Numerical study on oscillatory propagation dynamics and physics near the limits of planar freely propagating premixed flames

Abstract

An oscillatory phenomenon of the freely propagating premixed flames for dimethyl ether–air mixtures at near-limit conditions was systematically analyzed to understand the underlying physicochemical processes that control the oscillation and extinction of hot-lean flames. The flame oscillation dynamics and extinction mechanism were first investigated using the analyses of flame oscillating structure and phase function. It was found that the leading cool-flame front and tailing hot-flame front present a significant separation during the oscillatory propagation, and the formation of flame oscillation was the consequence of the interaction between the two flame fronts which can be explained as the competition of reactions between low-, intermediate-, and high-temperature pathways. Meanwhile, due to the excessive heat losses of the lower half period, the flame will eventually extinguish in the divergent oscillating process. Furthermore, the key processes in the flame oscillation were further revealed by chemical explosive mode analysis-based diagnosis. The results show that the oscillations are primarily originated from the species and reactions of the intermediate-temperature pathway, including H2O2 and CH2O, and chain-branching/-termination reactions. These key processes eventually lead to the flame oscillatory extinction presenting an O(2) frequency.