The results of the study have been compared with the water-ZrO2 nanofluid and pure-water as the working fluid. The equipment used in this experiment is a vertical triangular sub-channel, equipped by primary cooling system, heat exchanger and a secondary cooling system. As a heating source used three vertical cylinders that have a uniform heat flux with a pitch to diameter ratio (P/D) 01:16. Cooling is used is water-Al2O3 colloid at 0.05 wt. %. Heat transfer from heating to cooling would occur in natural or forced convection. However, in this study will be discussed only natural convection heat transfer. The results showed that the natural convection heat transfer of water-Al2O3 nanofluid in a triangular sub-channels depending on the position. The results of the correlation as follows,
DEFF Research Database (Denmark)
Hosseini, R.; Kolaei, Alireza Rezania; Alipour, M.; Rosendahl, Lasse
2012-01-01
In this work, the natural convection heat transfer from a long vertical electrically heated cylinder to an adjacent air gap is experimentally studied. The aspect and diameter ratios of the cylinder are 55.56 and 6.33, respectively. The experimental measurements were obtained for a concentric...
International Nuclear Information System (INIS)
Convective heat transfer at exterior building surfaces has an impact on the design and performance of building components such as double-skin facades, solar collectors, solar chimneys and ventilated photovoltaic arrays, and also affects the thermal climate and cooling load in urban areas. In this study, an overview is given of existing correlations of the exterior convective heat transfer coefficient (CHTC) with the wind speed, indicating significant differences between these correlations. As an alternative to using existing correlations, the applicability of CFD to obtain forced CHTC correlations is evaluated, by considering a cubic building in an atmospheric boundary layer. Steady Reynolds-averaged Navier-Stokes simulations are performed and, instead of the commonly used wall functions, low-Reynolds number modelling (LRNM) is used to model the boundary-layer region for reasons of improved accuracy. The flow field is found to become quasi independent of the Reynolds number at Reynolds numbers of about 105. This allows limiting the wind speed at which the CHTC is evaluated and thus the grid resolution in the near-wall region, which significantly reduces the computational expense. The distribution of the power-law CHTC-U10 correlation over the windward and leeward surfaces is presented (U10 = reference wind speed at 10 m height). It is shown that these correlations can be accurately determined by simulations with relatively low wind speed values, which avoids the use of excessively fine grids for LRNM, and by using only two or three discrete wind speed values, which limits the required number of CFD simulations.
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
Directory of Open Access Journals (Sweden)
J. Szajnar
2010-07-01
Full Text Available The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace, casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power windings of the AC specific frequency. Thissolution allowed to obtain a homogeneous microstructure of the continuous casting of cast iron EN-GJL-200 species.
Wood, Toby S; Stellmach, Stephan
2012-01-01
Regions of stellar and planetary interiors that are unstable according to the Schwarzschild criterion, but stable according to the Ledoux criterion, are subject to a form of oscillatory double-diffusive (ODD) convection often called "semi-convection". In this series of papers, we use an extensive suite of three-dimensional (3D) numerical simulations to quantify the transport of heat and composition by ODD convection, and ultimately propose a new 1D prescription that can be used in stellar and planetary structure and evolution models. The first paper in this series demonstrated that under certain conditions ODD convection spontaneously transitions from an initially homogeneously turbulent state into a staircase of convective layers, which results in a substantial increase in the transport of heat and composition. Here, we present simulations of ODD convection in this layered regime, we describe the dynamical behavior of the layers, and we derive empirical scaling laws for the transport through layered convecti...
Lopez, Jose M; Avila, Marc
2015-01-01
The flow of fluid confined between a heated rotating cylinder and a cooled stationary cylinder is a canonical experiment for the study of heat transfer in engineering. The theoretical treatment of this system is greatly simplified if the cylinders are assumed to be of infinite length or periodic in the axial direction, in which cases heat transfer occurs only through conduction as in a solid. We here investigate numerically heat transfer and the onset of turbulence in such flows by using both periodic and no-slip boundary conditions in the axial direction. We obtain a simple linear criterion that determines whether the infinite-cylinder assumption can be employed. The curvature of the cylinders enters this linear relationship through the slope and additive constant. For a given length-to-gap aspect ratio there is a critical Rayleigh number beyond which the laminar flow in the finite system is convective and so the behaviour is entirely different from the periodic case. The criterion does not depend on the Pra...
'Butterfly effect' in porous Bénard convection heated from below
International Nuclear Information System (INIS)
Transition from steady to chaos for the onset of Bénard convection in porous medium was analyzed. The governing equation is reduced to ordinary differential equation and solved using built in MATLAB ODE45. The transition from steady to chaos take over from a limit cycle followed by homoclinic explosion
Convective Heat Transfer Augmentation by Flexible fins in Laminar Channel Pulsating flow
Joshi, Rakshitha U; Bhardwaj, Rajneesh
2015-01-01
Fluid-structure interaction (FSI) of thin flexible fins coupled with convective heat transfer has applications in energy harvesting and in understanding functioning of several biological systems. We numerically investigate FSI of the thin flexible fins involving large-scale flow-induced deformation as a potential heat transfer enhancement technique. An in-house, strongly-coupled fluid-structure interaction (FSI) solver is employed in which flow and structure solvers are based on sharp-interface immersed boundary and finite element method, respectively. We consider twin flexible fins in a heated channel with laminar pulsating cross flow. The vortex ring past the fin sweep higher sources of vorticity generated on the channel walls out into the downstream - promoting the mixing of the fluid. The moving fin assists in convective mixing, augmenting convection in bulk and at the walls; and thereby reducing thermal boundary layer thickness and improving heat transfer at the channel walls. The thermal augmentation is...
Second law analysis of laminar forced convection in a rotating curved duct
Directory of Open Access Journals (Sweden)
Razavi Esmail Seyed
2015-01-01
Full Text Available In this paper, flow characteristics, heat transfer and entropy generation in a rotating curved duct are studied numerically. The continuity, Navier-Stokes and energy equations are solved using control volume method. The effects of Dean number, non-dimensional wall heat flux, and force ratio (the ratio of Coriolis to centrifugal forces on the entropy generation due to friction and heat transfer irreversibility and also overall entropy generation are presented. Optimal thermal operating conditions (based on dimensionless parameters are determined from the viewpoint of thermodynamics second law. The comparison of numerical results at different force ratios indicates that for any fixed Dean number or non-dimensional heat flux, the minimal frictional entropy generation occurs when the Coriolis and centrifugal forces have the same value but in the opposite direction. For a specific non-dimensional heat flux, there is a force ratio with maximum heat transfer irreversibility which depends on Dean number. Based on optimal analysis, the optimal force ratio with minimal total entropy generation depends on heat flux and Dean number.
Jiang, Jing; Huang, Xinjian; Wang, Lishi
2016-04-01
Detection of nanoparticle (NP) collision events at ultramicroelectrode (UME) has emerged as a new methodology for the investigation of single NP in recent years. Although the method was widely employed, some fundamental knowledge such as how the NP moves to and interacts with the UME remain less understood. It was generally recognized that the recorded rate of collision was determined by diffusion that should follow Fick's first law. However, significant lower collision frequency compared with that of predicted by theory were frequently reported. Experiments carried out by us suggest that the collision frequency will increase dramatically if forced convection (stir or flow injection) is applied during detection. Furthermore, the collision frequency gradually increases to a maximum and then decreases, along with the increase of the convection intensity. This phenomenon is interpreted as follows: (a) there are two steps for a freely moving NP to generate a detectable collision signal. The first step is the move of NP from bulk solution to the surface of the UME which is mass transfer limited; the second step is the landing of NP on the surface of UME which is affected by many factors and is the critical step; (b) there is a barrier that must be overcame before the contact between freely moving NP and UME. Forced convection with moderate intensity can not only increase the mass transfer rate but also help to overcome this barrier and thus enhance the collision frequency; (c) the landing of NP on the surface of UME can be suppressed by stronger convections, because NP will be swept away by hydrodynamic force. PMID:26802274
Natural convection in water along a vertical plate with constant surface heat flux
International Nuclear Information System (INIS)
An experimental investigation of natural convection in water along a vertical plate with constant surface heat flux is described. The results show the effects of the variation of the physical properties on heat transfert especially in the transition regime. In this regime, all the exprimental datas are correlated by a single law which includes the variation of viscosity and the critical Rayleigh number
Heat output to the medium of supercritical parameters with mixed convection in horizontal tubes
Energy Technology Data Exchange (ETDEWEB)
Robakidze, L.V.; Khasanov-Agayev, L.R.; Miropolskiy, Z.L.
1983-01-01
Results are presented of experimental study of heat emission in a horizontal tube with supply of heat (1 approx. = 4,000 mm, d = 16 mm) with flow of carbon dioxide of supercritical pressure under conditions of mixed convection in the region of significant influence of the thermal-gravitation effects.
Effects of Hall current on convective heat generating fluid in slip flow regime
Energy Technology Data Exchange (ETDEWEB)
Singh, S.S.; Ram, P.C. (Kenyatta Univ., Nairobi (KE). Dept. of Mathematics); Stower, G.X. (Jomo Kenyatta Univ. College of Agriculture and Technology, Nairobi (KE). Dept. of Mathematics and Computer Science)
1992-08-01
The problem of free convection flow of a viscous heat generating rarefied gas is considered for the case when a strong magnetic field is imposed perpendicularly to the plane of flow. Analytical expressions for the velocity field and temperature are obtained, and the influence of the Hall currents m and the heat source parameter {delta} on the velocity field and temperature are discussed. (Author).
Experimental study of cooling BIPV modules by forced convection in the air channel
International Nuclear Information System (INIS)
Highlights: • An experimental setup for studying the effects of forced convection on cell temperature. • The induced velocity within the forced convection channel significantly affects the PV cooling. • Correlations for the Ross coefficient, module temperature, efficiency, and power output. • Prediction of the thermal behavior of the PV module in BIPV configurations. - Abstract: The efficiency of photovoltaic systems depends mainly on the cell temperature. Frequently, the PV collectors are installed on the top of the building. One cost effective method to regulate the temperature of rooftop integrated photovoltaic panels is to provide an open air channel beneath the panel. The cell temperature of these PV modules is very much influenced by the capability of ventilating this channel. The ventilation may be modified by different factors such as the wind velocity, the air gap size, and the forced convection induced by a fan or by a conventional air conditioning system. This paper describes an experimental setup to study the influence of the air gap size and the forced ventilation on the cell temperature (and consequently on the electrical efficiency of the PV module) of a BIPV configuration, for different values of the incident solar radiation, ambient temperatures, and aspect ratios, as well as for several forced ventilation conditions. Semi empirical correlations for the Ross coefficient, module temperature, electrical efficiency, and power output are proposed, showing a good agreement with respect to experimental measurements. A critical channel aspect ratio close to 0.11 can be considered to minimize overheating of PV devices. For a duct velocity Vv = 6 m/s, a power output increase of 19% is observed over the natural ventilation case (Vv = 0.5 m/s)
Convective transport due to poloidal electric fields during electron cyclotron heating in IMS
International Nuclear Information System (INIS)
Steady state hollow density profiles, observed during electron cyclotron resonant heating in the Interchangeable Module Stellarator (IMS), are shown to be consistent with a transport model that includes convection in the particle balance equation. The factor of two difference in the confinement time between a hollow profile and a fairly flat profile is due to particle convection and is in good agreement with that calculated from the equilibrium profiles. It is observed that the poloidal electric fields are greater for the hollow profile than for the less hollow case, indicating that they are most likely the cause of the convection. (author). Letter-to-the-editor. 13 refs, 3 figs
Scientific Electronic Library Online (English)
I. K., Adegun; F. L., Bello-Ochende.
2004-09-01
Full Text Available A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated i [...] ndependently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary condition of uniform wall temperature in the flow direction is considered. A special discritization method is employed to solve the problem associated with near boundary grid points. Results for mean and total mean Nusselt numbers for various values of Reynolds number ,Re; Rayleigh number , Ra ; Geometric ratio ,r g ; Aspect ratio, rA ; Radiation-Conduction parameter , . ; Optical thickness, J ; Rotational Reynolds number ,Ro and Emissivity, epsilon ; are presented. For the range of parameters considered, results show that radiation and rotation enhance heat transfer. It is also indicated in the results that heat transfer from the surface of the circle exceeds that of the rectangle. Optimum heat transfer and fluid bulk temperature are attained when the duct is vertically positioned. The Parameter ranges of 0.2 # r g # 0.84, 0 # PeRa # 7.3 x 10(5) and r g rA # 1 demarcate the extent of the validity of the numerical solution.
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Woloszyn, Monika; Pompeo, C.
2013-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in...
J. M. Jawdat; Hashim, I.; Momani, S.
2012-01-01
The effect of nanofluids on chaotic convection in a fluid layer heated from below was studied in this paper for low Prandtl number based on the theory of dynamical systems. A low-dimensional, Lorenz-like model was obtained using Galerkin-truncated approximations. The fourth-order Runge-Kutta method was employed to solve the nonlinear system. The results show that inhibition of chaotic convection can be observed when using nanofluids.
Lauret, Philippe; Miranville, Frédéric; Boyer, Harry; Garde, Francois; Adelard, Laetitia
2006-01-01
This paper deals with the application of Bayesian methods to the estimation of two convective heat transfer coefficients of a roof-mounted radiant barrier system (RBS). As part of an empirical validation of the thermal model of the roofing complex, a parametric sensitivity analysis highlighted the importance of convective coefficients in the thermal behavior of a roofing complex. A parameter estimation method is then used in order to find the values of the coefficients that lead to an improve...
Specialists' meeting on evaluation of decay heat removal by natural convection
International Nuclear Information System (INIS)
Decay heat removal by natural convection (DHRNC) is essential to enhancing the safety of liquid metal fast reactors (LMFRs). Various design concepts related to DHRNC have been proposed and experimental and analytical studies have been carried out in a number of countries. The purpose of this Specialists' Meeting on 'Decay Heat Removal by Natural Convection' organized by the International Working Group on Fast Reactors IAEA, is to exchange information about the state of the art related to methodologies on evaluation of DHRNC features (experimental studies and code developments) and to discuss problems which need to be solved in order to evaluate DHRNC properly and reasonably. The following main topical areas were discussed by delegates: Overview; Experimental studies and code validation; Design study. Two main DHR systems for LMFR are under consideration: (i) direct reactor auxiliary cooling system (DRACS) with immersed DFIX in main vessel, intermediate sodium loop and sodium-air heat exchanger; and (ii) auxiliary cooling system which removes heat from the outside surface of the reactor vessel by natural convection of air (RVACS). The practicality and economic viability of the use of RVACS is possible up to a modular type reactor or a middle size reactor based on current technology. For the large monolithic plant concepts DRACS is preferable. The existing experimental results and the codes show encouraging results so that the decay heat removal by pure natural convection is feasible. Concerning the objective, 'passive safety', the DHR by pure natural convection is essential feature to enhance the reliability of DHR
International Nuclear Information System (INIS)
Coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas, which changes drastically the heat and mass transfer across the liquid layer. Two experiments are considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow looking like petals or rays appears when the aspect ratio length/depth is small, and like concentric rings in the case of large values of the aspect ratio. The lateral confinement selects the azimuthal length wave. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be 'weak', even for the largest values of the Marangoni number (Ma â‰… 1.3 * 105). In the case of mercury, the thermo-capillary effect is reduced to zero, due to impurities at the surface, which have special trajectories we describe and compare to a simpler experiment. The only buoyancy forces induces an un-stationary, weakly turbulent flow as soon as the heating power exceeds 4 W (â‰… 4.5 * 103, calculated with h = 1 mm). The last part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number, the buoyancy force, with the help of the literature. Results concerning heat transfer, especially the exponent of the law Nusselt number vs. heating power, are compared with available data. (author)
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1). PMID:25750751
Solar drying of whole mint plant under natural and forced convection
Directory of Open Access Journals (Sweden)
Y.I. Sallam
2015-03-01
Full Text Available Two identical prototype solar dryers (direct and indirect having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s?1. The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day. The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10?11 and 1.33 × 10?11 m2 s?1.
Evaluation of T-111 forced-convection loop tested with lithium at 13700C
International Nuclear Information System (INIS)
A T-111 alloy (Ta--8 percent W--2 percent Hf) forced-convection loop containing molten lithium was operated 3000 h at a maximum temperature of 13700C. Flow velocities up to 6.3 m/s were used. The results obtained in this forced-convection loop are very similar to those observed in lower velocity thermal-convection loops of T-111 containing lithium. Weight changes were determined at 93 positions around the loop. The maximum dissolution rate occurred at the maximum wall temperature of the loop and was less than 1.3 ? m/year. Mass transfer of hafnium, nitrogen, and, to a lesser extent, carbon occurred from the hotter to cooler regions. Exposed surfaces in the highest temperature region were found to be depleted in hafnium to a depth of 60 ? m with no detectable change in tungsten content. There was some loss in room-temperature tensile strength for specimens exposed to lithium at 13700C, attributable to depletion of hafnium and nitrogen and to attendant grain growth. (U.S.)
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients in an electrically heated 2*6 rod bundle cooled by water. The objective of the study was to develop an understanding of the thermal-hydraulic phenomena at low flows in a rod bundle subjected to lateral power skews and to develop a transient data base for evaluating themal-hydraulic computer programs. Test conditions included 150 second and 45 second flow coastdown transients, a Reynolds number range of 300 to 1300, and power skews of 1:0 and 2:1. A model incorporating the enhanced turbulent mixing due to thermal plume interactions was used to improve the COBRA-WC code predictions at large ratios of Gr/Re/sup 2/. A preliminary criterion was developed for the inception of recirculation within the bundle. 10 refs
Directory of Open Access Journals (Sweden)
Moh'd A. Al-Nimr
2004-06-01
Full Text Available Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha, Eckert number (Ec, Prandtl number (Pr, Joule heating parameter (R and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž on the entropy generation characteristics is analyzed. The dimensionless governing equations in Cartesian coordinate were solved by an implicit finite difference technique. The solutions were carried out for Ha2=0.5-3, Ec=0.01-0.05, Pr=1-5 and ÃŽÂ¸Ã¢ÂˆÂž=1.1-2.5. It was found that, the entropy generation increased with increasing Ha, Ec and R. While, increasing the free stream temperature parameter, and Prandtl number tend to decrease the local entropy generation.
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Scientific Electronic Library Online (English)
M., Bernardi; V., Silveira Jr.; V. R. N., Telis; A. L., Gabas; J., Telis-Romero.
2009-06-01
Full Text Available The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating secti [...] ons with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Directory of Open Access Journals (Sweden)
M. Bernardi
2009-06-01
Full Text Available The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating sections with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.
On the Interaction of Internal Gravity Waves with Magnetic Field II. Convective Forcing
Rogers, T M
2010-01-01
We present results from numerical simulations of the interaction of internal gravity waves (IGW) with magnetic fields in the radiative interior of the Sun. In this second paper, the waves are forced self-consistently by an overlying convection zone and a toroidal magnetic field is imposed in the stably stratified layer just underneath convection zone. Consistent with the results of previous analytic and simple numerical calculations, we find a strong wave-field interaction, in which waves are reflected in the field region. The wave-field interaction and wave reflection depend on the field strength as well as adopted values of the diffusivities. In some cases wave reflection leads to an increased mean flow in the field region. In addition to reproducing some of the features of our simpler models, we find additional complex behaviour in these more complete and realistic calculations.
Indian Academy of Sciences (India)
Ke Wu; Le Wang; Yi-Bo Yu; Zhi-Yi Huang; Pei Liang
2013-12-01
Heat dissipation enhancement of LED luminaries is of great significance to the large-scale application of LED. Luminaries-level structure improvement by the method of boring through-hole is adopted to intensify heat dissipation. Furthermore, the natural convection heat transfer process of LED luminaries is simulated by computational fluid dynamics (CFD) model before and after the structural modification. As shown by computational results, boring through-hole is beneficial to develop bottomto-top natural convection, eliminate local circumfluence, and finally form better flow pattern. Analysis based on field synergy principle shows that boring through-hole across LED luminaries improves the synergy between flow field and temperature field, and effectively decreases the thermal resistance of luminaries-level heat dissipation structure. Under the same computational conditions, by luminaries-level structure improvement the highest temperature of heat sink is decreased by about 8Â° C and the average heat transfer coefficient is increased by 45.8%.
Energy Technology Data Exchange (ETDEWEB)
Bose, Sumantra; Palo, Daniel R.; Paul, Brian
2007-07-24
Diffusion bonding cycle times can be a large cost factor in the production of metal microchannel devices. The challenge is to significantly minimize this cost by reducing the bonding cycle time through rapid and uniform heating and cooling within the bonding process. Heating rates in diffusion bonding processes are typically limited by the need to minimize thermal gradients during bonding. A novel method is described which takes advantage of the internal flow passages within microchannel devices for convective heat transfer during the bonding process. The internal convective heating (ICH) technique makes use of heated inert gas to provide the microchannel assembly with rapid and uniform heat input. This paper will demonstrate the ability to effectively diffusion bond microchannel laminae using the ICH method by investigating the leakage rates.
Numerical studies of convective heat transfer in an inclined semiannular enclosure
Wang, Lin-Wen; Yung, Chain-Nan; Chai, An-Ti; Rashidnia, Nasser
1989-01-01
Natural convection heat transfer in a two-dimensional differentially heated semiannular enclosure is studied. The enclosure is isothermally heated and cooled at the inner and outer walls, respectively. A commercial software based on the SIMPLER algorithm was used to simulate the velocity and temperature profiles. Various parameters that affect the momentum and heat transfer processes were examined. These parameters include the Rayleigh number, Prandtl number, radius ratio, and the angle of inclination. A flow regime extending from conduction-dominated to convection-dominated flow was examined. The computed results of heat transfer are presented as a function of flow parameter and geometric factors. It is found that the heat transfer rate attains a minimum when the enclosure is tilted about +50 deg with respect to the gravitational direction.
Numerical studies of convective heat transfer in an inclined semi-annular enclosure
Wang, L.-W.; Chai, A.-T.; Yung, C.-N.; Rashidnia, N.
1989-01-01
Natural convection heat transfer in a two-dimensional differentially heated semiannular enclosure is studied. The enclosure is isothermally heated and cooled at the inner and outer walls, respectively. A commercial software based on the SIMPLER algorithm was used to simulate the velocity and temperature profiles. Various parameters that affect the momentum and heat transfer processes were examined. These parameters include the Rayleigh number, Prandtl number, radius ratio, and the angle of inclination. A flow regime extending from conduction-dominated to convection-dominated flow was examined. The computed results of heat transfer are presented as a function of flow parameter and geometric factors. It is found that the heat transfer rate attains a minimum when the enclosure is tilted about +50 deg with respect to the gravitational direction.
Enhancement of Natural Convection Heat Transfer from Perforated Fin
Mr. Saurabh ,D. Bahadure , Mr. G. D. Gosavi
2014-01-01
A comprehensive theoretical and experimental study was carried out on the thermal performance of a pin fin heat sink. An experimental model was shows that have the capability of predicting influence of effective surface area of pin fin on thermal heat transfer coefficient. Pin fin array are used in many applications to enhance heat transfer and also shows enhancement of heat transfer coefficient for different material of fin. Several different type of experiment te...
El-Amin, Mohamed
2013-01-01
In this paper, the effects of thermal dispersion and variable viscosity on the non-Darcy free, mixed, and forced convection heat transfer along a vertical flat plate embedded in a fluid-saturated porous medium are investigated. Forchheimer extension is employed in the flow equation to express the non-Darcy model. The fluid viscosity varies as an inverse linear function of temperature. The coefficient of thermal diffusivity has been assumed to be the sum of the molecular diffusivity and the dynamic diffusivity due to mechanical dispersion. Similarity solutions of the governing equations, for an isothermally heated plate, are obtained. Effects of the physical parameters, which govern the problem, on the rate of heat transfer in terms of Nusselt number, the slip velocity, and the boundary layer thickness, for the two cases Darcy and non-Darcy, are shown on graphs or entered in tables. Â© 2013 by Begell House, Inc.
International Nuclear Information System (INIS)
Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma
Directory of Open Access Journals (Sweden)
Shestakov Igor A.
2015-01-01
Full Text Available The article shows the results of mathematical modeling of convective heat transfer in the low-temperature storage of liquefied natural gas. Regime of natural convection in an enclosure with different intensity of the heat flux at the external borders are investigated. Was examined two-dimensional nonstationary problem within the model of Navier-Stokes in dimensionless variables “vorticity - stream function - temperature”. Distributions of hydrodynamic parameters and temperatures that characterize the basic regularities of the processes are obtained. Circulating flows are determined and carried out the analysis of vortices formation mechanism and the temperature distribution in solution at conditions of natural convection when the Grashof number (Gr = 106. A significant influence of heat transfer rate on solutions boundary on flow structure and temperature field in LNG storage tanks.
Revisiting the Bulk Relation for Heat Flux in the Free Convection Limit
Kitamura, Yuji; Ito, Junshi
2015-08-01
We modify the velocity applied to the bulk relation for surface heat flux using turbulent kinetic energy, such that the effect of horizontal flow induced by unresolved free convection is incorporated. Numerical experiments with a large-eddy simulation (LES) and a single-column model (SCM) are examined for an ideal convective boundary layer. The surface fluxes obtained from both models are compared to investigate the effect of the velocity correction. It is confirmed that the surface heat flux calculated with the velocity correction is relatively consistent between the LES and SCM, even for a free convection case. Furthermore, the proposed method provides an evaluation of the surface heat flux that is insensitive to the model resolution, unlike the conventional method.
Local Convective Boiling Heat Transfer And Pressure Drop Of Nanofluid In Narrow Rectangular Channels
Boudouh, Mounir; Gualous, Hasna Louahlia; De Labachelerie, Michel
2010-01-01
Abstract This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannels. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 ?m. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper-water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using...
A new glass interim storage with heat removal by natural convection
International Nuclear Information System (INIS)
In order to meet COGEMA's strong operating and safety requirements, SGN proposed simple innovative solutions for the design of a new high level vitrified waste interim storage facility. The design is based on the use of steel jackets placed around vertical storage wells in vaults where the heat is removed by natural convection. Heat transfer in the annular space created by the steel jackets is enhanced. Efficient heat removal allows for a compact and cost-effective design. (author)
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per
2013-01-01
Night-time ventilation is a promising approach to reduce the energy needed for cooling buildings without reducing thermal comfort. Nevertheless actual building simulation tools have showed their limits in predicting accurately the efficiency of night-time ventilation, mainly due to inappropriate models for convection. In a full-scale test room, the heat transfer was investigated during 12 h of discharge by night-time ventilation. A total of 34 experiments have been performed, with different ventilation types (mixing and displacement), air change rates, temperature differences between the inlet air and the room, and floor emissivities. This extensive experimental study enabled a detailed analysis of the convective and radiative flow at the different surfaces of the room. The experimentally derived convective heat transfer coefficients (CHTC) have been compared to existing correlations. For mixing ventilation, existing correlations did not predict accurately the convective heat transfer at the ceiling due to differences in the experimental conditions. But the use of local parameters of the air flow showed interesting results to obtain more adaptive CHTC correlations. For displacement ventilation, the convective heat transfer was well predicted by existing correlations. Nevertheless the change of floor emissivity influenced the CHTC at the surface of interest.
Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer
Energy Technology Data Exchange (ETDEWEB)
Akpinar, E. Kavak [Mechanical Engineering Department, Firat University, 23279 Elazig (Turkey)]. E-mail: eakpinar@firat.edu.tr
2005-09-15
In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 deg. C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 x 12.5 x 25 mm) and cylindrical shaped pumpkin slices (35 x 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m{sup 2} C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.
Planform structure and heat transfer in turbulent free convection over horizontal surfaces
Theerthan, S. Ananda; Arakeri, Jaywant H.
2000-04-01
This paper deals with turbulent free convection in a horizontal fluid layer above a heated surface. Experiments have been carried out on a heated surface to obtain and analyze the planform structure and the heat transfer under different conditions. Water is the working fluid and the range of flux Rayleigh numbers (Ra) covered is 3×107-2×1010. The different conditions correspond to Rayleigh-Bénard convection, convection with either the top water surface open to atmosphere or covered with an insulating plate, and with an imposed external flow on the heated boundary. Without the external flow the planform is one of randomly oriented line plumes. At large Rayleigh number Ra and small aspect ratio (AR), these line plumes seem to align along the diagonal, presumably due to a large scale flow. The side views show inclined dyelines, again indicating a large scale flow. When the external flow is imposed, the line plumes clearly align in the direction of external flow. The nondimensional average plume spacing, Ra?1/3, varies between 40 and 90. The heat transfer rate, for all the experiments conducted, represented as Ra?T-1/3, where ?T is the conduction layer thickness, varies only between 0.1-0.2, showing that in turbulent convection the heat transfer rates are similar under the different conditions.
Roberts, J. H.; Nimmo, F.
2007-12-01
Rapid strike-slip motion is predicted to be a consequence of diurnal tidal stresses in most satellites of the outer solar system with short orbital timescales [1]. Such motion can lead to near-surface heating through friction or viscous dissipation [2]. Here we discuss the effect of near-surface shear heating on convection in the underlying ice shells of icy satellites [3], with a focus on Enceladus and a possible origin of the south polar thermal anomaly [4]. We present models of convection in spherical ice shells including both spatially variable volumetric tidal heating [5] and regional shear heating localized in the top 5 km at either the pole or the equator. We observe that the presence of the near-surface heating strongly controls the convective pattern, increasing the wavelength, and promoting the formation of a hot upwelling beneath the shear zone. Our results suggest that localized near- surface heating may result in a degree-1 convective planform in an ice shell of a thickness that may be appropriate for a differentiated Enceladus (d Tobie, G., A. Mocquet, C. Sotin, Icarus, 177 534-549. [6] Nimmo, F., R. T. Pappalardo, Nature, 441, 614-616.
Energy Technology Data Exchange (ETDEWEB)
Kim, Myoung Jun; Lee, Hee Joon [Kookmin Univ., Seoul (Korea, Republic of); Moon, Joo Hyung; Bae, Youngmin; Kim, Youngin [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2014-05-15
For the long operation of secondary passive cooling system, however, water level goes down by evaporation in succession at emergency cooling tank. At the end there would be no place to dissipate heat from condensation heat exchanger. Therefore, steam cooling heat exchanger is put on the top of emergency cooling tank to maintain appropriate water level by collecting evaporating steam. Steam cooling heat exchanger is installed inside an air chimney and evaporated steam is cooled down by air natural convection. In this study, thermal sizing of steam cooling heat exchanger under air natural convection was conducted by TSCON program for the design of experimental setup as shown in Fig. 2. Thermal sizing of steam cooling heat exchanger tube under air natural convection was conducted by TSCON program for the design of experimental setup. 25 - 1' tubes which has a length 1687 mm was determined as steam cooling heat exchanger at 2 kW heat load and 100 liter water pool in emergency cooling tank (experimental limit condition). The corresponding width of two tubes is 50 mm and has 5 by 5 tube array for heat exchanger.
International Nuclear Information System (INIS)
For the long operation of secondary passive cooling system, however, water level goes down by evaporation in succession at emergency cooling tank. At the end there would be no place to dissipate heat from condensation heat exchanger. Therefore, steam cooling heat exchanger is put on the top of emergency cooling tank to maintain appropriate water level by collecting evaporating steam. Steam cooling heat exchanger is installed inside an air chimney and evaporated steam is cooled down by air natural convection. In this study, thermal sizing of steam cooling heat exchanger under air natural convection was conducted by TSCON program for the design of experimental setup as shown in Fig. 2. Thermal sizing of steam cooling heat exchanger tube under air natural convection was conducted by TSCON program for the design of experimental setup. 25 - 1' tubes which has a length 1687 mm was determined as steam cooling heat exchanger at 2 kW heat load and 100 liter water pool in emergency cooling tank (experimental limit condition). The corresponding width of two tubes is 50 mm and has 5 by 5 tube array for heat exchanger
International Nuclear Information System (INIS)
A boundary condition dissection method is developed for the solution of boundary value heat conduction problems with position-varying convective coefficients imposed at the boundaries. The method is postulated on the fact that a condition imposed on the boundary of a heat conduction problem may be realized in practice by using conditions that are not of the imposed kind. Thus, a Robin condition with position varying convective coefficient can be dissected as a linear combination of boundary heat flux and temperature, and either of these conditions can be used in place of the Robin condition in formulating an auxiliary problem. This auxiliary problem can be solved for temperature, and finally this temperature is forced to satisfy the actual Robin conditions imposed at the boundaries. Three basic problems are solved in this paper, and by means of two examples the boundary condition dissection method is shown to yield temperature results that are more accurate than those by the finite difference method, which has been conventionally used in solving the heat conduction problems. (orig.)
Numerical Simulation of Convective Heat and Mass Transfer in a Two-Layer System
Myznikova, B. I.; Kazaryan, V. A.; Tarunin, E. L.; Wertgeim, I. I.
The results are presented of mathematical and computer modeling of natural convection in the “liquid-gas” two-layer system, filling a vertical cylinder surrounded by solid heat conductive tract. The model describes approximately the conjugate heat and mass transfer in the underground oil product storage, filled partially by a hydrocarbon liquid, with natural gas layer above the liquid surface. The geothermal gradient in a rock mass gives rise to the intensive convection in the liquid-gas system. The consideration is worked out for laminar flows, laminar-turbulent transitional regimes, and developed turbulent flows.
Bounds on heat transport in Bénard-Marangoni convection
Hagstrom, George; Doering, Charles R.
2010-04-01
For Pearson’s model of Bénard-Marangoni convection, the Nusselt number Nu is proven to be bounded as a function Marangoni number Ma according to Nu?0.838×Ma2/7 for infinite Prandtl number and according to Nu?Ma1/2 uniformly for finite Prandtl number. The analysis is also used to raise the lower bound for the critical Marangoni number for energy stability of the conduction solution from 56.77 to 58.36 when the Prandtl number is infinite.
Chakraborty, Swastika; Saha, Upal; Maitra, Animesh
2015-07-01
The response of atmospheric heat fluxes and sea surface temperatures on the convective precipitation over the tropics has been an important area of research in recent decades. A long-term observation (1979-2008) of the increase in convective precipitation in relation to the latent and sensible heat fluxes on a tropical location, Kolkata, has been investigated in the present study. Invigoration of convective precipitation has been caused by vertically integrated divergence of moisture flux, rise in sea surface temperatures, convective cloud cover and surface evaporation rate over the tropical region. A convective precipitation estimation (CPE) index is proposed, considering the Bowen ratio, surface evaporation rate, sea surface temperature and temperatures at 500 hpa pressure level during the pre-monsoon season (March-May), to estimate the amount of convective precipitation over the tropics using multiple linear regression technique is also another aim of this study. A good agreement is obtained between the results from the proposed model and the MERRA observations during the years 2009-2013.
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Natural convection heat transfer phenomena on a horizontal cylinder have been studied experimentally in order to investigate the applicability of analogy experimental methodology using a copper electroplating system and to visualize the local heat transfer rates depending on the angular position and the diameter of the horizontal cylinder. In the copper electroplating system, the copper ion produced at the anode moves by convection and diffusion to the cathode and reduces at the cathode, representing the heat transfer. By using aluminum cathode with a distinguishable color, the amount of copper plated could visualize the amount of heat transferred depending on the angular position of the cylinder. The diameter of the cylinder is varied from 0.01m to 0.15m, which correspond to Rayleigh numbers in the range of 1.73x107 to 5.69x1011. The test results are in good agreement with existing heat transfer correlations
AN INTERNAL CONVECTIVE HEATING TECHNIQUE FOR DIFFUSION BONDING ARRAYED MICROCHANNEL ARCHITECTURES
Energy Technology Data Exchange (ETDEWEB)
Paul, Brian; Bose, Sumantra; Palo, Daniel R.
2010-01-08
Diffusion bonding cycle times can be a large factor in the production cost of metal microchannel devices. The challenge is to significantly minimize bonding cycle times through rapid heating and cooling within the bonding process. A novel method is described which takes advantage of the internal flow passages within microchannel devices for convective heat transfer during the bonding process. The internal convective heating (ICH) technique makes use of heated inert gas to provide the microchannel assembly with rapid and uniform heat input. Results demonstrate that the ICH technique is feasible, capable of producing microchannels with higher dimensional integrity and shorter bonding cycle times than traditional vacuum hot press methods. Results suggest that this may be due to smaller thermal gradients within microchannel devices during the ICH bonding cycle.
HEAT TRANSFER ENHANCEMENT IN PLATES BY NATURAL CONVECTION WITH AND WITHOUT VERTICAL CONFINING WALLS
Directory of Open Access Journals (Sweden)
Mahendra P Nimkar*
2015-01-01
Full Text Available Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with and without vertical confining walls. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of horizontal plate with respect to vertical plate and gap of horizontal plate from bottom to top. The present study aims to determine the heat transfer characteristics, along the plate for the selection of optimum dimension for design purpose. Further, the influences of aspect ratio on the performance characteristics of heat transfer will be studied.
Determining convective heat transfer coefficient using phoenics software package
Energy Technology Data Exchange (ETDEWEB)
Kostikov, A.; Matsevity, Y. [Institute of Mechanical Engineering Problems of National Academy of Sciences of Ukraine, Kharkov (Ukraine)
1997-12-31
The two methods of determination of such important quantity of heat exchange on a body surface using PHOENICS are suggested in the presentation. The first method consists in a post-processing of results of conjugate heat transfer problem solved by PHOENICS. The second one is solving an inverse heat conduction problem for solid body using PHOENICS. Comparative characteristic of these two methods is represented. (author) 4 refs.
International Nuclear Information System (INIS)
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*105 and 5.4*105. A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends