Chapter 12 Radiation Heat Transfer12-42EA 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. Assumptions1Steady operating conditions exist 2The surfaces are opaque, diffuse, and gray. 3Convection heat transfer is not considered.4There is no heat loss through the side surfaces.PropertiesThe emissivities are ε= 1 for the ceiling and ε= 0.8 for the floor. The emissivity of insulated (or reradiating) surfaces is also 1. AnalysisThe 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 T2= 90°F ε2= 0.8 Ceiling: 12 ft ×12 ft T1= 90°F ε1= 1 Insulated side surfacess 9 ft2123131221111RRRREEQbb+⎟⎟⎠⎞
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