Air enters a 25-cm-diameter 12-m-long underwater duct at 50°C and 1 atm at a mean velocity of 7 m/s, and is cooled by the water outside. If the average heat transfer coefficient is 85 W/m2?K and the tube temperature is nearly equal to the water temperature of 10°C, determine the exit temperature of air and the rate of heat transfer. Evaluate air properties at a bulk mean temperature of 30°C. Is this a good assumption?

Hot air flows with a mass flow rate m = 0.09 kg/s through an uninsulated sheet metal duct of diameter D = 0.15 m, which is in the crawlspace of a house. The hot air enters at 110°C and, after a distance of L 5 m, cools to 80°C. The heat transfer coefficient between the duct surface and the ambient air at To 0°C is known to be h, = 9 W / m²K. (a) Calculate the heat loss from the duct over the length L. (b) Determine the heat flux and the surface temperature at x = L. (c) Determine the heat flux and the surface temperature at x = 0. Assume, for air at Tm 368 K: c, 1011 J/kg.K; for air at Tm.L = 353 K: k= 0.030 W/m.K, u = 209x10' N.s/m2, Pr = 0.70%; for air at Tm,0 = 383 K: k = 0.033 W/m.K, u=223x10" N.s/m2, Pr = 0.69. Show all calculations. Cold Air Hot air in -L-