Heat Transfer in Absorbing and Emitting Media (Gaseous Radiation)
The radiation exchange between an absorbing and emitting gas and a solid surface is considerably more complex than exchanges between solid surfaces through transparent medium. The specific features of the gaseous radiation have been discussed in this chap
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Heat Transfer in Absorbing and Emitting Media (Gaseous Radiation)
12.1
Introduction
Monatomic and diatomic gases such as argon, helium, oxygen, nitrogen, and hydrogen are transparent (diathermic) to the thermal radiation and their capacity to radiate or absorb the thermal radiation is insignificant except at extremely high temperatures. Molecules such as carbon dioxide, carbon monoxide, sulfur dioxide, water vapour, and hydrocarbon gases are capable of emitting and absorbing the heat radiation. Carbon dioxide and water vapour are formed when combustion of hydrocarbon fuels takes place and the study of these gases is of practical importance. Methane and carbon dioxide are regarded as green house gases and are the cause of global warming. The radiation exchange between a gas and a solid surface is considerably more complex than exchanges between solid surfaces. The specific features of the gaseous radiation are being discussed in this chapter. However, the discussion is basically confined to the behavior of carbon dioxide, water vapour, and their mixtures.
12.2
Specific Features of Gaseous Radiation
12.2.1 Selective Emitters Most of the solids possess continuous radiation spectra, i.e., they emit and absorb rays of wave length zero to infinity. But the gases emit and absorb radiation in certain narrow wavelength regions called bands, Fig. 12.1. Outside these bands, these gases are practically transparent and their emissive power is zero. Thus, the gases are selective absorbers and emitters. These bands, their width, and numbers are different for different gases. Typically for carbon dioxide, these bands are
© Springer Science+Business Media Singapore 2017 R. Karwa, Heat and Mass Transfer, DOI 10.1007/978-981-10-1557-1_12
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12 Heat Transfer in Absorbing and Emitting Media (Gaseous Radiation)
Fig. 12.1 Emissive power of a gas versus a blackbody Blackbody
Eλ
Radiation bands for a gas
λ1
Dk1 ¼ 2:36 3:02 lm;
Dk2 ¼ 4:01 4:80 lm;
λ2
λ
λ3
Dk3 ¼ 12:5 16:5 lm;
and for the water vapour, these bands are Dk1 ¼ 1:7 2:0 lm; Dk2 ¼ 2:24 3:27 lm; Dk4 ¼ 12:0 30:0 lm:
Dk3 ¼ 4:8 8:5 lm;
12.2.2 Beer’s Law In opaque solids, the emission and absorption of the heat radiation occur only in thin surface layers but in gases, the emission and absorption occur over their volume. As the radiation passes through a gas, reduction in its intensity takes place. This reduction depends on the number of molecules encountered, i.e., the thickness of the gas volume and the partial pressure of the gas. The absorption of the radiation in the gas layers has been expressed in a mathematical form. Let a monochromatic beam of radiation intensity (Iλ)0 impinges on the gas volume and passes through it, refer Fig. 12.2. Then the decrease in the intensity due to the absorption in the gas layer at any plane xx has been found to be proportional to the thickness dx of the layer and the intensity of radiation (Iλ)x at that plane. dðIk Þx / ðIk Þx dx ¼ ak ðIk Þx dx
ð12:1Þ
where αλ is called monochromatic absorption coefficient. Th
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