Observation of two different cool flame regimes of diethyl ether in a counterflow burner

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This short communication reports, for the first time, the existence of two different self-sustaining cool flame regimes of diethyl ether (DEE) in a diffusion counterflow burner: a weaker autoignition-assisted cool flame near the fuel burner and a normal diffusion cool flame near the stagnation plane. The results show that the normal diffusion cool flame extinction limit increases monotonically with the fuel mole fraction, while the autoignition-assisted cool flame approaches a plateau and can exist at a fuel mole fraction below the normal diffusion cool flame. It is shown that both flame regimes are governed by the same low-temperature chain-branching reaction pathway of DEE. By using in situ laser diagnostics, entrainment of unburned fuel stream to oxygen stream at the outer edge of the fuel burner is identified as the governing physical mechanism causing a partially premixed self-sustained hollow cool flame structure. The results reveal that when a fuel with high low-temperature reactivity, two different cool flame regimes can be observed in a counterflow flame experiment. Future studies with high-reactivity fuels in a counterflow burner must ensure to distinguish between the two self-sustaining cool flame regimes. Moreover, the existence of these different cool flame regimes needs to be examined so that they would not trigger an uncontrolled combustion phasing in advanced engines fed with high low-temperature reactivity fuels.