The defroster of an automobile functions by discharging warm air on the inner surface of the windshield. To prevent condensation of water vapor on the surface, the temperature of the air and the surface convection coefficient (∞T∞,i,h¯i) must be large enough to maintain a surface temperature Ts,i that is at least as high as the dewpoint (Ts,i≥Tdp).

 

Consider a windshield of length L= 800 mm and thickness t= 6 mm and driving conditions for which the vehicle moves at a velocity of V= 70 mph in ambient air at ∞T∞,0=-1.5ºC. From laboratory experiments performed on a model of the vehicle, the average convection coefficient on the outer surface of the windshield is known to be correlated by an expression of the form  N⁢u¯L=0.030⁢ R⁢eL0.8P⁢r1/3, where R⁢eL≡V⁢L/ν. Air properties may be approximated as k= 0.023 W/m·K, ν= 12.5 × 10^-6 m²/s, and P⁢r= 0.71. If Tdp=10ºC and ∞T∞,i= 50ºC.

Use the overall heat-transfer resistance presented by the external air and the glass itself to determine the heat flux in W/m² if the internal glass surface is maintained at the dew point.

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