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Radiation Heat Transfer Between Real Surfaces

Radiation Heat Transfer Between Real Surfaces. P M V Subbarao Professor Mechanical Engineering Department IIT Delhi. A means for basic life on earth……. The total (hemispherical) energy emitted by a body, regardless of the wavelengths, is given by:. where ε is the emissivity of the body,

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Radiation Heat Transfer Between Real Surfaces

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  1. Radiation Heat Transfer Between Real Surfaces P M V Subbarao Professor Mechanical Engineering Department IIT Delhi A means for basic life on earth……..

  2. The total (hemispherical) energy emitted by a body, regardless of the wavelengths, is given by: • where ε is the emissivity of the body, • A is the surface area, • T is the temperature, and • σ is the Stefan-Boltzmann constant, equal to 5.67×10-8 W/m2K4. • Emissivity is a material property, ranging from 0 to 1, which measures • how much energy a surface can emit with respect to an ideal emitter (ε = 1) at the same temperature

  3. The total (hemispherical) emissive power is, then, given byë Define total (hemispherical) emissivity, at a defined temperature Here, e can be interpreted as either the emissivity of a body, which is wavelength independent, i.e., e is constant, or as the average emissivity of a surface at that temperature. A surface whose properties are independent of the wavelength is known as a gray surface. The total emissive power of a real surface is given by

  4. Emission from Colour Surface The total (hemispherical) emissive power of a colour surface is, then, given byë

  5. Coating of Frost : Water on Mars?

  6. Hemi - Spherical Emissive PowerGeometry The emissive power of a surface is distributed in a hemispherical space. Black body Emissive Intensity The emissive power of an ideal (Black) surface is equally distributed in a hemispherical space.

  7. Real Surface emission The radiation emitted by a real surface is spatially distributed: Directional Emissive Intensity: Directional Emissivity:

  8. Directional Emissivity

  9. Real Surface Directional emission The radiation emitted by a real surface is spatially distributed amond various wave lengths (colours) : Directional Monochromatic Emissive Intensity: Directional Monochromatic Emissivity: Total emissive Power of a real surface (W/m2):

  10. Vectors in Spherical Geometry Zenith Angle : q Azimuthal Angle : f (f, q,r)

  11. dAn dw dA1 Definition of Solid Angle Differential solid angle is defined by a region between the rays of a sphere. It is measured as the ratio of the element of areas dAn on the sphere to the square of the sphere’s radius

  12. Differential Solid Angle due to Spherical Differential Volume

  13. Emissive Intensity • Solid Angle: Emissive intensity Monochromatic Emissive intensity

  14. Projection of dAn normal to direction of radiation: Spectral Radiation flux

  15. The specrtal heat flux associated with emission into hypothetical hemisphere above dA1 Total (hemispherical) heat flux associated with dA1

  16. Relation to Emission Spectral emissive power is defined as the rate at which radiation of wavelength l is emitted in all directions form a surface per unit wavelength dl about l. The total hemispherical emissive power:

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