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

5 Results
5.6 Recommendations and Future Tasks
The study at hand is focused on the combustion of single droplets and droplet
arrays under microgravity conditions. While studying these droplet setups under
idealized conditions, the understanding of particular physical and chemical
processes could be improved. Nevertheless, convective flow around the
droplets also has a major influence on heat and mass transfer, and the interaction
of droplets is significant in sprays. Both effects could, in part, be
accounted for by the experimental part of this study. Besides, an extended,
two-dimensional model was developed in cylindrical coordinates to accommodate
those aspects. Its implementation into the software package Cosilab ®
and the multiphysics and reaction engineering environments of COMSOL ®
[80–83] was carried out successfully. However, neither implementation was
capable of providing results comparable to those of the one-dimensional
model with respect, in particular, to NO x formation. This shortcoming was
due to numerical instabilities, on the one hand, and combustion chemistry
too complex for the CFD framework, on the other hand. In addition, droplet
burnout within a droplet array is achieved at different instants, as the droplets
vaporize and burn at different rates depending on their individual state of preheating
and combustion. However, a droplet of zero radius cannot be represented
by the numerical model, and thus calculations have to be stopped,
once the size of a droplet drops below a certain threshold. Then, the interface
conditions have to be changed before continuing simulation of the remaining
droplets.
Nonetheless, such a two-dimensional model is capable of representing interactions
in droplet arrays. The additional spatial dimension allows to model
forced ignition even more realistically. It is also possible to study the influence
of convective flow on single droplets or along a droplet array. Various droplet
arrays can be investigated regarding droplet size and inter-droplet distance.
Additional findings with regard to flame shape and temperature distribution
can provide a deeper insight into the complex processes within spray flames.
In general, further work needs to be carried out to develop a comprehensive
understanding of NO x formation in partially pre-vaporized droplet and
spray flames. It should focus on the ambient setting, including temperature,
pressure, and atmospheric composition, as well as secondary effects of vapor-
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