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Radiation Heat Transfer

Radiation Heat Transfer. EGR 4345 Heat Transfer. Blackbody Radiation. Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in all directions – no directional distribution – it’s diffuse

MikeCarlo
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Radiation Heat Transfer

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  1. Radiation Heat Transfer EGR 4345 Heat Transfer

  2. 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

  3. 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.”

  4. 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.

  5. 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.

  6. 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

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

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

  9. Surface Emission

  10. Surface Emission Spectral, directional emissivity Total, directional emissivity Spectral, hemispherical emissivity

  11. Surface Emission Spectral, hemispherical emissivity Substituting spectral emissive power

  12. Surface Emission Total, directional emissivity Normal emissivity predictable Total, hemispherical emissivity

  13. Spectral, normal emissivity

  14. Total, normal emissivity

  15. Total, normal emissivity

  16. Absorption, Reflection, and Transmission

  17. Absorptivity Spectral, directional absorptivity Spectral, hemispherical absorptivity Total, hemispherical absorptivity

  18. Reflectivity Spectral, directional reflectivity Spectral, hemispherical reflectivity Total, hemispherical reflectivity

  19. Reflectivity

  20. Transmissivity Spectral, hemispherical transmissivity Total, hemispherical reflectivity

  21. Special Considerations • Semitransparent medium  +  +  = 1 Opaque  +  = 1

  22. Kirchhoff’s Law  =  No real surface can have an emissive power exceeding that of a black surface at the same temperature.

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