Thermodynamics HW Solutions 165

Thermodynamics HW Solutions 165 - medium are to be...

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Chapter 3 Steady Heat Conduction 3-24 A power transistor dissipates 0.2 W of power steadily in a specified environment. The amount of heat dissipated in 24 h, the surface heat flux, and the surface temperature of the resistor are to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat is transferred uniformly from all surfaces of the transistor. Analysis ( a ) The amount of heat this transistor dissipates during a 24-hour period is Air, 30 ° C Power Transistor 0.2 W kWh 0.0048 = = = Δ = Wh 8 . 4 h) W)(24 2 . 0 ( t Q Q ( b ) The heat flux on the surface of the transistor is 2 2 2 m 0001021 . 0 m) m)(0.004 005 . 0 ( 4 m) 005 . 0 ( 2 4 2 = + = + = π DL D A s 2 W/m 1959 = = = 2 m 0001021 . 0 W 2 . 0 s A Q q ( c ) The surface temperature of the transistor can be determined from C 193 ° = ° = + = ⎯→ = ) m 21 C)(0.00010 . W/m (18 W 2 . 0 ) ( 2 2 s s s s hA Q T T T T hA Q 3-25 A circuit board houses 100 chips, each dissipating 0.07 W. The surface heat flux, the surface temperature of the chips, and the thermal resistance between the surface of the board and the cooling
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Unformatted text preview: medium are to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat transfer from the back surface of the board is negligible. 2 Heat is transferred uniformly from the entire front surface. Analysis ( a ) The heat flux on the surface of the circuit board is Chips T s Q & T ∞ 2 m 0216 . m) m)(0.18 12 . ( = = s A 2 W/m 324 = × = = 2 m 0216 . W ) 07 . 100 ( s A Q q & & ( b ) The surface temperature of the chips is C 72.4 ° = ° × ° = + = ⎯→ ⎯ − = ∞ ∞ ) m 0216 . )( C . W/m 10 ( W ) 07 . 100 ( + C 40 ) ( 2 2 s s s s hA Q T T T T hA Q & & ( c ) The thermal resistance is C/W 4.63 ° = ° = = ) m 0216 . ( C) . W/m 10 ( 1 1 2 2 s conv hA R 3-8...
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This note was uploaded on 01/14/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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