A numerical study on near-limit extinction dynamics of dimethyl ether spherical diffusion flame

Abstract

The near-extinction oscillatory dynamics and underlying mechanism of the dimethyl ether (DME) spherical diffusion flames in the hot- and cool-flame conditions were studied by the numerical approach with detailed chemistry and transport models. It was found that the self-sustaining spherical cool diffusion flame could be readily obtained in a wide range of parameters, and additionally, the flammability limit could be considerably extended by the cool-flame chemistry. Strong oscillations that can trigger flame extinguishment prior to the steady-state extinction turning point were observed in either the hot or cool flame regime. The DME hot flame near extinction was governed by a single oscillatory mode with a fixed frequency (1 Hz) that was irrelevant with the ambient oxygen level. By contrast, the cool-flame extinction was controlled by dual oscillatory modes which had rather distinct frequencies, and the oscillatory period of the high-frequency mode increased significantly when approaching the extinguishment. Additionally, the transient dynamics of cool flame near extinction was much more complicated, due to the existence of strong coupling between the high-frequency and low-frequency oscillatory modes which was affected by the phase difference. The governing reactions that controlled the oscillatory extinction in hot- and cool-flame regions were revealed using a logarithmic sensitivity index. It was found that the hot-flame oscillatory extinction was controlled by the competition of high-temperature exothermic/endothermic reactions with the chain branching/termination reactions involving small molecules. The cool-flame oscillation was controlled by the low-temperature branching and termination competition in the negative temperature coefficient regime.