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

4.6 Model Validation
Table 4.1: Validation Test Cases for Vaporization and Burning Rate. All experimental results,
acting as references, are for the fuel n-decane (C 10 H 22 ) [465]. The initial droplet
diameter is set to D 0,sim = 1.0mm for all simulation runs, here [298].
Ambient Experiment Simulation
temperature diameter range vap./burn. rate vap./burn. rate
Test case T ∞ in K D 0,exp in mm k exp in mm 2 s −1 k sim in mm 2 s −1
Vaporization 773 0.72 – 1.39 0.42 – 0.55 0.43
Burning 943 0.91 – 1.51 0.85 – 1.03 1.10
Burning 973 0.88 – 1.25 0.94 – 1.06 1.10
Burning 1093 0.97 – 1.54 1.05 – 1.44 1.13
Burning 1123 1.29 1.38 1.15
thermore, taking into account all experimental data available, data scattering
is large and seems, in part, to be subject to the experiment setup [298].
For validation, the combustion related parameters are set almost identical
to the ones used in the numerical simulations contributing to the results of
Chapter 5. Only the initial droplet diameter D 0 is adjusted to the target value
of the experimental data, which is D 0,exp ≡ D 0,sim = 1.0mm. Table 4.1 compares
the experimental and numerical results of the vaporization and burning rates
k. The results agree very well in a qualitative and quantitative manner, with
the numerical results lying in the upper range of the experimental data. The
experimental results tend to be lower due to radiant heat losses from the flame
and due to soot formation. Generally, both effects cause a decrease of flame
temperature and k-value. As reported by Nakanishi et al. [306] and Xu et al.
[465, 467], an increase of the ambient temperature enhances Stefan flow, soot
formation, and soot oxidation, resulting in three different burning regimes.
The second of these regimes is characterized by the most significant increase
of the soot formation rate. The third of them reveals a dominance of soot oxidation.
Here, transition from the second to the third of these regimes can be
observed at temperatures around T ∞ = 1093 K by comparing the experimental
burning rate k exp with the numerical burning rate k sim (Tab. 4.1) [298].
Figure 4.10 confirms that a variation of D 0 according to the validation test
cases of Table 4.1 does not result in a significant gain of insight. This is due
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