Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 19

We may consider that a similar mechanism as shown in Figure 1.9 is still valid
in HiTAC furnaces. However, from the viewpoint of the material being heated and
the furnace walls, it is not necessary to consider the heat conduction in a solid due
to the uniform heating resulting from the small deviation in temperature distribution.
That is a characteristic feature of heating in HiTAC furnaces. Furthermore, as is
described in a later section, the characteristic length of radiation heat transfer is
large and the energy, which travels as an electromagnetic wave, can be transferred
from frontside to backside of a furnace directly. Therefore, because wide wall
surfaces are usually maintained at a nearly uniform temperature in HiTAC, heating
by radiation dominates the heat transfer in the furnace. The direct convection heat
transfer to the material being heated plays only a supplementary role.
Nevertheless, convection heat transfer in HiTAC is important as an initiator of
heat transfer in the furnace. Since flames in HiTAC are not like conventional lumi-
nous flames but are blurred with low luminosity, as shown on the cover photograph
of this topic, we cannot expect effective radiation heat transfer from flames to
materials being heated. However, the convection heat transfer from gas to walls
works as the starting point for heat transfer in the furnace.
A simulation dealing with reaction and heat transfer in three dimensions is
described in Section 3.4 in this topic; we show another example here. This simulation
was carried out assuming that the flame temperature was constant, and each element
in the furnace, such as a wall surface, flame zone, and material being heated, was
treated as a distributed constant. The dependence of radiation heat transfer on
wavelength was considered. The flame gas absorptivity was expressed using the
band absorption of CO 2 and H 2 O by taking the respective mole fractions into account.
Wall surface was regarded as a selective absorption surface, assuming typical refrac-
tory. As a result, it acted as a low absorption body for far-infrared radiation having
a wavelength longer than 4 µm, and as a blackbody for a wavelength shorter than
2 µm. The furnace was a small rectangular furnace (length 2.5 × width 1.2 × height
0.8 m, Al 2 O 3 w all thickness 0.3 m), in which a flame was positioned in the center
and oriented longitudinally and the material being heated was placed 0.3 m above
the bottom surface.
Some of the results are shown in Figure 1.10 . As the emissivity and absorptivity
of flame become smaller, the ratio of the radiation heat transfer from flame to wall
to the convection heat transfer to the wall becomes smaller. On the contrary, the
contribution of radiation heat transfer in the total heat transfer to the material being
heated increases. This is in contrast to our expectation when the flame is in low
luminosity. This is a result of the increase of radiation heat transfer from walls,
which are heated by the convection, to the materials being heated. Therefore, the
thermal field in a furnace is retained in equilibrium by radiation heat transfer. This
has a long characteristic transfer distance, when the flame luminosity is low. It means
that the field is dominated by radiant heat, and the necessary heating time is shortened
by the change of luminous flames into nonluminous ones.
The increase of radiant heat transfer between walls means the rapid redistribution
of the heat, transferred by local convection, to other walls. Therefore, the radiant
heat from a wall covers most of the heat flux to the material being heated, although