Thermodynamics HW Solutions 554

Thermodynamics HW Solutions 554 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-24 A circuit board is cooled by air. The surface temperatures of the electronic components at the leading edge and the end of the board are to be determined. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. 4 Any heat transfer from the back surface of the board is disregarded. 5 Air is an ideal gas with constant properties. Air 20 ° C 5 m/s Circuit board 15 W 15 cm Properties Assuming the film temperature to be approximately 35 ° C, the properties of air are evaluated at this temperature to be (Table A-15) 7268 . 0 Pr /s m 10 655 . 1 C W/m. 0265 . 0 2 5 - = × = ° = υ k Analysis ( a ) The convection heat transfer coefficient at the leading edge approaches infinity, and thus the surface temperature there must approach the air temperature, which is 20 ° C. ( b ) The Reynolds number is 4 2 5 10 532 . 4 /s m 10 655 . 1 m) m/s)(0.15 5 ( Re × = × = υ = x
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