The roof of a refrigerated truck compartment is of composite construction, consisting of a layer of foamed urethane insulation (t2 = 50 mm, ki = 0.026 W/m ∙ K sandwiched between aluminum alloy panels (11 = 5 mm, kp = 180 W/m ∙ K). The length and width of the roof are L = 10 m and W = 3.5 m, respectively, and the temperature of the inner surface is Ts,i = - 10°C. Consider conditions for which the truck is moving at a speed of V= 105 km/h, the air temperature is T∞ = 32°C, and the solar irradiation is Gs = 750 W/m2. Turbulent flow mil) be assumed over the entire length of the roof.

(a) For equivalent values of the solar absorptivity and the emissivity of the outer surface (as = 8 = 0.5), estimate the average temperature Ts,o of the outer surface. What is the corresponding heat load imposed on the refrigeration system?

(b) A special finish (as = 0.15, ε = 0.8) may be applied to the outer surface. What effect would such an application have on the surface temperature and the heat load?

(c) If, with as = ε = 0.5, the roof is not insulated (12 = 0), what are the corresponding values of the surface temperature and the heat load?

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