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

5.2 Combustion of Partially Pre-Vaporized Droplets
tracted. The amount of fuel referred to is either the reacting fuel mass, that is
consumed until simulation end (Eq. (4.46)), or the initial mass of the droplet.
In general, NO x emissions are almost constant for Ψ< 0.5. This tendency suggests
that combustion behavior is similar in this range. As the radial profile
of the fuel mass fraction Y C10 H 22
(r ) is steep in the gas phase, the amount of
flammable, premixed gas is low. Most of the pre-vaporized fuel is oxidized
in a diffusion flame. However, at large degrees of pre-vaporization (Ψ> 0.5),
the amount of fuel under flammable, partially premixed conditions increases.
Immediately after ignition, the flame type tends more and more towards a premixed
flame. The NO x emissions decrease. Besides those effects due to fuel
premixing and pre-vaporization, the temporal and spatial scales are important
in terms of NO x formation, as well. Their impact is discussed in detail in
the context of the verification of the ignition position (Chap. 5.2.2).
As can be seen in Figure 5.3, it is not relevant to distinguish between the two
fuel masses referred to within one and the same ignition method, as long as
the pre-vaporization rate is low (Ψ< 0.5). However, this becomes important
for larger values of Ψ. Comparing the two referencing methods, the emission
index EI NOx generally decreases with an increase in Ψ, which is due to an overall
decreasing NO x production. Still, it decreases at a slower rate, when referring
to the reacting fuel mass instead of to the initial droplet mass. This trend
is not an indicator of insufficient convergence. Vaporized fuel simply diffuses
outwards in the radial direction. Thus, with an increase of Ψ, more gaseous
fuel is shifted towards regions with an equivalence ratio remaining below the
lower flammability limit. This fuel is lost as far as the spherical droplet flame
is concerned. However, given an appropriate design of the combustion zones
within a technical application, it can be consumed in the subsequent combustion
zone at a lower partial pressure of oxygen, resulting in a significant
overall NO x reduction.
5.2.2 Impact of Ignition Position
Generally, heat introduction to initiate ignition is conducted within a region
spanned by the flammability limits and their outer proximities, taking a wide
variation of droplet pre-vaporization Ψ as a basis. Two feasible approaches are
outlined in the related model description of Chapter 4.3. The first approach
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