# Lecture 27 Radiation - 1 MECE 4364 Heat Transfer Prof. Dong...

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MECE 4364 Heat Transfer Prof. Dong Liu Department of Mechanical Engineering University of Houston 1 Lecture 27 – Nov 23, 2010 Lecture 27 Radiation 2

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Lecture 27 Irradiation ± Irradiation ± Radiation incident on a surface is called irradiation ± Spectral Intensity, I λ ,i ( λ , θ , φ ) ± a quantity used to specify the incident radiant heat flux (W/m 2 ) within a unit solid angle about the direction of incidence (W/m 2 -sr) and within a unit wavelength interval about a prescribed wavelength (W/m 2 -sr- μ m) and the projected area of the receiving surface (dA 1 cos θ ) 3 Emitted radiation, I λ ,e v.s. Lecture 27 Radiosity ± Radiosity ± accounts for all radiation leaving a surface ² emission ² reflection ± Spectral Intensity, I λ ,e+r ± emitted plus reflected radiation intensity ± The integration of the spectral heat flux is called the spectral radiosity ± spectral emission + reflection (heat flux) over all possible directions ± The total heat flux from the surface due to irradiation over all wavelengths and directions total radiosity q λ = J () = I , e + r , θ , φ cos sin d d 0 π 2 0 2 W m 2 μ m q = J = I , e + r , , cos sin d d d 0 2 0 2 = J d 0 0 W m 2 4
Lecture 27 Surface Properties ± Real surfaces do not behave like ideal black bodies ± non-ideal surfaces are characterized by factors (< 1) - the ratio of the non-ideal performance to the ideal black body performance ± these factors can be a function of wavelength (spectral dependence) and direction (angular dependence) ± Non-Ideal Radiation Factor ± emissivity, ε ± Non-Ideal Irradiation ± absorptivity, α ± reflectivity, ρ ± transmissivity, τ 5 Lecture 27 Emissivity ± Emissivity ± Ratio of the emission of a real body to the ideal emission of a black body ± can be ² a function of wavelength (spectral dependence) and direction (angular dependence) ² a function of wavelength (spectral dependence) averaged over all directions ² average over all wavelengths and a function of direction (angular dependence) ± Spectral, Directional Emissivity ± Spectral, Hemispherical Emissivity (directional average) ± Total, Directional Emissivity (spectral average) ε λ , θ , , φ , T () = I , e , , , T ( ) I , b , T 2 2 , 2 00 , ,, 0 ,, c o ss i n , ,2 , , c o s s i n e bb Id d ET TT d IT π λθ λλ λθφ θ θ φ λε λπ == = ∫∫ , , T = I e , , T ( ) I b T 6

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Lecture 27 Absorption/Reflection/Transmission ± Spectral, Directional Absorptivity ± Spectral, Directional Reflectivity ± Spectral, Hemispherical transmittivity ± Semi-Transparent Materials ± Opaque Materials () ( ) ,, , , iabs i I I λ λθ λθφ αλθφ = 7 ( ) , , ire f i I I ρλθφ = τ ()= G , tr ( ) G α + ρ + = 1 + + = 1 and + = 1 + = 1 and Lecture 27 Black Body ± Black Body ± an idealization providing limits on radiation emission and absorption by matter - for a prescribed temperature and wavelength, no surface can emit more than a black body Î ideal emitter
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## This note was uploaded on 01/27/2011 for the course MECE 4364 taught by Professor Lipinglui during the Winter '10 term at University of Houston.

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Lecture 27 Radiation - 1 MECE 4364 Heat Transfer Prof. Dong...

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