On the Formation of Nitrogen Oxides During the Combustion of ...

5.3 Influence of Ambient Preheating
40
g kg −1
Fraction of droplet emissions ΓEI
80
60
40
20
100
%
Γ EINOx in air
Γ EINOx in exhaust
Droplet EI NOx
in exhaust
Initial temperature T g,0
32
24
16
8
Droplet emission index NOx EINOx
0
1600 1800 2000 2200 K
0
Figure 5.18: Impact of Gas Atmosphere on the Contribution of Droplet NO x Production to
the Overall NO x Production. The droplet emission index EI NOx is indicated for
comparison (cf. Figs. 5.2 and 5.17).
ing initial air temperatures T g ,0 were also selected for the case of the hot air
atmosphere to allow a direct comparison. Droplet lifetime is related to the
dimensionless time t /∆t sim . The resulting flame stand-off ratio ζ shows similar
characteristics for both cases. Deviations within each set of curves are
small, which is due to a low impact of the temperature level. For t /∆t sim ranging
from 0.0 to 0.1, the initial stages of unsteady droplet burning are observable
(cf. Fig. 5.10). The stage of quasi-steady droplet burning appears subsequently
until t /∆t sim reaches 0.8, and ζ remains almost constant with the
flame position following the shrinking droplet. At the end of droplet lifetime
(t /∆t sim > 0.8), the droplet shrinks increasingly fast with the flame being unable
to follow this trend, resulting in a rapid increase of ζ. However, the most
essential aspect of Figure 5.19 with regard to NO x formation is the discrepancy
in the absolute values of ζ for air and exhaust gas. The time-averaged flame
stand-off ratio ζ of droplets burning in exhaust ranges from 48 to 70% above
the one of droplets burning in air. Figure 5.20 compares this parameter for the
two cases, underlaid with the mole fraction X O2 of excessive oxygen from the
initial premixed flame. Assuming that the exhaust gas of this flame only consists
of excessive oxygen and inert components, it can be regarded as a diluted
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