A rectangular forced air heating duct is suspended from the ceiling of a basement whose air and walls are at a temperature of T00 = Tsur = 5°C. The duct is 15 m long, and its cross section is 350 mm X 200 mm.
(a) For an uninsulated duct whose average surface tem- perature is 50°C, estimate the rate of heat loss from the duct. The surface emissivity and convection coefficient are approximately 0.5 and 4 W/m2 · K, respectively.
(b) If heated air enters the duct at 58°C and a velocity of 4 m /s and the heat loss corresponds to the result of part (a), what is the outlet temperature? The density and specific heat of the air may be assumed to be p= 1.10 kg/m3 and cp = 1008 J/kg · K, respectively.