Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 43

285mm
FIGURE 2.17 Range of LIF measurement (285 mm square).
the OH distribution per unit area is strong, to make the combustion reaction active.
However, Figures 2.18b t hrough e show the results of simultaneous measurement of
both the OH distribution and the acetone distribution for one laser pulse, in the case
of the distributed combustion flame. In either figure, the OH distributes outside the
fuel jet diffusion is the state of a thin film in the vicinity of the burners, to form a
diffusion flame. The OH distribution spreads out gradually at the lower portions of
the flame stream.
As for the acetone distribution, since the wavelength range (340 to 450 nm) of
the fluorescence signals is nearly the same, a distribution that is conjectured as a PAH
and is visualized at the same time is observed at the downstream portion of flame.
This phenomenon can be interpreted as follows. When simultaneous measurements
are carried out in the vicinity of the fuel nozzles while the in-furnace temperature
changes in the range between 900 and 1100˚C under the condition of using LPG as
the fuel and setting the air ratio at 1.2, the results obtained show that the signal intensity
at the downstream portion of the flame increases as the temperature rises (see Figure
2.19 ). Based on this information, it can be considered that the fuel is jetted into the
inside of the furnace to cause a thermal decomposition, resulting in the fuel being
reformed to produce PAH, a precursor of soot. Furthermore, a flow of the preheated
air is jetted between two flows of the fuel jet diffusion. When attention is paid to the
OH distribution in this airflow jetted area, the distribution contacts the airflow at the
lower portion of the flame stream in the case of condition (e), compared with conditions
(b) through (d). As for the fuel jet diffusion, the acetone is added only to the nozzle
set below the air nozzle. When comparing the signals from these two nozzles, it can
be observed that the fuel is thermally decomposed at a distance of about 50 mm
downstream from the nozzle. Under condition (d), where the NO x concentration
becomes maximum in the F2 mode, the PAH signals are taken into the preheated
airflow in a shorter distance from the nozzle and the distribution steadily disappears.
This phenomenon is different from those under other conditions. Furthermore, the OH
distribution is intensified at the side contacting the airflow, and a feature is observed
that the combustion reaction is carried out at a higher portion of the flame stream,
compared with those under different conditions.
Entrainment of the in-furnace exhaust gas to the fuel jet diffusion is examined on
the basis of the results obtained by measuring spontaneous emission spectra in the
combustion flame. The spontaneous emission spectrum of the radical is proportional
to the density of the radical in an excitation state. In an emission spectrum of the flame