Hot air flows from a furnace through a 0.15 m diameter, thin walled steel duct with a velocity of 3 m/sec. the duct passes through crawlspace of a house, and its un-insulated exterior surface is exposed to quiescent air and surroundings of 0℃. the duct outer surface has an emissivity of 0.5. a) determine the heat loss per unit duct length and the duct wall temperature at the location in the duct for which the mean air temperature is 70℃. b) if the duct is wrapped with a 25 mm thick layer of 85% magnesia insulation, k = 0.05 w/m.°k, and having surface emissivity of = 0.60, what are the duct wall temperature, the outer surface temperature and the heat loss per unit length. use the following correlations for nusselt numbers,