A cylindrical billet of steel, 50 cm long with an outside diameter of 3 cm is initially held at 500°C. The billet is plunged into an oil bath at 100°C.
The average convective coefficient, h, between the oil and the steel is 500 W/m2K.
Find the time it takes to cool the center of the billet to a temperature of 250°C, assuming: (a) Lumped Capacitance (b) Billet is not lumped.