Laminar flame speeds and oxidation kinetics of benzene-air and toluene-air flames

Using the counterflow twin-flame technique, the laminar fame speeds of benzene-air and toluene-air mixtures were determined experimentally over an extensive range of equivalence ratios at room temperature and atmospheric pressure. The minimization of stretch effects in the determination of the flame speed was accomplished through both linear and nonlinear extrapolations of the stretched reference flame speed to vanishing stretch, with the nonlinearly extrapolated values being typically 2 cm/s smaller. The laminar flame speeds of toluene were found to be lower than those of benzene, typically by 5-6 cm/s. Numerical simulations of the flame speeds were performed using two reaction mechanisms developed by Emdee, Brezinsky, and Glassman (EBG) and by Lindstedt and Skevis (LS), respectively. Predictions by using both mechanisms were close and were substantially lower than the experimental values. A modified EBG mechanism is then proposed, which adopts a recently measured rate constant of phenyl + O-2 --> phenoxy + O and takes into account the pressure fall-off for the rate coefficients of several key radical-radical recombination reactions. It is shown that these modifications substantially improve the flame-speed predictions, while minimally affect the available flow-reactor data of benzene and toluene oxidation. The present analysis suggests that the major oxidation pathways of benzene and toluene proposed in the flow-reactor study of Emdee, Brezinsky, and Glassman can account for their oxidation in flames and, therefore, serve as a foundation for a comprehensive model. It further indicates that the development of such a model requires additional studies on the reaction kinetics of phenol and cyclic C-5 species.