Repeat Prob. 20€“90E assuming the fan fails and thus the entire heat generated inside the duct must be rejected to the ambient air by natural convection through the outer surfaces of the duct. Evaluate air properties at a film temperature of 120°F and 1 atm pressure. Is this a good assumption?

Repeat Prob. 20€“90E

The components of an electronic system dissipating 180 W are located in a 4-ft-long horizontal duct whose cross section is 6 in × 6 in. The components in the duct are cooled by forced air, which enters at 85°F at a rate of 22 cfm and leaves at 100°F. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 80°F, determine

(a) The heat transfer from the outer surfaces of the duct to the ambient air by natural convection

(b) The average temperature of the duct. Evaluate air properties at a film temperature of 100°F and 1 atm pressure. Is this a good assumption?

Transcribed Image Text:

Natural convection 100°F 80°F 180 W 4 ft 85°F 22 cfm