The roof of a refrigerated truck compartment is of composite construction, consisting of a layer of foamed urethane insulation \(\left(t_{2}=50 \mathrm{~mm}, k_{i}=0.026 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\) sandwiched between aluminum alloy panels \(\left(t_{1}=5 \mathrm{~mm}\right.\), \(\left.k_{p}=180 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\right)\). The length and width of the roof are \(L=12 \mathrm{~m}\) and \(\mathrm{W}=3.5 \mathrm{~m}\), respectively, and the temperature of the inner surface is \(T_{s, i}=-10^{\circ} \mathrm{C}\). Consider conditions for which the truck is moving at a speed of \(V=\) \(110 \mathrm{~km} / \mathrm{h}\), the air temperature is \(T_{\infty}=30^{\circ} \mathrm{C}\), and the solar irradiation is \(G_{S}=900 \mathrm{~W} / \mathrm{m}^{2}\). Turbulent flow may be assumed over the entire length of the roof.

(a) For equivalent values of the solar absorptivity and the emissivity of the outer surface \(\left(\alpha_{S}=\varepsilon=\right.\) 0.6), estimate the average temperature \(T_{s, o}\) of the outer surface. What is the corresponding heat load imposed on the refrigeration system?

(b) A special finish \(\left(\alpha_{S}=0.2, \varepsilon=0.8\right)\) 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 \(\alpha_{S}=\varepsilon=0.6\), the roof is not insulated \(\left(t_{2}=\right.\) 0 ), what are the corresponding values of the surface temperature and the heat load?

Transcribed Image Text:

V. T Frank & Dave's Frozen Desserts for Those Wh Don't Count Calories m The -E,