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

5 Results
4.0
g kg −1
Emission index of NOx EINOx
3.0
2.0
1.0 With heat extraction (ref. reacting mass)
With heat extraction (ref. droplet mass)
Without heat extraction (ref. reacting mass)
Without heat extraction (ref. droplet mass)
0.0
0.0 0.2 0.4 0.6 0.8 1.0
Pre-vaporization rate Ψ
Figure 5.3: Emission Index of NO x as a Function of Pre-Vaporization Rate. The initial
droplet diameter of the illustrated simulation runs is set to D 0 = 100µm. The
positions of heat introduction and extraction are set to the constant values of
r m,in = 0.8×10 −3 m and r m,ex = 1.4×10 −3 m, respectively [298].
gas phase is discretized with N = 200 grid points, twice as many as used for
the configuration discussed in Chapter 5.1. Apart from that, the production of
NO x remains negligible in the outer radius of the gas phase because of moderate
ambient temperatures of the burning regime investigated. The emissions
caused by mere droplet combustion are equivalent to the total emissions here
(cf. Eqs. (5.1) and (5.2), i.e. m NOx ,atm≈0) [297, 298].
Baessler et al. [31, 32] reported that pre-vaporization of fuel spray becomes
beneficial with respect to NO x formation only if the degree of vaporization
is above a minimum limit and combustion is run in the lean regime. If insufficient
fuel is vaporized before ignition, the NO x emissions stay almost
unaffected. This correlation is clearly observable in the results reprinted in
Figure 2.3. The numerical simulations discussed here reproduce this trend
for single droplets, and its characteristics are reflected in detail in Figure 5.3.
Squares and circles indicate setups in which heat is introduced into the computational
domain to enforce ignition and where it is also extracted after a
characteristic time to keep the energy balance (cf. Figs. 4.2 and 4.3). For comparison,
the triangles mark emissions if heat is introduced only but not ex-
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