Lecture 26 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 26 – Nov 18, 2010 Lecture 26 Radiation 2
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Lecture 26 Solid Angle measures how large an object appears to an observer looking from a point which is subtended by the object. ± A small object nearby may subtend the same solid angle as a larger object farther away ± Example: The eclipse!! Solid Angle 3 2 sin n dA r d d θ θ φ = 2 sin n dA d d d r ω θ θ φ = = Lecture 26 Spectral Intensity Spectral radiation intensity, I λ ,e ( λ , θ , φ ) The rate at which radiant energy is emitted at the wavelength λ in the ( θ , φ ) direction per unit area of the emitting surface normal to this direction per unit solid angle about this direction per unit wavelength interval d λ about λ the rate at which emission from dA 1 passes through dA n ( )( ) , 1 , , cos e dq I dA d d λ λ θ φ θ ω λ = 4 Consider the rate at which emission from dA 1 passes through dA n where 1 cos n dA dA θ = This is how dA 1 appears along θ , φ Unit: W/m 2 -sr- μ m
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