Through the use of an ozone-assisted counterflow burner, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. Comparisons of experimental results with numerical calculations based upon a detailed chemical kinetic model show a large discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H_2O_2 is formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.
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