GATE 2020 ME (ME2) – Question 61
Water flows through a 3 cm internal-diameter, 20 m long tube heated electrically with circumferentially and axially uniform heat flux. Inlet and outlet mean temperatures are 10°C and 70°C; mass flow is 720 kg/hour. Neglect wall thermal resistance; internal heat-transfer coefficient is 1697 W/(m²·K) and cₚ = 4.179 kJ/(kg·K). Find inner wall temperature at the outlet (°C), to one decimal.
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Correct answer: 85.1 to 86.1
Explanation
Heat input is $\dot m c_p\Delta T=0.2(4179)(60)=50148$ W. Uniform flux is50148/(π·0.03·20)=26604 W/m². Thus exit wall temperature is70+q″/h≈**85.7°C**. With uniform heat flux, the wall-to-bulk difference is constant, while bulk temperature rises along the tube.