In order to defog your windshield, you turn on the defroster which blows hot air

Question

In order to defog your windshield, you turn on the defroster which blows hot air over the surface. The warm air that is blowing over the windshield surface at 50 degree C with a convection coefficient of = 60 W/m^2.K. The glass windshield is 4 mm thick. The outside temperature is 10 degree C and the outside air has a convection coefficient of = 150 W/m^2.K. Find: Thermal Resistance (R") from to Heat flux from to Temperature on the inner surface of the windshield Temperature on the outer surface of the windshield

Explanation / Answer

a)

The total resistance is given by

R'' = R1 + R2 + R3

R'' = 1/hin + L/k + 1/hout

k is the thermal conductivity of glass (0.8)

R'' = 17/600

b)

Heat flux

q = (Tin-Tout)/R''

q = 40*600/17

q = 1411.77 W/m2

c)

R1 = 1/hin = 1/60

Now we can write

Tin - Ts,in = q*R1

Tin - Ts,in = 1411.77/60

Ts,in = 26.5 C

d)

R3 = 1/150

Ts,out-Tout = q*R3

Ts,out-Tout = 1411.77/150

Ts,out = 10+9.4

Ts,out = 19.4 C

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