Thermodynamics HW Solutions 936

Thermodynamics HW Solutions 936 - Chapter 12 Radiation Heat...

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Chapter 12 Radiation Heat Transfer 12-43 Two perpendicular rectangular surfaces with a common edge are maintained at specified temperatures. The net rate of radiation heat transfers between the two surfaces and between the horizontal surface and the surroundings are to be determined. Assumptions 1 Steady operating conditions exist 2 The surfaces are opaque, diffuse, and gray. 3 Convection heat transfer is not considered. Properties The emissivities of the horizontal rectangle and the surroundings are ε = 0.75 and ε = 0.85, respectively. Analysis We consider the horizontal rectangle to be surface 1, the vertical rectangle to be surface 2 and the surroundings to be surface 3. This system can be considered to be a three-surface enclosure. The view factor from surface 1 to surface 2 is determined from 27 . 0 75 . 0 6 . 1 2 . 1 5 . 0 6 . 1 8 . 0 12 2 1 = = = = = F W L W L (Fig. 12-6) W = 1.6 m (2) L 2 = 1.2 m L 1 = 0.8 m A 1 (1) A 2 T 3 = 290 K ε 3 = 0.85 T 2 = 550 K ε 2 = 1 T 1 =400 K ε 1 =0.75 (3) The surface areas are 2 2 2 1 m 92 . 1 ) m 6 . 1 )( m 2 . 1 ( m 28 . 1 ) m 6 . 1 )( m 8 . 0 ( = = = = A A 2 2 2 3 m 268 . 3 6 . 1 2 . 1 8 . 0 2 8 . 0 2 . 1 2 = × + + × × = A Note that the surface area of the surroundings is determined assuming that surroundings forms flat surfaces
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