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

5 Results
D 0 being varied between 25 and 200µm. The atmosphere composition utilized
is again taken from the exhaust gas of a perfectly premixed flame with
φ = 0.8, leading to well-burned ambiance with sufficient oxygen remaining
for the succeeding droplet combustion. Results of droplet-caused NO x emissions
are shown in Figure 5.21. These calculated values lie on a straight line
through the origin of the plot. This tendency is in disagreement with the results
of Bracco [55]. He hypothesized that the NO x formation rate (normalized
by the fuel burning rate) increases with the square of the droplet diameter.
However, Bracco’s results are based on a rudimentary combustion model and
a number of simplifications (see also Chap. 2). The integral I of the NO source
term ˙ω NO , for instance, is assumed by Bracco to be quasi-constant throughout
droplet combustion. This assumption is fairly inaccurate. On the one hand, it
is inconsistent with other assumptions and results of Bracco’s own work. On
the other hand, this approach would require the NO source term ˙ω NO to balance
the decrease of the droplet diameter squared due to vaporization and
combustion [54, 55, 298].
60
g
50
40
30
20
10
Droplet NO x (ref.: droplet mass)
0
0.0 x 10 −4 0.5 x 10 −4 1.0 x 10 −4 1.5 x 10 −4 2.0 x 10 −4 m
kg −1 Initial droplet diameter D 0
Emission index of NOx EINOx
Figure 5.21: Emission Index of NO x for an n-Decane Droplet in Hot Exhaust Gas as a Function
of the Initial Droplet Diameter. The emission index EI NOx is calculated by
referring to the initial mass of the fuel droplet [298].
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