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

5 Results
combustion in exhaust gas, a set of exhaust gas concentrations and temperatures
has to be used as an initial condition.
Figure 5.17 depicts NO x emissions caused by the droplet exclusively (Eq. (5.1))
as well as the total NO x emissions of the computational domain accounting
for the absolute temperature level. These emissions are identical for both
cases of air and exhaust gas atmosphere, as long as T g ,0 remains moderate
(T g ,0 < 1800 K). However, in either case the values of “Droplet NO x ” drop off
from the ones of “Total NO x ” with an increase in T g ,0 . The availability of the
oxidant O 2 is one limiting factor of NO x formation, too. A further resistance
to NO x formation is thermal ballast, resulting in depressed maximum temperatures
(cf. Fig. 5.2). Consequently, the NO x emissions of droplets burning in
exhaust fall short of the ones burning in air of the same temperature.
As a reduction of droplet-caused NO x in a technical application is a priori
supposed to noticeably lower the total NO x emissions, droplet-caused NO x
must be as a significant portion of the total NO x emissions. The fraction
of droplet emissions Γ EI is a suitable measure in this context. According to
Equation (5.8), it is the ratio of the droplet and total emission indices:
Γ EI = EI NO x ,droplet
. (5.8)
EI NOx ,tot
Figure 5.18 depicts the fraction of droplet emissions Γ EI as a function of the
initial temperature T g ,0 . Droplet emissions dominate in both atmospheres for
temperatures up to 1800 K but decrease with increasing temperatures. While
the fraction Γ EI remains at a high level for air over the whole temperature
range, it declines rapidly for exhaust gas around 2000 K. At an initial temperature
of T g ,0 = 2178 K, which corresponds to φ = 0.875, it drops below 10%.
Hence, a reduction of droplet-caused NO x in exhaust gas for φ > 0.875 may
be less important, when aiming for a reduction of the total NO x emissions.
Nonetheless, reducing NO x emissions of droplets in exhaust gas for φ ≤ 0.8
tends to be efficient. The droplet emission index EI NOx is indicated in accordance
with Figures 5.2 and 5.17 for comparison.
For the sake of completeness, the flame stand-off ratio ζ (Eq. (5.5)) is shown
in Figure 5.19 for droplets burning in the two distinctive atmospheres. In the
case of the exhaust atmosphere, extreme φ-values were selected for this plot
(lean, rich, and maximum of NO x production; cf. Fig. 5.2). The correspond-
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