The flat roof on the refrigeration compartment of a food deli very truck is of length L = 5 m and width W = 2 m. It is fabricated from thin sheet metal to which a fiberboard insulating material of thickness t = 25 mm and thermal conductivity k = 0.05 W/m · K is bonded. During normal operation, the truck moves at a velocity of V = 30 m/s in air at T8 = 27°C, with a rooftop solar irradiation of G s = 900 W/m2 and with the interior surface temperature maintained at Ts,i = - 13oC.
(a) The owner has the option of selecting a roof coating from one of the three paints listed in Table A.12 (Parsons Black, Acrylic White. or Zinc Oxide White). Which should be chosen and why?
(b) For the preferred paint of part (a), determine the steady-state value of the outer surface temperature Ts,o. The boundary layer is tripped at the leading edge of the roof, and turbulent flow may be assumed to exist over the entire roof. Properties of the air may be taken to be v = 15 x 10-6 m 2/s, k = 0.026 W/m · K, and Pr = 0.71.
(c) What is the load (W) imposed on the refrigeration system by heat transfer through the roof? (d) Explore the effect of the truck velocity on the outer surface temperature and the heat load.
