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CHAPTER 6.0: Fundamentals of Thermal Radiation - Coggle Diagram
CHAPTER 6.0:
Fundamentals of Thermal Radiation
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.
where,
frequency, ν
wavelength, λ
speed, c
Blackbody Radiation
A blackbody ─ the maximum amount of radiation that can be emitted by a surface at a given temperature.
A blackbody emits radiation energy uniformly
The radiation emitted by a blackbody( Stefan-Boltzmann law)
σ=5.67 X 10-8 W/m2·K4.
The spectral blackbody emissive power
The wavelength at which the peak occurs is given by Wien’s displacement law as
Blackbody radiation function
Radiative Properties
radiation is considered to be a surface phenomenon for such materials.
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
The emissivity of real surface varies with
-the temperature of the surface
-the wavelength
-the direction of the emitted radiation
Spectral directional emissivity ─ the most elemental emissivity of a surface at a given temperature.
The total directional emissivity
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
When radiation strikes a surface, part of it:
is absorbed
is reflected
is transmitted
The first law of thermodynamics requires that the sum of the absorbed, reflected, and transmitted radiation be equal to the incident radiation.
For opaque surfaces, τ=0, and thus
These definitions are for total hemispherical properties.
Spectral directional absorptivity
Spectral hemispherical absorptivity
Spectral directional reflectivity
Spectral hemispherical reflectivity
Spectral hemispherical transmissivity
Absorptivity
Reflectivity
Transmissivity
Kirchhoff's Law
This relation is valid when 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
