On multiple criticality in hydrocarbon ignition

The counterflow ignition of nitrogen-diluted methane by heated air was computationally studied, with particular interest on the role of thermal- versus radical-induced runaway. Results show that for situations with low methane concentrations, the characteristic ignition-extinction S-curve has two ignition turning points, implying that ignition could occur in two stages as the air temperature is increased. It is further shown that the first ignition event, occurring at the lower temperature, is controlled by radical runaway and does not involve thermal feedback, while the second ignition event does require thermal feedback. The crucial reaction that effects the folding of the first ignition turning point is CH₃ + HO₂ → CH₃O + OH, which converts the nominally inactive HO₂to two reactive radicals. The analogy of this reaction with H + HO₂ → OH + OH in the ignition of hydrogen is noted, and the importance of HO₂ in inducing ignition in general is discussed. The study in addition emphasizes the need to have accurate knowledge of the rate constants of the key reactions, such as that involving HO₂ branching, as moderate differences or uncertainties in their values could lead to qualitatively different descriptions of the combustion response, including the presence/absence of the radical-induced ignition event.