Athick steel slab (rho= 7800 kg/m3 , cp= 480 j/kg k, k= 50 w/m k) is initially at 300 °c and is cooled by water jets impinging on one of its surfaces. the temperature of the water is 25 °c, and the jets maintain an extremely large, approximately uniform convection coefficient at the surface. assuming that the surface is maintained at the temperature of the water throughout the cooling, how long will it take for the temperature to reach 50 °c at a distance of 25 mm from the surface?

Along 8-cm diameter steam pipe whose external surface temperature is 900c connects two buildings. the pipe is exposed to ambient air at 70c with a wind speed of 50 km/hr blowing across the pipe. determine the heat loss from the pipe per unit length. (b) air at 500c enters a section of a rectangular duct (15 cm x 20 cm) at an average velocity of 7 m/s. if the walls of the duct are maintained at 100c. a) the length of the tube for an exit temperature of the air to be 40 0c. b)the rate of heat transfer from the air. c) the fan power needed to overcome the pressure drop in this section of the duct.