Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 22

(a)
blackbody emittance
(b)
wavelength conversion
(c)
(d)
10
Wave Length
FIGURE 1.11 Radiation energy exchange by wavelength conversion.
as a temperature rise. The radiant intensity from the wall is determined by the product
of its emissivity and the emissive power of a blackbody at the same temperature as
the wall shown in Figure 1.11b . Although the combustion gas absorbs a part of the
radiation emitted from the walls selectively as shown in Figure 1.11c , most of the
radiation energy converted into different wavelengths reaches the opposite wall as
shown in Figure 1.11d . Since the radiation energy absorbed in the combustion gas
is small (2 to 5% at most in the example case), and the average transmission distance
is about 1.4 m, the exchange of heat in the furnace is dominated by the radiation
heat transfer between walls.
Only a fraction of the energy radiated in a short wavelength from a flame in
HiTAC is trapped by semi-transparent combustion gas due to its low absorptivity.
Once it reaches an opposing wall, it is converted into radiation with a long wave-
length spectrum, and the radiation energy is retained in the furnace, except for a
portion that is reflected from the furnace, and contributes to the heating of surround-
ing materials.