5. 5.60 Spheres of 40-mm diameter heated to a uniform temperature of 400°C are suddenly removed from the oven and placed in a forced-air bath operating at 25°C with a convection coefficient of 300...

Extracted text: 5. 5.60 Spheres of 40-mm diameter heated to a uniform temperature of 400°C are suddenly removed from the oven and placed in a forced-air bath operating at 25°C with a convection coefficient of 300 W/m2 K on the sphere surfaces. The thermophysical properties of the sphere material are p = 3000 kg/m', c = 850 J/kg · K, %3D and k = 15 W/m ·K. %3D (a) How long must the spheres remain in the air bath for 80% of the thermal energy to be removed? (b) The spheres are then placed in a packing carton that prevents further heat transfer to the environment. What uniform temperature will the spheres eventu- ally reach?