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

2.1 Classification of Combustion Processes
Since significant droplet slip velocities are known to cause a transition from
the stoichiometric envelope flame to a wake flame, the influence of this flame
type transition on NO formation is also investigated on a single droplet flame
by a numerical study [38–41].
Even though a lean premixed combustion zone resembles an optimum in
terms of low NO x emissions in all of the studies outlined above, combustion
stability will suffer due to homogeneity of the mixture: The associated lean
flammability limit lies at an equivalence ratio of φ ≈ 0.5 for most hydrocarbon
fuels [241, 443]. In nonuniform systems, however, the mean equivalence
ratio can be considerably less than this lean limit, as locally rich zones will
maintain combustion. Such a system is safeguarded against blowout near the
lean flammability limit; a lean premixed system is not [18, 295, 375]. Other
issues of lean flames are flashback [19, 119, 120, 226] and combustion instabilities.
Fritz [139] and Kröner [218], for instance, first reported on combustion
induced vortex breakdown, which is a type of sudden flame flashback
that is driven by the interaction of turbulence and chemistry. Sattelmayer and
Polifke [377–379], Lieuwen et al. [252], and Nguyen [313], for instance, investigated
the role of unmixedness and equivalence ratio fluctuations in LP and
LPP combustion with respect to combustion induced instabilities. In this context,
heat release oscillations of large amplitude excite pressure oscillations of
some of the combustor’s acoustic modes that couple with the reactants feed
lines, and thus drive the combustion instabilities [26].
2.1.3 Droplet Combustion
Power generation by means of hydrocarbon combustion will keep its predominant
role for the decades to come despite the increasing share of renewable
energy sources in the global energy mix [24, 421, 450]. In particular, combustion
of liquid fuels will remain a major energy source in the transport sector
due to its high energy density. As outlined in Chapters 1.4 and 2.1.2, lean,
partially premixed spray flames are discussed for novel combustion concepts
to allow a further abatement of NO x emissions. The relevant knowledge is
based to a large extent on combustion research of manageable and observable
droplet setups. Partially pre-vaporized droplets at moderate temperatures, ignited
by an external energy source, are representative here [102, 296–298].
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