Hot air at 60°C leaving the furnace of a house enters a 12-m-long section of a sheet metal duct of rectangular cross section 20 cm × 20 cm at an average velocity of 4 m/s. The thermal resistance of the duct is negligible, and the outer surface of the duct, whose emissivity is 0.3, is exposed to the cold air at 10°C in the basement, with a convection heat transfer coefficient of 10 W/m²ˆ™K. Taking the walls of the basement to be at 10°C also, determine

(a) The temperature at which the hot air will leave the basement and

(b) The rate of heat loss from the hot air in the duct to the basement. Evaluate air properties at a bulk mean temperature of 50°C. Is this a good assumption?

Transcribed Image Text:

10°C ho = 10 W/m2-K 12 m Hot alr 60°C Air duct 20 cm x 20 cm 4 m/s E = 0.3