Thermodynamics HW Solutions 560

Thermodynamics HW Solutions 560 - relation in laminar flow...

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Chapter 7 External Forced Convection 7-30 Air is blown over an aluminum plate mounted on an array of power transistors. The number of transistors that can be placed on this plate is 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 Heat transfer from the back side of the plate is negligible. 5 Air is an ideal gas with constant properties. 6 The local atmospheric pressure is 1 atm. Properties The properties of air at the film temperature of (T s + T )/2 = (65+35)/2 = 50 ° C are (Table A-15) Transistors Air V = 4 m/s T = 35 ° C L = 25 cm T s = 65 ° C 7228 . 0 Pr /s m 10 798 . 1 C W/m. 02735 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is 617 , 55 /s m 10 798 . 1 m) m/s)(0.25 (4 Re 2 5 = × = = L L V which is less than the critical Reynolds number ( 51 0 5
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Unformatted text preview: relation in laminar flow for Nusselt number, heat transfer coefficient and the heat transfer rate are determined to be C . W/m 37 . 15 ) 5 . 140 ( m 25 . C W/m. 02735 . 5 . 140 ) 7228 . ( ) 617 , 55 ( 664 . Pr Re 664 . 2 3 / 1 5 . 3 / 1 5 . = = = = = = = Nu L k h k hL Nu L W 28.83 = C 35)-)(65 m C)(0.0625 . W/m 37 . 15 ( ) ( m 0.0625 = m) m)(0.25 25 . ( 2 2 2 = = = = s s conv s T T hA Q wL A & Considering that each transistor dissipates 3 W of power, the number of transistors that can be placed on this plate becomes 4 = = 8 . 4 W 6 W 8 . 28 n This result is conservative since the transistors will cause the flow to be turbulent, and the rate of heat transfer to be higher. 7-22...
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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