$1-$ The steel balls $($AISI$304)$ have a uniform temperature of $850$C after leaving the furnace. For the purpose of surface heat treatment, the temperature of the surface which is located at a distance of $2$ mm from its outer surface should reach $100$C within $20$ minutes. Balls with a diameter of $20$mm are placed in an oil flow at a temperature of $40$C$.$ Is it desirable to have a displacement heat transfer coefficient that the oil flow should create on the balls? Determine the amount of energy transferred from each bullet to the oil during this time