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

5.2 Combustion of Partially Pre-Vaporized Droplets
to diffusion (i.e. during homogenization, Fig. 5.12) and convection (i.e. the gas
sampling process).
Figure 5.13 depicts two trends of the NO x emissions over the pre-vaporization
rate Ψ: If the correction factor f EINOx is taken into consideration, there is no
straightforward linear correlation of the emission index EI NOx (Ψ ≤ 0.5378),
but it remains at a quasi-constant level. If the correction factor f EINOx is not
taken into consideration, an almost linear decrease of EI NOx can be observed
with an increase of Ψ, which is in compliance with Moesl et al. [296]. Furthermore,
the corrected values of the NO x emissions are in good qualitative
agreement with Figure 2.3 and 5.3 for sprays and single droplets, respectively.
Thus, the experimental work at hand confirms the findings of the numerical
simulations of Chapter 5.2: A substantial droplet pre-vaporization is required
before NO x emissions can be reduced.
The CO concentrations indicated by using the two FT-IR standards match
perfectly for the lower Ψ-values and still very well for Ψ = 0.5378 (see also
Fig. 5.13). This conformity was expected but is also an indication of high accuracy
and data reproducibility of the exhaust gas analysis. The overall exponential
increase in CO is due to the formation of a triple flame 4 with an
increase in Ψ. The development of the triple flame is significantly triggered
by the presence of a combustible gas phase along the droplet array and flame
spread through this region as well as thermal expansion thereof. While the degree
of fuel vaporization Ψ increases, fuel-rich regions are gradually formed
along the droplet array. These rich regions are consumed by the approaching
triple flame, and thus tend to be an excessive source of CO production
[211, 253, 344, 452]. This particular development of the triple flame structure
was also confirmed through visual observation of the flame spread sequence
(Fig. 5.14) and numerical work conducted by Kikuchi and coworkers
[206, 208].
Figure 5.14 shows sequences of flame spread associated with the emissions
of the three experiment runs of Figure 5.13. The images are taken from highspeed
recording and cover the field of view from the third to the fifth droplet.
The flame front travels from left to right. In all cases, a dim blue flame marks
the flame front, propagating into the unburned region. A yellow, luminous
4 According to van Oijen and de Goey [452], a triple flame is a flame structure generated by flame propagation in
a partially premixed system. Typical triple flames comprise a rich and lean premixed flame front and a diffusion
flame, all of them intercepting in the triple point. Triple flames are also denoted as tribrachial flames.
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