抄録
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 H_2O and CO_2 decrease while those of H_2 and CO increase, and the concentrations of benzene and toluene and those of C_2-C_5 hydrocarbon intermediates increase apparently. The reaction flux analysis indicates that the self-recombination of propargyl radical (C_3H_3) and the cross reaction between C_3H_3 and allyl radical (a-C_3H_5) are the dominant pathway leading from small aliphatics to C_6H_6. The consumption of C_6H_6 is primarily resulted from the attack by OH and H radicals. The larger amount of C_6H_6 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 C_6H_6, which also contributes to the higher C_6H_6 concentration in the richer flames.
