A decorative plastic film on a copper sphere of 10 mm diameter in an oven at 750C. Upon removal from the oven, the sphere is subjected to an air stream at 1 atm. , and 230C having a velocity of 10 meter/sec. Estimate how long it will take to coal the sphere to 350C? Density of copper is 8933 kg/m3 , thermal conductivity k = 399 W/m. K and specific heat Cp = 387 J/kg. K. for Air at an average temperature of T[infinity] = 296 K has viscosity µ= 181x10-7 N. s/m2 , kinematic viscosity ν = 15.36x10-6 m2/s and thermal conductivity k = 0.0258 W/m. K and Prandtl No. Pr = 0.709 and air at surface temperature Ts = 328 K will have viscosity µs = 197x10-7 N. s/m2