Abstract:
Chain branching and heat release processes and their influence on the burning velocity of pre-mixed rich and near-stoichiometric flames of methanol with air were studied by numerical simulation and sensitivity analysis. The phenomenon of super-adiabatic temperatures in these flames due to the kinetic mechanism of methanol combustion was first detected. Comparison of the results of simulation of the structure of methanol and formaldehyde flames showed that the formation of water in super-equilibrium concentrations in flames does not necessarily lead to superadiabatic temperatures, as believed earlier. It was first found that decreasing the dilution of the $\mathrm{CH}_3\mathrm{OH}/\mathrm{O}_2/\mathrm{N}_2$ combustible mixture with nitrogen at a constant equivalence ratio enhances the superadiabatic temperature effect. According to simulation results, in a rich near-limit methanol flame, the role of the $\mathrm{H}+\mathrm{O}_2=\mathrm{O}+\mathrm{OH}$ and $\mathrm{O}+\mathrm{H}_2=\mathrm{H}+\mathrm{OH}$ is negligible due to their low rate. At relatively low temperatures, branching occurs mainly in reactions involving $\mathrm{HO}_2$ and $\mathrm{H}_2\mathrm{O}_2$ peroxide compounds, whose concentration is orders of magnitude higher than the concentration of the main carriers of the chain $\mathrm{H}$, $\mathrm{O}$, and $\mathrm{OH}$. From the sensitivity analysis it follows that the methanol flame speed positively affects mainly the reactions of the formation of chain carriers and negatively affects the reactions in which chain carriers are consumed. The stages introducing the main contribution to heat release, but not involved in the formation and consumption of radicals have small sensitivity coefficients.
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