4. A stainless steel ball (=8055kg/m3 and Cp=480 J/kgC ) of diameter D=15cm is removed from the oven at a unlform temperature of 350C. The ball is then subjected to the flow of air at 1atm pressure and 30C with a velocity of 6m/s. The surface temperature of the ball eventually drops to 250C. Determine the average convection heat transfer coefficient during this cooling process and estimate how long this process has taken. (25.12W/m2C,23.75min)