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

3.3 Numerical Study of the Fluid Dynamics Within the Combustion Chamber
due to combustion discussed above. On the one hand, gas is extracted from
the combustion chamber through the sample probes, and on the other hand,
“fresh” air is entrained from outside through the opening in the bottom of the
combustion chamber [293].
In order to clarify and quantify the fluid dynamics of the sampling process
in the combustion chamber, a second CFD study was conducted. Due to the
preceding combustion process and the implemented trigger logic (Fig. 3.20),
flame extinction and homogenization of the combustion products within the
volume of the combustion chamber is presumed for the start of exhaust gas
sampling in a first estimate. In reality, however, some pockets of fresh, unconsumed
air remain, for instance, in the corners of the combustion chamber.
Still, this CFD study was conducted independently of the solution of the
previous study on combustion. As the sampling process itself also aims at
avoiding contamination effects due to fresh air entrainment, the numerical
results were used in an iterative optimization of the probe locations by minimizing
the amount of fresh air collected. Negative effects are possible for the
succeeding exhaust gas analysis if high amounts of fresh air are collected as a
consequence of direct jets between the open bottom of the combustion chamber
and the sample probes. Other negative effects are possible if the probes
are arranged either too close to or too far from the combustion zone. Furthermore,
the fresh air content calculated was used as a verification value in the
correction procedure for the dilution of the physical gas samples, which is a
crucial step in post-processing of the gas analysis. The additional variable χ
is introduced for this purpose into the CFD model, monitoring the local volume
fraction of fresh air. At the start of exhaust gas sampling (t = 0s), it takes
the value zero for the whole combustion chamber, whereas it tends to one for
pure fresh air from outside the combustion chamber.
The numerical model of this second process is similar to the above CFD model
used on the combustion process. However, as no modeling of vaporization
and combustion is needed here, it is simplified in several aspects, which reduces
the requirements for computational power. Additionally, symmetry
is implemented as a boundary condition in the longitudinal middle plane
(z = 0mm) of the combustion chamber, the plane as shown in Figure 3.22.
Consequently, only two sample probes are part of this second numerical domain.
The boundary condition for the gas extraction through the sample
probes is realized in CEL by the temporal specification of mass flow (down-
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