On the nature of the synergistic effect in flames of methane and formaldehyde mixtures with air

This paper is devoted to the study of the nature of the synergistic effect in flames of methane- and formaldehyde-air mixtures. Combustion of mixtures of various fuels is of great practical and fundamental interest. It is found that the addition of formaldehyde to a rich methane/air flame at a constant concentration of methane first reduces its propagation speed and then begins to increase it. The synergism mechanism in this case is due to the predominant and complete formaldehyde consumption due to its greater reactivity and its negative impact on the rate of methane consumption. As a result of predominant combustion of one of the fuels, there are two spatially separated heat release zones in the flame. Heat release in the first the zone is mainly due to the oxidation of formaldehyde and formyl radical, and in the second zone to in the recombination of methyl radicals. An analysis of the flame speed sensitivity shows that the key reactions affecting the flame speed are the stages of formation of radicals (mainly hydroxyl) or products that lead to their formation. Reactions that make the main contribution to heat release generally do not affect the flame speed. It is found that the interaction of two fuels $\mathrm{CH}_4$ and $\mathrm{CH}_2\mathrm{O}$ in the mixture with air leads to a marked increase in the super-adiabatic temperature effect.