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,

Acommercial grade cubical freezer, 4 m on a side, has a composite wall consisting of an exterior sheet of 5.0-mm thick plain carbon steel (kst= 60.5 w/m k), an intermediate layer of 100-mm thick polyurethane insulation (kins 0.02 w/m k), and an inner sheet of 5.0- mm thick aluminium alloy (kal polyurethane insulation and both metallic sheets are each characterized by a thermal contact resistance of r 2.5 x 104 m2 k/w. (a) what is the steady-state cooling load that must be maintained by the refrigerator under conditions for which the outer and inner surface temperatures are 25°c and -5°c, respectively? (b) for power saving purpose, which wall material should be increased/reduced in. thickness in order to reduce 50% of the cooling load found in part (a)? redesign the thickness of the proposed material. 177 w/m-k). adhesive interfaces between the q=575.93 w