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

3 Experiments on Droplet Array Combustion
of typical hydrocarbon fuels of approximately 2000 K, substantial amounts of
CO and H 2 exist in equilibrium with CO 2 and H 2 O. However, by assuming that
CO 2 and H 2 O are the only combustion products, the total heat of production
is overpredicted. This effect was pointed out by the authors of the mechanism
[459], and it is illustrated in Figure A.1 for C 10 H 22 . Here, the temperature
difference in the flame and post-flame zone adds up to 170K, comparing
the single-step mechanism used with the detailed kinetics of Zhao et al.
[474]. Consequently, the results of this first numerical simulation may only
be consulted for a first estimate of flame spread and are not suitable for any
quantitative NO x studies, such as those involving NO x post-processing. In addition,
there are significant heat losses in the physical experiment due to radiation
that are not accounted for within this basic CFD study. In this respect, a
rough estimate of the real temperature rise due to combustion can be derived
from the measurement readings plotted in Figures D.1 through D.3. Despite
the thermocouple positions some distance away from the flame zone and the
inertia of the system, the temperature increase observed remains low. Consequently,
as expected, there is a noticeable difference between physical experiment
and the CFD study. Furthermore, Figure 3.22 clearly illustrates the fluid
flow through the combustion chamber opening. In contrast to the present design,
an idealized combustion chamber setup would foresee symmetry so that
an even exhaust gas distribution could be maintained throughout the combustion
process. However, this obvious design recommendation was beyond
the scope of this design study.
3.3.2 Fluid Dynamics During Exhaust Gas Sampling
The second relevant gas exchange process of the combustion chamber is the
exhaust gas sampling itself, where evacuated sample cylinders collect the
combustion products. Position and orientation of the related sample probes
inside the combustion chamber are important here, in particular in combination
with the individual sampling approach for every single combustion run.
Therefore, four symmetrically aligned probes were defined in the experiment
specifications to allow a spatially uniform gas collection from the combustion
chamber. The probe orifices were positioned as close as possible to the combustion
zone, which facilitated a direct collection of the combustion products.
Nevertheless, the ensuing gas exchange is even more complex than the one
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