Two-dimensional Direct Numerical Simulation of Opposed-jet Hydrogen/air Flames: Transition from a Diffusion to an Edge Flame

In an opposed-jet diffusion flame experiment, under certain conditions, after the extinction of the diffusion flame, an edge flame can be obtained. This ring-shaped edge flame was first reported in 1959 by Potter and Butler but received little attention. It was reported again recently in a numerical and an experimental work and is responsible for an interesting transition between two distinct burning flames (multiple solutions). Motivated by our previous numerical results, obtained with simplified kinetics and some recently reported experimental data, we performed direct numerical simulations of this transition to investigate the underlying physical mechanisms. The appearance of an edge flame after the extinction of the diffusion flame, the hysteresis reported in the experiments, and the existence of multiple vigorously burning flames at identical conditions are all captured by our simulations. Our numerical results show that, in the absence of an inert coflow curtain, when the diffusion flame disk is extinguished, an edge flame forms and propagates in the mixing layer. After the formation of this edge flame, even when the applied strain rate is reduced to the initial subcritical value, the diffusion flame disk does not reappear, because the local fluid velocity still exceeds the propagation speed of the edge flame. This hysteresis has significant implications in the common submodel that utilizes the strain rate as a parameter to determine local reignition in flamelet models; it indicates that a subcritical strain rate is not a sufficient condition for the reignition of a diffusion flame. Further investigation of this phenomenon is clearly needed to refine submodels of local extinction and reignition in the flamelet models for turbulent diffusion flames. The opposedjet configuration provides a convenient platform to analyze edge flames which are stabilized aerodynamically in a two-dimensional geometry, thus making matching two-dimensional direct numerical simulations effective.