Reconsider Prob. 8–89. Using EES (or other) software, investigate the effect of air velocity and the surface emissivity on the exit temperature of air and the rate of heat loss. Let the air velocity vary from 1 m/s to 10 m/s and the emissivity from 0.1 to 1.0. Plot the exit temperature and the rate of heat loss functions of the air velocity and emissivity, and discuss the results.

Data from problem 89

Hot air at 608C 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/m²? 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

(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 508C. Is this a good assumption?

Transcribed Image Text:

10°º℃ h, = 10 W/m².K 12 m Hot air 60°C 4 m/s Air duct 20 cm x 20 cm ε = 0.3