Thermodynamics HW Solutions 247

Thermodynamics HW Solutions 247 - Chapter 3 Steady Heat...

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Chapter 3 Steady Heat Conduction 3-129 The walls and the roof of the house are made of 20-cm thick concrete, and the inner and outer surfaces of the house are maintained at specified temperatures. The rate of heat loss from the house through its walls and the roof is to be determined, and the error involved in ignoring the edge and corner effects is to be assessed. Assumptions 1 Steady operating conditions exist. 2 Heat transfer at the edges and corners is two-or three- dimensional. 3 Thermal conductivity of the concrete is constant. 4 The edge effects of adjoining surfaces on heat transfer are to be considered. Properties The thermal conductivity of the concrete is given to be k = 0.75 W/m °C. Analysis The rate of heat transfer excluding the edges and corners is first determined to be A total 2 m =− −+ = (. ) ) ) ( . ) . 12 04 12 04 4 12 04 6 02 4037 & () ) ( . ) . , Q kA L TT = ° −°= total 2 W/m. C m m C W 12 075 02 15 3 18 167 The heat transfer rate through the edges can be determined using the shape factor relations in Table 3-5, m 04 . 26
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