A solid iron sphere of diameter 5 cm, initially at a uniform
temperature of 700 °C is exposed to a cool air
stream at 100 °C. The heat transfer coefficient between the air
stream and the surface of the iron
sphere is 80 W/ m 2 C. a) Check whether the lumped system is
applicable. b) If applicable, determine the
time required for the temperature of the sphere to reach 300 °C.
For iron, k = 60 W/ m °C, density =
7,800 kg/ m 3 and specific heat = 460 J/ kg °C.