To defrost ice on the outer surface of an automobile windshield, warm air is blo

Question

To defrost ice on the outer surface of an automobile windshield, warm air is blown over the inner surface. Consider the automobile wind shield with an overall height of 0.5 m. The average temperature of the defroster air is 40 degree and the air velocity over the windshield surface is 20 m/s. The temperature of inside surface of the wind shield is 10 degree C. Assuming that the windshield is a flat plate, determine the convection heat transfer coefficient for the inner surface of the wind shield (The transition Reynolds number for parallel flow over a flat plat is 5 times 105)

Explanation / Answer

Take kinematic viscosity of air, as 15.68*10-6 m2/s, and Prandtl number Pr = 0.707 and thermal conductivity k =0.0257 W/m-K

Reynolds number, Re = UL/

Re = 20 * 0.5 / (15.68*10-6) = 637755.

This is more than 5 x 105. Hence, the flow is turbulent.

Nusselt no, Nu = hL/k = 0.032 Re0.8Pr0.43

Nu = hL/k = 0.032 * 6377550.80.7070.43 = 1213.7

So, h = 1213.7 * 0.0257/0.5 = 62.4 W/m2K

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