## 3 Results
### Numerical Variation of the Fuel Composition  
The measured higher soot concentration in the surrogate flames compared to the Jet A-1 flames is at odds with the lower benzene concentration there. To try to explain this behavior, another fuel variation was considered using the DLR and the Creek mechanism for numerical calculations. To understand the influence of the surrogate composition on the formation of soot precursors and thus on soot formation, calculations were carried out with different fuel compositions using the DLR mechanism. The mechanisms were chosen in each case depending on the available species.  
For this purpose, five different surrogates containing (tri)methylbenzene, cycloalkane or alkane were considered. In addition to the previously used mixture (42.67 vol.-% \(N\)-decane, 33.02 vol.-% isooctane and 24.31 vol.-% toluene), the toluene was replaced by  
Figure 6: Comparison between measured data of Jet A-1 flame at \(\Phi\) = 2.1 and v = 5 cm/s and numerical calculations.  
cyclohexane in the second mixture. The third mixture consist out of pure \(N\)-decane. In a second study, trimethylbenzene/n-decane and toluene/n-decane mixtures were investigated to gain insight into direct formation of different aromatics.  
The results in Figure 7 were calculated with the DLR mechanism and Figure 7 shows the concentration profiles for the short-chained methane, ethylene and acetylene. The profiles of methane and acetylene are almost identical for all fuel compositions and the profiles of ethylene are similar, with differences in the concentrations before and at the maximum. This observation fits the experimental data, which also show very similar profiles for kerosene and the surrogate, except for ethylene.  
Figure 7 shows the comparison (now plotted logarithmically) for the 1-4 ring aromatic compounds benzene, naphthalene, anthracene and pyrene. PAH are known for their role in soot formation and pyrene is often considered a direct soot precursor in numerical models. The comparison now shows clear differences between the mixture with toluene to the other mixtures. For all PAH considered, the concentrations of the surrogate with toluene are 1-2 orders of magnitude higher than those of the other two blends. This would also lead to higher soot concentrations in these blends. This indicates that different sooting behavior of the three fuels is not due to the formation of short-chained soot precursors, but to the different decomposition of the fuel directly to PAHs.  
In the second study, the Creck mechanism was used for the calculations and the results are shown in Figure 8. Figure 8 shows the calculated benzene concentrations where the mixture with toluene has a maximum benzene concentration almost twice that of the mixture with trimethylbenzene. In Figure 8, the comparison of pyrene concentrations is plotted. Here, it is reversed and the mixture with trimethylbenzene has significantly higher maximum pyrene concentrations. Given the known role of pyrene in soot formation, it can be assumed that this flame would also show higher soot concentrations. One explanation for the higher pyrene concentrations in the flames with the trimethylbenzene mixture may be the known behavior of trimethylbenzene leading to rapid formation of polycyclic aromatic hydrocarbons by a number of different mechanisms [37]. It is also known that the formation of PAH is very sensitive to fuel composition and a variety of different reaction pathways can occur [38]. It is therefore difficult to infer the soot properties of the fuel and the amount of soot in the flame from individual measured aromatic concentrations.  
Figure 7: Comparison of numerically calculated species profiles for different mixture compositions.