An 8-m-long, uninsulated square duct of cross section 0.2 m x 0.2 m and relative roughness 10-3 passes through the attic space of a house. hot air enters the duct at 1 atm and 80c at a volume flow rate of 0.15 m^3 /s. the duct surface is nearly isothermal at 60c. determine the rate of heat loss from the duct to the attic space and the pressure difference between the inlet and outlet sections of the duct. evaluate air properties at a bulk mean temperature of 80c. is this a good assumption?