Chapter 1, Problem 1.40P

Chips of width $L=15\text{ mm}$ on a side are mounted to asubstrate that is installed in an enclosure whose wallsand air are maintained at a temperature of $T_{sut}=\text{25}°\text{C}$ .The chips have an emissivity of $\in =0.60$ and a maximum allowable temperature of $T_s=\text{85}°\text{C}$ .

(a) If heat is rejected from the chips by radiation andnatural convection, what is the maximum operatingpower of each chip? The convection coefficientdepends on the chip-to-air temperature differenceand may be approximated as $h=C{\left(T_s-T_{\infty }\right)}^{1/4}$ , where $C=4.2\text{W}/{\text{m}}^{\text{2}}\cdot {\text{K}}^{\text{5}/\text{4}}$ .
(b) If a fan is used to maintain airflow through theenclosure and heat transfer is by forced convection, with $h=250\text{W}/{\text{m}}^{\text{2}}\cdot \text{K}$ , what is the maximum operating power?