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

5.5 Final Evaluation of Results
Second, the combustion of partially pre-vaporized droplets was studied carrying
out numerical experiments, too. Here, the atmosphere was air with an
initial temperature in the range of 500 to 700 K. As this temperature is too low
for auto-ignition, the droplets were ignited using an external energy source
after the pre-vaporization time t Ψ was reached. The energy introduced into
the numerical domain was methodically re-extracted upon onset of reaction
and before temperatures leading to substantial NO x formation were attained.
These numerical simulations yielded an almost constant NO x production up
to a pre-vaporization rate of around 50%. If more than half of the droplet mass
is vaporized before ignition takes place (Ψ> 0.5), lean, partially premixed areas
develop and NO x emissions decrease with an increase in Ψ. During the
period of heat introduction, the droplet burning behavior is dominated by the
influence of the volumetric heat source. Thus, it is crucial to choose energy
and location of the ignition source in a way that the influence on total NO x
generation is negligible. Compared to previous studies, the precise supply
and removal of energy by a source term turned out to be a beneficial feature to
control ignition. As a result of improving the ignition method, the amount of
heat introduced can be kept constant from case to case. This generic ignition
procedure is energetically neutral. It allows to study NO x formation without
artificial, thermal effects stemming from ignition. Nonetheless, modeling ignition
will remain a general problem in droplet combustion modeling. This
becomes apparent when comparing different ignition approaches.
In summary, the overall agreement – in a qualitative manner – between experiments,
numerical simulations, and analytical approximations is very good.
There is also a very good reproducibility within the experimental results. The
model, as employed in the numerical simulations, is in good agreement with
literature. However, due to the different setups of spherically symmetric single
droplets and linear droplet arrays, it is hard to develop correlations between
numerical and experimental results. Significant heat losses in the experiment
contribute to an absolute deviation in the NO x values by a factor of 5 to 10.
The strong coupling between droplet ignition and NO x formation highlights
the need for a reasonable modeling of the ignition position for pre-vaporized
droplets. It is not possible to use the single droplet model for modeling the
interaction of droplets in sprays.
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