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

2 Combustion Theory
of partial pre-vaporization on NO x emissions in “globally premixed” sprays.
Based on his experimental results, Cooper postulates an analytical model stating
that NO x emissions are due to a contribution of vapor-phase combustion
and droplet combustion at stoichiometric conditions. However, the influence
of a flame holding device installed primarily to sustain recirculation zones
and, thus, stabilize the combustion process itself on the attainable mixture
quality proved to be substantial. Consequently, Cooper considers the interrelationships
between droplet collection efficiency, reatomization efficiency,
and blockage of the flame holder as well as the initial droplet size distribution
with respect to NO x formation. The model postulated requires knowledge
of the following combustor conditions: temperature, pressure, fuel type,
equivalence ratio, and residence time, plus preparation characteristics of the
initial fuel-air mixture, including droplet size distribution and degree of vaporization.
Cooper himself admits the lack of supporting experimental data
that hinders the application of his procedures developed for predicting NO x
emissions [86, 87]. Baessler [31] uses kerosene to assess the influence of the
pre-vaporization rate Ψ on NO x emissions; an approach he has in common
with Cooper. However, the setup of Baessler differs from Cooper’s in that turbulence
level and preheating temperature are lower. On the other hand, the
achievable range of Ψ is much more widespread in the work of Baessler, particularly
for low values of Ψ with a high liquid fuel mass fraction (see Fig. 2.3).
Due to the broad basis of experimental results, Baessler reports a constant
progression of NO x emissions starting from low degrees of vaporization followed
by a linearly decreasing dependence for high degrees [31, 32].
Beck [38] utilizes a swirl-stabilized burner in his studies and supposes this stabilization
mechanism to have a significant impact on the flame structure. In
order to provide an incomplete liquid fuel pre-vaporization, two liquid fuel injectors
are installed in the experimental setup. The first injector is located far
upstream of the combustor and generates a fully pre-vaporized and premixed
fuel-air mixture, whereas the second injector is located at the combustor inlet.
The droplet slip observed is fundamentally different compared to the setups
of Cooper and Baessler because the gas-phase velocity fluctuates significantly
along the droplet trajectories. Therefore, Beck expects his setup to best represent
the characteristics of a gas turbine combustor. The results obtained by experimental
investigation suggest that a variation of the pre-vaporization rate
Ψ may be abstracted as the superposition of a droplet and premixed flame.
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