Chapter12 - Chapter 12 Radiation Radiation heat transfer is...

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Copyright 2003 John Baldwin 1 ChEn 323 – Chapter 12 – Slide 1 Chapter 12 – Radiation Radiation heat transfer is unique because it requires no matter (or medium) for transmission The primary objective here is to consider: § The generation of heat radiation § Nature of heat radiation § How radiation interacts with matter Topics: § Fundamental concepts § Radiation Intensity § Blackbody radiation § Surface emission § § Kirchhoff’s Law § The gray surface § Environmental radiation ChEn 323 – Chapter 12 – Slide 2 Fundamental Concepts Consider a hot solid in a vacuum where T S > T SUR (figure at right). All forms of matter emit radiation. For normal solids, the radiation emission is a surface phenomenon. materials (e.g., gases, glass, etc.) it is a volumetric phenomenon.
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Copyright 2003 John Baldwin 2 ChEn 323 – Chapter 12 – Slide 3 Fundamental Concepts – Continued Radiation can be viewed as the propagation of electromagnetic waves: Radiation encompasses a range of wavelengths. Magnitude of emission for a particular material varies with wavelength. Radiation is directional: n l c = Wavelength Frequency Speed of light in medium ChEn 323 – Chapter 12 – Slide 4 Radiation Intensity Definitions; § q ~ Zenith Angle § f ~ Azimuthal Angle § Differential solid angle d w gives rise to: Consider the geometry (angles): f q q w f q q f q w d d d d d r dA dA r dA d n n n sin sin 2 2 = = so , So, direction. ) , ( to normal is where Plane Angle: d a ~ radians (rad) Solid Angle: d w ~ steradians (sr)
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Copyright 2003 John Baldwin 3 ChEn 323 – Chapter 12 – Slide 5 Spectral Intensity Spectral Intensity, I λ ,e , is 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 and per unit wavelength interval d λ about λ . The units are (W/m 2 sr μ m). Then, ( 29 ( 29 ( 29 ( 29 ∫ ∫ = = = p p l l l l l l l f q q q f q l f q q q f q l w q f q l l l l w q f q l 2 0 2 / 0 , , 1 , 1 , sin cos , , sin cos , , cos , , cos , , d d I q d d I q d d dA I dq dq d dq d d dA dq I e e e A e : is flux the Then gives ent Rearrangem dA through passes and dA leaves h wavelengt which at rate ~ where n 1 ChEn 323 – Chapter 12 – Slide 6 Spectral Intensity – Continued ( 29 ( 29 ∫ ∫ ∫ ∫ = = = = = 0 2 / 0 2 0 2 / 0 2 0 2 / 0 , 2 sin 2 sin sin cos , , l l p q q p f q q w f q q q f q l l p p p p p l l d q q d d d d d d I q h e ) (obviously is hs wavelengt all over flux heat total The : is hemisphere the for angle solid the equation; last the Repeating
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Copyright 2003 John Baldwin 4 ChEn 323 – Chapter 12 – Slide 7 Emission The spectral hemispherical emissive power, E
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This note was uploaded on 03/26/2008 for the course CHEN 323 taught by Professor Johnbaldwin during the Fall '03 term at Texas A&M.

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Chapter12 - Chapter 12 Radiation Radiation heat transfer is...

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