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

6 Summary and Conclusions
tion, and exhaust gas formation. Nitrogen oxide emissions of partially prevaporized
droplets were finally studied, and the burning behavior was analyzed.
Special attention was given to the interaction of both these aspects with
the ignition model.
Recalling the findings of this research, pre-vaporization is beneficial to avoid
NO x formation. The major points observed are given as follows:
• Different stages of droplet burning could be confirmed, which is in line
with results from literature. The ignition phase of a droplet is dominated
by the ignition parameters and crucial to the overall NO x production.
• After ignition, partially pre-vaporized droplets generally burn similarly to
droplets in hot atmospheres. However, for high pre-vaporization rates, a
significant amount of flammable premixed gas accumulates around the
droplet. In this case, the flame burns in a rather premixed regime before
it turns its mode into a diffusion flame.
• The accurate prediction of NO x emissions is sensitive against heat introduction
if an ignition model is required. Heat extraction from the exhaust
gas regions of burning droplets shows only a minor impact on flame position
and NO x emissions.
• Within the burning regimes investigated, a minimum of 50% droplet prevaporization
is required to perceive a significant NO x abatement.
Against this background, the influence of the most essential combustion parameters
on NO x formation shows:
• A strong dependence on the ambient temperature T ∞ .
• A dominance of droplet-caused NO x up to 1800 K, followed by a rapid
decrease around 2000 K, and a dominance of the gas phase at higher T ∞ .
• A linear dependence of droplet-caused NO x for a variation of D 0 in a hot
exhaust gas atmosphere.
• An overall similar progress for Ψ confirmed by experiments and numerical
studies, as reported by Baessler [31] for sprays.
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