Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 84

influential factors to determine physical and chemical processes in a turbulent flame
even in steady operation. Unmixedness is a degree of turbulent mixing, which tends
to disappear with the dissipation of turbulence. Therefore, fluctuations in temperature
indicate the existence of high and low temperature lumps of fluid passing through
an observation point.
We tried to estimate flame uniformity and stability by observing the fluctuations
in optical emissions from flames. Figure 2.58 shows an experimental rig and mea-
suring devices consisting of a monochromator and a high-speed ultraviolet video
camera. The emission radiated from a flame was first focused by the first lens and
then collimated by the second lens prior to the light signal entering the monochro-
mator. This makes images multiplex and avoids image distortion through the mono-
chromator. The third lens to construct a monochromatic image of the flame, which
is observed by the high-speed video camera, focuses the parallel beams emerging
from the exit slit of the monochromator. The use of monochromator as a dispersion
device offers significant advantages over the band-pass filter from the point of view
of optical band-pass width and its easy variability.
The measured spontaneous molecular emission bands are C 2 (at 471 and 510
nm), CH (at 431 nm) and OH (at 306 nm). These chemical species are among the
important radicals that provide an important role during combustion. The two C 2
bands were used to calculate the vibrational temperature as well as the distribution
of temperatures of two dimensions within the flame based on a two-line method.
Figure 2.59 shows the calculated temperature profiles obtained with normal air
at room temperature and highly preheated air having low oxygen concentration. The
temperature profiles obtained spectroscopically indicate that the flame front exists
only within the combustion reaction zone and is due to the emission radiated from
C 2 molecules. A comparison between the two figures reveals that the temperature
gradient becomes smaller with diluted air combustion.

Wave length
C 2 (471nm and 516nm)
CH(431nm) OH(306nm) Thermocouple array

Band Pass:15nm/75nm

8cm

High speed UV
video camera
(128 frames sec -1 )
or CCD camera

Monochro
-mator

Thermocouple

LPG (Propane) 0.06m 3 /h

5cm

Vent

Regenerator

4-way valve

Air (+N 2 ) 15m 3 /h

LPG (Propane)

Microcomputer

FIGURE 2.58 Regenerative combustion system with thermocouple array and high-speed
spectro video camera.