14
Radiation Transfer through Gases
14.1 Gas Radiation Properties
A volume of gases, such as CO 2 , H 2 O (water vapor), CO, NO, NH 3 , SO 2 , HCl,
the hydrocarbons, and the alcohols, can emit and absorb energy at a given
temperature and pressure. It is important to determine gas radiation properties such as emissivity and absorptivity for combustion furnace designs [1–5].
The combustion products (CO 2 , water vapor, CO, NO, and NH 3 ) have radiation properties but air (oxygen and nitrogen gases), helium, and hydrogen
have no radiation properties (transparent to radiation). Assume that there is
no scattering effect, in order to simplify the analysis. In general, gas emissivity
and absorptivity increase with pressure and volume, but decrease with temperature; gas emissivity and absorptivity vary with wavelength [1] as shown
in Figure 14.1. Gases absorb and emit radiation in rather narrow wavelength
bands rather than in the continuous spectrum exhibited by solid surfaces. For
real gas, gas absorptivity is not the same as emissivity. But under gray gas
assumption, absorptivity can be equivalent to emissivity.
Figure 14.2 shows that gas radiation properties (carbon dioxide, water
vapor) increase with their partial pressure and geometric mean bean length
(four times volume divided by surface area), decrease with temperature [1].
The results were obtained by applying hemispherical gas radiation to an element area at the center of the base, as shown in Figure 14.3. Several other
geometries that contain gases are also sketched in the figure. In general, gas
emissivity and absorptivity are relatively low, approximately equivalent to
an order of magnitude 0.1.
ε c = ε c (P c L, T g ) ∼
(14.1)
= 0.1
ε w = ε w (P w L, T g ) ∼
(14.2)
= 0.1.
At 1 atm total pressure, CO 2 has partial pressure P c and water vapor has
partial pressure P w . L, geometric mean bean length, is defined as
4V
4V
∼
L =
= 0.9
(14.3)
A s
A s
where V is the volume and A s is surface area.
275
Radiation Transfer through Gases
14.1 Gas Radiation Properties
A volume of gases, such as CO 2 , H 2 O (water vapor), CO, NO, NH 3 , SO 2 , HCl,
the hydrocarbons, and the alcohols, can emit and absorb energy at a given
temperature and pressure. It is important to determine gas radiation properties such as emissivity and absorptivity for combustion furnace designs [1–5].
The combustion products (CO 2 , water vapor, CO, NO, and NH 3 ) have radiation properties but air (oxygen and nitrogen gases), helium, and hydrogen
have no radiation properties (transparent to radiation). Assume that there is
no scattering effect, in order to simplify the analysis. In general, gas emissivity
and absorptivity increase with pressure and volume, but decrease with temperature; gas emissivity and absorptivity vary with wavelength [1] as shown
in Figure 14.1. Gases absorb and emit radiation in rather narrow wavelength
bands rather than in the continuous spectrum exhibited by solid surfaces. For
real gas, gas absorptivity is not the same as emissivity. But under gray gas
assumption, absorptivity can be equivalent to emissivity.
Figure 14.2 shows that gas radiation properties (carbon dioxide, water
vapor) increase with their partial pressure and geometric mean bean length
(four times volume divided by surface area), decrease with temperature [1].
The results were obtained by applying hemispherical gas radiation to an element area at the center of the base, as shown in Figure 14.3. Several other
geometries that contain gases are also sketched in the figure. In general, gas
emissivity and absorptivity are relatively low, approximately equivalent to
an order of magnitude 0.1.
ε c = ε c (P c L, T g ) ∼
(14.1)
= 0.1
ε w = ε w (P w L, T g ) ∼
(14.2)
= 0.1.
At 1 atm total pressure, CO 2 has partial pressure P c and water vapor has
partial pressure P w . L, geometric mean bean length, is defined as
4V
4V
∼
L =
= 0.9
(14.3)
A s
A s
where V is the volume and A s is surface area.
275