A rectangular forced air heating duct is suspended from the ceiling of a basement whose air and walls are at a temperature of Too = 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 temperature 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 ρ = 1.10 kg/m3 and cp = 1008J/kg ∙ K, respectively.