Structure of incipiently sooting partially premixed ethylene counterflow flames

We perturbed an atmospheric-pressure ethylene diffusion flame by progressively adding oxygen in the fuel stream, while holding constant peak temperature and stoichiometric mixture fraction. The resulting partially premixed flames presented a well-defined double-flame structure, with a (lightly) sooting region sandwiched between a premixed flame component and a diffusion flame one. Temperature measurements were performed using fine thermocouples and thin filament pyrometry, whereas species concentrations profiles of CO2, CO, N2, O2, H2 and C1-C12 species, including aromatics, were determined by gas sampling through a quartz microprobe followed by GC-MS analysis. In addition to the diffusion flame, two of the flames, at equivalence ratio, Φ = 6.5 and Φ = 5.0, were probed in detail with these diagnostic techniques. A fourth flame at Φ = 3.0 was examined only qualitatively because of excessive soot presence. The premixed flame component of the dual flame structure and the diffusion flame one are coupled both thermally and chemically. Soot formation increases with lowering equivalence ratio and increasing temperature, a trend that is consistent with that of purely premixed strained flames stabilized against a hot nitrogen counterflow, as confirmed computationally by comparing profiles of a critical soot precursor such as benzene. However, partially premixed flames, probably as a result of the back diffusion of key radicals such as H and OH from the diffusion flame component, have a lower tendency to soot as compared to purely premixed ones. Comparison of measurements with computational results using two detailed chemical kinetic mechanisms show good agreement for major species once the velocity boundary conditions are properly determined via 2-D modeling of the flow within the burner. In one case, the agreement is also good for some soot precursors such as benzene, despite mismatches in some critical intermediates. Reaction path analysis suggests an increasingly larger contribution of the C3 path to benzene formation with the lowering of the equivalence ratio. The database with the measurements of primary reactants, products and intermediates, including critical soot precursors up to 3-ring aromatics is available to developers of chemical reaction mechanisms.