An 8-m-long, uninsulated square duct of cross section 0.2 m X0.2 m and relative roughness 10-3 passes through the attic space ofa house. Hot air enters the duct at 1 atm and 91oC at a volume flowrate of 0.15 m3/s. The duct surface is nearly isothermal at 60oC.Determine the rate of heat loss from the duct to the attic spaceand the pressure difference between the inlet and outlet sectionsof the duct. Evaluate air properties at a bulk mean temperature of80oC. Is this a good assumption? Taking a bulk mean fluidtemperature of 80oC based on the problem statement (this assumesthat the air does not loose much heat to the attic), the propertiesof air are: cp = 1008 J/kg∙oC k = 0.02953 W/m∙oC ν = 2.097 × 10-5m2/s Ï = 0.9994 kg/m3 Pr = 0.7154
