Experimental and modelling study of ¹CH₂ in premixed very rich methane flames

Stoichiometric and very rich (1.5 ≤ ɸ ≤ 1.9) laminar flat flames of methane have been investigated using nonintrusive laser diagnostics. Premixed CH₄ + O₂ + N₂ flames were stabilized at a pressure of 30 ± 0.3 Torr. Temperature profiles were obtained using laser-induced fluorescence of OH. Absolute concentration profiles of singlet methylene, ¹CH₂, were measured by intracavity laser absorption spectroscopy. Uncertainties of the relative and absolute concentrations of singlet methylene were evaluated to be about ±10% and ±30%, respectively. These new experimental data were compared with predictions of three detailed kinetic mechanisms: GRI-mech. 3.0, Aramco mech. 1.3, and the model under development in Lund. In the last mechanism 78 rate constants of reactions along the pathway CH₃ → ¹CH₂ → ³CH₂ → CH were reviewed and updated. No adjustment or tuning of the rate expressions to accommodate experimental results was attempted. GRI-mech. significantly overpredicts singlet methylene concentrations in all flames. Aramco mech. and the present model are in good agreement with the measurements in stoichiometric flame, while in all rich flames only the present mechanism reproduces spatial profiles of ¹CH₂. Detailed analysis of the behaviour of these models revealed that omission of the reaction ¹CH₂ + M = ³CH₂ + M is the main reason of the discrepancy between predictions of the Aramco 1.3 and GRI-mech. 3.0 and experimental ¹CH₂ concentrations in rich flames.