Simulation Models for High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 193

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Tg= 1923K

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FIGURE 3.10 Radiation intensity of a blackbody.

energy is called absorptivity, a λ , reflectivity, τ λ , and transmissivity, d λ . The interre-
lation among them can be written as

a λ + τ λ + d λ = 1 (3.17)

As a blackbody absorbs all of the incident radiant energy, we have a λ = 1, τ λ =
d λ = 0. Therefore, we can calculate radiative heat transfer between blackbodies using
only their surface temperature, because we need not take into account the wavelength
dependency of radiation energy.
When the absorptivity, a λ , of a material is constant and smaller than unity, it is
called a graybody. The monochromatic emissive power, E λ , of a graybody is pro-
portional to the monochromatic emissive power of a blackbody, and the ratio between
the two defines the monochromatic emissivity, ε λ .

E λ = ε λ E b λ (3.18)

The monochromatic absorptivity of the material is also proportional to that of
a blackbody and the monochromatic absorptivity a is equal to the monochromatic
emissivity ε for graybodies of the same temperature.

a = ε (3.19)

A similar relation is valid for the emissivity and absorptivity,