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Journal of heat transfer: Transactions of the ASME |
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Ajouter le résultat dans votre panierA novel methodology for combined parameter and function estimation problems / Hosein Molavi in Journal of heat transfer, Vol. 132 N° 12 (Décembre 2010)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121301-1/11]
Titre : A novel methodology for combined parameter and function estimation problems Type de document : texte imprimé Auteurs : Hosein Molavi, Auteur ; Ali Hakkaki-Fard, Auteur ; Ramin K. Rahmani, Auteur Année de publication : 2010 Article en page(s) : pp. [121301-1/11] Note générale : Physique Langues : Anglais (eng) Mots-clés : Box-Kanemasu method Conjugate gradient method Function estimation Parameter estimation Index. décimale : 536 Chaleur. Thermodynamique Résumé : This article presents a novel methodology, which is highly efficient and simple to implement, for simultaneous retrieval of a complete set of thermal coefficients in combined parameter and function estimation problems. Moreover, the effect of correlated unknown variables on convergence performance is examined. The present methodology is a combination of two different classical methods: The conjugate gradient method with adjoint problem (CGMAP) and Box–Kanemasu method (BKM). The methodology uses the benefit of CGMAP in handling function estimation problems and BKM for parameter estimation problems. One of the unique features about the present method is that the correlation among the separate unknowns does not disrupt the convergence of the problem. Numerical experiments using measurement errors are performed to verify the efficiency of the proposed method in solving the combined parameter and function estimation problems. The results obtained by the present approach show that the combined procedure can efficiently and reliably estimate the values of the unknown thermal coefficients.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] A novel methodology for combined parameter and function estimation problems [texte imprimé] / Hosein Molavi, Auteur ; Ali Hakkaki-Fard, Auteur ; Ramin K. Rahmani, Auteur . - 2010 . - pp. [121301-1/11].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121301-1/11]
Mots-clés : Box-Kanemasu method Conjugate gradient method Function estimation Parameter estimation Index. décimale : 536 Chaleur. Thermodynamique Résumé : This article presents a novel methodology, which is highly efficient and simple to implement, for simultaneous retrieval of a complete set of thermal coefficients in combined parameter and function estimation problems. Moreover, the effect of correlated unknown variables on convergence performance is examined. The present methodology is a combination of two different classical methods: The conjugate gradient method with adjoint problem (CGMAP) and Box–Kanemasu method (BKM). The methodology uses the benefit of CGMAP in handling function estimation problems and BKM for parameter estimation problems. One of the unique features about the present method is that the correlation among the separate unknowns does not disrupt the convergence of the problem. Numerical experiments using measurement errors are performed to verify the efficiency of the proposed method in solving the combined parameter and function estimation problems. The results obtained by the present approach show that the combined procedure can efficiently and reliably estimate the values of the unknown thermal coefficients.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121302-1/6]
Titre : Analytical solution to nonlinear thermal diffusion : kirchhoff versus cole–hopf transformations Type de document : texte imprimé Auteurs : Peter Vadasz, Auteur Année de publication : 2010 Article en page(s) : pp. [121302-1/6] Note générale : Physique Langues : Anglais (eng) Mots-clés : Nonlinear conduction Kirchhoff transformation Cole-hopf transformation Nonlinear thermal diffusion Temperature dependent properties Index. décimale : 536 Chaleur. Thermodynamique Résumé : The Kirchhoff transformation is the classical method of solution to the nonlinear thermal diffusion problem with temperature dependent properties. It essentially converts the nonlinear problem into a linear one if the thermal diffusivity is approximately constant. Unfortunately, with the only exception of an exponential dependence of the thermal conductivity on temperature, all other thermal conductivity functions produce an inconvenient form for the inverse transform. This paper shows that the Kirchhoff transformation is a particular consequence of the more general Cole–Hopf transformation. However, the classical presentation of the Kirchhoff transformation in terms of a definite integral is more restrictive than the result obtained from the Cole–Hopf transformation and it is this restrictiveness that causes the practical inconvenience in the form of the inverse transform. It is shown that a more compact and practically convenient form of the inverse transform can be obtained by using directly the result from the Cole–Hopf transformation, hence, making its application more attractive.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Analytical solution to nonlinear thermal diffusion : kirchhoff versus cole–hopf transformations [texte imprimé] / Peter Vadasz, Auteur . - 2010 . - pp. [121302-1/6].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121302-1/6]
Mots-clés : Nonlinear conduction Kirchhoff transformation Cole-hopf transformation Nonlinear thermal diffusion Temperature dependent properties Index. décimale : 536 Chaleur. Thermodynamique Résumé : The Kirchhoff transformation is the classical method of solution to the nonlinear thermal diffusion problem with temperature dependent properties. It essentially converts the nonlinear problem into a linear one if the thermal diffusivity is approximately constant. Unfortunately, with the only exception of an exponential dependence of the thermal conductivity on temperature, all other thermal conductivity functions produce an inconvenient form for the inverse transform. This paper shows that the Kirchhoff transformation is a particular consequence of the more general Cole–Hopf transformation. However, the classical presentation of the Kirchhoff transformation in terms of a definite integral is more restrictive than the result obtained from the Cole–Hopf transformation and it is this restrictiveness that causes the practical inconvenience in the form of the inverse transform. It is shown that a more compact and practically convenient form of the inverse transform can be obtained by using directly the result from the Cole–Hopf transformation, hence, making its application more attractive.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121401-1/9]
Titre : Measurement of entropy generation in microscale thermal-fluid systems Type de document : texte imprimé Auteurs : Meghdad Saffaripour, Auteur ; Richard Culham, Auteur Année de publication : 2010 Article en page(s) : pp. [121401-1/9] Note générale : Physique Langues : Anglais (eng) Mots-clés : Microscale thermal-fluid systems Microchannel heatsinks Entropy generation measurement Index. décimale : 536 Chaleur. Thermodynamique Résumé : A new nonintrusive and whole field method for the measurement of entropy generation in microscale thermal-fluid devices is presented. The rate of entropy generation is a measure of the thermodynamic losses or irreversibilities associated with viscous effects and heat transfer in thermal-fluid systems. This method provides the entropy generation distribution in the device, thus enabling the designers to find and modify the areas producing high energy losses characterized by large entropy production rates. The entropy generation map is obtained by postprocessing the velocity and temperature distribution data, measured by micro particle image velocimetry and laser induced fluorescence methods, respectively. The velocity and temperature measurements lead to the frictional and thermal terms of entropy generation. One main application of this method is optimizing the efficiency of microchannel heatsinks, used in cooling of electronic devices. The minimum amount of entropy generation determines the optimum design parameters of heatsinks, leading to highest heat removal rates and at the same time, the lowest pressure drop across the heatsink. To show the capability of this technique, the entropy generation field in the transition region between a 100 µm wide and a 200 µm wide rectangular microchannel is measured. This method is used to measure thermal and frictional entropy generation rates in three different flow area transition geometries. The results can be used to determine which geometry has the highest thermal and hydraulic efficiencies.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Measurement of entropy generation in microscale thermal-fluid systems [texte imprimé] / Meghdad Saffaripour, Auteur ; Richard Culham, Auteur . - 2010 . - pp. [121401-1/9].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121401-1/9]
Mots-clés : Microscale thermal-fluid systems Microchannel heatsinks Entropy generation measurement Index. décimale : 536 Chaleur. Thermodynamique Résumé : A new nonintrusive and whole field method for the measurement of entropy generation in microscale thermal-fluid devices is presented. The rate of entropy generation is a measure of the thermodynamic losses or irreversibilities associated with viscous effects and heat transfer in thermal-fluid systems. This method provides the entropy generation distribution in the device, thus enabling the designers to find and modify the areas producing high energy losses characterized by large entropy production rates. The entropy generation map is obtained by postprocessing the velocity and temperature distribution data, measured by micro particle image velocimetry and laser induced fluorescence methods, respectively. The velocity and temperature measurements lead to the frictional and thermal terms of entropy generation. One main application of this method is optimizing the efficiency of microchannel heatsinks, used in cooling of electronic devices. The minimum amount of entropy generation determines the optimum design parameters of heatsinks, leading to highest heat removal rates and at the same time, the lowest pressure drop across the heatsink. To show the capability of this technique, the entropy generation field in the transition region between a 100 µm wide and a 200 µm wide rectangular microchannel is measured. This method is used to measure thermal and frictional entropy generation rates in three different flow area transition geometries. The results can be used to determine which geometry has the highest thermal and hydraulic efficiencies.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp.[121402-1/9]
Titre : 3D Integrated water cooling of a composite multilayer stack of chips Type de document : texte imprimé Auteurs : Fabio Alfieri, Auteur ; Manish K. Tiwari, Auteur ; Igor Zinovik, Auteur Année de publication : 2010 Article en page(s) : pp.[121402-1/9] Note générale : Physique Langues : Anglais (eng) Mots-clés : 3D chip stack Micropin-fun Integrated water cooling Conjugate heat transfer modeling Variable properties Thin porous medium Nonthermal equilibrium Index. décimale : 536 Chaleur. Thermodynamique Résumé : New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 µm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] 3D Integrated water cooling of a composite multilayer stack of chips [texte imprimé] / Fabio Alfieri, Auteur ; Manish K. Tiwari, Auteur ; Igor Zinovik, Auteur . - 2010 . - pp.[121402-1/9].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp.[121402-1/9]
Mots-clés : 3D chip stack Micropin-fun Integrated water cooling Conjugate heat transfer modeling Variable properties Thin porous medium Nonthermal equilibrium Index. décimale : 536 Chaleur. Thermodynamique Résumé : New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 µm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121501-1/6]
Titre : Heat transfer characteristics of oscillating heat pipe with water and ethanol as working fluids Type de document : texte imprimé Auteurs : Haizhen Xian, Auteur ; YongPing Yang, Auteur Année de publication : 2010 Article en page(s) : pp. [121501-1/6] Note générale : Physique Langues : Anglais (eng) Mots-clés : Oscillating heat pipe Heat transfer characteristics Water Ethanol Index. décimale : 536 Chaleur. Thermodynamique Résumé : In this paper, experiments were conducted to achieve a better understanding of the oscillating heat pipe (OHP) operating behavior with water and ethanol as working fluid. The experimental results showed that there existed a necessary temperature difference between the evaporator and the condenser section to keep the heat pipe working. The maximum effective conductivity of the water OHP reached up to 259 kW/m K, while that of the ethanol OHP is of 111 kW/m K. Not all the OHPs are operated in the horizontal operation mode. The heat transfer performance of the ethanol OHP was obviously affected by the filling ratio and the inclination angle but the influence law is irregular. The effect of the filling ratio and the inclination angle of the water OHP were smaller than that of the ethanol one. The heat transfer performance of the OHP was improved with increase of operating temperature. The startup characteristics of the OHP depended on the establishment of the integral oscillating process, which was determined by the operating factors. The startup temperature of the ethanol OHP varied from 40°C to 50°C and that of the water, OHP varied from 40°C to 60°C without considering the horizontal operating mode. The water OHP showed a better performance and more stable heat transfer characteristics than the ethanol OHP, which had no obvious advantages of the startup capability as well.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Heat transfer characteristics of oscillating heat pipe with water and ethanol as working fluids [texte imprimé] / Haizhen Xian, Auteur ; YongPing Yang, Auteur . - 2010 . - pp. [121501-1/6].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121501-1/6]
Mots-clés : Oscillating heat pipe Heat transfer characteristics Water Ethanol Index. décimale : 536 Chaleur. Thermodynamique Résumé : In this paper, experiments were conducted to achieve a better understanding of the oscillating heat pipe (OHP) operating behavior with water and ethanol as working fluid. The experimental results showed that there existed a necessary temperature difference between the evaporator and the condenser section to keep the heat pipe working. The maximum effective conductivity of the water OHP reached up to 259 kW/m K, while that of the ethanol OHP is of 111 kW/m K. Not all the OHPs are operated in the horizontal operation mode. The heat transfer performance of the ethanol OHP was obviously affected by the filling ratio and the inclination angle but the influence law is irregular. The effect of the filling ratio and the inclination angle of the water OHP were smaller than that of the ethanol one. The heat transfer performance of the OHP was improved with increase of operating temperature. The startup characteristics of the OHP depended on the establishment of the integral oscillating process, which was determined by the operating factors. The startup temperature of the ethanol OHP varied from 40°C to 50°C and that of the water, OHP varied from 40°C to 60°C without considering the horizontal operating mode. The water OHP showed a better performance and more stable heat transfer characteristics than the ethanol OHP, which had no obvious advantages of the startup capability as well.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp.[121601-1/6]
Titre : Imaging of surface-tension-driven convection using liquid crystal thermography Type de document : texte imprimé Auteurs : T. W. Dutton, Auteur ; L. R. Pate, Auteur ; D. K. Hollingsworth, Auteur Année de publication : 2010 Article en page(s) : pp.[121601-1/6] Note générale : Physique Langues : Anglais (eng) Mots-clés : Marangoni flow Surface-tension-driven flow Liquid crystal thermography Index. décimale : 536 Chaleur. Thermodynamique Résumé : Surface-tension forces can drive fluid motion within thin liquid layers with a free surface. Spatial variations in the temperature of the free surface create surface tractions that drive cellular motions. The cells are most commonly hexagonal in shape and they scale on the thickness of the fluid layer. This investigation documents the formation of cells in the liquid film in the presence of a uniform-heat-flux lower boundary condition. Liquid crystal thermography was used to image the cells and measure the temperature distribution on the lower surface of the liquid layer. A 1.1 mm deep pool of silicone oil was supported on a 50 µm thick electrically heated metal foil. The oil was retained inside an independently heated acrylic ring mounted on the top surface of the foil and a dry-ice cooling plate served as the low-temperature sink above the free surface of the oil. Color images of hexagonal convection cells were captured using liquid crystal thermography and a digital image acquisition and processing system. The temperature distribution inside a typical cell was measured using thermographic image analysis. Experimental issues, such as the use of an independently heated retaining ring to control the height of the liquid film and the utility of a flux-based Marangoni number are discussed.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Imaging of surface-tension-driven convection using liquid crystal thermography [texte imprimé] / T. W. Dutton, Auteur ; L. R. Pate, Auteur ; D. K. Hollingsworth, Auteur . - 2010 . - pp.[121601-1/6].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp.[121601-1/6]
Mots-clés : Marangoni flow Surface-tension-driven flow Liquid crystal thermography Index. décimale : 536 Chaleur. Thermodynamique Résumé : Surface-tension forces can drive fluid motion within thin liquid layers with a free surface. Spatial variations in the temperature of the free surface create surface tractions that drive cellular motions. The cells are most commonly hexagonal in shape and they scale on the thickness of the fluid layer. This investigation documents the formation of cells in the liquid film in the presence of a uniform-heat-flux lower boundary condition. Liquid crystal thermography was used to image the cells and measure the temperature distribution on the lower surface of the liquid layer. A 1.1 mm deep pool of silicone oil was supported on a 50 µm thick electrically heated metal foil. The oil was retained inside an independently heated acrylic ring mounted on the top surface of the foil and a dry-ice cooling plate served as the low-temperature sink above the free surface of the oil. Color images of hexagonal convection cells were captured using liquid crystal thermography and a digital image acquisition and processing system. The temperature distribution inside a typical cell was measured using thermographic image analysis. Experimental issues, such as the use of an independently heated retaining ring to control the height of the liquid film and the utility of a flux-based Marangoni number are discussed.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121701-1/7]
Titre : An experimental investigation of performance and exergy analysis of a counterflow vortex tube having various nozzle numbers at different inlet pressures of air, oxygen, nitrogen, and argon Type de document : texte imprimé Auteurs : Volkan Kirmaci, Auteur ; Onuralp Uluer, Auteur ; Kevser Dincer, Auteur Année de publication : 2010 Article en page(s) : pp. [121701-1/7] Note générale : Physique Langues : Anglais (eng) Mots-clés : Vortex tube (RHVT) Exergy Cooling Heating Index. décimale : 536 Chaleur. Thermodynamique Résumé : An experimental investigation has been carried out to determine the thermal behavior of cooling fluid as it passes through a vortex tube and the effects of the orifice nozzle number and the inlet pressure on the heating and cooling performance of the counterflow type vortex tube (RHVT). Experiments have been performed using oxygen (O2), nitrogen (N2), and argon (Ar). Five orifices have been fabricated and used during the experimental study with different nozzle numbers of 2, 3, 4, 5, and 6. The orifices used at these experiments are made of the polyamide plastic material. The thermal conductivity of polyamide plastic material is 0.25 W/m K. To determine the energy separation, the inlet pressure values were adjusted from 150 kPa to 700 kPa with 50 kPa increments for each one of the orifices and each one of the studied fluids. The vortex tube that was used during the experiments has L/D ratio of 15 and the cold mass fraction was held constant at 0.5. As a result of the experimental study, it is determined that the temperature gradient between the cold and hot exits is decreased depending on the orifice nozzle number increase. Exergy analyses have been realized for each one of the studied fluids under the same inlet pressures with the experiments (Pi=150–700). The exergy efficiency of the vortex tube is more affected by inlet pressure than nozzle number.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] An experimental investigation of performance and exergy analysis of a counterflow vortex tube having various nozzle numbers at different inlet pressures of air, oxygen, nitrogen, and argon [texte imprimé] / Volkan Kirmaci, Auteur ; Onuralp Uluer, Auteur ; Kevser Dincer, Auteur . - 2010 . - pp. [121701-1/7].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121701-1/7]
Mots-clés : Vortex tube (RHVT) Exergy Cooling Heating Index. décimale : 536 Chaleur. Thermodynamique Résumé : An experimental investigation has been carried out to determine the thermal behavior of cooling fluid as it passes through a vortex tube and the effects of the orifice nozzle number and the inlet pressure on the heating and cooling performance of the counterflow type vortex tube (RHVT). Experiments have been performed using oxygen (O2), nitrogen (N2), and argon (Ar). Five orifices have been fabricated and used during the experimental study with different nozzle numbers of 2, 3, 4, 5, and 6. The orifices used at these experiments are made of the polyamide plastic material. The thermal conductivity of polyamide plastic material is 0.25 W/m K. To determine the energy separation, the inlet pressure values were adjusted from 150 kPa to 700 kPa with 50 kPa increments for each one of the orifices and each one of the studied fluids. The vortex tube that was used during the experiments has L/D ratio of 15 and the cold mass fraction was held constant at 0.5. As a result of the experimental study, it is determined that the temperature gradient between the cold and hot exits is decreased depending on the orifice nozzle number increase. Exergy analyses have been realized for each one of the studied fluids under the same inlet pressures with the experiments (Pi=150–700). The exergy efficiency of the vortex tube is more affected by inlet pressure than nozzle number.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121702-1/11]
Titre : A Semi-empirical heat transfer model for forced convection in pin-fin heat sinks subjected to nonuniform heating Type de document : texte imprimé Auteurs : S. S. Feng, Auteur ; T. Kim, Auteur ; T. J. Lu, Auteur Année de publication : 2010 Article en page(s) : pp. [121702-1/11] Note générale : Physique Langues : Anglais (eng) Mots-clés : Fuite volume method Heat spreading Nonuniform heating Pin-fin heat sink Index. décimale : 536 Chaleur. Thermodynamique Résumé : This paper presents a cost effective semi-empirical analytical model for convective heat transfer in pin-fin heat sinks subjected to nonuniform heating set by a circular hot gas impinging jet. Based on empirical correlations taken from the open literature, temperature variations in the heat sink are obtained from the finite volume solution of the semi-empirical model. Based on a purpose-built experimental setup, measurements of a substrate temperature are performed using an infrared camera. These, along with the convective fluid temperature measured at the exit of the pin-fin array, are compared against analytical model predictions, with overall good agreement achieved. Subsequently, the influences of the convection Reynolds number, substrate thickness, and thermal conductivity of material on the distribution of substrate temperature are quantified by the validated model. It is demonstrated that the present model is capable of predicting local thermal behaviors such as the footprints of the pin fins. In addition, with the spreading resistance captured accurately, the model can be used for the design optimization of pin-fin/substrate systems subjected to nonuniform heating.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] A Semi-empirical heat transfer model for forced convection in pin-fin heat sinks subjected to nonuniform heating [texte imprimé] / S. S. Feng, Auteur ; T. Kim, Auteur ; T. J. Lu, Auteur . - 2010 . - pp. [121702-1/11].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121702-1/11]
Mots-clés : Fuite volume method Heat spreading Nonuniform heating Pin-fin heat sink Index. décimale : 536 Chaleur. Thermodynamique Résumé : This paper presents a cost effective semi-empirical analytical model for convective heat transfer in pin-fin heat sinks subjected to nonuniform heating set by a circular hot gas impinging jet. Based on empirical correlations taken from the open literature, temperature variations in the heat sink are obtained from the finite volume solution of the semi-empirical model. Based on a purpose-built experimental setup, measurements of a substrate temperature are performed using an infrared camera. These, along with the convective fluid temperature measured at the exit of the pin-fin array, are compared against analytical model predictions, with overall good agreement achieved. Subsequently, the influences of the convection Reynolds number, substrate thickness, and thermal conductivity of material on the distribution of substrate temperature are quantified by the validated model. It is demonstrated that the present model is capable of predicting local thermal behaviors such as the footprints of the pin fins. In addition, with the spreading resistance captured accurately, the model can be used for the design optimization of pin-fin/substrate systems subjected to nonuniform heating.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121703-1/17]
Titre : Laminar boundary layer development around a circular cylinder : fluid flow and heat-mass transfer characteristics Type de document : texte imprimé Auteurs : A. Alper Ozalp, Auteur ; Ibrahim Dincer, Auteur Année de publication : 2010 Article en page(s) : pp. [121703-1/17] Note générale : Physique Langues : Anglais (eng) Mots-clés : Heat transfer Fluid flow Mass transfer Confined flow Friction coefficient Moisture diffusivity Moisure transfer Coefficient Drying Index. décimale : 536 Chaleur. Thermodynamique Résumé : This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Red=40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version 11.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (beta=0.200–0.800) on the flow and heat transfer mechanism and clarifying the combined roles of beta and moisture diffusivity (D=1×10−8–1×10−5 m2/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (Cf) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nuave=3.66, 4.05, 4.97, and 6.51 for beta=0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (thetas) and maximum Cf locations (thetaCf−max) downstream to the positions of thetas=54.10, 50.20, 41.98, and 37.30 deg and thetaCf−max=51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of beta=0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (hm) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (hm−bf) is dominant to that of the front face values (hm−ff), with the interpreting ratios of [overline h]m−bf/[overline h]m=0.50 and 0.57 and [overline h]m−ff/[overline h]m=1.50 and 1.43 for beta=0.200 and 0.800.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Laminar boundary layer development around a circular cylinder : fluid flow and heat-mass transfer characteristics [texte imprimé] / A. Alper Ozalp, Auteur ; Ibrahim Dincer, Auteur . - 2010 . - pp. [121703-1/17].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121703-1/17]
Mots-clés : Heat transfer Fluid flow Mass transfer Confined flow Friction coefficient Moisture diffusivity Moisure transfer Coefficient Drying Index. décimale : 536 Chaleur. Thermodynamique Résumé : This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Red=40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version 11.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (beta=0.200–0.800) on the flow and heat transfer mechanism and clarifying the combined roles of beta and moisture diffusivity (D=1×10−8–1×10−5 m2/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (Cf) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nuave=3.66, 4.05, 4.97, and 6.51 for beta=0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (thetas) and maximum Cf locations (thetaCf−max) downstream to the positions of thetas=54.10, 50.20, 41.98, and 37.30 deg and thetaCf−max=51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of beta=0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (hm) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (hm−bf) is dominant to that of the front face values (hm−ff), with the interpreting ratios of [overline h]m−bf/[overline h]m=0.50 and 0.57 and [overline h]m−ff/[overline h]m=1.50 and 1.43 for beta=0.200 and 0.800.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121901-1/9]
Titre : Performance of aluminum and carbon foams for air side heat transfer augmentation Type de document : texte imprimé Auteurs : Patrick T. Garrity, Auteur ; James F. Klausner, Auteur ; Renwei Mei, Auteur Année de publication : 2010 Article en page(s) : pp. [121901-1/9] Note générale : Physique Langues : Anglais (eng) Mots-clés : Porous foam Heat transfer augmentation Heat exchanger performance Index. décimale : 536 Chaleur. Thermodynamique Résumé : The air side heat transfer performance of three aluminum foam samples and three modified carbon foam samples are examined for comparison with multilouvered fins often found in compact heat exchangers. The aluminum foam samples have a bulk density of 216 kg/m3 with pore sizes of 0.5, 1, and 2 mm. The modified carbon foam samples have bulk densities of 284, 317, and 400 kg/m3 and machined flow passages of 3.2 mm in diameter. The samples were placed in a forced convection arrangement using a foil heater as the heat source and ambient air as the sink. A constant heat flux of 9.77 kW/m2 is applied throughout the experiments with the mean air velocity ranging from 1 to 6 m/s as the control parameter. The steady volume-averaged momentum equation and a two-equation nonequilibrium heat transfer model are employed to extract the volumetric heat transfer coefficients. Pressure drop measurements are correlated with the Darcy–Forcheimer relation. Empirical heat transfer correlations for the aluminum and carbon foam samples are provided. Using a hypothetical heat exchanger considering only the thermal resistance between the ambient air and the outer tube wall, the air side performance for each sample is modeled based on the local heat transfer coefficients and friction factors obtained from experiments. The performance of each sample is evaluated based on a coefficient of performance (COP, defined as the ratio of the total heat removed to the electrical input of the blower), compactness factor (CF, defined as the total heat removed per unit volume), and power density (PD, defined as the total heat removed per unit mass). Results show the carbon foam samples provide significant improvement in CF but the COP and PD are considerably lower than that for comparable multilouvered fin heat exchangers.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Performance of aluminum and carbon foams for air side heat transfer augmentation [texte imprimé] / Patrick T. Garrity, Auteur ; James F. Klausner, Auteur ; Renwei Mei, Auteur . - 2010 . - pp. [121901-1/9].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [121901-1/9]
Mots-clés : Porous foam Heat transfer augmentation Heat exchanger performance Index. décimale : 536 Chaleur. Thermodynamique Résumé : The air side heat transfer performance of three aluminum foam samples and three modified carbon foam samples are examined for comparison with multilouvered fins often found in compact heat exchangers. The aluminum foam samples have a bulk density of 216 kg/m3 with pore sizes of 0.5, 1, and 2 mm. The modified carbon foam samples have bulk densities of 284, 317, and 400 kg/m3 and machined flow passages of 3.2 mm in diameter. The samples were placed in a forced convection arrangement using a foil heater as the heat source and ambient air as the sink. A constant heat flux of 9.77 kW/m2 is applied throughout the experiments with the mean air velocity ranging from 1 to 6 m/s as the control parameter. The steady volume-averaged momentum equation and a two-equation nonequilibrium heat transfer model are employed to extract the volumetric heat transfer coefficients. Pressure drop measurements are correlated with the Darcy–Forcheimer relation. Empirical heat transfer correlations for the aluminum and carbon foam samples are provided. Using a hypothetical heat exchanger considering only the thermal resistance between the ambient air and the outer tube wall, the air side performance for each sample is modeled based on the local heat transfer coefficients and friction factors obtained from experiments. The performance of each sample is evaluated based on a coefficient of performance (COP, defined as the ratio of the total heat removed to the electrical input of the blower), compactness factor (CF, defined as the total heat removed per unit volume), and power density (PD, defined as the total heat removed per unit mass). Results show the carbon foam samples provide significant improvement in CF but the COP and PD are considerably lower than that for comparable multilouvered fin heat exchangers.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122201-1/11]
Titre : Calibration of a computational model to predict mist/steam impinging jets cooling with an application to gas turbine blades Type de document : texte imprimé Auteurs : Ting Wang, Auteur ; T. S. Dhanasekaran, Auteur Année de publication : 2010 Article en page(s) : pp. [122201-1/11] Note générale : Physique Langues : Anglais (eng) Mots-clés : Impinging jets Mist cooling Heat transfer enhancement Two-phase flow Gas turbine blade cooling Index. décimale : 536 Chaleur. Thermodynamique Résumé : In heavy-frame advanced turbine systems, steam is used as a coolant for turbine blade cooling. The concept of injecting mist into the impinging jets of steam was experimentally proved as an effective way of significantly enhancing the cooling effectiveness in the laboratory under low pressure and temperature conditions. However, whether or not mist/steam cooling is applicable under actual gas turbine operating conditions is still subject to further verification. Recognizing the difficulties of conducting experiments in an actual high-pressure, high-temperature working gas turbine, a simulation using a computational fluid dynamic (CFD) model calibrated with laboratory data would be an opted approach. To this end, the present study conducts a CFD model calibration against the database of two experimental cases including a slot impinging jet and three rows of staggered impinging jets. The calibrated CFD model was then used to predict the mist cooling enhancement at the elevated gas turbine working condition. Using the experimental results, the CFD model has been tuned by employing different turbulence models, computational cells, and wall y+ values. In addition, the effects of different forces (e.g., drag, thermophoretic, Brownian, and Saffman's lift force) are also studied. None of the models is a good predictor for all the flow regions from near the stagnation region to far-field downstream of the jets. Overall speaking, both standard k-epsilon and Reynolds stress model (RSM) turbulence models perform better than other models. The RSM model has produced the closest results to the experimental data due to its capability of modeling the nonisotropic turbulence shear stresses in the 3D impinging jet fields. The simulated results show that the calibrated CFD model can predict the heat transfer coefficient of steam-only case within 2–5% deviations from the experimental results for all the cases. When mist is employed, the prediction of wall temperatures is within 5% for a slot jet and within 10% for three-row jets. The predicted results with 1.5% mist at the gas turbine working condition show the mist cooling enhancement of 20%, whereas in the laboratory condition, the enhancement is predicted as 80%. Increasing mist ratio to 5% increased the cooling enhancement to about 100% at the gas turbine working condition.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Calibration of a computational model to predict mist/steam impinging jets cooling with an application to gas turbine blades [texte imprimé] / Ting Wang, Auteur ; T. S. Dhanasekaran, Auteur . - 2010 . - pp. [122201-1/11].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122201-1/11]
Mots-clés : Impinging jets Mist cooling Heat transfer enhancement Two-phase flow Gas turbine blade cooling Index. décimale : 536 Chaleur. Thermodynamique Résumé : In heavy-frame advanced turbine systems, steam is used as a coolant for turbine blade cooling. The concept of injecting mist into the impinging jets of steam was experimentally proved as an effective way of significantly enhancing the cooling effectiveness in the laboratory under low pressure and temperature conditions. However, whether or not mist/steam cooling is applicable under actual gas turbine operating conditions is still subject to further verification. Recognizing the difficulties of conducting experiments in an actual high-pressure, high-temperature working gas turbine, a simulation using a computational fluid dynamic (CFD) model calibrated with laboratory data would be an opted approach. To this end, the present study conducts a CFD model calibration against the database of two experimental cases including a slot impinging jet and three rows of staggered impinging jets. The calibrated CFD model was then used to predict the mist cooling enhancement at the elevated gas turbine working condition. Using the experimental results, the CFD model has been tuned by employing different turbulence models, computational cells, and wall y+ values. In addition, the effects of different forces (e.g., drag, thermophoretic, Brownian, and Saffman's lift force) are also studied. None of the models is a good predictor for all the flow regions from near the stagnation region to far-field downstream of the jets. Overall speaking, both standard k-epsilon and Reynolds stress model (RSM) turbulence models perform better than other models. The RSM model has produced the closest results to the experimental data due to its capability of modeling the nonisotropic turbulence shear stresses in the 3D impinging jet fields. The simulated results show that the calibrated CFD model can predict the heat transfer coefficient of steam-only case within 2–5% deviations from the experimental results for all the cases. When mist is employed, the prediction of wall temperatures is within 5% for a slot jet and within 10% for three-row jets. The predicted results with 1.5% mist at the gas turbine working condition show the mist cooling enhancement of 20%, whereas in the laboratory condition, the enhancement is predicted as 80%. Increasing mist ratio to 5% increased the cooling enhancement to about 100% at the gas turbine working condition.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122202-1/8]
Titre : Experimental investigation on the heat transfer of a leading edge impingement cooling system for low pressure turbine vanes Type de document : texte imprimé Auteurs : Pedro de la Calzada, Auteur ; Jose Javier Alvarez, Auteur Année de publication : 2010 Article en page(s) : pp. [122202-1/8] Note générale : Physique Langues : Anglais (eng) Mots-clés : Turbine cooling Heat transfer Impingement cooling Liquid crystal Index. décimale : 536 Chaleur. Thermodynamique Résumé : Impingement cooling through jet holes is a very attractive cooling system for heat rejection at high heat loaded areas as the leading edge of turbine vanes. Although some correlations and tools are available to dimension such systems, the variety and complexity of the flow features present in those systems still require experimental validation of real engine designs. Among the experimental techniques possible to be used, transient liquid crystal method offers good resolution as well as sufficient accuracy. Under this investigation, an impingement cooling system for the leading edge of a contrarotating power turbine (PT) representative of a small turboshaft engine was investigated experimentally. The PT vane features a very thin leading edge with high curvature and side channels rapidly turning backward. Constraints on cooling flow consumption and distribution led to a leading edge configuration with two rows of staggered jets. This particular configuration was experimentally investigated for three different Reynolds numbers around the design point by using a transient liquid crystal technique, which allows the measurement of surface distribution of heat transfer coefficient at the area of interest. Heat transfer results are presented in terms of surface distributions, impingement rows stagnation line local distributions, streamwise distributions along planes over the impingement stagnation points, span averaged streamwise local distributions, and surface averaged values. These results are then compared with available correlations from existing literature showing good matching for both maximum and averaged values. The results are also used as baseline data to discuss some of the flow features that can have effect on the heat transfer on this particular configuration.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Experimental investigation on the heat transfer of a leading edge impingement cooling system for low pressure turbine vanes [texte imprimé] / Pedro de la Calzada, Auteur ; Jose Javier Alvarez, Auteur . - 2010 . - pp. [122202-1/8].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122202-1/8]
Mots-clés : Turbine cooling Heat transfer Impingement cooling Liquid crystal Index. décimale : 536 Chaleur. Thermodynamique Résumé : Impingement cooling through jet holes is a very attractive cooling system for heat rejection at high heat loaded areas as the leading edge of turbine vanes. Although some correlations and tools are available to dimension such systems, the variety and complexity of the flow features present in those systems still require experimental validation of real engine designs. Among the experimental techniques possible to be used, transient liquid crystal method offers good resolution as well as sufficient accuracy. Under this investigation, an impingement cooling system for the leading edge of a contrarotating power turbine (PT) representative of a small turboshaft engine was investigated experimentally. The PT vane features a very thin leading edge with high curvature and side channels rapidly turning backward. Constraints on cooling flow consumption and distribution led to a leading edge configuration with two rows of staggered jets. This particular configuration was experimentally investigated for three different Reynolds numbers around the design point by using a transient liquid crystal technique, which allows the measurement of surface distribution of heat transfer coefficient at the area of interest. Heat transfer results are presented in terms of surface distributions, impingement rows stagnation line local distributions, streamwise distributions along planes over the impingement stagnation points, span averaged streamwise local distributions, and surface averaged values. These results are then compared with available correlations from existing literature showing good matching for both maximum and averaged values. The results are also used as baseline data to discuss some of the flow features that can have effect on the heat transfer on this particular configuration.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122401-1/8]
Titre : Entropy generation analysis for nanofluid flow in microchannels Type de document : texte imprimé Auteurs : Jie Li, Auteur ; Clement Kleinstreuer, Auteur Année de publication : 2010 Article en page(s) : pp. [122401-1/8] Note générale : Physique Langues : Anglais (eng) Mots-clés : Computer simulation Nanofluid flow Entropy minimization Microchannel geometry Microchannel operation Index. décimale : 536 Chaleur. Thermodynamique Résumé : Employing a validated computer simulation model, entropy generation is analyzed in trapezoidal microchannels for steady laminar flow of pure water and CuO-water nanofluids. Focusing on microchannel heat sink applications, local and volumetric entropy rates caused by frictional and thermal effects are computed for different coolants, inlet temperatures, Reynolds numbers, and channel aspect ratios. It was found that there exists an optimal Reynolds number range to operate the system due to the characteristics of the two different entropy sources, both related to the inlet Reynolds number. Microchannels with high aspect ratios have a lower suitable operational Reynolds number range. The employment of nanofluids can further minimize entropy generation because of their superior thermal properties. Heat transfer induced entropy generation is dominant for typical microheating systems while frictional entropy generation becomes more and more important with the increase in fluid inlet velocity/Reynolds number.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Entropy generation analysis for nanofluid flow in microchannels [texte imprimé] / Jie Li, Auteur ; Clement Kleinstreuer, Auteur . - 2010 . - pp. [122401-1/8].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122401-1/8]
Mots-clés : Computer simulation Nanofluid flow Entropy minimization Microchannel geometry Microchannel operation Index. décimale : 536 Chaleur. Thermodynamique Résumé : Employing a validated computer simulation model, entropy generation is analyzed in trapezoidal microchannels for steady laminar flow of pure water and CuO-water nanofluids. Focusing on microchannel heat sink applications, local and volumetric entropy rates caused by frictional and thermal effects are computed for different coolants, inlet temperatures, Reynolds numbers, and channel aspect ratios. It was found that there exists an optimal Reynolds number range to operate the system due to the characteristics of the two different entropy sources, both related to the inlet Reynolds number. Microchannels with high aspect ratios have a lower suitable operational Reynolds number range. The employment of nanofluids can further minimize entropy generation because of their superior thermal properties. Heat transfer induced entropy generation is dominant for typical microheating systems while frictional entropy generation becomes more and more important with the increase in fluid inlet velocity/Reynolds number.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122402-1/6]
Titre : Influence of inter- and intraband transitions to electron temperature decay in noble metals after short-pulsed laser heating Type de document : texte imprimé Auteurs : Patrick E. Hopkins, Auteur Année de publication : 2010 Article en page(s) : pp. [122402-1/6] Note générale : Physique Langues : Anglais (eng) Mots-clés : Intraband transition Electron-phonon coupling factor Short-pulsed laser heating Electronic band structure Fermi smearing Two temperature model Index. décimale : 536 Chaleur. Thermodynamique Résumé : This work examines the effects of photonically induced interband excitations from the d-band to states at the Fermi energy on the electron temperature decay in noble metals. The change in the electron population in the d-band and the conduction band causes a change in electron heat capacity and electron-phonon coupling factor. In noble metals, due to the large d-band to Fermi energy separation, the contributions to electron heat capacity and electron-phonon coupling factor of intra- and interband transitions can be separated. The two temperature model describing electron-phonon heat transfer after short-pulsed laser heating is solved using the expressions for heat capacity and electron-phonon coupling factor after intra- and interband excitations, and the predicted electron temperature change of the intra- and interband excited electrons are examined. A critical fluence value is defined that represents the absorbed fluence needed to fill all available states at a given photon energy above the Fermi level. At high absorbed laser fluences and pulse energies greater than the interband transition threshold, the interband and intraband contributions to thermophysical properties differ and are shown to affect temporal electron temperature profiles.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Influence of inter- and intraband transitions to electron temperature decay in noble metals after short-pulsed laser heating [texte imprimé] / Patrick E. Hopkins, Auteur . - 2010 . - pp. [122402-1/6].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122402-1/6]
Mots-clés : Intraband transition Electron-phonon coupling factor Short-pulsed laser heating Electronic band structure Fermi smearing Two temperature model Index. décimale : 536 Chaleur. Thermodynamique Résumé : This work examines the effects of photonically induced interband excitations from the d-band to states at the Fermi energy on the electron temperature decay in noble metals. The change in the electron population in the d-band and the conduction band causes a change in electron heat capacity and electron-phonon coupling factor. In noble metals, due to the large d-band to Fermi energy separation, the contributions to electron heat capacity and electron-phonon coupling factor of intra- and interband transitions can be separated. The two temperature model describing electron-phonon heat transfer after short-pulsed laser heating is solved using the expressions for heat capacity and electron-phonon coupling factor after intra- and interband excitations, and the predicted electron temperature change of the intra- and interband excited electrons are examined. A critical fluence value is defined that represents the absorbed fluence needed to fill all available states at a given photon energy above the Fermi level. At high absorbed laser fluences and pulse energies greater than the interband transition threshold, the interband and intraband contributions to thermophysical properties differ and are shown to affect temporal electron temperature profiles.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122403-1/7]
Titre : Formation mechanism and characteristics of a liquid microlayer in microchannel boiling system Type de document : texte imprimé Auteurs : Yaohua Zhang, Auteur ; Yoshio Utaka, Auteur ; Yuki Kashiwabara, Auteur Année de publication : 2010 Article en page(s) : pp. [122403-1/7] Note générale : Physique Langues : Anglais (eng) Mots-clés : Microlayer Microchannel Phase change Laser extinction method Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experiments were performed using the laser extinction method to measure the thickness of the liquid film formed by growing flattened bubbles in a microchannel for gap sizes of 0.5 mm, 0.3 mm, and 0.15 mm. Water, ethanol, and toluene were used as test fluids. A high-speed camera was also used to simultaneously measure the bubble growth process. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of the interface was divided into two regions: region I, where the velocity is small and the thickness increases linearly with increasing velocity, and region II, where the thickness is almost constant or decreased slightly with increasing velocity. Furthermore, a nondimensional correlation for investigating the effects of test materials and gap sizes on microlayer thickness is presented. An analysis of the results showed that the boundaries of the two regions correspond to a Weber number of approximately 110, and in the region where the Weber number was smaller than 110, the thickness of the microlayer was thinner for the liquid whose value of rho0.62nu0.42sigma−0.62 was relatively small. However, for the region where Weber number was larger than 110, the smaller the kinematic viscosity of the liquid, the thinner the microlayer became.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Formation mechanism and characteristics of a liquid microlayer in microchannel boiling system [texte imprimé] / Yaohua Zhang, Auteur ; Yoshio Utaka, Auteur ; Yuki Kashiwabara, Auteur . - 2010 . - pp. [122403-1/7].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122403-1/7]
Mots-clés : Microlayer Microchannel Phase change Laser extinction method Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experiments were performed using the laser extinction method to measure the thickness of the liquid film formed by growing flattened bubbles in a microchannel for gap sizes of 0.5 mm, 0.3 mm, and 0.15 mm. Water, ethanol, and toluene were used as test fluids. A high-speed camera was also used to simultaneously measure the bubble growth process. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of the interface was divided into two regions: region I, where the velocity is small and the thickness increases linearly with increasing velocity, and region II, where the thickness is almost constant or decreased slightly with increasing velocity. Furthermore, a nondimensional correlation for investigating the effects of test materials and gap sizes on microlayer thickness is presented. An analysis of the results showed that the boundaries of the two regions correspond to a Weber number of approximately 110, and in the region where the Weber number was smaller than 110, the thickness of the microlayer was thinner for the liquid whose value of rho0.62nu0.42sigma−0.62 was relatively small. However, for the region where Weber number was larger than 110, the smaller the kinematic viscosity of the liquid, the thinner the microlayer became.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122501-1/9]
Titre : Prediction of local heat transfer in a vertical cavity using artificial neutral networks Type de document : texte imprimé Auteurs : M. Ebrahim Poulad, Auteur ; D. Naylor, Auteur ; A. S. Fung, Auteur Année de publication : 2010 Article en page(s) : pp. [122501-1/9] Note générale : Physique Langues : Anglais (eng) Mots-clés : Local heat transfer Artifical neural network Mach-Zehnder interferometry Free convection Index. décimale : 536 Chaleur. Thermodynamique Résumé : A time-averaging technique was developed to measure the unsteady and turbulent free convection heat transfer in a tall vertical enclosure using a Mach–Zehnder interferometer. The method used a combination of a digital high speed camera and an interferometer to obtain the local time-averaged heat flux in the cavity. The measured values were used to train an artificial neural network (ANN) algorithm to predict the local heat transfer. The time-averaged local Nusselt number is needed to study local phenomena, e.g., condensation in windows. Optical heat transfer measurements were made in a differentially heated vertical cavity with isothermal walls. The cavity widths were W=12.7 mm, 32.3 mm, 40 mm, and 56.2 mm. The corresponding Rayleigh numbers were about 3×103, 5×104, 1×105, and 2.7×105, respectively, and the enclosure aspect ratio (H/W) ranged from A=18 to 76. The test fluid was air and the temperature differential was about 15 K for all measurements. ALYUDA NEUROINTELLIGENCE (version 2.2) was used to generate solutions for the time-averaged local Nusselt number in the cavity based on the experimental data. Feed-forward architecture and training by the Levenberg–Marquardt algorithm were adopted. The ANN was designed to suit the present system, which had 4–13 inputs and one output. The network predictions were found to be in a good agreement with the experimental local Nusselt number values.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Prediction of local heat transfer in a vertical cavity using artificial neutral networks [texte imprimé] / M. Ebrahim Poulad, Auteur ; D. Naylor, Auteur ; A. S. Fung, Auteur . - 2010 . - pp. [122501-1/9].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122501-1/9]
Mots-clés : Local heat transfer Artifical neural network Mach-Zehnder interferometry Free convection Index. décimale : 536 Chaleur. Thermodynamique Résumé : A time-averaging technique was developed to measure the unsteady and turbulent free convection heat transfer in a tall vertical enclosure using a Mach–Zehnder interferometer. The method used a combination of a digital high speed camera and an interferometer to obtain the local time-averaged heat flux in the cavity. The measured values were used to train an artificial neural network (ANN) algorithm to predict the local heat transfer. The time-averaged local Nusselt number is needed to study local phenomena, e.g., condensation in windows. Optical heat transfer measurements were made in a differentially heated vertical cavity with isothermal walls. The cavity widths were W=12.7 mm, 32.3 mm, 40 mm, and 56.2 mm. The corresponding Rayleigh numbers were about 3×103, 5×104, 1×105, and 2.7×105, respectively, and the enclosure aspect ratio (H/W) ranged from A=18 to 76. The test fluid was air and the temperature differential was about 15 K for all measurements. ALYUDA NEUROINTELLIGENCE (version 2.2) was used to generate solutions for the time-averaged local Nusselt number in the cavity based on the experimental data. Feed-forward architecture and training by the Levenberg–Marquardt algorithm were adopted. The ANN was designed to suit the present system, which had 4–13 inputs and one output. The network predictions were found to be in a good agreement with the experimental local Nusselt number values.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122701-1/8]
Titre : Analytical solution under two-flux approximation to radiative heat transfer in absorbing emitting and anisotropically scattering medium Type de document : texte imprimé Auteurs : Xin-Lin Xia, Auteur ; Dong-Hui Li, Auteur ; Feng-Xian Sun, Auteur Année de publication : 2010 Article en page(s) : pp. [122701-1/8] Note générale : Physique Langues : Anglais (eng) Mots-clés : Anisotropically scattering Radiative heat transfer Analytical solution Two-flux approximation Index. décimale : 536 Chaleur. Thermodynamique Résumé : Radiative transfer in absorbing, emitting, and highly anisotropically scattering media is widely encountered in high temperature applications such as pulverized coal firing furnaces and high temperature thermal protection materials. Efficient and effective solution methods for the transfer process are very crucial, especially in thermal radiation related reverse problems and optimization designs. In this study, the analytical solution for radiative heat transfer in an absorbing, emitting, and anisotropically scattering slab between two parallel gray walls are derived under the two-flux approximation. Explicit expression for the radiative heat flux in a slab is obtained under two-flux approximation. The reliability and adaptability of an analytical solution is examined in case studies by comparing with the Monte Carlo results. Comparative studies indicate that the analytical solution can be used in radiative transfer calculation in an absorbing emitting and anisotropically scattering slab. It is much more applicable in a forward and isotropic scattering slab than in an absorbing one, especially in a forward scattering slab. Because of simplicity and high computing efficiency with the analytical solution, it may be useful in reverse radiative transfer problems, in optimization design, and in developing some numerical schemes on radiative heat transfer.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] Analytical solution under two-flux approximation to radiative heat transfer in absorbing emitting and anisotropically scattering medium [texte imprimé] / Xin-Lin Xia, Auteur ; Dong-Hui Li, Auteur ; Feng-Xian Sun, Auteur . - 2010 . - pp. [122701-1/8].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122701-1/8]
Mots-clés : Anisotropically scattering Radiative heat transfer Analytical solution Two-flux approximation Index. décimale : 536 Chaleur. Thermodynamique Résumé : Radiative transfer in absorbing, emitting, and highly anisotropically scattering media is widely encountered in high temperature applications such as pulverized coal firing furnaces and high temperature thermal protection materials. Efficient and effective solution methods for the transfer process are very crucial, especially in thermal radiation related reverse problems and optimization designs. In this study, the analytical solution for radiative heat transfer in an absorbing, emitting, and anisotropically scattering slab between two parallel gray walls are derived under the two-flux approximation. Explicit expression for the radiative heat flux in a slab is obtained under two-flux approximation. The reliability and adaptability of an analytical solution is examined in case studies by comparing with the Monte Carlo results. Comparative studies indicate that the analytical solution can be used in radiative transfer calculation in an absorbing emitting and anisotropically scattering slab. It is much more applicable in a forward and isotropic scattering slab than in an absorbing one, especially in a forward scattering slab. Because of simplicity and high computing efficiency with the analytical solution, it may be useful in reverse radiative transfer problems, in optimization design, and in developing some numerical schemes on radiative heat transfer.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122901-1/7]
Titre : A Computational fluid dynamics study on the effect of carbon particle seeding for the improvement of solar reactor performance Type de document : texte imprimé Auteurs : Nesrin Ozalp, Auteur ; Anoop Kanjirakat, Auteur Année de publication : 2010 Article en page(s) : pp. [122901-1/7] Note générale : Physique Langues : Anglais (eng) Mots-clés : Computational flow dynamics Solar reactor Carbon seeding Solar cracking Hydrogen Index. décimale : 536 Chaleur. Thermodynamique Résumé : This study focuses on a technique, referred to as “solar cracking” of natural gas for the coproduction of hydrogen and carbon as byproduct with zero emission footprint. Seeding a solar reactor with micron-sized carbon particles increases the conversion efficiency drastically due to the radiation absorbed by the carbon particles and additional nucleation sites formed by carbon particles for heterogeneous decomposition reaction. The present study numerically tries to investigate the above fact by tracking carbon particles in a Lagrangian framework. The results on the effect of particle loading, particle emissivity, injection point location, and effect of using different window screening gases on a flow and temperature distribution inside a confined tornado flow reactor are presented.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] A Computational fluid dynamics study on the effect of carbon particle seeding for the improvement of solar reactor performance [texte imprimé] / Nesrin Ozalp, Auteur ; Anoop Kanjirakat, Auteur . - 2010 . - pp. [122901-1/7].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [122901-1/7]
Mots-clés : Computational flow dynamics Solar reactor Carbon seeding Solar cracking Hydrogen Index. décimale : 536 Chaleur. Thermodynamique Résumé : This study focuses on a technique, referred to as “solar cracking” of natural gas for the coproduction of hydrogen and carbon as byproduct with zero emission footprint. Seeding a solar reactor with micron-sized carbon particles increases the conversion efficiency drastically due to the radiation absorbed by the carbon particles and additional nucleation sites formed by carbon particles for heterogeneous decomposition reaction. The present study numerically tries to investigate the above fact by tracking carbon particles in a Lagrangian framework. The results on the effect of particle loading, particle emissivity, injection point location, and effect of using different window screening gases on a flow and temperature distribution inside a confined tornado flow reactor are presented.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [124501-1/4]
Titre : On the analysis of the aerodynamic heating problem Type de document : texte imprimé Auteurs : A. Ozer Arnas, Auteur ; Daisie D. Boettner, Auteur ; Gunnar Tamm, Auteur Année de publication : 2010 Article en page(s) : pp. [124501-1/4] Note générale : Physique Langues : Anglais (eng) Mots-clés : Aerodynamic heating Convection Flat plate Index. décimale : 536 Chaleur. Thermodynamique Résumé : A complete analytical solution to the problem of aerodynamic heating is lacking in heat transfer textbooks, which are used for undergraduate and graduate education. There are many issues that are very important from a convective heat transfer point of view. In practice, poor analyses lead to poor design, thus faulty manufacturing. Since, over the years analysis has given way to numerical studies, the instructors do not take the necessary time to go through analytical details. Thus the students just use the results without any awareness of how to get them and the inherent limitations of the analytical solution. The only intent of this paper, therefore, is to present the detailed analytical study of the aerodynamic heating problem.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] On the analysis of the aerodynamic heating problem [texte imprimé] / A. Ozer Arnas, Auteur ; Daisie D. Boettner, Auteur ; Gunnar Tamm, Auteur . - 2010 . - pp. [124501-1/4].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [124501-1/4]
Mots-clés : Aerodynamic heating Convection Flat plate Index. décimale : 536 Chaleur. Thermodynamique Résumé : A complete analytical solution to the problem of aerodynamic heating is lacking in heat transfer textbooks, which are used for undergraduate and graduate education. There are many issues that are very important from a convective heat transfer point of view. In practice, poor analyses lead to poor design, thus faulty manufacturing. Since, over the years analysis has given way to numerical studies, the instructors do not take the necessary time to go through analytical details. Thus the students just use the results without any awareness of how to get them and the inherent limitations of the analytical solution. The only intent of this paper, therefore, is to present the detailed analytical study of the aerodynamic heating problem.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [124502-1/4]
Titre : On the condition for thermal rectification using bulk materials Type de document : texte imprimé Auteurs : David B. Go, Auteur ; Mihir, Sen, Auteur Année de publication : 2010 Article en page(s) : pp. [124502-1/4] Note générale : Physique Langues : Anglais (eng) Mots-clés : Rectification Conduction Composite material Analytical Index. décimale : 536 Chaleur. Thermodynamique Résumé : In conduction, thermal rectification occurs when a material or structure transfers heat asymmetrically. In this work, it is shown that, in the bulk, a necessary condition for thermal rectification is that the thermal conductivity of the material or structure be a function of both space and temperature, and that this function be nonseparable. Practically, this can be achieved using a composite structure consisting of many materials although it could also be achieved with a properly developed single material.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...] [article] On the condition for thermal rectification using bulk materials [texte imprimé] / David B. Go, Auteur ; Mihir, Sen, Auteur . - 2010 . - pp. [124502-1/4].
Physique
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 132 N° 12 (Décembre 2010) . - pp. [124502-1/4]
Mots-clés : Rectification Conduction Composite material Analytical Index. décimale : 536 Chaleur. Thermodynamique Résumé : In conduction, thermal rectification occurs when a material or structure transfers heat asymmetrically. In this work, it is shown that, in the bulk, a necessary condition for thermal rectification is that the thermal conductivity of the material or structure be a function of both space and temperature, and that this function be nonseparable. Practically, this can be achieved using a composite structure consisting of many materials although it could also be achieved with a properly developed single material.
DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&ONLINE=YES&smode= [...]
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