Chapter 11 Fundamentals of Thermal Radiation 11-45 The variations of emissivity of two surfaces are given. The average emissivity, absorptivity, and reflectivity of each surface are to be determined at the given temperature. AnalysisFor the first surface:λμμλ1308900291T=⎯→⎯=(.m)(3000 K) = 9000 mKfThe average emissivity of this surface is 0.28=−+=−ε+ε=ελλ)890029.01)(9.0()890029.0)(2.0()1()(1121ffTThe absorptivity and reflectivity are determined from Kirchhoff’s law 0.720.28=−=α−=ρ⎯→⎯=ρ+α=α=ε28.0111K)3000(at For the second surface: 1308900291T→m)(3000 K) = 9000 mKfελλ, μm3 0.80.20.10.9The average emissivity of this surface is
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