Thermodynamics HW Solutions 935

Thermodynamics HW Solutions 935 - Chapter 12 Radiation Heat...

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Chapter 12 Radiation Heat Transfer 12-42E A room is heated by lectric resistance heaters placed on the ceiling which is maintained at a uniform temperature. The rate of heat loss from the room through the floor is to be determined. Assumptions 1 Steady operating conditions exist 2 The surfaces are opaque, diffuse, and gray. 3 Convection heat transfer is not considered. 4 There is no heat loss through the side surfaces. Properties The emissivities are ε = 1 for the ceiling and ε = 0.8 for the floor. The emissivity of insulated (or reradiating) surfaces is also 1. Analysis The room can be considered to be three-surface enclosure with the ceiling surface 1, the floor surface 2 and the side surfaces surface 3. We assume steady-state conditions exist. Since the side surfaces are reradiating, there is no heat transfer through them, and the entire heat lost by the ceiling must be gained by the floor. Then the rate of heat loss from the room through its floor can be determined from T 2 = 90 ° F ε 2 = 0.8 Ceiling: 12 ft × 12 ft T 1 = 90 ° F ε 1 = 1 Insulated side surfacess 9 ft 2 1 23 13 12 2 1 1 1 1 R R R R E E Q b b +
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