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

3 Experiments on Droplet Array Combustion
of the combustion chamber and the total mass flow given as an outlet boundary
condition. Moreover, it approaches the total mass flow at a late stage of
the sampling process, which is a positive indication of having found a favorable
combination of combustion chamber volume, sample volume, and probe
layout. Furthermore, the fresh air content χ is also shown in Figure 3.23. It is a
measure of air entrainment into the combustion chamber due to exhaust gas
sampling and does not include “unconsumed” air that was already present in
the combustion chamber before the start of exhaust gas sampling. After 68%
of the sampling time and having collected 88% of the total mass of the gas
sample, the local fresh air content at the probe orifice rises above the fresh air
content averaged for the whole combustion chamber. Nevertheless, the final
gas sample comprises 81% of gas as present in the combustion chamber after
termination of the combustion process, and only 19% of fresh air.
Figure 3.24 shows contours of the sampling process by highlighting the fresh
air content χ. The coordinate system is identical to Figures 3.7 and 3.22.
Four parallel planes are depicted (z = 0, 4, 8, and 12mm). The color blue
(χ = 0.0) indicates “combustion gases” that were present in the combustion
chamber before the initiation of the sampling process, whereas the color red
(χ= 1.0) stands for fresh air. Time t = 0s corresponds to the start of exhaust
gas sampling. For the first time steps (t /∆t sampling < 0.3), the gas collection is
indirectly observable by the rise of fresh air through the open bottom of the
combustion chamber. It consecutively replaces the sampled exhaust. After
t /∆t sampling = 0.3 and having collected 44% of the total mass, the first fresh air
is indicated inside the sample probes (cf. Figs. 3.23 and 3.24). White streamlines
additionally visualize the flow field in the plane of the sample probes
(z = 12mm).
The flow regime within the combustion chamber, and in particular in the critical
regions of opening (i.e. the remaining gap at the bottom of the combustion
chamber) and outlet (i.e. the gas sample probes), may be characterized by the
Reynolds number according to Equation (3.21). On the one hand, the opening
is critical due to its geometry and the main flow of fresh air that is oriented in
inwards. Here, the Reynolds number can be calculated using the kinematic
viscosity of air at ambient conditions of ν= 15×10 −6 m 2 s −1 and the hydraulic
diameter D h , as stated in Equation (3.22) with the cross-sectional area A and
its wetted perimeter Π. On the other hand, the outlet is critical in respect of a
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