Environmental Engineering Reference
In-Depth Information
2.3.6 R ADIATION
There are two kinds of combustion flames: luminous flames and nonluminous flames.
While the former includes suspended solid particles and gray approximation can be
applied to its radiation effect, the latter has to be analyzed under a nongray condition
(different consideration of wavelength bands involved in radiation and those not
involved in it; see Figures 2.71 a nd 2.72 ) , because its radiation is mainly gas radiation
from CO 2 and H 2 O. Because radiation has to be handled in actual practice, naturally,
as a three-dimensional phenomenon, the analysis of nonluminous flame radiation
is, however, not easy and it requires some kind of simplification. Further, the
overlapping of radiation wavelength bands has to be taken into account regarding a
gas body containing CO 2 and H 2 O, and hence it is insufficient for obtaining the total
radiation effect by simply adding up individual heat transfer effects of CO 2 and H 2 O,
each calculated separately.
In other words, to analyze the radiation effect of gas or nonluminous flames of
a gas body containing CO 2 and H 2 O, it is necessary to calculate the absorption
coefficient for each of the radiation wavelength bands, considering the overlapping
of the radiation from CO 2 and that from H 2 O (see Figure 2.73 ). Radiation heat
transfer of gas or flames alone is unrealistic because radiation effects of solid surfaces
are inevitably involved in the heat transfer inside a furnace with highly preheated
air combustion. Overlapping with the radiation wavelength bands of furnace walls
as shown in Figure 2.74 , therefore, has also to be taken into account in the analysis
and, accordingly, analysis procedures will be different depending on whether gray
or nongray approximation is applied to the furnace walls.
100
CO 5%
N 95%
2
Incident from nongray gas layer
2
80
60
40
20
0
0
2000
4000
6000
8000
cm -1
Wavelength, ν
FIGURE 2.71 Wavelength characteristics of CO 2 .
Although the nongray radiation effects were studied using conventional analyt-
ical techniques, many of the study results related to cases where temperature and
partial pressure of the gas were uniform and few looked into cases where they were
unevenly distributed. But in actual cases, temperature and the partial pressure are
 
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