Engine oil flows through a 25-mm-diameter tube at a rate of 0.5 kg/so The oil enters the tube at a temperature of 25°C, while the tube surface temperature is maintained at 100°C.

(a) Determine the oil outlet temperature for a 5-m and for a 100-m long tube. For each case, compare log mean temperature difference to the arithmetic mean temperature difference.

(b) For 5 < L < 100 m, compute and plot the average Nusselt number NU D and the oil outlet temperature as a function of L.