Quantification of molecular structure impact on combustion properties for synthetic diesel fuel: 2,6,10-trimethyldodecane

Global combustion characteristics of synthetic diesel fuel, 2,6,10-trimethyldodecane (TMD) has been investigated experimentally by measuring extinction limits of diffusion flames at 1 atm and reflected shock ignition delays at 20 atm. Derived cetane number (DCN) of TMD is measured as 59.1, thus similar global combustion characteristics have been speculated between TMD and the previously studied S-8 POSF 4734 and its surrogate. Identical high temperature reacitivities have been found both in diffusion flame extinction and ignition delay times. However, it has been found that S-8 POSF 4734 surrogate has the faster ignition delay times than TMD at temperatures below 870 K. To elucidate this difference, chemical function group analysis has been performed and it has identified that the methyl to methylene ratio plays important role in the low temperature reactions, particularly for high DCN fuel mixtures. To verify this, a mixture of n-hexadecane and iso-cetane has been also tested by measuring both diffusion flame extinction limits and reflected shock ignition delay times. The result confirms the importance of methy to methylene ratio as a matching condition. The behavior of TMD and the n-hexadecane/iso-cetane mixture for both diffusion flame extinctions and reflected shock ignition delays over the entire temperature conditions considered are identical. Further numerical analysis has been conducted based on ignition delay calculations for the stoichiometric fuel/air mixture at 850 K and 20 atm. The numerical results demonstrate that the ignition delay times of all tested n-alkane/iso-alkane mixtures can be correlated with the methyl to methylene ratio, which controls the initial fuel oxidation pathways in the formation of QOOH and OH radicals. The results in this study suggest that TMD needs more careful assessment of its global reactivity when only the DCN is used. The measured DCN fails to distinguish the unique characteristics of low temperature reaction of the tested fuel when it approaches a numerical value of 60.