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

1 Introduction
In thermal engines the time between atomization and ignition often is too
short to achieve full premixing, or premixed flames are not even desirable
because their heat release characteristics are incompatible with the engine
requirements. Often, droplet sizes and vaporization rates allow only small
amounts of fuel to be vaporized before ignition, and a nonpremixed flame is
observed around the droplets or the droplet cloud. In diffusion-dominated
single droplet combustion, vaporized fuel from the droplets is transported
from one side into the spherical flame zone, whereas oxygen (O 2 ) diffuses from
the opposite direction. It is observed that the reaction zone stabilizes near
stoichiometric conditions. Under these conditions the exothermic reaction
sequence of the combustion process produces the highest temperature rise.
However, this naturally leads to undesired thermal NO (nitric oxide) formation
and imposes limitations on the NO x (principally NO and NO 2 ) emission levels
that can be reached without full pre-vaporization [253, 298, 443, 451, 458, 461].
1.2 Droplet and Spray Combustion
Spray flames are generally characterized by the coexistence of premixed and
nonpremixed flames [241, 242, 244, 263]. Figure 1.1 illustrates the processes of
liquid fuel injection and spray combustion. Liquid fuel is injected into a combustion
chamber, where the fuel jet is atomized. The resulting droplets vaporize
partially. The mixture of fuel vapor and air can auto-ignite or be ignited
by an ignition source, depending on local temperature, pressure, and equivalence
ratio. Provided a sufficient amount of flammable mixture and heat of
reaction is given, a flame zone forms around single droplets or droplet groups
[72, 73, 305]. In order to maintain reaction, reactants continually need to diffuse
into the flame zones from the droplets and gas phase, respectively, or the
flame front itself needs to propagate towards unconsumed reactants. Because
of its exothermic nature, the combustion reaction causes a rise in temperature.
Furthermore, fuel consumption steepens the concentration gradients,
and thus drives diffusive transport in the vicinity of the droplet. Consequently,
the droplets burn in a mixed atmosphere of fuel vapor, air, and hot exhaust
gas, and vaporization of the surrounding droplets is accelerated due to the
increase in temperature.
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