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.
 Fall '12
 DrMuzychka
 Heat Transfer

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