Laminar premixed flame propagation and non-premixed ignition of toluene and xylenes

The laminar flame speeds and nonpremixed ignition temperatures of toluene and the xylene isomers were experimentally determined at atmospheric and elevated pressures. The experimental situations were then simulated using the kinetic models of Metcalfe et al. (M-mechanism) and Narayanaswamy et al. (N-mechanism). The experimental results show that the toluene flame propagates faster than the xylene flames at all pressures, while the computation analyses show that this is primarily due to the fuel chemistry. For the nonpremixed stagnation ignition, experimental results show that toluene has lower ignition temperatures than the xylenes, and numerical analysis identifies that it is mainly due to the higher fuel diffusivity of toluene. Furthermore, the ignition temperatures of o-xylene are lower than those of m- and p-xylenes, and are suggested to be due to the formation of dimethylphenyl radicals with isolated H abstraction sites, which react with O₂ for further chain branching. The computed ignition temperatures for all fuels are consistently higher than the experimental values.