Thermodynamics HW Solutions 635

# Thermodynamics HW Solutions 635 - the flow is laminar Using...

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Chapter 7 External Forced Convection 7-103 A circuit board houses 80 closely spaced logic chips on one side. All the heat generated is conducted across the circuit board and is dissipated from the back side of the board to the ambient air, which is forced to flow over the surface by a fan. The temperatures on the two sides of the circuit 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 Air is an ideal gas with constant properties. 7 The pressure of air is 1 atm. Properties Assuming a film temperature of 40 ° C, the properties of air are (Table A-15) 7255 . 0 Pr /s m 10 702 . 1 C W/m. 02662 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is T 1 [] 4 2 5 10 051 . 7 /s m 10 702 . 1 m) (0.18 m/s ) 60 / 400 ( Re × = × = υ = L V L T 2 which is less than the critical Reynolds number. Therefore,
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Unformatted text preview: the flow is laminar. Using the proper relation for Nusselt number, heat transfer coefficient is determined to be T ∞ =30 ° C 400 m/min C . W/m 43 . 23 ) 4 . 158 ( m 18 . C W/m. 02662 . 4 . 158 ) 7255 . ( ) 10 051 . 7 ( 664 . Pr Re 664 . 2 3 / 1 5 . 4 3 / 1 5 . ° = ° = = = × = = = Nu L k h k hL Nu L The temperatures on the two sides of the circuit board are C 39.52 C 39.48 ° = ° × + ° = + = → − = ° = ° × + ° = + = → − = ∞ ∞ m) m)(0.18 C)(0.12 W/m. 16 ( m) W)(0.003 06 . 80 ( C 48 . 39 ) ( m) m)(0.18 C)(0.12 . W/m 43 . 23 ( W ) 06 . 80 ( C 30 ) ( 2 1 2 1 2 2 2 s s s s kA L Q T T T T L kA Q hA Q T T T T hA Q & & & & 7-90...
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