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Abstract
Continuation computations were performed to generate the S-curve as well as the flammability limit. Transient simulations using detailed fuel chemistry and transport models were conducted to reproduce the oscillatory extinction process. It was found that the cool spherical diffusion flame (SDF) had a much extended extinction limit than the hot SDF. The cool SDF with double-reaction-zone structure had non-stoichiometric and partially premixed combustion features. Oscillation can induce extinction prior to the steady-state bifurcation point, especially for the cool flame which exhibited much stronger and more complex oscillations due to its inherent strong coupling with transports or solid boundary. The oscillatory extinction of hot dimethyl ether SDF was governed by a single-oscillatory mode, while the cool SDF extinction by dual oscillatory modes with two distinct frequencies. However, the dual oscillatory modes exhibited only inside the flame front; the high-frequency mode vanished outside the flame zone. The hot SDF oscillatory extinction was caused by competing coupling of HRR with flux mixing, i.e. oscillations will break the thermal balance by enhancing flux mixing and inhibiting heat production which further leads to flame extinguishment. However, the cool-SDF oscillations near the extinction were driven by the thermokinetic type heat production in the NTC region.
Acknowledgements
The present research was supported by the National Natural Science Foundation of China (grant number 51706027), the Natural Science Foundation of Chongqing (grant number: cstc2019jcyj-msxmX0584), and the Fundamental Research Funds for the Central Universities (grant number: 2020CDJGFND012).
Disclosure statement
No potential conflict of interest was reported by the author(s).