Experimental and modeling study of the effects of equivalence ratio on the benzene formation chemistry of one-dimensional laminar premixed n-heptane flames

In order to gain a better understanding of the soot formation mechanism involved in the combustion process of hydrocarbon fuels, laminar premixed n-heptane flames with different equivalence ratios (1.20, 1.60 and 1.80) are studied by using synchrotron photoionization and molecular-beam mass spectrometry (PI-MBMS) techniques at low pressure(30Torr). To isolate the influence from flame temperature, the composition of unburned mixtures is adjusted such that the temperature profiles are approximately equivalent. Mole fraction profiles of major and intermediate species were derived and compared among different flames. Parallel modeling study based on a modified mechanism is conducted. The predicted mole fraction profiles of concerned species agree reasonably well with experimental measurements. The results show that, as equivalence ratio increases, the equilibrium mole fractions of H₂O and CO₂ decrease while those of H₂ and CO increase, and the concentrations of benzene and toluene and those of C₂-C ₅ hydrocarbon intermediates increase apparently. The reaction flux analysis indicates that the self-recombination of propargyl radical (C ₃H₃) and the cross reaction between C₃H ₃ and allyl radical (a-C₃H₅) are the dominant pathway leading from small aliphatics to C₆H₆. The consumption of C₆H₆ is primarily resulted from the attack by OH and H radicals. The larger amount of C₆H₆ formed in richer flames should be attributed mainly to the higher concentrations of benzene precursors. In addition, the weakening influence of the OH-attack and O-attack reaction tends to slow down the consumption of C₆H ₆, which also contributes to the higher C6H6 concentration in the richer flames.