The effect of fuel additives on the autoignition dynamics of rich methanol/air mixtures

The use of H₂O₂, CH₂O, H₂O, CH₄, C₂H₅OH, CHOCHO and CH₃CHO as additives in rich methanol/air mixtures is investigated for the control of ignition delay and of the super adiabatic temperature phenomenon (SAT) during autoignition. Since H₂O₂ acts as an oxidizer, effectively it shifts the process to smaller equivalence ratios, which none of the other additives can do and decreases drastically ignition delay and suppresses SAT. CH₂O is the second most influential additive, resulting in a milder decrease of ignition delay and an even milder suppression of SAT. Of the remaining additives considered, only those containing the methyl group are shown to influence ignition delay. The effect of additives on SAT is shown to depend on the amount of oxygen in the additive molecule. A detailed investigation of the influence of H₂O₂ and CH₂O additives leads to the conclusion that, for the cases of pure fuel and CH₂O addition, a chemical runaway develops that is supported by the reactions of methanol and CH₂O with HO₂ that form H₂O₂, which then dissociates to OH. H₂O₂ addition, though, obviates the need for a chemical runaway because it provides the system with H₂O₂ readily, thus leading to a much shorter ignition delay. In the post-ignition regime, CH₂O does not alter the dominance of the endothermic dissociation reactions that cause the SAT, while H₂O₂ reinforces exothermic reactions, thus suppressing SAT.