Due to its comparatively large thermal conductivity,water

Chapter 8, Problem 8.111

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Due to its comparatively large thermal conductivity,water is a preferred fluid for convection cooling. How-ever, in applications involving electronic devices, watermust not come into contact with the devices, whichwould therefore have to be hermetically sealed. To cir-cumvent related design and operational complexities andto ensure that the devices are not rendered inoperable bycontact with the coolant, a dielectric fluid is commonlyused in lieu of water. Many gases have excellent dielec-tric characteristics, and despite its poor heat transferproperties, air is the common choice for electronic cool-ing. However, there is an alternative, which involves aclass of perfluorinated liquidsthat are excellentdielectrics and have heat transfer properties superior tothose of gases.Consider the microchannel chip cooling applica-tion of 8.109 but now for a perfluorinatedliquid with properties of cp?1050 J/kg?K, k?0.065W/m?K, ??0.0012 N?s/m2, and Pr?15.(a) For channel dimensions of H?200?m, W?50?m, and S?20?m, a chip thermal conductivity ofkch?140 W/m?K and width L?10 mm, a chan- nel base temperature (x?0) of Ts?350 K, achannel inlet temperature of Tm,i?290 K, and aflow rate of m?1?10?4kg/s per channel, determinethe outlet temperature and the chip power dissipa-tion for the dielectric liquid.(b) Consider the foregoing conditions, but with air ata flow rate of m?1?10?6kg/s used as the coolant.Using properties of cp?1007 J/kg?K, k?0.0263W/m?K, and ??185?10?7N?s/m2, determinethe air outlet temperature and the chip power dissipation.