Semester I, 2013-2014 November 10, 2013

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SME 4463 HEAT TRANSFER SPACE - Kuala Lumpur TEST 4 Name: .................................................................................................... Matric No.: ..................................... Instruction: Please return this question paper together with your answers.

Question 1 (20 marks) An array of power transistors, each dissipating 5 Watts of power, are mounted on a 25 cm x 25 cm square aluminum plate. Air at temperature 35C is blown over the plate with a velocity of 4 m/s. The average temperature of the plate is not to exceed 65C. Heat transfer from the back of the plate can be neglected. Radiation heat loss from the plate can also be ignored. Determine: (a) the heat transfer coefficient h (in W/m2 C), (b) the rate of heat transfer by convection (in Watts), and (c) the number of transistors that can be placed on the plate. Properties of air: k = 0.02735 W/m C, v = 1.798 x 10-5 m2/s, Pr = 0.7228

Question 2 (20 marks) Air blows over a long and uninsulated steam pipe that has a diameter of 12 cm. The surface temperature of the pipe is found to be 90C. The air is at a pressure of 1 atm and temperature of 7C. It is blown over the pipe at a velocity of 18 m/s. (a) Determine the heat transfer coefficient h (in W/m2 C) and (b) Estimate the rate of heat transfer (in Watts) per meter length of the pipe. (c) What is the implication if the diameter of the pipe is made larger than 12 cm. Properties of air: k = 0.02724 W/m C, v = 1.784 x 10-5 m2/s, Pr = 0.7232