Cationic structure of premixed near-stoichiometric CH4/O2/Ar flames at atmospheric pressure: New insights from mass spectrometry, quantum chemistry, and kinetic modeling

Abstract

The spatial distributions of naturally occurring cations in near-stoichiometric (equivalence ratios ϕ = 0.8 and 1.2) burner-stabilized methane/oxygen/argon flames at atmospheric pressure are studied by molecular beam mass spectrometry (MBMS) and kinetic modeling. The disturbing effect of a nickel sampling cone used in the measurements on the flame was comprehensively modeled using a two-dimensional direct numerical simulation of reacting flow near the axially symmetric probe along with a reduced chemical kinetic mechanism. The use of one-dimensional numerical simulation for predicting the flame structure perturbed by the metallic probe was firmly justified. We also proposed and applied a procedure to correct the measured spatial profiles of cations on the contribution of signals from the hydrates formed slightly upstream the reaction zone due to the sampling probe effects. With all these methodological advantages, we validated the ion chemistry mechanisms proposed earlier in the literature against the novel experimental data on the spatial distributions of the following cations: HCO+, CH3+, H3O+, C2H3O+, CH5O+, C3H3+. The kinetic mechanism published recently by Chen et al. [Combust. Flame 202 (2019) 208] for the charged species formed in methane flames was revised in order to improve its predictive ability. To this end, the highly accurate W2-F12 quantum chemical calculations were used to obtain the reliable formation enthalpies of all cations considered in the mechanism. In the case of C2H3O+ and C3H3+ cations, the calculated values turned out to be profoundly lower than those reported before. Apart from this, the theory also predicted another exit channel for H3O+ + acetylene reaction yielding HCO+ + CH4 instead of C2H3O+ + H2 proposed earlier. The corrections significantly improved the predictive ability of the ion chemistry mechanism. We also considered the most important directions for the further refinement of the mechanism.