Chapter 11 Fundamentals of Thermal Radiation 11-42 The variation of reflectivity of a surface with wavelength is given. The average reflectivity, emissivity, and absorptivity of the surface are to be determined for two source temperatures. AnalysisThe average reflectivity of this surface for solar radiation (= 5800 K) is determined to be TλμμλT=→=(.30 978746m)(5800 K) = 17400 mKfρλλ, μm3 0.950.35ρρρλλ()(. )) (. )(.)TfTff=+−−=−−∞102121110 35 0 9787460 95 1 0 9787460.362Noting that this is an opaque surface, τ =0 At = 5800 K: T0.638=−=−=⎯→⎯=+362.0111ραRepeating calculations for radiation coming from surfaces at = 300 K, TμλT=⎯→⎯=30 0001685m)(300 K) = 900 mKf10.95=−+=)0001685.01)(95.0()0001685.0)(35.0()(TAt = 300 K: T0.05=−=−=⎯=+95.0111and εα==0.05The temperature of the aluminum plate is close to room temperature, and thus emissivity of the plate will be equal to its absorptivity at room temperature. That is, εεααrooms0050638..which makes it suitable as a solar collector. (0and 1room==sfor an ideal solar collector) 11-43
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