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QUESTION 2 (a) A student calculates the total heat transfer from a spherical copper ball of diameter 18 cm initially at 200°C and its environment at a constant temperature of 25°C during the first 20 min of cooling is 3150 kJ. Is this result reasonable? Explain your answer. [C5, SP2][5 marks] (b) A 15 cm x 15 cm circuit board as shown in Figure Q2(b) dissipating 20 W of power uniformly is cooled by air, which approaches the circuit board at 20°C with a velocity of 6 m/s. Disregarding any heat transfer from the back surface of the board and assume the flow to be turbulent since the electronic components are expected to act as turbulators. Air 20°C 6 m/s Circuit board 20 W 50°C (ii) Calculate Reynolds number. 15 cm 15 cm Figure Q2(b) Circuit board (i) Determine the surface temperature of the electronic components at the leading edge of the board. [C3, SP3][2 marks] [C4, SP3][4 marks] (iii) Does the answer in (ii) meet the Reynolds number criterion for turbulent flow? Explain your answer. [C5, SP4][3 marks]
(iv) Determine the surface temperature of the electronic components at the end of the board. Use Nu = 0.0308 Rex08 Pr¹/3 [C4, SP3][8 marks] (v) State THREE (3) assumptions based on your calculation. [C2, SP1][3 marks]