Question

The rear window of an automobile is defogged by passing warm air over its inner surface. The warm air is at ${T_{\infty ,}}_i$ = 40°C and the corresponding convection coefficient is $h_i=30W/m^2-K$. The outside ambient air temperature is ${T_{\infty ,}}_o$ = -10°C and the associated convection coefficient is $h_o=65W/m^2-K$.
The outer surface temperature of 4 mm thick window glass (k = 1.2 W/m-K) is

• A. 4.77°C
• B. 7.94°C
• C. 20.76°C
• D. 18.87°C

Answer 1

The correct option is A 4.77°C

$HeatTransferRate=\frac{T_{\infty ,i}-T_{\infty ,o}}{\frac{1}{h_{i}A}+\frac{L}{k.A}+\frac{1}{h_{o}A}}=\frac{T_2-T_{\infty ,0}}{\frac{1}{h_o.A}}--------\left(1\right)$

$\Rightarrow \frac{40-(-10)}{\frac{1}{30}+\frac{0.004}{1.2}+\frac{1}{65}}=\frac{T_2-(-10)}{\frac{1}{65}}$

$\frac{50}{0.052}=(T_2+10)\times 65$

$T_2=4.77{}^{\circ }C$