IV. Radiation Heat Transfer IV. Radiation Heat Transfer
IV. Radiation Heat Transfer IV. Radiation Heat Transfer
IV. Radiation Heat Transfer IV. Radiation Heat Transfer
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<strong>IV</strong>. <strong>Radiation</strong> <strong>Heat</strong> <strong>Transfer</strong>c. the gray surface (cont.)A bTAAt equilibrium, the temperature ofA b is T and it emits energy A b σT 4 .It also absorbs energy A b σT 4 .We conclude that the irradiation,G, on any surface in theenclosure isG = σT 4 (<strong>IV</strong>.B.7)At equilibrium, surface A absorbs and emits perabsorption = AGα = emission = AE.(<strong>IV</strong>.B.8)Define the emissivity (emittance) by (E = total emissive power of realsurface)Eε = or E = εEb(<strong>IV</strong>.B.9)Eblesson 28<strong>IV</strong>. <strong>Radiation</strong> <strong>Heat</strong> <strong>Transfer</strong>c. The gray surface (cont.)A bTAThen for surface A at equilibriumAGα = AσT 4 α = AE = AσT 4 ε orα = ε. (<strong>IV</strong>.B.10)(<strong>IV</strong>.B.10) is called Kirchoff’s law. It was derived for equilibriumconditions but is frequently applied to nonequilibrium situations.A gray surface is one for which, at a given T,Eε ( λ,T)= λλEbλlesson 28= ε(T)= const.forallλ ⇒ ε(T)= α(T)= const.(<strong>IV</strong>.B.11)2
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