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Journal of heat transfer |
Dépouillements
Ajouter le résultat dans votre panierFlow boiling of water on nanocoated surfaces in a microchannel / Hai Trieu Phan in Journal of heat transfer, Vol. 134 N° 2 (Fevrier 2012)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 06 p.
Titre : Flow boiling of water on nanocoated surfaces in a microchannel Type de document : texte imprimé Auteurs : Hai Trieu Phan, Auteur ; Nadia Caney, Auteur ; Philippe Marty, Auteur Année de publication : 2012 Article en page(s) : 06 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boiling Contact angle Diamond-like carbon Heat transfer Microchannel flow Silicon compounds Titanium Two-phase flow Water Wetting Index. décimale : 536 Chaleur. Thermodynamique Résumé : The experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m2 s and the base heat flux varied from 30 to 80 kW/m2. Water enters the test channel under subcooled conditions. The samples are silicon oxide (SiOx), titanium (Ti), diamond-like carbon (DLC), and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26 deg, 49 deg, 63 deg, and 104 deg, respectively. The results show significant impacts of surface wettability on heat transfer coefficient. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Flow boiling of water on nanocoated surfaces in a microchannel [texte imprimé] / Hai Trieu Phan, Auteur ; Nadia Caney, Auteur ; Philippe Marty, Auteur . - 2012 . - 06 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 06 p.
Mots-clés : Boiling Contact angle Diamond-like carbon Heat transfer Microchannel flow Silicon compounds Titanium Two-phase flow Water Wetting Index. décimale : 536 Chaleur. Thermodynamique Résumé : The experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m2 s and the base heat flux varied from 30 to 80 kW/m2. Water enters the test channel under subcooled conditions. The samples are silicon oxide (SiOx), titanium (Ti), diamond-like carbon (DLC), and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26 deg, 49 deg, 63 deg, and 104 deg, respectively. The results show significant impacts of surface wettability on heat transfer coefficient. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 06 p.
Titre : A theoretical model for axial heat conduction effects during single-phase flow in microchannels Type de document : texte imprimé Auteurs : Ting-Yu Lin, Auteur ; Satish G. Kandlikar, Auteur Année de publication : 2012 Article en page(s) : 06 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boundary layers Heat conduction Laminar flow Microchannel flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : A model is developed to analyze the effect of axial conduction on heat transfer during single-phase flow in microchannels. The axial heat conduction in the wall introduces heat flow toward the inlet section resulting in an increase in the local fluid temperature and a corresponding increase in the wall temperature. Neglecting this effect while reducing the experimental data results in a lower value of the experimental Nusselt number. The model derived in this work takes into account this effect and offers a parameter to estimate the effect introduced by the axial heat conduction effect in the wall. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] A theoretical model for axial heat conduction effects during single-phase flow in microchannels [texte imprimé] / Ting-Yu Lin, Auteur ; Satish G. Kandlikar, Auteur . - 2012 . - 06 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 06 p.
Mots-clés : Boundary layers Heat conduction Laminar flow Microchannel flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : A model is developed to analyze the effect of axial conduction on heat transfer during single-phase flow in microchannels. The axial heat conduction in the wall introduces heat flow toward the inlet section resulting in an increase in the local fluid temperature and a corresponding increase in the wall temperature. Neglecting this effect while reducing the experimental data results in a lower value of the experimental Nusselt number. The model derived in this work takes into account this effect and offers a parameter to estimate the effect introduced by the axial heat conduction effect in the wall. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 07 p.
Titre : Theoretical and experimental study of a flexible wiretype Joule–Thomson microrefrigerator for use in cryosurgery Type de document : texte imprimé Auteurs : Adhika Widyaparaga, Auteur ; Masashi Kuwamoto, Auteur ; Naoya Sakoda, Auteur Année de publication : 2012 Article en page(s) : 07 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Coolants Cooling Equations of state Finite difference methods Flow simulation Heat exchangers Heat radiation Joule-Thomson effect Natural convection Refrigerators Stainless steel Temperature distribution Temperature measurement Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : We have developed a model capable of predicting the performance characteristics of a wiretype Joule–Thomson microcooler intended for use within a cryosurgical probe. Our objective was to be able to predict cold tip temperature, temperature distribution, and cooling power using only inlet gas properties as input variables. To achieve this, the model incorporated gas equations of state to account for changing gas properties due to heat transfer within the heat exchanger and expansion within the capillary. In consideration of inefficiencies, heat in-leak from free convection and radiation was also considered and the use of a 2D axisymmetric finite difference code allowed simulation of axial conduction. To validate simulation results, we have constructed and conducted experiments with two types of microcoolers differing in inner tube material, poly-ether-ether-ketone (PEEK) and stainless steel. The parameters of the experiment were used in the calculations. CO2 was used as the coolant gas for inlet pressures from 0.5 MPa to 2.0 MPa. Heat load trials of up to 550 mW along with unloaded trials were conducted. The temperature measurements show that the model was successfully able to predict the cold tip temperature to a good degree of accuracy and well represent the temperature distribution. For the all PEEK microcooler in a vacuum using 2.0 MPa inlet pressure, the calculations predicted a temperature drop of 57 K and mass flow rate of 19.5 mg/s compared to measured values of 63 K and 19.4 mg/s, therefore, showing that conventional macroscale correlations can hold well for turbulent microscale flow and heat transfer as long as the validity of the assumptions is verified. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Theoretical and experimental study of a flexible wiretype Joule–Thomson microrefrigerator for use in cryosurgery [texte imprimé] / Adhika Widyaparaga, Auteur ; Masashi Kuwamoto, Auteur ; Naoya Sakoda, Auteur . - 2012 . - 07 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 07 p.
Mots-clés : Coolants Cooling Equations of state Finite difference methods Flow simulation Heat exchangers Heat radiation Joule-Thomson effect Natural convection Refrigerators Stainless steel Temperature distribution Temperature measurement Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : We have developed a model capable of predicting the performance characteristics of a wiretype Joule–Thomson microcooler intended for use within a cryosurgical probe. Our objective was to be able to predict cold tip temperature, temperature distribution, and cooling power using only inlet gas properties as input variables. To achieve this, the model incorporated gas equations of state to account for changing gas properties due to heat transfer within the heat exchanger and expansion within the capillary. In consideration of inefficiencies, heat in-leak from free convection and radiation was also considered and the use of a 2D axisymmetric finite difference code allowed simulation of axial conduction. To validate simulation results, we have constructed and conducted experiments with two types of microcoolers differing in inner tube material, poly-ether-ether-ketone (PEEK) and stainless steel. The parameters of the experiment were used in the calculations. CO2 was used as the coolant gas for inlet pressures from 0.5 MPa to 2.0 MPa. Heat load trials of up to 550 mW along with unloaded trials were conducted. The temperature measurements show that the model was successfully able to predict the cold tip temperature to a good degree of accuracy and well represent the temperature distribution. For the all PEEK microcooler in a vacuum using 2.0 MPa inlet pressure, the calculations predicted a temperature drop of 57 K and mass flow rate of 19.5 mg/s compared to measured values of 63 K and 19.4 mg/s, therefore, showing that conventional macroscale correlations can hold well for turbulent microscale flow and heat transfer as long as the validity of the assumptions is verified. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p.
Titre : Identifying inefficiencies in unsteady pin fin heat transfer using orthogonal decomposition Type de document : texte imprimé Auteurs : Markus Schwänen, Auteur ; Andrew Duggleby, Auteur Année de publication : 2012 Article en page(s) : 10 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Blades Channel flow Cooling Flow instability Flow simulation Gas turbines Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Internal cooling of the trailing edge region in a gas turbine blade is typically achieved with an array of pin fins. In order to better understand the effectiveness of this configuration, high performance computations are performed on cylindrical pin fins with a spanwise distance to fin diameter ratio of 2 and height over fin diameter ratio of one. For validation purposes, the flow Reynolds number based on hydraulic channel diameter and bulk velocity (Re = 12,800) was set to match experiments available in the open literature. Simulations included a URANS and LES on a single row of pin fins where the URANS domain was 1 pin wide versus the LES with 3 pins. The resulting time-dependent flow field was analyzed using a variation of bi-orthogonal decomposition (BOD), where the correlation matrices were built using the internal energy in addition to the three velocity components. This enables a detailed comparison of URANS and LES to assess the URANS modeling assumptions as well as a flow decomposition with respect to the flow structure's influence on surface heat transfer. This analysis shows low order modes which do not contribute to turbulent heat flux, but instead increase the heat exchanger's global inefficiency. In the URANS study, the forth mode showed the first nonzero temperature basis function, which means that a considerable amount of energy is contained in flow structures that do not contribute to increasing endwall heat transfer. In the LES, the first non zero temperature basis function was the seventh mode. Both orthogonal basis function sets were evaluated with respect to each mode's contribution to turbulent heat exchange with the surface. This analysis showed that there exists one distinct, high energy mode that contributes to wall heat flux, whereas all others do not. Modifying this mode could potentially be used to improve the heat exchanger's efficiency with respect to pressure loss. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Identifying inefficiencies in unsteady pin fin heat transfer using orthogonal decomposition [texte imprimé] / Markus Schwänen, Auteur ; Andrew Duggleby, Auteur . - 2012 . - 10 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p.
Mots-clés : Blades Channel flow Cooling Flow instability Flow simulation Gas turbines Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Internal cooling of the trailing edge region in a gas turbine blade is typically achieved with an array of pin fins. In order to better understand the effectiveness of this configuration, high performance computations are performed on cylindrical pin fins with a spanwise distance to fin diameter ratio of 2 and height over fin diameter ratio of one. For validation purposes, the flow Reynolds number based on hydraulic channel diameter and bulk velocity (Re = 12,800) was set to match experiments available in the open literature. Simulations included a URANS and LES on a single row of pin fins where the URANS domain was 1 pin wide versus the LES with 3 pins. The resulting time-dependent flow field was analyzed using a variation of bi-orthogonal decomposition (BOD), where the correlation matrices were built using the internal energy in addition to the three velocity components. This enables a detailed comparison of URANS and LES to assess the URANS modeling assumptions as well as a flow decomposition with respect to the flow structure's influence on surface heat transfer. This analysis shows low order modes which do not contribute to turbulent heat flux, but instead increase the heat exchanger's global inefficiency. In the URANS study, the forth mode showed the first nonzero temperature basis function, which means that a considerable amount of energy is contained in flow structures that do not contribute to increasing endwall heat transfer. In the LES, the first non zero temperature basis function was the seventh mode. Both orthogonal basis function sets were evaluated with respect to each mode's contribution to turbulent heat exchange with the surface. This analysis showed that there exists one distinct, high energy mode that contributes to wall heat flux, whereas all others do not. Modifying this mode could potentially be used to improve the heat exchanger's efficiency with respect to pressure loss. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Titre : Forced convection heat transfer using high temperature and pressure water in an upward-inclined tube Type de document : texte imprimé Auteurs : Wu Gang, Auteur ; Qincheng Bi, Auteur ; Han Wang, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Forced convection High-pressure effects Mass transfer Natural convection Pipe flow Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : Within the range of pressure from 9 to 28 MPa, mass flux from 600 to 1500 kg/m2s, heat flux at inside wall from 200 to 600 kW/m2, and wall temperature up to 650 °C, experiments were conducted to research the forced convection heat transfer of water in an inclined upward tube with an inclination angle of 20 deg and an inner diameter of 26 mm. According to the experimental data, the effects of pressure and heat flux on heat transfer of water were analyzed in detail. In the subcritical pressure region, it was found that heat transfer characteristics of water are not uniform along the circumference of the inclined tube. Temperature of the top is always higher than that of the bottom, which can be attributed to the buoyancy effect in the inclined tube. In the supercritical pressure region, natural convection makes the low-density hot fluid gather at the top of the inclined tube; hence, heat transfer condition is deteriorated and wall temperature is increased. Furthermore, the criterions of Petukhov and Jackson were selected to judge the buoyancy effect in the inclined upward tube. The result seems acceptable but these criterions should be further improved to get a better applicability for an inclined tube. DEWEY : 536 ISSN : 0022-1481 En ligne : http://www.asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO00013400 [...] [article] Forced convection heat transfer using high temperature and pressure water in an upward-inclined tube [texte imprimé] / Wu Gang, Auteur ; Qincheng Bi, Auteur ; Han Wang, Auteur . - 2012 . - 08 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Mots-clés : Forced convection High-pressure effects Mass transfer Natural convection Pipe flow Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : Within the range of pressure from 9 to 28 MPa, mass flux from 600 to 1500 kg/m2s, heat flux at inside wall from 200 to 600 kW/m2, and wall temperature up to 650 °C, experiments were conducted to research the forced convection heat transfer of water in an inclined upward tube with an inclination angle of 20 deg and an inner diameter of 26 mm. According to the experimental data, the effects of pressure and heat flux on heat transfer of water were analyzed in detail. In the subcritical pressure region, it was found that heat transfer characteristics of water are not uniform along the circumference of the inclined tube. Temperature of the top is always higher than that of the bottom, which can be attributed to the buoyancy effect in the inclined tube. In the supercritical pressure region, natural convection makes the low-density hot fluid gather at the top of the inclined tube; hence, heat transfer condition is deteriorated and wall temperature is increased. Furthermore, the criterions of Petukhov and Jackson were selected to judge the buoyancy effect in the inclined upward tube. The result seems acceptable but these criterions should be further improved to get a better applicability for an inclined tube. DEWEY : 536 ISSN : 0022-1481 En ligne : http://www.asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO00013400 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012)
Titre : Flow boiling characteristics for R1234ze(E) in 1.0 and 2.2 mm circular channels Type de document : texte imprimé Auteurs : Cristiano Bigonha Tibiriçá, Auteur ; Gherhardt Ribatski, Auteur ; John Richard Thome, Auteur Année de publication : 2012 Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boiling Flow visualisation Heat transfer Microchannel flow Pattern formation Pipe flow Refrigerants Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I.D. (internal diameter) stainless steel tubes for tests with R1234ze(E), a new refrigerant developed as a substitute for R134a with a much lower global warming potential (GWP). The experiments were performed for these two tube diameters in order to investigate a possible transition between macro and microscale flow boiling behavior. The experimental campaign includes mass velocities ranging from 50 to 1500 kg/m2 s, heat fluxes from 10 to 300 kW/m2, exit saturation temperatures of 25, 31 and 35 °C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high speed videos. Heat transfer coefficient, critical heat flux and flow pattern data were obtained. R1234ze(E) demonstrated similar thermal performance to R134a data when running at similar conditions. DEWEY : 536 ISSN : 0022-1481 En ligne : http://www.asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO00013400 [...] [article] Flow boiling characteristics for R1234ze(E) in 1.0 and 2.2 mm circular channels [texte imprimé] / Cristiano Bigonha Tibiriçá, Auteur ; Gherhardt Ribatski, Auteur ; John Richard Thome, Auteur . - 2012.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012)
Mots-clés : Boiling Flow visualisation Heat transfer Microchannel flow Pattern formation Pipe flow Refrigerants Index. décimale : 536 Chaleur. Thermodynamique Résumé : Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I.D. (internal diameter) stainless steel tubes for tests with R1234ze(E), a new refrigerant developed as a substitute for R134a with a much lower global warming potential (GWP). The experiments were performed for these two tube diameters in order to investigate a possible transition between macro and microscale flow boiling behavior. The experimental campaign includes mass velocities ranging from 50 to 1500 kg/m2 s, heat fluxes from 10 to 300 kW/m2, exit saturation temperatures of 25, 31 and 35 °C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high speed videos. Heat transfer coefficient, critical heat flux and flow pattern data were obtained. R1234ze(E) demonstrated similar thermal performance to R134a data when running at similar conditions. DEWEY : 536 ISSN : 0022-1481 En ligne : http://www.asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO00013400 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Titre : Heat transfer to suspensions of microencapsulated phase change material flowing through minichannels Type de document : texte imprimé Auteurs : Frank Dammel, Auteur ; Peter Stephan, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Channel flow Flow simulation Heat transfer Suspensions Viscosity Index. décimale : 536 Chaleur. Thermodynamique Résumé : The heat transfer to water-based suspensions of microencapsulated phase change material (MEPCM) flowing laminarly through rectangular copper minichannels was investigated both experimentally and numerically. The MEPCM-particles had an average size of 5 µm and contained as phase change material n-eicosane, which has a theoretical melting temperature of 36.4 °C. Water and suspensions with particle mass fractions of 10% and 20% were considered. While the experiments result in rather global values such as wall temperatures at certain points, suspension in- and outlet temperatures, and the pressure drop, the numerical simulations allow additionally a more detailed insight, for example, into the temperature distribution in the flowing suspension. The results show that MEPCM suspensions are only advantageous in comparison to water in a certain range of parameter combinations, where the latent heat is exploited to a high degree. The available latent heat storage potential, which depends on the particle fraction in the suspension and on the mass flow rate, has to be in the same order of magnitude as the supplied heat. Moreover, the mean residence time of the particles in the cooling channels must not be considerably shorter than the characteristic time for heat conduction perpendicular to the flow direction. Otherwise, the particles in the center region of the flow leave the cooling channels with still solid cores, and their latent heat is not exploited. Furthermore, the benefit of the added MEPCM particles depends on the inlet temperature, which has to be slightly below the theoretical melting temperature, and on the subcooling temperature after the heat supply, which has to be sufficiently low to guarantee that the entire phase change material solidifies again before it re-enters the cooling channels. The suspensions showed Newtonian behavior in the viscosity measurement. The actual pressure drop determined in the experiments is smaller than the pressure drop estimation based on the measured viscosities. The difference between the two values increases with increasing particle mass fraction. This shows that the particles are not evenly distributed in the flowing suspension, but that there is a particle-depleted layer close to the channel walls. This reduces the required pumping power, but makes it even more important to provide conditions, in which a sufficiently large amount of the supplied heat is conducted to the center region of the channels. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Heat transfer to suspensions of microencapsulated phase change material flowing through minichannels [texte imprimé] / Frank Dammel, Auteur ; Peter Stephan, Auteur . - 2012 . - 08 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Mots-clés : Channel flow Flow simulation Heat transfer Suspensions Viscosity Index. décimale : 536 Chaleur. Thermodynamique Résumé : The heat transfer to water-based suspensions of microencapsulated phase change material (MEPCM) flowing laminarly through rectangular copper minichannels was investigated both experimentally and numerically. The MEPCM-particles had an average size of 5 µm and contained as phase change material n-eicosane, which has a theoretical melting temperature of 36.4 °C. Water and suspensions with particle mass fractions of 10% and 20% were considered. While the experiments result in rather global values such as wall temperatures at certain points, suspension in- and outlet temperatures, and the pressure drop, the numerical simulations allow additionally a more detailed insight, for example, into the temperature distribution in the flowing suspension. The results show that MEPCM suspensions are only advantageous in comparison to water in a certain range of parameter combinations, where the latent heat is exploited to a high degree. The available latent heat storage potential, which depends on the particle fraction in the suspension and on the mass flow rate, has to be in the same order of magnitude as the supplied heat. Moreover, the mean residence time of the particles in the cooling channels must not be considerably shorter than the characteristic time for heat conduction perpendicular to the flow direction. Otherwise, the particles in the center region of the flow leave the cooling channels with still solid cores, and their latent heat is not exploited. Furthermore, the benefit of the added MEPCM particles depends on the inlet temperature, which has to be slightly below the theoretical melting temperature, and on the subcooling temperature after the heat supply, which has to be sufficiently low to guarantee that the entire phase change material solidifies again before it re-enters the cooling channels. The suspensions showed Newtonian behavior in the viscosity measurement. The actual pressure drop determined in the experiments is smaller than the pressure drop estimation based on the measured viscosities. The difference between the two values increases with increasing particle mass fraction. This shows that the particles are not evenly distributed in the flowing suspension, but that there is a particle-depleted layer close to the channel walls. This reduces the required pumping power, but makes it even more important to provide conditions, in which a sufficiently large amount of the supplied heat is conducted to the center region of the channels. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 13 p.
Titre : Gas microflows in the slip flow regime : a critical review on convective heat transfer Type de document : texte imprimé Auteurs : Colin, Stéphane, Auteur Année de publication : 2012 Article en page(s) : 13 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Aerodynamics Convection Knudsen flow Microchannel flow Slip flow Viscosity Index. décimale : 536 Chaleur. Thermodynamique Résumé : Accurate modeling of gas microvection is crucial for a lot of MEMS applications (microheat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature microsensors, micropumps, and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum, and dosing valves…). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10−2–10−1. In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Gas microflows in the slip flow regime : a critical review on convective heat transfer [texte imprimé] / Colin, Stéphane, Auteur . - 2012 . - 13 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 13 p.
Mots-clés : Aerodynamics Convection Knudsen flow Microchannel flow Slip flow Viscosity Index. décimale : 536 Chaleur. Thermodynamique Résumé : Accurate modeling of gas microvection is crucial for a lot of MEMS applications (microheat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature microsensors, micropumps, and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum, and dosing valves…). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10−2–10−1. In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Titre : Submerged jet impingement boiling of water under subatmospheric conditions Type de document : texte imprimé Auteurs : Ruander Cardenas, Auteur ; Vinod Narayanan, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boiling Heat transfer Jets Surface roughness Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : An experimental study of jet impingement boiling is presented for water under saturated and subcooled conditions. Unique to this study is the documentation of boiling curves of a submerged water jet under subatmospheric conditions. Data are reported at a fixed nondimensional nozzle-to-surface distance of H/dj = 6 and for a fixed surface-to-nozzle diameter ratio, dsurf/dj, of 23.8. Saturated jet impingement experiments are performed at three subatmospheric pool pressures of 0.176 bar, 0.276 bar, and 0.478 bar with corresponding saturation temperatures of 57.3 °C, 67.2 °C, and 80.2 °C. At each pressure, jet impingement boiling at varying Reynolds numbers are characterized and compared with pool boiling heat transfer. The effect of surface roughness and fluid subcooling is studied at the lowest pressure of 0.176 bar. Boiling curves indicate a strong dependence of heat flux on jet Reynolds number in the partially developed nucleate boiling region but only a weak dependence in the fully developed nucleate boiling region. At a fixed wall superheat, fluid subcooling is found to shift the boiling curve to the left thereby enhancing heat transfer performance. Critical heat flux is found to increase with increases in pressure, surface roughness, and Reynolds number. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Submerged jet impingement boiling of water under subatmospheric conditions [texte imprimé] / Ruander Cardenas, Auteur ; Vinod Narayanan, Auteur . - 2012 . - 08 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Mots-clés : Boiling Heat transfer Jets Surface roughness Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : An experimental study of jet impingement boiling is presented for water under saturated and subcooled conditions. Unique to this study is the documentation of boiling curves of a submerged water jet under subatmospheric conditions. Data are reported at a fixed nondimensional nozzle-to-surface distance of H/dj = 6 and for a fixed surface-to-nozzle diameter ratio, dsurf/dj, of 23.8. Saturated jet impingement experiments are performed at three subatmospheric pool pressures of 0.176 bar, 0.276 bar, and 0.478 bar with corresponding saturation temperatures of 57.3 °C, 67.2 °C, and 80.2 °C. At each pressure, jet impingement boiling at varying Reynolds numbers are characterized and compared with pool boiling heat transfer. The effect of surface roughness and fluid subcooling is studied at the lowest pressure of 0.176 bar. Boiling curves indicate a strong dependence of heat flux on jet Reynolds number in the partially developed nucleate boiling region but only a weak dependence in the fully developed nucleate boiling region. At a fixed wall superheat, fluid subcooling is found to shift the boiling curve to the left thereby enhancing heat transfer performance. Critical heat flux is found to increase with increases in pressure, surface roughness, and Reynolds number. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 07 p.
Titre : Prediction and measurement of thermal transport across interfaces between isotropic solids and graphitic materials Type de document : texte imprimé Auteurs : Pamela M. Norris, Auteur ; Justin L. Smoyer, Auteur ; John C. Duda, Auteur Année de publication : 2012 Article en page(s) : 07 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Carbon nanotubes Gold Graphene Heat conduction Phonons Thermal conductivity Thermoreflectance Thin films Index. décimale : 536 Chaleur. Thermodynamique Résumé : Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Prediction and measurement of thermal transport across interfaces between isotropic solids and graphitic materials [texte imprimé] / Pamela M. Norris, Auteur ; Justin L. Smoyer, Auteur ; John C. Duda, Auteur . - 2012 . - 07 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 07 p.
Mots-clés : Carbon nanotubes Gold Graphene Heat conduction Phonons Thermal conductivity Thermoreflectance Thin films Index. décimale : 536 Chaleur. Thermodynamique Résumé : Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p.
Titre : Numerical investigation of heat transfer in rectangular microchannels under H2 boundary condition during developing and fully developed laminar flow Type de document : texte imprimé Auteurs : V. V. Dharaiya, Auteur ; S. G. Kandlikar, Auteur Année de publication : 2012 Article en page(s) : 10 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boundary layers Computational fluid dynamics Flow simulation Heat sinks Heat transfer Laminar flow Numerical analysis Index. décimale : 536 Chaleur. Thermodynamique Résumé : Study of fluid flow characteristics at microscale is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in electronic chip cooling, heat exchangers, MEMS, and microfluidic devices. Due to short lengths employed in microchannels, entrance header effects can be significant and need to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (alpha = a/b) ranging from 0.1 to 10, are numerically simulated using CFD software tool fluent. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of nondimensional axial length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions are validated for existing data set for four wall heat flux case. Large numerical data sets are generated in this work for rectangular cross-sectional microchannels with heating on three walls, two opposing walls, one wall, and two adjacent walls under H2 boundary condition. This information can provide better understanding and insight into the transport processes in the microchannels. Although the results are seen as relevant in microscale applications, they are applicable to any sized channels. Based on the numerical results obtained for the whole range, generalized correlations for Nusselt numbers as a function of channel aspect ratio are presented for all the cases. The predicted correlations for Nusselt numbers can be very useful resource for the design and optimization of microchannel heat sinks and other microfluidic devices. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Numerical investigation of heat transfer in rectangular microchannels under H2 boundary condition during developing and fully developed laminar flow [texte imprimé] / V. V. Dharaiya, Auteur ; S. G. Kandlikar, Auteur . - 2012 . - 10 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p.
Mots-clés : Boundary layers Computational fluid dynamics Flow simulation Heat sinks Heat transfer Laminar flow Numerical analysis Index. décimale : 536 Chaleur. Thermodynamique Résumé : Study of fluid flow characteristics at microscale is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in electronic chip cooling, heat exchangers, MEMS, and microfluidic devices. Due to short lengths employed in microchannels, entrance header effects can be significant and need to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (alpha = a/b) ranging from 0.1 to 10, are numerically simulated using CFD software tool fluent. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of nondimensional axial length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions are validated for existing data set for four wall heat flux case. Large numerical data sets are generated in this work for rectangular cross-sectional microchannels with heating on three walls, two opposing walls, one wall, and two adjacent walls under H2 boundary condition. This information can provide better understanding and insight into the transport processes in the microchannels. Although the results are seen as relevant in microscale applications, they are applicable to any sized channels. Based on the numerical results obtained for the whole range, generalized correlations for Nusselt numbers as a function of channel aspect ratio are presented for all the cases. The predicted correlations for Nusselt numbers can be very useful resource for the design and optimization of microchannel heat sinks and other microfluidic devices. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Titre : Wetting mode evolution of steam dropwise condensation on superhydrophobic surface in the presence of noncondensable gas Type de document : texte imprimé Auteurs : Xuehu Ma, Auteur ; Sifang Wang, Auteur ; Zhong Lan, Auteur Année de publication : 2012 Article en page(s) : 09 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Air Condensation Contact angle Drops Heat transfer Hydrophobicity Steam Surface roughness Wetting Index. décimale : 536 Chaleur. Thermodynamique Résumé : It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Wetting mode evolution of steam dropwise condensation on superhydrophobic surface in the presence of noncondensable gas [texte imprimé] / Xuehu Ma, Auteur ; Sifang Wang, Auteur ; Zhong Lan, Auteur . - 2012 . - 09 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Mots-clés : Air Condensation Contact angle Drops Heat transfer Hydrophobicity Steam Surface roughness Wetting Index. décimale : 536 Chaleur. Thermodynamique Résumé : It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Titre : Water-heated pool boiling of different refrigerants on the outside surface of a horizontal smooth tube Type de document : texte imprimé Auteurs : Tailian Chen, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boiling Copper Flow measurement Heat transfer Pipe flow Refrigerants Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : Pool boiling heat transfer has been extensively studied over decades, but the effect of boundary heating conditions on boiling received little attention. In this work, heat transfer coefficients during pool boiling of five different refrigerants (R123, R245fa, R236fa, R134a, and R22) on the outside surface of a smooth copper tube were measured at the saturation temperature of 6.7 °C; water flows inside the tube and provides heat to the refrigerants to boil (thus, water-heated boiling). Measurements showed that the refrigerant of a higher vapor pressure has a higher heat transfer coefficient, with the exception that R22 performs nearly the same as R134a. A correlation previously developed for electrically-heated pool boiling on cylindrical tubes underpredicts by 30%–46% the heat transfer coefficients during water-heated boiling of the five refrigerants. Among the pool boiling correlations reviewed in this work, the Cooper correlation (for pool boiling on cylindrical tubes) predicts the boiling heat transfer coefficients of R22 and R245fa reasonably well (within ±8.5%), but not as well those of the other three refrigerants (underpredicts by nearly 30% for R134a and R236fa and overpredicts by nearly 40% for R123). It is found that the predicted boiling heat transfer coefficients of the five refrigerants by the modified Gorenflo correlation (simply adding a constant multiplier of 1.47 to the Gorenflo correlation) are in excellent agreement with their respective measurements. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Water-heated pool boiling of different refrigerants on the outside surface of a horizontal smooth tube [texte imprimé] / Tailian Chen, Auteur . - 2012 . - 08 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Mots-clés : Boiling Copper Flow measurement Heat transfer Pipe flow Refrigerants Water Index. décimale : 536 Chaleur. Thermodynamique Résumé : Pool boiling heat transfer has been extensively studied over decades, but the effect of boundary heating conditions on boiling received little attention. In this work, heat transfer coefficients during pool boiling of five different refrigerants (R123, R245fa, R236fa, R134a, and R22) on the outside surface of a smooth copper tube were measured at the saturation temperature of 6.7 °C; water flows inside the tube and provides heat to the refrigerants to boil (thus, water-heated boiling). Measurements showed that the refrigerant of a higher vapor pressure has a higher heat transfer coefficient, with the exception that R22 performs nearly the same as R134a. A correlation previously developed for electrically-heated pool boiling on cylindrical tubes underpredicts by 30%–46% the heat transfer coefficients during water-heated boiling of the five refrigerants. Among the pool boiling correlations reviewed in this work, the Cooper correlation (for pool boiling on cylindrical tubes) predicts the boiling heat transfer coefficients of R22 and R245fa reasonably well (within ±8.5%), but not as well those of the other three refrigerants (underpredicts by nearly 30% for R134a and R236fa and overpredicts by nearly 40% for R123). It is found that the predicted boiling heat transfer coefficients of the five refrigerants by the modified Gorenflo correlation (simply adding a constant multiplier of 1.47 to the Gorenflo correlation) are in excellent agreement with their respective measurements. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Titre : Investigations of biporous wick structure dryout Type de document : texte imprimé Auteurs : Qingjun Cai, Auteur ; Ya-Chi Chen, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Bubbles Carbon nanotubes Cooling Drops Evaporation Flow through porous media Flow visualisation Heat pipes Jets Mass transfer Pipe flow Two-phase flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Investigations of biporous wick structure dryout [texte imprimé] / Qingjun Cai, Auteur ; Ya-Chi Chen, Auteur . - 2012 . - 08 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 08 p.
Mots-clés : Bubbles Carbon nanotubes Cooling Drops Evaporation Flow through porous media Flow visualisation Heat pipes Jets Mass transfer Pipe flow Two-phase flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Titre : Flow modes and mode transitions for falling films on flat tubes Type de document : texte imprimé Auteurs : Xiaofei Wang, Auteur ; P. S. Hrnjak, Auteur ; S. Elbel, Auteur Année de publication : 2012 Article en page(s) : 09 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Drops External flows Film flow Flow instability Flow visualisation Jets Laminar to turbulent transitions Liquid films Pipe flow Two-phase flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Flow mode transitions for a liquid film falling over flat tubes are studied for a wide range of conditions. Hydraulic oil, ethylene glycol, ethylene-glycol-water mixtures (at three different volume ratios), and water are used as working fluids in experiments conducted under adiabatic conditions, without an imposed vapor flow. The flow modes are observed over a range of flow rate and tube spacing. The flow modes are more complex but similar to those manifested on round tubes, and include the sheet, sheet-jet, jet, jet-droplet, and droplet modes. However, the transitions at Ga1/4 <= 16 (mainly for hydraulic oil, ethylene glycol) are very different from those on round tubes: there is no sheet-jet mode during transition between the sheet and jet flow modes. Moreover, at all Ga, the Re range over which the jet mode occurs is narrower than that for round tubes, and the hysteresis in mode transitions is not as pronounced as for round tubes. The transitional Re increases slowly with tube spacing, also in contrast to observations of round-tube falling films. The flat-tube falling-film flow regimes are described, and new correlations are provided to relate the transitional Re number to Ga and geometry. By analyzing repeated experiments, it is shown that the standard deviation of the measurements from the correlations is less than 5.2%. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Flow modes and mode transitions for falling films on flat tubes [texte imprimé] / Xiaofei Wang, Auteur ; P. S. Hrnjak, Auteur ; S. Elbel, Auteur . - 2012 . - 09 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Mots-clés : Drops External flows Film flow Flow instability Flow visualisation Jets Laminar to turbulent transitions Liquid films Pipe flow Two-phase flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Flow mode transitions for a liquid film falling over flat tubes are studied for a wide range of conditions. Hydraulic oil, ethylene glycol, ethylene-glycol-water mixtures (at three different volume ratios), and water are used as working fluids in experiments conducted under adiabatic conditions, without an imposed vapor flow. The flow modes are observed over a range of flow rate and tube spacing. The flow modes are more complex but similar to those manifested on round tubes, and include the sheet, sheet-jet, jet, jet-droplet, and droplet modes. However, the transitions at Ga1/4 <= 16 (mainly for hydraulic oil, ethylene glycol) are very different from those on round tubes: there is no sheet-jet mode during transition between the sheet and jet flow modes. Moreover, at all Ga, the Re range over which the jet mode occurs is narrower than that for round tubes, and the hysteresis in mode transitions is not as pronounced as for round tubes. The transitional Re increases slowly with tube spacing, also in contrast to observations of round-tube falling films. The flat-tube falling-film flow regimes are described, and new correlations are provided to relate the transitional Re number to Ga and geometry. By analyzing repeated experiments, it is shown that the standard deviation of the measurements from the correlations is less than 5.2%. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Titre : Flow and heat transfer in microchannels with dimples and protrusions Type de document : texte imprimé Auteurs : Jibing Lan, Auteur ; Yonghui Xie, Auteur ; Di Zhang, Auteur Année de publication : 2012 Article en page(s) : 09 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Computational fluid dynamics Heat transfer Laminar flow Microchannel flow Navier-Stokes equations Numerical analysis Index. décimale : 536 Chaleur. Thermodynamique Résumé : Flow characteristics and heat transfer performances in a rectangular microchannel with dimples/protrusions are studied numerically in this research. The height and the width of the microchannel is 200 µm and 50 µm, respectively. The dimple/protrusion diameter is 100 µm, and the depth is 20 µm. The effects of Reynolds number, streamwise pitch, and arrangement pattern are examined. The numerical simulations are conducted using water as the coolant with the Reynolds number ranging from 100 to 900. The results show that dimple/protrusion technique in mcirochannel has the potential to provide heat transfer enhancement with low pressure penalty. The normalized Nusselt number is within the range from 1.12 to 4.77, and the corresponding normalized friction factor is within the range from 0.94 to 2.03. The thermal performance values show that the dimple + protrusion cases perform better than the dimple + smooth cases. The flow characteristics of the dimples/protrusions in microchannel are similar to those in conventional channel. Furthermore, from the viewpoint of energy saving, dimples/protrusions in microchannel behave better than those in conventional channel. Also from the viewpoint of field synergy principle, the synergy of the dimple + protrusion cases are much better than the dimple + smooth cases. Moreover, the synergy becomes worse with the increase in the Reynolds number and decrease in the streamwise pitch. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Flow and heat transfer in microchannels with dimples and protrusions [texte imprimé] / Jibing Lan, Auteur ; Yonghui Xie, Auteur ; Di Zhang, Auteur . - 2012 . - 09 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 09 p.
Mots-clés : Computational fluid dynamics Heat transfer Laminar flow Microchannel flow Navier-Stokes equations Numerical analysis Index. décimale : 536 Chaleur. Thermodynamique Résumé : Flow characteristics and heat transfer performances in a rectangular microchannel with dimples/protrusions are studied numerically in this research. The height and the width of the microchannel is 200 µm and 50 µm, respectively. The dimple/protrusion diameter is 100 µm, and the depth is 20 µm. The effects of Reynolds number, streamwise pitch, and arrangement pattern are examined. The numerical simulations are conducted using water as the coolant with the Reynolds number ranging from 100 to 900. The results show that dimple/protrusion technique in mcirochannel has the potential to provide heat transfer enhancement with low pressure penalty. The normalized Nusselt number is within the range from 1.12 to 4.77, and the corresponding normalized friction factor is within the range from 0.94 to 2.03. The thermal performance values show that the dimple + protrusion cases perform better than the dimple + smooth cases. The flow characteristics of the dimples/protrusions in microchannel are similar to those in conventional channel. Furthermore, from the viewpoint of energy saving, dimples/protrusions in microchannel behave better than those in conventional channel. Also from the viewpoint of field synergy principle, the synergy of the dimple + protrusion cases are much better than the dimple + smooth cases. Moreover, the synergy becomes worse with the increase in the Reynolds number and decrease in the streamwise pitch. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012)
Titre : Radiation properties of oxygen-enhanced normal and inverse diffusion flames Type de document : texte imprimé Auteurs : S. S. Krishnan, Auteur ; M. K. Saini, Auteur ; Y. Zheng, Auteur Année de publication : 2012 Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Chemically reactive flow Combustion Combustion equipment Diffusion Flames Heat transfer Laminar flow Organic compounds Pipe flow Radiative transfer Index. décimale : 536 Chaleur. Thermodynamique Résumé : Radiative heat transfer in oxygen-enhanced inverse flame configurations is an important area of study for fundamental combustion research and for terrestrial and spacecraft fire safety. Motivated by this, heat flux distributions, total radiative heat loss and spectral radiation intensities were investigated experimentally for oxygen-enhanced normal and inverse laminar ethane diffusion flames with increasing heat release rates. The oxygen mole fraction in the oxidizer was varied as 21%, 30%, 50%, and 100% with coflowing normal and inverse flame burners used to stabilize the flames. The inverse diffusion flames were essentially nonluminous while the normal diffusion flames with identical heat release rates were highly luminous. Oxygen enhancement led to reduced flame lengths, increased luminosities and increased total radiative heat loss and spectral radiation intensities for both normal and inverse diffusion flames. Using flame length as the characteristic length parameter, the normalized radiative heat flux distributions for flames approximately collapsed together, further establishing the effectiveness of the single point radiant output measurement technique. Radiative heat loss fractions of normal and inverse diffusion flames with varying oxygen concentrations in the oxidizer are compared. The radiation spectra of all flames included significant contributions from gas radiation from carbon dioxide and water vapor and the radiation spectra of the high oxygen concentration flames included contributions from soot radiation. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Radiation properties of oxygen-enhanced normal and inverse diffusion flames [texte imprimé] / S. S. Krishnan, Auteur ; M. K. Saini, Auteur ; Y. Zheng, Auteur . - 2012.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012)
Mots-clés : Chemically reactive flow Combustion Combustion equipment Diffusion Flames Heat transfer Laminar flow Organic compounds Pipe flow Radiative transfer Index. décimale : 536 Chaleur. Thermodynamique Résumé : Radiative heat transfer in oxygen-enhanced inverse flame configurations is an important area of study for fundamental combustion research and for terrestrial and spacecraft fire safety. Motivated by this, heat flux distributions, total radiative heat loss and spectral radiation intensities were investigated experimentally for oxygen-enhanced normal and inverse laminar ethane diffusion flames with increasing heat release rates. The oxygen mole fraction in the oxidizer was varied as 21%, 30%, 50%, and 100% with coflowing normal and inverse flame burners used to stabilize the flames. The inverse diffusion flames were essentially nonluminous while the normal diffusion flames with identical heat release rates were highly luminous. Oxygen enhancement led to reduced flame lengths, increased luminosities and increased total radiative heat loss and spectral radiation intensities for both normal and inverse diffusion flames. Using flame length as the characteristic length parameter, the normalized radiative heat flux distributions for flames approximately collapsed together, further establishing the effectiveness of the single point radiant output measurement technique. Radiative heat loss fractions of normal and inverse diffusion flames with varying oxygen concentrations in the oxidizer are compared. The radiation spectra of all flames included significant contributions from gas radiation from carbon dioxide and water vapor and the radiation spectra of the high oxygen concentration flames included contributions from soot radiation. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 13 p.
Titre : A review on critical heat flux enhancement with nanofluids and surface modification Type de document : texte imprimé Auteurs : Ho Seon Ahn, Auteur ; Moo Hwan Kim, Auteur Année de publication : 2012 Article en page(s) : 13 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Boiling Heat transfer Nanofluidics Surface phenomena Index. décimale : 536 Chaleur. Thermodynamique Résumé : Recently, there has been increasing interest in boiling nanofluids and their applications. Among the many articles that have been published, the critical heat flux (CHF) of nanofluids has drawn special attention because of its dramatic enhancement. This article includes recent studies on CHF increasing during the past decade by various researchers for both pool boiling and convective flow boiling applications using nanofluids as the working fluid. It presents a review of nanofluid critical heat flux research with the aim of identifying the reasons for its enhancement and the limitations of nanofluid applications based on various published reports. In addition, further research required to make use of the CHF enhancement caused by nanofluids for practical applications is discussed. Finally, the surface modification method with micro/nanostructures to increase the CHF is introduced and recommended as a useful way. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] A review on critical heat flux enhancement with nanofluids and surface modification [texte imprimé] / Ho Seon Ahn, Auteur ; Moo Hwan Kim, Auteur . - 2012 . - 13 p.
Heat transfer
Langues : Anglais (eng)
in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 13 p.
Mots-clés : Boiling Heat transfer Nanofluidics Surface phenomena Index. décimale : 536 Chaleur. Thermodynamique Résumé : Recently, there has been increasing interest in boiling nanofluids and their applications. Among the many articles that have been published, the critical heat flux (CHF) of nanofluids has drawn special attention because of its dramatic enhancement. This article includes recent studies on CHF increasing during the past decade by various researchers for both pool boiling and convective flow boiling applications using nanofluids as the working fluid. It presents a review of nanofluid critical heat flux research with the aim of identifying the reasons for its enhancement and the limitations of nanofluid applications based on various published reports. In addition, further research required to make use of the CHF enhancement caused by nanofluids for practical applications is discussed. Finally, the surface modification method with micro/nanostructures to increase the CHF is introduced and recommended as a useful way. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]
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