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Radiation Fundamental Concepts

Radiation Fundamental Concepts. EGR 4345 Heat Transfer. Thermal Radiation. Occurs in solids, liquids, and gases Occurs at the speed of light Has no attenuation in a vacuum Can occur between two bodies with a colder medium in between. Fundamental Concepts. T s > T sur q rad,net.

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Radiation Fundamental Concepts

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  1. RadiationFundamental Concepts EGR 4345 Heat Transfer

  2. Thermal Radiation • Occurs in solids, liquids, and gases • Occurs at the speed of light • Has no attenuation in a vacuum • Can occur between two bodies with a colder medium in between

  3. Fundamental Concepts Ts > Tsur q rad,net

  4. Types of Radiation • Two categories • Volumetric phenomenon – gases, transparent solids • Surface phenomenon – most solids and liquids • Thermal radiation – emitted by all substances above absolute zero • Includes visible & infrared radiation & some UV radiation.

  5. Fundamental Concepts

  6. Background • Electromagnetic radiation – energy emitted due to changes in electronic configurations of atoms or molecules • where l=wavelength (usually in mm), n=frequency • In a vacuum c=co=2.998x108 m/s • Other media: c= co /n where n=index of refraction

  7. Background, cont. • Radiation – photons or waves? • Max Planck (1900): each photon has an energy of • h=Planck’s constant=6.625 x 10-34 Js • Shorter wavelengths have higher energy

  8. Radiation Spectrum

  9. Radiation Properties • Magnitude of radiation varies with wavelength – it’s spectral. • Radiation is made up of a continuous, nonuniform distribution of monochromatic (single-wavelength) components. • Magnitude & spectral distribution vary with temp & type of emitting surface.

  10. Emission Variation with Wavelength

  11. Radiation Properties • Directional distribution – a surface doesn’t emit the same in all directions.

  12. Nomenclature

  13. Nomenclature

  14. Nomenclature

  15. Spectral Intensity • Spectral Intensity of the Emitted Radiation, I,e • Rate at which radiant energy is emitted at the wavelength, , in the (, ) direction, per unit solid angle about this direction, and per unit wavelength interval d  about 

  16. Spectral Intensity

  17. Spectral Intensity

  18. Heat Flux

  19. Emissive Power • Spectral, hemispherical emissive power – E • Rate at which radiation of wavelength  is emitted in all directions from a surface per unit wavelength d  about  and per unit surface area • Flux based on actual surface area (not projected) • Hemispherical often not used as emission is in all directions from surface • Total emissive power, E

  20. Emissive Power

  21. Emissive Power • Diffuse emitter – intensity of the emitted radiation independent of direction

  22. Spectral Irradiation • Spectral Irradiation, G  • Rate at which radiation of wavelength  is incident on a surface per unit area of the surface and per unit wavelength d  about  • Total Irradiation, G

  23. Spectral Irradiation

  24. Spectral Radiosity • Spectral Radiosity, J  • Rate at which radiation of wavelength  is leaves a unit area of surface, per unit wavelength interval d  about  • Total Radiosity, J

  25. Spectral Radiosity

  26. Blackbody Radiation • Blackbody – a perfect emitter & absorber of radiation • Emits radiation uniformly in all directions – no directional distribution – it’s diffuse • Joseph Stefan (1879)– total radiation emission per unit time & area over all wavelengths and in all directions: • s=Stefan-Boltzmann constant =5.67 x10-8 W/m2K4

  27. Planck’s Distribution Law • Sometimes we’re interested in radiation at a certain wavelength • Spectral blackbody emissive power (Ebl) = “amount of radiation energy emitted by a blackbody at an absolute temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength l.”

  28. Planck’s Distribution Law • Emitted radiation varies continuously with  • At any  the magnitude of the emitted radiation increases with increasing temperature • The spectral region in which the radiation is concentrated depends on temperature, with comparatively more radiation appearing at the shorter  as the temperature increases • Sun – approximated as a blackbody at 5800 K, radiation is mostly in the visible region

  29. Planck’s Distribution Law • For a surface in a vacuum or gas • Other media: replace C1 with C1/n2 • Integrating this function over all l gives us the equation for Eb.

  30. Radiation Distribution • Radiation is a continuous function of wavelength • Radiation increases with temp. • At higher temps, more radiation is at shorter wavelengths. • Solar radiation peak is in the visible range.

  31. Wien’s Displacement Law • Peak can be found for different temps using Wien’s Displacement Law: • Note that color is a function of absorption & reflection, not emission

  32. Blackbody Radiation Function • We often are interested in radiation energy emitted over a certain wavelength. • This is a tough integral to do!

  33. Blackbody Radiation Function • Use blackbody radiation function, Fl • If we want radiation between l1 & l2,

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