Extinction of methane/air counterflow partially premixed flames

Flame interactions of partially premixed flames (PPFs) were experimentally and numerically studied using counterflow configurations with CH₄/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 -→ CH₃ + H₂O and CH₃ + O → CO + H₂ + 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.