Laminar flame propagation and nonpremixed stagnation ignition of toluene and xylenes

The laminar premixed flame propagation and nonpremixed ignition of toluene and xylenes at atmospheric and elevated pressures were studied. Toluene showed higher flame speeds than the xylenes at all pressures and this difference was due to the fuel kinetic effect. Flame chemistry analysis revealed that increased methylation generated more stable benzyl- like radicals as the H abstraction in the benzylic C-H bonds was energetically favored. The toluene flame speeds simulated by both M- and N-mechanisms generally agreed with the experimental measurements at all pressures with somewhat higher values by the N-mechanism. The experimental ignition temperatures showed that at all pressures the trend was toluene < o- xylene < m-xylene ≈p-xylene. The higher ignition temperature of m- xylene than toluene was mainly due to the lower fuel diffusivity while fuel chemical effect only played a secondary role. The lower ignition temperatures of o-xylene than m- and p-xylenes was due to the number of dimethylphenyl radicals with isolated H abstraction sites which may react with the oxygen molecule for the secondary chain branching. O-xylene could produce more dimethylphenyl radicals and thus exhibit higher fuel reactivity.