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hw3 - 25ºC and with a convection coefficient of 8 W/m 2...

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ME 324 HW #3 14.03.2011 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. 2. A long plastic rod of 30-mm diameter ( k = 0.3 W/m K and c p = 1040 kJ/m 3 K) is uniformly heated in an oven as preparation for a pressing operation. For best results, the temperature in the rod should not be less than 200ºC. (1) To what uniform temperature should the rod be heated in the oven if, for the worst case, the rod sits on a conveyor for 3 min while exposed to convection cooling with ambient air at
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Unformatted text preview: 25ºC and with a convection coefficient of 8 W/m 2 K? (2) A further condition for good results is a maximum-minimum temperature difference of less than 10ºC. Is this condition satisfied? 3. A large slab of aluminum having a thickness of 5 cm is initially at a temperature of i T = 70°C, and both surfaces of the plate are suddenly exposed to a convection environment at  T = 20°C with h = 800 W/m 2 °C. Find the temperature difference between the center and the plate surface at 30 seconds later. 4. A very thick plane wall is made of common brick (see Table A.3 for its thermophysical properties). It is initially at a uniform temperature of 20ºC. Now it is suddenly exposed to a convection environment at 30ºC with the convection heat transfer coefficient of 50 W/m 2 K. What will be the surface temperature of the plate after 1000s?...
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