Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 21

flame two and a half times as long when the oxygen concentration in the air is
reduced to 5% with the addition of nitrogen and the flame volume also increased
dramatically.
A large volume reaction zone recognized by a mild temperature rise with low
luminosity is a typical feature of HiTAC, which is clearly different from ordinary
combustion burning with luminous flames. When discussing the heat transfer in
HiTAC furnaces, we must take this low luminosity flame into account. Specifically,
the primary heating mode in HiTAC furnaces is radiation heat transfer to the mate-
rials being heated from the walls, which have been heated by the convection heat
transfer of the nonluminous combustion gas. Therefore, despite this undesirable
property of HiTAC flame, the utilization of the furnace wall effectively increases
the direct exchange area for the material being heated. This is another most important
role of the radiation heat transfer in HiTAC furnaces.
1.2.3.3 Effect of Wall as Wavelength Conversion Body in High
Temperature Air Combustion
A typical material for furnace walls has physical properties acting as a selective
absorption body for radiation energy. They function as an absorber for short wave-
lengths (<2 µm) and as a reflector for long wavelengths (>5 µm) for typical properties
of the material frequently used in furnaces. Although nonmetallic heat insulation
materials often possess the properties mentioned above, their emissivity lies in the
range of 0.8 to 0.95 and increases with an increase in temperature. The emissivity
generally depends on the surface roughness as well as on the fine-scale temperature
distribution of the material surface. In contrast, iron or copper as a material being
heated has emissivity of 0.85 to 0.95 for iron oxide and 0.55 to 0.65 for copper
oxide, respectively, since its surface is an oxide. Accordingly, high emissivity mate-
rial being heated, which is placed in high temperature surroundings, acts as an
absorber for short wavelength and a weak reflector for long wavelengths. On the
other hand, regarding energy exchange between gas and walls, the short wavelength
energy radiated from combustion gas is mostly absorbed by lower temperature walls
which have high absorptivity in short wavelength. Because gas is considered a
transparent medium in long wavelengths except for band absorption spectra of H 2 O
or CO 2 , the net exchange of radiant energy between walls balances each other
because of the small deviation in wall temperature. Therefore, radiant energy is
transferred from walls to materials being heated according to their respective tem-
peratures. This is because both materials have high emissivity at long wavelength.
These characteristics in terms of wavelength produce a desirable function of wall
working as a wavelength conversion body in the radiation heat transfer.
The mechanism of wavelength conversion in the vicinity of 1200˚C is schemat-
ically shown in Figure 1.11 . The radiation energy from combustion gas is emitted
mainly from CO 2 and H 2 O and sometimes from hydrocarbon contents, such as CH 4 ,
as band emissions are usually distributed in the wavelength range over 2 µm, as
shown in Figure 1.11a . Then, it is absorbed by the walls having the absorptivity
indicated by the dotted line in Figure 1.11b , and it is converted into heat recognized