Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 20

6

5

for HiTAC by
gaseous fuel

4

3

2

x10

1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Radiation emittance/absorptance of flame

FIGURE 1.10 Radiation and convection heat transfer in furnaces.

the direct radiation heat transfer from flame to the material being heated is small
compared with the heat transfer in a conventional combustion furnace. The increase
from the effect described above can be considered as the effective increase in the
exchange area, or configuration coefficient, of radiation heat transfer.
As explained above, in HiTAC where a flame is nonluminous, the convection
heat transfer to the wall triggers the radiation heat transfer to the material being
heated. The precise convection heat transfer coefficient depends mainly on the
orientation and velocity of the flow. At the same time, it is more effective to agitate
the flow and to avoid the development of a boundary layer on the solid wall surface
by disposing refractory as a spacer. The material being heated against the flow is
more effective than constructing the furnace with simple plane walls. Furthermore,
the obstacles in the flow often work as heat accumulators and can be used as
wavelength conversion bodies, which are discussed later, although it is difficult to
estimate the convection heat transfer coefficient in a complicated flow.

1.2.3.2 Radiant Heat Transfer of High Temperature Air

Combustion

The uniformity of temperature distribution comes from the dispersed reaction zone
accompanied by a relatively mild temperature rise because combustion air or fuel
is diluted with burned product before combustion. Accordingly, the reaction rate
decreases, resulting in a long flame or a reaction zone if it is invisible. Generally,
the length of a turbulent jet flame is kept nearly constant, because the momentum
increase of a fuel jet generates enhanced mixing by entraining larger amounts of
surrounding air. However, under low oxygen concentration atmosphere, the flame
length will increase by the decreased reaction rate. Example calculations predict a