15 radiaon heat transfer radiaon from real surfaces

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Unformatted text preview: not simple to do, but it produces a very simple result! •  Once again note that T must be in Kelvin! 12 Radia%on Heat Transfer •  Band Emission –  We frequently wish to evaluate radia%ve transfer proper%es or emission over a por%on of the spectral distribu%on. This is done using tabulated data for the following equa%ons: λ2 ∫E Fλ1 → λ 2 = € λ1 λ ,b dλ − 0 ∫E 0 σT 4 λ ,b dλ = F0 → λ 2 − F0 → λ1 –  There are two other tabulated band emission func%ons, these will be referred to as necessary for problems. 13 Radia%on Heat Transfer 14 Example 1 •  Consider a blackbody at 500 K. Calculate the –  the total emissive power, –  the peak wavelength of emission, and –  the frac%on of thermal radia%on emiCed in the range of +/ ­ 20% of peak wavelength. 15 Radia%on Heat Transfer •  Radia%on from real surfaces is less than ideal. We now define an important property of surfaces: emissivity. Two defini%ons are: ε (T ) = ε ( λ, T ) = € E (T ) E b (T ) E ( λ, T ) E b ( λ, T ) Total Hemispherical Emissivity Spectral Hemispherical Emissivity • There are other defini%ons which involve direc%onal effects. We typically use Total or spectral proper%es. 16 Radia%on Heat Transfer •  Emissivity varies according to direc%on, wavelength, and temperature. •  Emissivity is oeen reported as the “total normal emissivity”, εn. We frequently approximate total emissivity using the “normal” value. 17 Radia%on Heat Transfer •  Typical values of the total normal emissivity of materials. 18 Radia%on Heat Transfer •  Absorp%on, Reflec%on, and Transmission Gabs Gλ Gref ρ(λ) = Gλ G τ ( λ) = tr Gλ α ( λ) = − absorptivity Gλ = Gλ ,ref + Gλ ,abs + Gλ ,tr − reflectivity α + ρ + τ = 1 semi − transparent α + ρ =1 opaque − tr...
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This document was uploaded on 02/14/2014 for the course ENGR 6901a at Memorial University.

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