Page 157 - Industrial Ventilation Design Guidebook
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4.3 HEAT AND MASS TRANSFER 1 I 9
where
When these are derived with respect to the wavelength, and the wavelength
value, with the maximum value of radiation intensity, is solved for, the result
is Wien's law:
According to Wien's law, the wavelength representing the maximum point de-
creases with increasing temperature (Fig. 4.29).
The visible region of the spectrum lies between the wavelengths of 0.4 and
0.7 jxm. When the temperature of a body is increased, its color changes to-
ward smaller wavelengths—in other words, from the red region of the spec-
trum to the blue region.
4.3.5.2 Emissivity and Absorption
Suppose two objects are in a hollow (Fig. 4.30): object A, which is black,
and object B, which is gray (a body that does not absorb all the incoming radi-
ation). The energy and mass are in balance when the temperatures of A, B,
and C are equal. In the balanced state the radiation emitted by the bodies is
equal to the radiation received.
The radiation density in C is in constant balance at all points and in all di-
rections (for a given frequency). The radiation density is
FIGURE 4.29 Radiation intensity of a blackbody as a function of wavelength (temperature parameter).
