Asteel sphere with a diameter of 7.6 cm is to be hardened by first heating it to a uniform temperature of 870c and then quenching it in a large bath of water maintained at a temperature of 38c. the fluid motion in the quenching bath is such that it produces a convective heat transfer coefficient of h = 566 w/m2 · k. determine (a) the time required to cool the center of the sphere to 204c, and (b) the elapsed time when the surface of the sphere has cooled to 750c. and property values: k = 43 w/m · k, cp = 628 j/kg k, and = 7840 kg/m3 .

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.