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Design and test of carbon nanotube biwick structure for high-heat-flux phase change heat transfer / Qingjun Cai in Journal of heat transfer, Vol. 132 N° 5 (Mai 2010) 

Public ISBD [article]  in Journal of heat transfer > Vol. 132 N° 5 (Mai 2010) . - pp. [052403-1/8] Titre : Design and test of carbon nanotube biwick structure for high-heat-flux phase change heat transfer Type de document : texte imprimé Auteurs : Qingjun Cai, Auteur ; Chung-Lung Chen, Auteur Article en page(s) : pp. [052403-1/8] Note générale : Physique Langues : Anglais (eng) Mots-clés : Thermometer  Carbon  nanotube  Wick  structure  Temperature  measurement  Capillarity  Cooling Index. décimale : 536 Chaleur. Thermodynamique Résumé : With the increase in power consumption in compact electronic devices, passive heat transfer cooling technologies with high-heat-flux characteristics are highly desired in microelectronic industries. Carbon nanotube (CNT) clusters have high thermal conductivity, nanopore size, and large porosity and can be used as wick structure in a heat pipe heatspreader to provide high capillary force for high-heat-flux thermal management. This paper reports investigations of high-heat-flux cooling of the CNT biwick structure, associated with the development of a reliable thermometer and high performance heater. The thermometer/heater is a 100-nm-thick and 600 µm wide Z-shaped platinum wire resistor, fabricated on a thermally oxidized silicon substrate of a CNT sample to heat a 2×2 mm2 wick area. As a heater, it provides a direct heating effect without a thermal interface and is capable of high-temperature operation over 800°C. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation versus temperature after the annealing process is applied. The thermally oxidized layer on the silicon substrate is around 1-µm-thick and pinhole-free, which ensures the platinum thermometer/heater from the severe CNT growth environments without any electrical leakage. For high-heat-flux cooling, the CNT biwick structure is composed of 250 µm tall and 100 µm wide stripelike CNT clusters with 50 µm stripe-spacers. Using 1×1 cm2 CNT biwick samples, experiments are completed in both open and saturated environments. Experimental results demonstrate 600 W/cm2 heat transfer capacity and good thermal and mass transport characteristics in the nanolevel porous media. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&smode=strresults& [...] [article] Design and test of carbon nanotube biwick structure for high-heat-flux phase change heat transfer [texte imprimé] / Qingjun Cai, Auteur ; Chung-Lung Chen, Auteur . - pp. [052403-1/8]. Physique Langues : Anglais (eng) in Journal of heat transfer > Vol. 132 N° 5 (Mai 2010) . - pp. [052403-1/8] Mots-clés : Thermometer  Carbon  nanotube  Wick  structure  Temperature  measurement  Capillarity  Cooling Index. décimale : 536 Chaleur. Thermodynamique Résumé : With the increase in power consumption in compact electronic devices, passive heat transfer cooling technologies with high-heat-flux characteristics are highly desired in microelectronic industries. Carbon nanotube (CNT) clusters have high thermal conductivity, nanopore size, and large porosity and can be used as wick structure in a heat pipe heatspreader to provide high capillary force for high-heat-flux thermal management. This paper reports investigations of high-heat-flux cooling of the CNT biwick structure, associated with the development of a reliable thermometer and high performance heater. The thermometer/heater is a 100-nm-thick and 600 µm wide Z-shaped platinum wire resistor, fabricated on a thermally oxidized silicon substrate of a CNT sample to heat a 2×2 mm2 wick area. As a heater, it provides a direct heating effect without a thermal interface and is capable of high-temperature operation over 800°C. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation versus temperature after the annealing process is applied. The thermally oxidized layer on the silicon substrate is around 1-µm-thick and pinhole-free, which ensures the platinum thermometer/heater from the severe CNT growth environments without any electrical leakage. For high-heat-flux cooling, the CNT biwick structure is composed of 250 µm tall and 100 µm wide stripelike CNT clusters with 50 µm stripe-spacers. Using 1×1 cm2 CNT biwick samples, experiments are completed in both open and saturated environments. Experimental results demonstrate 600 W/cm2 heat transfer capacity and good thermal and mass transport characteristics in the nanolevel porous media. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&smode=strresults& [...]

Investigations of biporous wick structure dryout / Qingjun Cai in Journal of heat transfer, Vol. 134 N° 2 (Fevrier 2012) 

Public ISBD [article]  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 [...]