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Thermodynamics HW Solutions 168

Thermodynamics HW Solutions 168 - Chapter 3 Steady Heat...

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Chapter 3 Steady Heat Conduction 3-29 The roof of a house with a gas furnace consists of 3-cm thick concrete that is losing heat to the outdoors by radiation and convection. The rate of heat transfer through the roof and the money lost through the roof that night during a 14 hour period are to be determined. Assumptions 1 Steady operating conditions exist. 2 The emissivity and thermal conductivity of the roof are constant. Properties The thermal conductivity of the concrete is given to be k = 2 W/m ⋅° C. The emissivity of both surfaces of the roof is given to be 0.9. Q & T sky = 100 K T air =10 ° C T in =20 ° C L =15 cm Analysis When the surrounding surface temperature is different than the ambient temperature, the thermal resistances network approach becomes cumbersome in problems that involve radiation. Therefore, we will use a different but intuitive approach. In steady operation, heat transfer from the room to the roof (by convection and radiation) must be equal to the heat
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