Effect of flame stretch on the transition boundary of “explosive transition of deflagration”

Recent investigations have shown that mixtures with Le > 1 are characterized by a temperature limit for sustaining a one-dimensional, stretch-free, highly preheated premixed flame. This limit is attributed to the chemical reactions occurring in the preheat zone under such extreme conditions. Beyond this, volumetric autoignition, termed as “explosive deflagration transition (ETD),” can occur. A previous two-dimensional simulation [Morii et al., “Two-dimensional laboratory-scale DNS for knocking experiment using n-heptane at engine-like condition,” Combust. Flame 223, 330–336 (2021)] for an nC7H16/O2/Ar mixture, which quantitatively reproduced an experiment, shows that knock onset aligns with ETD. In this study, we used a counterflow configuration to systematically vary the residence time of highly preheated premixed flames and investigated the flame stretch effects on ETD. A conceptual framework showed that the reaction progress in a zero-dimensional ignition system is equivalent to that in a one-dimensional stretched flame with Le = 1. This framework was extended through simulations of highly preheated stretched flames for a lean C3H8/O2/He mixture, as a representative mixture with Le > 1, consistent with ETD conditions. The three reaction front modes were identified: (I) conventional premixed flames, (II) flames influenced by preheat-zone reactions, and (III) a front governed by autoignition chemistry. Further analysis expanded the concept of Zel'dovich's spontaneous ignition front into a propagating stretched flame. Then, the mode (III) was revealed to correspond to the spontaneous ignition front. The transition from mode (I) to mode (II) marked the boundary of ETD, with the boundary temperature increasing with increasing stretch rate. These results suggest potential knock limit extension due to the stretch effect in engine combustion.