Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 106

can be concluded that the assumption that the absorption coefficient is constant
regardless of density change is contradictory to reality.
The biggest problem is that CO 2 and H 2 O, which actually emit the radiation,
are nongray gases, and that some part of the radiation energy emitted from solid
surfaces to which gray approximation is applicable passes through these gas without
being absorbed in some wavelength bands. This phenomenon is the same when the
mean effective thickness is larger. During the radiation analysis, due attention should
be paid to it as a fundamentally different point from the cases of the gray gas
approximation.
In some practical cases where simplified analyses are acceptable, the gray gas
approximation may be applied, but it may not be easy to calculate furnace temper-
ature distribution together with the amount of radiation heat transfer. Although it is
necessary to typify the objects of analysis to some extent in accordance with the
practical requirements, it is not desirable to apply conditions such as linear radiation
approximation or gray approximation simply for the reasons of the analysis tech-
nique employed.
Regarding then applications of the nongray or gray characteristics to solid
surfaces in a furnace, whereas gray conditions may be generally applicable to bright
metal surfaces despite their low emissivity, nongray conditions should be applied to
the analysis of objects such as molten glass. Apparent emissivity of objects with
rough surfaces such as furnace walls and oxidized metal surfaces tends to be con-
siderably greater than the value of the component material itself due to reflection
effects of the surface roughness. Thus, the nongray characteristics of the material
are attenuated, and the gray approximation becomes applicable.
As described above, gray approximation is applicable to solid surfaces in the
case of industrial furnaces, especially metal heating furnaces, but glass melting
furnaces and the like require application of nongray properties to the objects to be
heated; otherwise, practically useful results cannot be obtained.
When high temperature air combustion is used for an industrial furnace, the flames
will be nonluminous flames especially with gaseous fuels, and a homogeneous high
temperature field will be formed inside the furnace in most cases. Although the final
temperature of the heated objects is not much different from the general furnace
temperature in the case of metal heating furnaces, the furnace temperature is low near
the heated surfaces because the temperature of the heated surfaces is low in the case
of boilers or petrochemical furnaces. Because the temperature of the heated objects is
not very high, either, in the case of metal annealing furnaces or at the entry side of
metal heating furnaces, the furnace temperature falls to some extent there.
For the purpose of radiation analysis envisaging acceleration of heat transfer in
industrial furnaces, it is essential therefore to introduce analysis techniques capable
of dealing with furnace temperature distribution with any combustion processes
including the highly preheated air combustion. This point should not be overlooked
in developing the computer software. Influences of distributions of temperature and
partial pressure on nongray gas radiation are shown in Tables 2.5 and 2.6 in com-
parison with those in the case of even temperature and partial pressure. These
examples make clear that the difference cannot be overlooked, that is, the heat flux
of either CO 2 or H 2 O fluctuates, depending on the distribution pattern of temperature