Chemical functional group descriptor for ignition propensity of large hydrocarbon liquid fuels

The chemical functional group approach is investigated to verify the fundamental applicability of low-dimensional descriptors in the prediction of global combustion behavior, as described by homogeneous reflected shock ignition delay times. Three key chemical functional groups, CH2, CH3 and benzyl-type, are used to represent n-alkyl, iso-alkyl, and aromatic functionalities, respectively. To examine whether such descriptors can appropriately reflect the influences of these functionalities on ignition delay, Quantitative Structure-Property Relationship (QSPR) regression analysis is performed with the formulation of analytical models based on a fundamental Arrhenius-type description. The models are trained using literature measurements of reflected shock ignition delay times for stoichiometric fuel/air mixtures at 20 atm. Sensitivity analyses applied to the QSPR regression models show that the CH2 functional group dominates chemical kinetic behaviors at low temperature, while the chemical kinetic impacts of CH2, CH3, and benzyl-type functional groups all diminish as temperature increases. Further analyses of constant-volume adiabatic ignition delay predictions using detailed chemical kinetic models demonstrate influences of n-alkyl, iso-alkyl, and aromatic functionalities at both low and high temperature, consistent with those found for the QSPR regression models. Finally, 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy is used to directly quantify the chemical functional group compositions of both petroleum-derived and alternative jet fuels. Combining the QSPR model with NMR spectra interpretation, the applicability of current approach as an expeditious tool to accurately characterize the ignition propensity of real transportation fuels is demonstrated by comparison with experimental measurements.