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In: Mechanical Engineering

A white van is parked in the sun. The direct irradiance (Gdir) from the sun is...

A white van is parked in the sun. The direct irradiance (Gdir) from the sun is 1065 W/m2 with an incidence angle of 20o from perpendicular to the roof of the van (area 3 m x 1.8 m). The roof has an absorptivity of 0.9 to solar radiation and emissivity of 0.2 to the surroundings (which have an effective temperature of 25oC). A gentle breeze of 25oC air blows over the van, with a speed of 6 m/s along the length of the van roof (3 m).

Assuming the bottom side of the roof is well insulated, calculate:

a) The convective heat transfer coefficient for the top surface of the roof of the van, assuming it is an isothermal flat plate and using the properties of air at 25oC.

b) The irradiance and the total radiative energy absorbed by the roof (assume Kirchoff's law applies for diffuse radiative transfer between the surroundings and the roof of the van).

c) The temperature of the roof of the van and its radiosity.

d) A supplier offers to apply a reflective coating that reduces the absorptivity at solar wavelengths to 0.7 but reduces the emissivity at ambient conditions to 0.05. Would you recommend this as a solution to reduce heat transfer to the van? Why or why not?

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Expert Solution

samet mamat answered 2 months ago
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