Effects of fuel branching on the propagation of octane isomers flames

Laminar flame speeds of mixtures of air with 3-methylheptane/air, 2,5-dimethylhexane/air, and iso-octane/air, were determined for a wide range of equivalence ratios in the counterflow configuration at atmospheric pressure, and an unburned mixture temperature of 353 K. The results were compared against those obtained in recent investigations for n-octane/air and 2-methylheptane/air flames. It was determined that n-octane/air flames propagate the fastest, followed by 2- and 3-methylheptane/air, 2,5-dimethylhexane/air, and iso-octane/air flames, confirming that the overall reactivity decreases as the extent of fuel branching increases. The experimental data were modeled using a combination of recently developed kinetic models. Detailed sensitivity and reaction path analyses were performed in order to provide further insight into the high temperature oxidation chemistry of the octane isomers. It was determined that for all octane isomers, flame propagation is largely sensitive to H2/CO and C1C4 kinetics, while the effects of fuel-related reactions were found to be minor. Additionally, the analysis of the computed flame structures revealed that the low reactivity of branched isomers could be attributed to the production of unreactive, H-scavenging, resonantly stabilized intermediates such as propene, allyl, and iso-butene. Although fuel specific reactions do not exert a first order effect on flame propagation, the products of the initial fuel decomposition affect the concentrations of C1C4 intermediates, whose subsequent reactions are responsible for the observed differences in the overall flame reactivity among all isomers.