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New prediction methods for CO2 evaporation inside tubes: Part II - An updated general flow boiling heat transfer model based on flow patterns

CHENG, Lixin; RIBATSKI, Gherhardt; THOME, John R.
Fonte: PERGAMON-ELSEVIER SCIENCE LTD Publicador: PERGAMON-ELSEVIER SCIENCE LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.36%
Corresponding to the updated flow pattern map presented in Part I of this study, an updated general flow pattern based flow boiling heat transfer model was developed for CO2 using the Cheng-Ribatski-Wojtan-Thome [L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes, Int. J. Heat Mass Transfer 49 (2006) 4082-4094; L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, Erratum to: ""New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside tubes"" [Heat Mass Transfer 49 (21-22) (2006) 4082-4094], Int. J. Heat Mass Transfer 50 (2007) 391] flow boiling heat transfer model as the starting basis. The flow boiling heat transfer correlation in the dryout region was updated. In addition, a new mist flow heat transfer correlation for CO2 was developed based on the CO2 data and a heat transfer method for bubbly flow was proposed for completeness sake. The updated general flow boiling heat transfer model for CO2 covers all flow regimes and is applicable to a wider range of conditions for horizontal tubes: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m(2) s, heat fluxes from 1.8 to 46 kW/m(2) and saturation temperatures from -28 to 25 degrees C (reduced pressures from 0.21 to 0.87). The updated general flow boiling heat transfer model was compared to a new experimental database which contains 1124 data points (790 more than that in the previous model [Cheng et al....

Estudo teórico-experimental da transferência de calor e da perda de pressão em um dissipador de calor baseado em microcanais; A theoretical and experimental study on heat transfer and pressure drop in a heat sink based on microchannels

Nascimento, Francisco Júlio do
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 28/05/2012 PT
Relevância na Pesquisa
66.3%
A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico em um dissipador de calor baseado em microcanais. Este tipo de dissipador de calor tem sido usado para a intensificação da troca de calor em sistemas compactos e de alto desempenho. A intensificação da troca de calor promovida pelo escoamento em microcanais é acompanhada de um incremento na perda de pressão, portanto o estudo destes dois parâmetros é essencial para o entendimento dos fenômenos relacionados e fundamental para o desenvolvimento de ferramentas de projeto para dissipadores de calor baseados em microcanais. Inicialmente, um levantamento bibliográfico extenso sobre a ebulição convectiva em microcanais de reduzido diâmetro foi realizado. Este estudo da literatura trata de critérios de transição entre micro- e macro-escala, padrões de escoamento, métodos de previsão do coeficiente de transferência de calor e perda de pressão. Atenção específica foi dada a estudos de dissipadores de calor baseados em microcanais. Com base nesta análise da literatura, uma bancada experimental foi confeccionada para que dados experimentais de transferência de calor e perda de pressão pudessem ser levantados a partir de um dissipador de calor de microcanais. O dissipador de calor fabricado para este estudo é constituído de 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento...

Transferencia de calor em um regenerador rotativo com perda de carga estabelecida nos dutos da matriz; Heat transfer in a rotary regenerator with fixed pressure drop

Paulo Cesar Mioralli
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 18/02/2009 PT
Relevância na Pesquisa
66.3%
O processo de transferência de calor em um regenerador rotativo foi analisado a partir de valores estabelecidos para a perda de carga nos canais da matriz. A troca térmica no equipamento foi maximizada através da obtenção de um valor ótimo de porosidade para esta perda de carga fixada. Os dutos da matriz foram admitidos triangulares eqüiláteros. Um programa computacional foi confeccionado para a simulação do regenerador, no qual a velocidade dos fluidos e o coeficiente de transferência de calor foram obtidos através de correlações levando em consideração a região de entrada dos dutos. Estes resultados foram utilizados para a obtenção do calor transferido no regenerador através de uma resolução numérica. A influência do bloqueio parcial do escoamento nos canais da matriz, causado por parte da estrutura do equipamento, e os efeitos dos vazamentos sobre o calor transferido, devido às folgas de trabalho do regenerador, também foram investigados. Uma análise de escala foi efetuada e expressões para a estimativa da porosidade ótima e do calor máximo transferido no equipamento foram obtidas. Baseado nesta análise, parâmetros adimensionais foram introduzidos e foi verificado que a análise de escala é capaz de prever a ordem de grandeza da porosidade ótima e o correspondente calor máximo transferido. Por fim...

Estudo experimental da transferencia de calor no escoamento bifasico intermitente horizontal; Experimental study of the heat transfer in the intermittent horizontal two-phase flow pattern

Ivan Noville Rocha Correa Lima
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 31/07/2009 PT
Relevância na Pesquisa
66.31%
O presente trabalho tem por objetivo central a medição experimental do coeficiente de transferência de calor bifásico no padrão gás-líquido horizontal intermitente, por ser este um padrão muito comum nas operações de produção de petróleo. Para alcançar esse objetivo, foi construído um aparato experimental consistindo de uma seção de testes alimentada por uma mistura pré-aquecida de ar e água, a diferentes pares de vazões e temperaturas de entrada. Para realizar a troca térmica, o tubo foi circundado por uma jaqueta de água gelada submetida a diversas vazões e temperaturas de entrada, formando, com o tubo central, uma seção de troca térmica de correntes paralelas. A metodologia experimental consistiu em medir a taxa de transferência de calor do fluido interno para o fluido externo e, a partir da diferença média de temperatura entre o fluido quente e a parede, determinar o coeficiente de transferência de calor entre o fluido quente e a parede, para cada elenco de condições de operação. O procedimento foi realizado tanto para os ensaios em escoamento monofásico como para o escoamento bifásico. Os resultados para o coeficiente de transferência de calor do fluido quente foram comparados às correlações e aos modelos existentes e apresentaram resultados satisfatórios; The main objective of the present work is the experimental measurement of the two-phase heat transfer coefficient in the intermittent horizontal gas-liquid flow pattern. This flow pattern is very common in oil production pipelines. In order to reach this objective...

Turbulent heat transfer and pressure drop in pinned annular regions

Nieckele,Angela Ourivio; Saboya,Francisco Eduardo Mourão
Fonte: The Brazilian Society of Mechanical Sciences Publicador: The Brazilian Society of Mechanical Sciences
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2000 EN
Relevância na Pesquisa
66.31%
Experiments were performed to determine average heat transfer coefficients and friction factors for turbulent flow through annular ducts with pin fins. The measurements were carried out by means of a double-pipe heat exchanger. The total number of pins attached to the inner wall of the annular region was 560. The working fluids were air, flowing in the annular channel, and water through the inner circular tube. The average heat transfer coefficients of the pinned air-side were obtained from the experimental determination of the overall heat transfer coefficients of the heat exchanger and from the knowledge of the average heat transfer coefficients of the circular pipe (water-side), which could be found in the pertinent literature. To attain fully developed conditions, the heat exchanger was built with additional lengths before and after the test section. The inner circular duct of the heat exchanger and the pin fins were made of brass. Due to the high thermal conductivity of the brass, the small tube thickness and water temperature variation, the surface of the internal tube was practically isothermal. The external tube was made of an industrial plastic which was insulated from the environment by means of a glass wool batt. In this manner...

Non-uniform Heat Transfer During Air-Blast Freezing of a Fruit Pulp Model in Multilayer Boxes

Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
66.3%
Effective heat transfer coefficients were measured using an aluminum test body and compared with the results obtained from a Gnielinski correlation for air-blast freezing of a fruit pulp model in multilayer boxes, with the internal airflow through rectangular ducts and the hydraulic diameter as characteristic dimensions. The quantities of products inside the boxes were varied, and the inlet air velocities and temperature profiles during freezing were measured. The inlet air velocities were applied in dimensionless Gnielinski correlations to estimate the local heat transfer coefficient values. The experimental and predicted heat transfer coefficient values were used to determine an average convective heat transfer coefficient weighted by the heat transfer area. The results from this methodology were used in an analytically derived procedure for freezing-time estimates and then compared with experimental results. The average effective heat transfer coefficient underestimated freezing times and demonstrated a higher level of accuracy than the Gnielinski correlation when applied to boxes containing smaller product amounts. For experiments with greater quantity of products, the use of average heat transfer coefficients from the Gnielinski correlation yielded errors lower than 20%. Based on boundary layer theory...

On the heat transfer characteristics of liquid-gas taylor flows in minichannels

Howard, James A
Fonte: University of Limerick Publicador: University of Limerick
Tipo: info:eu-repo/semantics/doctoralThesis; all_ul_research; ul_published_reviewed; ul_theses_dissertations
ENG
Relevância na Pesquisa
66.31%
peer-reviewed; Heat transfer enhancement is of significant importance to many thermal management applications. Multi phase flows are an effective way to meet increasing thermal demands. This work investigates heat transfer enhancement potential associated with slug or Taylor flows within minichannels. The primary focus is upon understanding the mechanisms leading to enhanced heat transfer and the effects of using different liquid phases and varying flow parameters. High speed images were used to obtain mean bubble velocity, liquid film thickness and void fraction measurements within a 1/16" diameter channel. Pressure drop data was obtained using a differential pressure transducer while infrared thermography was used to capture high resolution experimental wall temperatures subjected to a constant heat flux boundary condition. Experiments were conducted for a range of liquid slug lengths and void fractions, ensuring results spanned both the thermal entrance and thermally developed flow regions. Flow dynamics, pressure drop and heat transfer characteristics were investigated using water and oils as the liquid phase while air was used as the segmenting phase throughout. An added novel aspect to this work is incorporating microencapsulated phase change material (MPCM) suspensions into conventional liquid-gas flows. Interfacial tension between the phases and hence...

Numerical Investigation on the Heat Transfer Enhancement Using Micro/Nano Phase-Change Particulate Flow

Xing, Keqiang
Fonte: FIU Digital Commons Publicador: FIU Digital Commons
Tipo: Artigo de Revista Científica Formato: application/pdf
Relevância na Pesquisa
66.31%
The introduction of phase change material fluid and nanofluid in micro-channel heat sink design can significantly increase the cooling capacity of the heat sink because of the unique features of these two kinds of fluids. To better assist the design of a high performance micro-channel heat sink using phase change fluid and nanofluid, the heat transfer enhancement mechanism behind the flow with such fluids must be completely understood. A detailed parametric study is conducted to further investigate the heat transfer enhancement of the phase change material particle suspension flow, by using the two-phase non-thermal-equilibrium model developed by Hao and Tao (2004). The parametric study is conducted under normal conditions with Reynolds numbers of Re=600-900 and phase change material particle concentrations ¡Ü0.25 , as well as extreme conditions of very low Reynolds numbers (Re < 50) and high phase change material particle concentration (0.5-0.7) slurry flow. By using the two newly-defined parameters, named effectiveness factor and performance index, respectively, it is found that there exists an optimal relation between the channel design parameters, particle volume fraction, Reynolds number, and the wall heat flux. The influence of the particle volume fraction...

Heat transfer and thermal storage in fixed and fluidized beds of phase change materials

Izquierdo Barrientos, Maria Asunción
Fonte: Universidade Carlos III de Madrid Publicador: Universidade Carlos III de Madrid
Tipo: Tese de Doutorado
ENG
Relevância na Pesquisa
66.34%
Thermal energy storage is a key technology for energy conservation since many energy sources are intermittent in nature. Latent heat storage is considered one of the most efficient ways of storing thermal energy because, unlike sensible heat storage, it provides a high-energy storage density with a small temperature swing. There are available many storage techniques, including sensible and latent heat storage or a combination of both. Fixed and fluidized beds may be feasible technologies when the storing materials may be encapsulated in cans, spheres or microencapsulated in highly porous structures with protecting envelopes. This PhD thesis deals with thermal storage and heat transfer in fixed and fluidized beds with phase change materials (PCMs). The behavior of a bed with granular PCM as a thermal storage system is studied. Charging and discharging experiments are carried out and models for the transient response of the bed are developed for fixed and fluidized bed configurations. Moreover, a model for the heat transfer coefficient between the bed of PCM and an immersed surface is presented and validated with experimental measurements. The experimental studies are conducted in a cylindrical bed filled with granular PCM and with air as the working fluid. The bed has an internal diameter of 200 mm. The granular PCMs used consist of paraffin...

Experimental heat transfer coefficients between a surface and fixed and fluidized beds with PCM

Izquierdo-Barrientos, María Asunción; Sobrino, Celia; Almendros-Ibáñez, José Antonio
Fonte: Elsevier Publicador: Elsevier
Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article
Publicado em /03/2015 ENG
Relevância na Pesquisa
66.3%
This work presents an experimental study to determine the capacity of a phase change material (PCM) in granular form to be used in fixed and bubbling fluidized beds for thermal energy storage. The experimental measurements are focused on determination of the heat transfer coefficient between a heated surface immersed in the bed and the granular PCM. The flow rate is varied to quantify its influence on the heat transfer coefficient. The PCM used is Rubitherm GR50 with a phase change temperature of approximately 50° C. The PCM is available in two different particle sizes, 0.54 mm and 1.64 mm, of which the finer is used in the fluidized bed and the coarser is used in the fixed bed. In addition, the results obtained for the PCM are compared with the heat transfer coefficients measured for sand, a material commonly used for thermal storage. In comparing the heat transfer coefficients for fixed and fluidized beds, the heat transfer coefficients in the fluidized bed with PCM are nearly three times higher than those for the fixed bed at the same gas flow rate. This increase in the heat transfer is a result of two main factors: first, the continuous renewal of PCM particles from the heated surface when they are fluidized...

Subcooled flow boiling heat transfer of ethanol aqueous solutions in vertical annulus space

Sarafraz, M.M.; Peyghambarzadeh, S.M.; Vaeli, N.
Fonte: Association of the Chemical Engineers Publicador: Association of the Chemical Engineers
Tipo: Artigo de Revista Científica
Publicado em //2012 EN
Relevância na Pesquisa
66.32%
The subcooled flow boiling heat-transfer characteristics of water and ethanol solutions in a vertical annulus have been investigated up to heat flux 132kW/m2. The variations in the effects of heat flux and fluid velocity, and concentration of ethanol on the observed heat-transfer coefficients over a range of ethanol concentrations implied an enhanced contribution of nucleate boiling heat transfer in flow boiling, where both forced convection and nucleate boiling heat transfer occurred. Increasing the ethanol concentration led to a significant deterioration in the observed heat-transfer coefficient because of a mixture effect, that resulted in a local rise in the saturation temperature of ethanol/water solution at the vapor-liquid interface. The reduction in the heat-transfer coefficient with increasing ethanol concentration is also attributed to changes in the fluid properties (for example, viscosity and heat capacity) of tested solutions with different ethanol content. The experimental data were compared with some well-established existing correlations. Results of comparisons indicate existing correlations are unable to obtain the acceptable values. Therefore a modified correlation based on Gnielinski correlation has been proposed that predicts the heat transfer coefficient for ethanol/water solution with uncertainty about 8% that is the least in comparison to other well-known existing correlations.; M.M. Sarafraz...

Optimal paths for minimizing lost available work during heat transfer processes with a generalized heat transfer law

Xia,Shaojun; Chen,Lingen; Sun,Fengrui
Fonte: Sociedade Brasileira de Física Publicador: Sociedade Brasileira de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2009 EN
Relevância na Pesquisa
66.3%
A common of finite-time heat transfer processes between high- and low-temperature sides with a generalized heat transfer law [q ∝ (Δ(Tn ))m] are studied in this paper. The optimal heating and cooling configurations for minimizing lost available work are derived for the fixed initial and final temperatures of the working fluid of the system (low-temperature side). Optimal paths are compared with the common strategies of constant heat flux, constant source (reservoir) temperature and the minimum entropy generation operation by numerical examples. The condition corresponding to the minimum lost available work strategy is that corresponding to a constant rate of lost available work, not only valid for Newton's heat transfer law [q ∝ ΔT] but also valid for the generalized convective heat transfer law [q ∝ (ΔT)m ]. The obtained results are more general and can provide some theoretical guidelines for the designs and operations of practical heat exchangers.

Experimental studies on nucleate pool boiling heat transfer to ethanol/MEG/DEG ternary mixture as a new coolant

Sarafraz, M.M.; Peyghambarzadeh, S.M.; Alavi Fazel, S.A.
Fonte: Association of the Chemical Engineers of Serbia Publicador: Association of the Chemical Engineers of Serbia
Tipo: Artigo de Revista Científica
Publicado em //2012 EN
Relevância na Pesquisa
66.3%
In this paper, nucleate pool boiling heat transfer coefficient of ternary mixtures of ethanol, monoethylene glycol (MEG) and diethylene glycol (DEG) as a new coolant with higher heat transfer coefficient has been investigated. Therefore, at varied concentrations of MEG and DEG and also at different heat fluxes, pool boiling heat transfer coefficients, have been experimentally measured. Results demonstrated the higher heat transfer coefficient in comparison with Water/MEG/DEG ternary mixture. In particular, at high heat fluxes, for ethanol/MEG/DEG mixture, higher boiling heat transfer coefficient is reported. Besides, experimental data were compared to well-known existing correlations. Results of this comparison express that the most accurate correlation for predicting the heat transfer coefficient of ethanol/MEG/DEG is modified Stephan - Preußer which has been obtained in our earlier work.; M.M. Sarafraz, S.M. Peyghambarzadeh, S.A. Alavi Fazel

Heat transfer, pressure drop, and dissolved gas effect during flow boiling in microchannels

Steinke, Mark
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado Formato: 625812 bytes; 3355869 bytes; 121742 bytes; 2 bytes; 2061 bytes; 698 bytes; 5612 bytes; 49 bytes; 625812 bytes; 3355869 bytes; application/pdf; application/pdf; text/plain; text/plain; text/plain; application/octet-stream; application/octet-stream; applica
EN_US
Relevância na Pesquisa
66.32%
Microchannels are being considered in many advanced heat transfer applications, including automotive, fuel cells, and electronics cooling. However, there are a number of fundamental issues still unresolved with respect to heat transfer and fluid mechanics perspective. An experimental investigation of the heat transfer, pressure drop, and flow patterns during flow boiling in microchannels is performed. Six parallel microchannels with a mean hydraulic diameter of 207 micron are manufactured and tested. Flow patterns have been observed in the channels under diabatic conditions. Observations suggest that the conventional flow patterns also occur in microchannels, however, the Reynolds number range is significantly lower in microchannels than in conventional channels (hydraulic diameter of 3 mm or higher). The effect of dissolved gas in water has also been investigated. A novel method for the removal of dissolved air has been applied and used to achieve several different levels of degassing. It was found that if the water is degassed to oxygen levels of 5.4 ppm, 3.2 ppm, and 1.8 ppm, behaves as predicted by correlations. The water that contained dissolved gas with an oxygen level of 8.0 ppm first exhibits a decrease in heat transfer and then an enhancement. The range of parameters are: mass flux - 160 to 1827 kg/m2s...

Experimental study of pool boiling heat transfer enhancement over microchanneled surfaces

Cooke, Dwight
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
EN_US
Relevância na Pesquisa
66.36%
Pool boiling is of interest in heat transfer applications because of its potential for removing large amount of heat resulting from the latent heat of evaporation and little pressure drop penalty for circulating coolant through the system. However, the heat transfer performance of pool boiling systems is still not comparable to the cooling ability provided by enhanced microchannels operating under single-phase conditions. This investigation focuses on the bubble dynamics and heat transfer on plain and structured microchanneled surfaces under various heat fluxes in an effort to understand the underlying heat transfer mechanism through the use of a high speed camera. In a preliminary study, silicon chips have been tested in the nucleate boiling regime, and beneficial microchannel geometries have been identified. It is determined that heat transfer enhancement occurs because of (i) an increase in surface area and (ii) an improvement in the heat transfer mechanism through the channels functioning as liquid conduits for three side heating. The range for channel size in which the greatest enhancement occurs has been identified as being 200 - 400 µm width and 300 - 500 µm depth. The second study has been investigated with copper chips, with improvements to the test setup for accurate measurement of surface temperature. Ten chips...

Single-phase liquid flow and heat transfer in plain and enhanced silicon microchannels

Steinke, Mark
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado Formato: 22657 bytes; 18658976 bytes; application/pdf; application/pdf
EN_US
Relevância na Pesquisa
66.31%
Microscale heat transfer and microfluidics have become increasingly important to overcome some very complex engineering challenges. The use of very small passages to gain heat transfer enhancement is a well documented method for achieving high heat flux dissipation. However, some interesting experimental results have caused researchers to question if the conventional theories for fluid flow and heat transfer are valid in the microscale passages. However, there is no significant physical basis for the discrepancies with singlephase liquid flows when the passage is scaled to the microscale. The present work identifies the sources of the discrepancies reported in literature and provides a method to correct for them. In the course of this pursuit, a new experimental facility is developed to generate highly accurate experimental data for single-phase flow of water. The new experimental data are used to highlight the sources of discrepancies and illustrate a course of action to correct for them. Finally, a novel method for creating even greater heat transfer enhancement has been realized. Small offset fins have been fabricated in silicon microchannels in order to create a constantly developing flow in the microchannel heat exchanger and thus heat transfer enhancement. A new parameter based upon the heat flux dissipated and the pressure drop required is developed to aid in the comparison between these enhanced silicon microchannels and plain geometry silicon microchannels. The result is an order of magnitude increase in thermal performance with a marginal increase in overall pressure drop.

SIMULATION OF HEAT TRANSFER FROM CANOPY SURFACES USING LOW-REYNOLDS NUMBER K-ε MODEL

Pillai, Sivaraja Subramania; Yoshie, Ryuichiro
Fonte: JUEE Press Publicador: JUEE Press
Tipo: info:eu-repo/semantics/article; info:eu-repo/semantics/publishedVersion; Formato: application/pdf
Publicado em 20/09/2015 ENG
Relevância na Pesquisa
66.31%
This study focuses on the Convective Heat Transfer Coefficient (CHTC) from urban building surfaces by numerical simulation. The heat transfer effects because of various geometrical and physical properties of urban areas exhibits a differential heating and uncomfortable environment compared to rural regions called as Urban Heat Island (UHI) phenomena. Investigation of Convective heat transfer coefficient becomes more important in the study of urban heat island phenomena. Experimental simulation of urban area with various urban canopy cases in thermally stratified wind tunnel is employed for the heat transfer kind of investigations in urban area. But, it is not an easy task in wind tunnel experiments to evaluate local CHTC, which vary on individual canyon surfaces transfer such as building roof, walls and ground. Numerical simulation validated by wind tunnel experiments can be an alternative for the prediction of CHTC from building surfaces in an urban area. In our study, Water evaporation technique used in wind tunnel experiment for the evaluation of convective heat transfer coefficient and naphthalene sublimation technique conducted by other researchers are used to validate the low-Reynolds-number k-ε model which was used for the evaluation of CHTC from surfaces. The calculated CFD results showed good agreement with both water evaporation technique and naphthalene sublimation experimental results. It is found that the low-Reynolds-number k-ε model is reliable for the investigations pertaining to heat transfer from urban canopy.

Finite-time exergy with a finite heat reservoir and generalized radiative heat transfer law

Xia,Shaojun; Chen,Lingen; Sun,Fengrui
Fonte: Sociedad Mexicana de Física Publicador: Sociedad Mexicana de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2010 EN
Relevância na Pesquisa
66.36%
The problem of the maximum work that can be extracted from a system consisting of one finite heat reservoir and one subsystem with the generalized radiative heat transfer law [q ∞ Δ (Tn)] is investigated in this paper. Finite-time exergy is derived for a fixed duration and a given initial state of the subsystem by applying optimal control theory. The optimal subsystem temperature configuration for the finite-time exergy consists of three segments, including the initial and final instantaneous adiabatic branches and the intermediate heat transfer branch. Analyses for special examples show that the optimal configuration of the heat transfer branch with Newton's heat transfer law [q ∞ Δ (T)] is that the temperatures of the reservoir and the subsystem change exponentially with time and the temperature ratio between them is a constant; The optimal configuration of the heat transfer branch with the linear phenomenological heat transfer law [q ∞ Δ (T-1)] is such that the temperatures of the reservoir and the subsystem change linearly and non-linearly with time, respectively, and the difference in reciprocal temperature between them is a constant. The optimal configuration of the heat transfer branch with the radiative heat transfer law [q ∞ Δ (T4)] is significantly different from those with the former two different heat transfer laws. Numerical examples are given...

Effect of heat transfer on the performance of a thermoelectric heat pump driven by a thermoelectric generator

Chen,Lingen; Meng,Fankai; Sun,Fengrui
Fonte: Sociedad Mexicana de Física Publicador: Sociedad Mexicana de Física
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2009 EN
Relevância na Pesquisa
66.33%
A model of a thermoelectric heat pump driven by a thermoelectric generator with external heat transfer irreversibility is proposed. The performance of the combined thermoelectric heat pump device obeying Newton's heat transfer law is analyzed using the combination of finite time thermodynamics and non-equilibrium thermodynamics. Two analytical formulae for heating load versus working electrical current, and the coefficient of performance (COP) versus working electrical current, are derived. For a fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the heating load and the COP of the combined thermoelectric heat pump device. For a fixed total number of thermoelectric elements, the ratio of the number of thermoelectric elements of the generator to the total number of thermoelectric elements is also optimized for maximizing both the heating load and the COP of the combined thermoelectric heat pump device. The influences of thermoelectric element allocation and heat transfer area allocation are analyzed by detailed numerical examples. The optimum working electrical currents for maximum heating load and maximum COP at different total numbers of thermoelectric elements and different total heat transfer areas are provided...

Use of artificial roughness to enhance heat transfer in solar air heaters - a review

Kumar,Thakur Sanjay; Thakur,N S; Kumar,Anoop; Mittal,Vijay
Fonte: Journal of Energy in Southern Africa Publicador: Journal of Energy in Southern Africa
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2010 EN
Relevância na Pesquisa
66.33%
Improvement in the thermo hydraulic performance of a solar air heater can be done by enhancing the heat transfer. In general, heat transfer enhancement techniques are divided into two groups: active and passive techniques. Providing an artificial roughness on a heat transferring surface is an effective passive heat transfer technique to enhance the rate of heat transfer to fluid flow. In this paper, reviews of various artificial roughness elements used as passive heat transfer techniques, in order to improve thermo hydraulic performance of a solar air heater, is done. The objective of this paper is to review various studies, in which different artificial roughness elements are used to enhance the heat transfer rate with little penalty of friction. Correlations developed by various researchers with the help of experimental results for heat transfer and friction factor for solar air heater ducts by taking different roughened surfaces geometries are given in tabular form. These correlations are used to predict the thermo hydraulic performance of solar air heaters having roughened ducts. The objective is to provide a detailed review on heat transfer enhancement by using an artificial roughness technique. This paper will be very helpful for the researchers who are researching new artificial roughness for solar air heater ducts to enhance the heat transfer rate and comparing with artificial roughness already studied by various researchers.