1. Engine oil flows at a rate of 1 kg/s through a 5-mm- diameter straight tube. The oil has an inlet temperature of 45°C and it is desired to heat the oil to a mean tem- perature of 80°C at the exit of the tube. The surface of the tube is maintained at 150°C. Determine the required length of the tube. Hint: Calculate the Reynolds num- bers at the entrance and exit of the tube before proceed- ing with your analysis.
2. Air at p 1 atm enters a thin-walled (D 5-mm diameter) long tube (L 2 m) at an inlet temperature ofTm,i 100°C. A constant heat flux is applied to the air from the tube surface. The air mass flow rate is.
m 135 X 10-6 kg/s.
(a) If the tube surface temperature at the exit is Ts,o 160°C, determine the heat rate entering the tube. Evaluate properties at T 400 K.
(b) If the tube length of part (a) were reduced to L 0.2 m, how would flow conditions at the tube exit be affected? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)?
(c) If the flow rate of part (a) were increased by a factor of 10, would there be a difference in flow conditions at the tube exit? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)?