Laminar flame speeds and oxidation kinetics of iso-octane-air and n-heptane-air flames

Laminar flame speeds of iso-octane-air and n-heptane-air mixtures were determined experimentally over an extensive range of equivalence ratios at room temperature and atmospheric pressure, employing the counterflow twin flame configuration. Using both linear and nonlinear extrapolations, the effect of stretch was minimized by extrapolating the reference flame speed to vanishing stretch, with the nonlinearly extrapolated values being typically 2 cm/s smaller. The laminar flame speeds of iso-octane were found to be lower than those of n-heptane throughout the range of experimental equivalence ratios. Predictions, using a detailed kinetic model based on the works of Held et al. and of Curran, Pitz, and Westbrook, agreed quite well with experimental data, especially at lean equivalence ratios, while yielding somewhat lower values at stoichiometric to rich equivalence ratios. Since the n-heptane kinetics in the model were previously compared to flow-reactor data, the present model was also compared to an iso-octane oxidation flow-reactor experiment. The model accurately predicted the fuel decay profile as well as those of propene and iso-butene, which are the two major intermediates formed initially at flow-reactor conditions. The present analysis suggests that the major high-temperature reaction pathways proposed by Curran et al. accurately describe the high-temperature oxidation of iso-octane and indicates that the development of a comprehensive model requires additional studies on the reaction kinetics of propene and iso-butene.