Thermodynamics HW Solutions 305

Thermodynamics HW Solutions 305 - Chapter 4 Transient Heat...

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Chapter 4 Transient Heat Conduction 4-25 An electronic device is on for 5 minutes, and off for several hours. The temperature of the device at the end of the 5-min operating period is to be determined for the cases of operation with and without a heat sink. Assumptions 1 The device and the heat sink are isothermal. 2 The thermal properties of the device and of the sink are constant. 3 The heat transfer coefficient is constant and uniform over the entire surface. Properties The specific heat of the device is given to be C p = 850 J/kg. ° C. The specific heat of the aluminum sink is 903 J/kg. ° C (Table A-19), but can be taken to be 850 J/kg. ° C for simplicity in analysis. Analysis ( a ) Approximate solution This problem can be solved approximately by using an average temperature for the device when evaluating the heat loss. An energy balance on the device can be expressed as EE E E Qt E t m C T p in out generation device out generation device −+ = ⎯− + = ΔΔ Δ && Δ Electronic device 30 W or, ) ( 2 generation
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This note was uploaded on 01/19/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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