A decorative plastic film on copper sphere having 10 mm diameter is cured in an oven at 750C. Upon removal from the oven, the copper sphere is subjected to an air stream at a pressure, temperature and velocity of 1 bar 230C and 10 m/s respectively. How long it will take for the sphere to cool down to 350C? State the assumptions made and justify the method of analysis used.
For copper, the density, specific heat and thermal conductivity are, respectively, 8933 kg/m3, 388 J/kg-K and 350 W/m-K. The following correlation for force convection may be used:
Nud = 2 + [0.4 Re 
] Pr0.4
For air, kinematic viscosity, thermal conductivity and Prandti number at the mean film temperature under consideration are 15.53 ´ 10–6 m2/s, 0.025 W/m-K and 0.708 respectively.