Lecture 25 Radiation

Lecture 25 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 25 – Nov 16, 2010 Lecture 25 Radiation 2
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Lecture 25 Overview ± Radiation - Emission ± thermal radiation is the emission of electromagnetic waves when matter is at an absolute temperature greater than 0 K ± emission is due to the oscillations and transitions of many electrons that comprise the matter ² the oscillations and transitions are sustained by the thermal energy of the matter ± emission corresponds to heat transfer from the matter and hence to a reduction in the thermal energy stored in the matter ± Radiation - Absorption ± radiation may also be absorbed by matter ± absorption results in heat transfer to the matter and hence to an increase in the thermal energy stored in the matter 3 Lecture 25 Overview ± Emission ± emission from a gas or semi-transparent solid or liquid is a volumetric phenomenon ± emission from an opaque solid or liquid is a surface phenomenon ² emission originates from atoms & molecules within 1 μ m of the surface Dual Nature in some cases, the physical manifestations of radiation may be explained by viewing it as particles (A.K.A. photons or quanta); in other cases, radiation behaves as an electromagnetic wave radiation is characterized by a wavelength, λ and frequency, ν which are related through the speed at which radiation propagates in the medium of interest (solid, liquid, gas, vacuum) λ = c ν c = c o = 2.998 × 10 8 m/s in a vacuum 4
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Lecture 25 Electromagnetic Spectrum ± Electromagnetic Spectrum ± the range of all possible radiation frequencies ± thermal radiation is confined to the infrared , visible , and ultraviolet regions of the spectrum 0.1 < λ < 100 μ m Spectral Distribution radiation emitted by an opaque surface varies with wavelength spectral distribution describes the radiation over all wavelengths monochromatic/spectral components are associated with particular wavelengths 5 Lecture 25 Directional Considerations ± Emission ± Radiation emitted by a surface will be in all directions associated with a hypothetical hemisphere about the surface and is characterized by a directional distribution 6 Differential plane angle Differential solid angle
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Lecture 25 Directional Considerations ± Emission ± Direction may be represented in a spherical coordinate system characterized by the zenith or polar angle θ and the azimuthal angle ϕ . The area dA n through which the radiation passes, subtends a differential solid angle d ω when viewed from a point on dA 1 dA n unit element of surface on a hypothetical sphere and normal to the ( θ , ϕ ) direction d ω = dA n r 2 7 Lecture 25 ± Solid Angle Directional Considerations 2 sin n dA r d d θθφ = 2 sin n dA dd d r == 2 2 00 sin 2 sr hemi π ∫∫ the solid angle ω has units of steradians (sr) the solid angle ω hemi associated with a complete hemisphere 8
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Lecture 25 Spectral Intensity ± Spectral radiation intensity, I λ ,e ±
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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 25 Radiation - 1 MECE 4364 Heat Transfer Prof. Dong...

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