Determine the total average hemispherical emissivity and the emissive power of a surface that has a spectral hemispherical emissivity of 0.8 at wavelengths less than 1.5 ?m. 0.6 at wavelengths from 1.5 to 2.5 ?m, and 0.4 at wavelengths longer than 2.5 ?m. The surface temperature is 1111 K.
GIVEN
• A surface at temperature (T) = 1111 K
• Spectral hemispherical emittance (??)
FIND
(a) The total average hemispherical emittance, ? (T)
(b) The emissive power, E(T)
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
(a) The total average hemispherical emittance is
Interpolating
Therefore
(b)
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