Due to its comparatively large thermal conductivity, water is a preferred fluid for convection cooling. How- ever, in applications involving electronic devices, water must not come into contact with the devices, which would therefore have to be hermetically sealed. To cir- cumvent related design and operational complexities and to ensure that the devices are not rendered inoperable by contact with the coolant, a dielectric fluid is commonly used in lieu of water. Many gases have excellent dielec- tric characteristics, and despite its poor heat transfer properties, air is the common choice for electronic cool- ing. However, there is an alternative, which involves a class of peruorinated liquids that are excellent dielectrics and have heat transfer properties superior to those of gases.

Consider the microchannel chip cooling applica- tion of Problem 8.109 but now for a perfluorinated liquid with properties of cp = 1050 J/kg · K, k = 0.065 W/m · K, µ, = 0.0012 N · s/m2, and Pr = 15.

(a) For channel dimensions of H = 200 µ,m, W = 50 µ,m, and S = 20 µ,m, a chip thermal conductivity of d/2 kch = 140 W/m · K and width L = 10 mm, a channel base temperature (x = 0) of Ts = 350 K, a Cap (Adiabatic) channel inlet temperature of Tm,i = 290 K, and a

(a) For the operating conditions prescribed in Problem 8.107 and a chip thermal conductivity of kch = 140 W/m · K, determine the water outlet tempera- ture and the chip power dissipation. Heat transfer from the sides of the chip to the surroundings and from the side walls of a channel to the cap may be neglected. Note that the spacing between channels, 8 = S - W, is twice the spacing between the side wall of an outer channel and the outer surface of the chip. The channel pitch is S = L/N, where L = 10 mm is the chip width and N = 50 is the number of channels.

(b) The channel geometry prescribed in Problem 8.107 and considered in part (a) is not optimized, and larger heat rates may be dissipated by adjust- ing related dimensions. Consider the effect of reducing the pitch to a value of S = 100 µ,m, while retaining a width of W = 50 µ,m and a flow rate per channel of m· 1 = 10-4 kg/s.