Extinction of methane/air counterflow partially premixed flames

摘要

Flame interactions of partially premixed flames (PPFs) were experimentally and numerically studied using counterflow configurations with CH_4/air mixtures under atmospheric condition. The main aims of this study were to describe the flame interactions in terms of variations in global strain rate (K) and to understand flame extinction. Two premixed-fuel streams were characterized by equivalence ratios (Φ_(lean) and Φ_(rich)). Experiments were conducted under the following conditions: Φ_(lean) + Φ_(rich) = 2, 0.5 Φ_(lean) < 0.7 (i.e., 1.3 < Φ_(rich) < 1.5), and several K values. To describe the interactions, twin flames (TFs), also examined under corresponding Φ, were used as a criterion, because the Φ values were within the flammability limits. Experimental results of PPFs showed a different behavior than those of TFs: first, three different luminous layers were observed; second, thin single-layered flames, with a nearly constant flame width, were observed near extinction for several combinations of Φ; third, PPFs could exist for significantly higher K than TFs. Numerical results supported the experimental results and showed that two reaction paths, CH_4 + OH → CH_3 + H_2O and CH_3 + O → CO + H_2 + H, were remarkably intensified by flame interaction of PPF near extinction; the significance of the reaction paths was determined. In addition, it was found that excess CO production plays an important role in extinction of PPFs.