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

3 Experiments on Droplet Array Combustion
run. Furthermore, t min does not necessarily need to be as early as shown in
Figure 3.20. It can also be specified in such a way that it might chronologically
lie behind the achievement of the two temperature conditions. For the
sounding rocket campaign, the temperature conditions were uniformly set to
T 1 = T 2 = 508.15 K (= 235 ◦ C) [196, 197]. Figures D.1 through D.3 show the actual
experiment readings including the values specified for t min and t max .
3.3 Numerical Study of the Fluid Dynamics Within the Combustion
Chamber
The density decrease of the gas inside the combustion chamber, accompanied
by the temperature increase of the combustion process, is one of the most
crucial issues of the experiment. 4 It causes an estimated volume expansion
of 1.2 to 1.8. This expansion results in a discharge of some exhaust gas from
the open combustion chamber. Furthermore, “fresh” air is entrained into the
combustion chamber by exhaust gas sampling. As a volume of 200 ml exhaust
is to be collected for the gas analysis and the combustion chamber itself has a
volume of 378 ml, the losses are significant and need to be considered within
the analysis process and scientific interpretation of the results. Consequently,
a numerical study of the two gas exchange processes was conducted within
the design process of the experiment setup, reflecting the open combustion
chamber [293]. The coordinate system employed for studying both processes
is identical to the one introduced in Figure 3.7.
Computational fluid dynamics (CFD) was used to solve the fluid flow in a
three-dimensional full-scale model of the combustion chamber. The experimental
parameters of the TEXUS-46 sounding rocket flight, with five n-decane
droplets, were the basis for this study. The software package ANSYS ® CFX ®
11.0 was used for the numerical studies and ICEM CFD for mesh generation.
The discretization in CFX ® 11.0 is realized by the finite volume method
[21, 22]. Heat transfer by radiation is neglected due to the simplicity of the
droplet model of this CFD study, which results in noticeable deviations from
the physical experiment. However, this CFD study was conducted with the
4 Assuming, hypothetically, a perfectly premixed fuel-air mixture inside the combustion chamber and a preheating
temperature of 500K, the global equivalence ratio calculates to φ= 0.36, which is well below the lean
flammability limit. The equilibrium temperature for these conditions would be 1387 K [300, 443].
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