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FUNDAMENTALS OF THERMAL RADIATION CHAPTER 6 - Coggle Diagram
FUNDAMENTALS OF THERMAL RADIATION CHAPTER 6
Introduction
Electromagnetic waves or electromagnetic radiation represent the energy emitted by matter as a result of the changes in the electronic configurations of the atoms or molecules
Electromagnetic waves are characterized by their frequency ν or wavelength λ
c ─ the speed of propagation of a wave in that medium
Blackbody Radiation
A body at a thermodynamic (or absolute) temperature above zero emits radiation in all directions over a wide range of wavelengths
A perfect emitter and absorber of radiation
The radiation energy emitted by a blackbody per unit time and per unit surface area (Stefan Boltzmann law) σ=5.67 X 10-8 W/m2·K4
The wavelength at which the peak occurs is given by Wien’s displacement law as
The radiation energy emitted by a blackbody per unit area over a wavelength band from λ=0 to λ= λ1 is determined from
Radiative Properties
Metals, wood, and bricks, are opaque to thermal radiation, and radiation is considered to be a surface phenomenon for such materials
Thermal radiation is emitted or absorbed within the first few microns of the surface
Glass and water exhibit different behavior at different wavelengths:
Visible spectrum ─ semitransparent,
Infrared spectrum ─ opaque
Emissivity
Emissivity of a surface ─ the ratio of the radiation emitted by the surface at a given temperature to the radiation emitted by a blackbody at the same temperature
Spectral directional emissivity
The total directional emissivity (intensities integrated over all wavelengths)
The spectral hemispherical emissivity
The total hemispherical emissivity
Gray and Diffuse Surfaces
Diffuse surface ─ a surface which properties are independent of direction
Gray surface ─ surface properties are independent of wavelength
Absorptivity, Reflectivity, and Transmissivity
Absorptivity:
- Reflectivity:
- Transmissivity:
Spectral directional absorptivity
Spectral directional reflectivity
Spectral hemispherical absorptivity
Spectral hemispherical reflectivity
Spectral hemispherical transmissivity
The sum of the absorbed, reflected, and transmitted fractions of radiation energy must be equal to unity
For opaque surfaces τ=0, and thus
Kirchhoff’s Law
A small body of surface area As, emissivity ε, and absorptivity α at temperature T contained in a large
isothermal enclosure at the same temperature
The radiation incident on any part of the surface of the small body is equal to the radiation emitted by a blackbody at temperature T [ G=Eb(T)=σT4 ]
The radiation absorbed by the small body per unit of its surface area is
The radiation emitted by the small body is
The small body is in thermal equilibrium with the enclosure, the net rate of heat transfer to the body must be zero
The irradiation or the emitted radiation is independent of direction
The form of Kirchhoff’s law that involves no restrictions is the spectral directional form
