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

3 Experiments on Droplet Array Combustion
W m −1 K −1 . All these property values refer to the gaseous state, as liquid fuel is
not part of this CFD simulation.
Figure 3.22 illustrates the temporal evolution of the combustion process during
the initial second, starting from ignition. Left and right side of the figure
depict the vertical middle plane of the combustion chamber with z = 0mm
(cf. Fig. 3.7). The left side shows the temperature profile in the gaseous phase
with the five fuel sources being the locations of the lowest temperatures,
whereas the right side shows the velocity field indicated by the absolute velocity|v|=(vx
2+ v 2 y + v z 2) 2 1 . The velocity vector of the flow is visualized by white
streamlines. Being the main subject of this first part of the CFD study, the
discharge of hot exhaust gas from the open bottom of the combustion chamber
by convection is clearly indicated by the evolution of the temperature and
velocity fields. This process is particularly responsible for the temperature
field becoming more and more asymmetric along the axis of the droplet array.
Here, the initial time (t = 0s) is defined as the time when ignition is enforced
by the implemented heat release mechanism. Ignition of the first droplet occurs
at t = 29 ms with a local temperature rise to 2118 K. Starting from this
local kernel, the flame front propagates along the flammable gas layer, forming
a spherical flame around the fuel source. At t = 0.2 s, the flame ball around
the first fuel source is fully developed and a temperature of T = T max = 2515 K
is indicated. Subsequently, the flame spreads from fuel source to fuel source
(Fig. 3.22, from left to right). At t = 0.4s, the fourth droplet is ignited but not
yet enclosed by the flame. The ensuing flame propagation and volume expansion
due to combustion are clearly observable in combination with the contour
plot of absolute velocity |v|. At t = 0.6s, all fuel sources are ignited and
the flame stabilizes at a high burning rate k. Still, a small eddy of unburned
air prevails in the upper right corner of the droplet array holder before it dissipates
around t = 0.8 s. It is a consequence of flame propagation towards the
“cold, metallic containment” of the droplet array holder. Finally, the highest
velocities within the combustion chamber are observed at the opening in the
vicinity of the vertical props with|v| max = 0.595 m s −1 at t = 0.6s. After ignition
of the fourth and fifth fuel source, the flow field becomes uniform, as underlined
by the white streamlines.
The utilized single-step mechanism of Westbrook and Dryer [459] can inherently
only be an approximation, as it is trimmed mainly to predict correct
flame speeds S L . Thus, it is subject to some restrictions: At flame temperatures
108