Shock tube ignition measurements of 2,5 dimethylhexane

Synthetic liquid fuels derived by reforming biomass syngas, using Fischer-Tropsch process, are an important class of renewable fuels. These synthetic fuels, as well as conventional diesel and aviation fuels, contain large concentrations of lightly branched alkanes (Dooley et al., 2012). Better understanding the combustion process of synthetic fuels and their surrogate formulation requires chemical kinetics characterization of singly and doubly branched alkanes. However, very little work has been done to study the chemical kinetics of lightly methylated alkanes. In this study, we present shock tube ignition delay time measurements of 2,5 dimethylhexane over the temperature range of 1100-1500 K, pressures of 5 and 10 atm, and equivalence ratios of 0.5, 1 and 2 with Argon as the diluent. Ignition delay times were determined by using pressure history and OH∗chemiluminescence near 306 nm. The ignition delay times were shorter at the higher pressure (10 atm) over the entire temperature range. Fuel lean mixtures were most reactive and fuel rich mixtures were slowest to ignite at any given temperature and pressure. This is due to the fact that in the high temperature regime the rate of dominant chain branching reaction H + H2 → O + OH depends on the concentration of molecular oxygen. Measured ignition delay times were compared with those predicted by the recently published kinetic model of Sarathy et al. (Sarathy et al., 2011). The model over-predicted ignition delay times over the studied temperature range. Sensitivity analysis revealed that 2,5 dimethylhexane ignition is highly sensitive to propene chemistry which requires further refinement and validation. Modifications to the propene chemistry are suggested, and these result in improved agreement with the experimental data. Finally, an ignition delay correlation was derived by the regression analysis of our experimental data. The correlation provides a convenient method to show the dependence of fuel ignition on relevant parameters (temperature, pressure, oxygen concentration). τign =1.23∗10-5∗p-0.744∗ Xo2-1.59 ∗ exp (18.889∗103/T) [μsec].