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

5.2 Combustion of Partially Pre-Vaporized Droplets
5.2 Combustion of Partially Pre-Vaporized Droplets
In this second configuration, droplets are partially pre-vaporized in an atmosphere
of air at moderate ambient temperatures before initiation of the actual
combustion process. A temperature level of T ∞ = 500K, below the autoignition
point, is chosen as the reference here. The pressure is atmospheric
(p = 1bar). The following results are based on experimental as well as numerical
work. The experimental results are obtained with the setup as introduced
in Chapter 3 for linear droplet arrays. The numerical results are generated by
use of the single droplet model of Chapter 4.5, including ignition modeling by
an external heat source, which in turn is outlined in Chapter 4.3. For both,
experiments and numerical simulations, spherically symmetric droplets can
be presumed as well as absence of gravity and forced convection.
The pre-vaporization rate Ψ is the main parameter characterizing this droplet
combustion regime. It was introduced by Equation (1.1) and can be resumed
in compact notation by
Ψ=1− ρ l,Ψ π 6 D 3 Ψ
π
ρ l,0 6 D 3 . (5.4)
0
Here, a variation of density is introduced for the respective droplet masses.
5.2.1 Numerical Results on Single Droplet Combustion
The emission index EI NOx is shown in Figure 5.3 as a function of the prevaporization
rate Ψ. The results of this figure are obtained exclusively from
numerical simulations with droplets of an initial diameter of D 0 = 100µm.
Here, the numerical domain is modeled large enough so that Dirichlet boundary
conditions can be applied for temperature, pressure, and mass fractions at
r = R ∞ . The initial values of temperature T 0 and species mass fractions Y m,0
are uniform block profiles. Thus, boundary and initial conditions of temperature
are set to the reference value of 500 K. The respective species mass fractions
are initialized according to the composition of air at reference conditions
[190]. Heat introduction and extraction is enforced at fixed positions according
to Figure 4.3. As heat introduction during the ignition phase is limited to a
narrow region, steep temperature gradients are expected. For this reason, the
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