3. (30 point) Oil is transported through a 6 cm diameter horizontal pipe with 0.1 cm wall thickness of cast iron that is submerged in a lake. The oil enters

the pipe at 80 °C at a rate of 10 kg/s.Note that at the bottom of the lake the water is very still. Determine the length of pipe required to cool the oil to 50 °C if the lake is at 10 °C. The heat transfer coefficient for the oil in the pipe is 10 W/m? C.