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hw3-solution

# hw3-solution - 1 The heat transfer coefficient for air...

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1. The heat transfer coefficient for air flowing over a sphere is to be determined by observing the temperature-time history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm in diameter, is at 66ºC before it is inserted into an airstream having a temperature of 27ºC. A thermocouple on the outer surface of the sphere indicates 55ºC, 69 seconds after the sphere is inserted in the airstream. Assume, and then justify, that the sphere behaves as a spacewise isothermal object and calculate the heat transfer coefficient. ASSUMPTIONS: (1) Temperature of sphere is spatially uniform, (2) Negligible radiation exchange, (3) Constant properties. PROPERTIES: Table A-1 , for pure copper (333K): ρ = 8933 kg/m 3 , c p = 389 J/kg K, k = 398 W/m K. ANALYSIS: Assuming the uniform temperature over the sphere, we use the Lumped Capacitance Method. t cL ρ h t cV ρ hA T T T T c i exp exp where 002117 . 0

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hw3-solution - 1 The heat transfer coefficient for air...

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