Chapter 11 Fundamentals of Thermal Radiation 11-79 The variation of emissivity of an opaque surface at a specified temperature with wavelength is given. The average emissivity of the surface and its emissive power are to be determined. AnalysisThe average emissivity of the surface can be determined from εεεελλλλ()((Tffff=−−12311+)+2)λ22Twhere are blackbody radiation functions corresponding to . These functions are determined from Table 11-1 to be ffλ1and 1Tand λμμμλλ122014025660819217TT=⎯→⎯=→(.m)(1200 K) = 2400 mKfm)(1200 K) = 7200 mKf12ελλ, μm2 6 0.85 0 and 0.577=−+−+=)819217.01)(0.0()140256.0819217.0)(85.0()140256.0)(0.0(εThen the emissive flux of the surface becomes 2W/m67,853=×=εσ=−44284K)1200)(K.W/m1067.5)(577.0(TE11-80 The variation of transmissivity of glass with wavelength is given. The transmissivity of the glass for solar radiation and for light are to be determined. AnalysisFor solar radiation, T= 5800 K.The average transmissivity of the surface can be determined from τ
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