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becomes, the lower the NO
x
concentration, and, hence, under a constant air
-preheat-
ing temperature, incomplete combustion takes place and NO
x
emission decreases as
the flame strain rate rises. The flame temperature-dependency of NO
x
suggests that
the Zel'dovich mechanism is predominant.
the amount of the N
2
dilution (mass %). As more nitrogen is added, the flame
temperature and the NO
x
concentration decrease. NO
x
concentration is decreased to
10 ppm when air is diluted with nitrogen by 30%. Because the air and nitrogen are
heated considerably and the flame temperature is consequently high, flames can be
maintained despite the air dilution with nitrogen; in other words, the flames are
maintained even at a low oxygen concentration. Hence, this results in the reduction
of NO
x
concentration, presumably.
2.3.1.4 Distributions of Temperature and Concentrations of
Species
NO
x
(= NO + NO
2
), O
2
, CO and CO
2
along the stagnation streamline under the
condition of the air preheat temperature of 1100 K and a flame strain rate of 3000 s
-1
.
Since diffusion flames are very stable under this condition, it was used as the reference
condition in this study.
Figure 2.43
shows also the distributions of flame temperature
and the concentrations of the above-mentioned species near the flame extinction under
the condition of the air preheat temperature of 1100 K and the flame strain rate of
2.42
while the scales of ordinates are the same. This is because the distance between
1650
30
1600
NOx
25
Tf
1550
20
1500
1450
15
1400
10
1350
5
1300
1250
0
0
5
10
15
20
25
30
35
added N
2
, %
FIGURE 2.41
Effect of dilution of N
2
on the NO
x
concentration and flame temperature.
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