Effects of Flow Strain on Triple Flame Propagation HONG G. IM* Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA and JACQUELINE H. CHEN Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA The primary objective of this study is to determine the effect of strain rate and scalar dissipation rate on the instantaneous local displacement speed at the triple flame edge. This is accomplished by performing direct numerical simulations of a hydrogen-air triple flame subjected to an unsteady strain field induced by a pair of counter-rotating vortices. It is observed that the triple flames maintain a positive displacement speed when the vortex strength is weak, such that they penetrate into the channel between the vortices. For the stronger vortex cases, the intense compressive strain field induced by the vortex pair yields a negative displacement speed and partial quenching of the leading edge of the flame in an extreme case. The displacement speed variations are analyzed in terms of curvature, and effective Karlovitz and Damko ¨hler numbers. It is found that the triple flame tip speed is predominantly governed by the curvature-induced compressive strain rather than by scalar dissipation rate. As a result, the displacement speed measured at the triple flame tip exhibits a strong correlation with flame stretch and curvature, and not with scalar dissipation rate. The correlation with flame stretch is similar to results found in earlier studies of turbulent premixed flames, suggesting that the propagation aspects of triple flames are the same as for a premixed flame. The trailing diffusion flame essentially has minimal impact on the propagation of the leading edge. A secondary observation is that for real chemical systems, ambiguity in the definition of the “leading edge” can lead to significant differences in the propagation response to strain. For instance, the displacement speed measured at the maximum heat release location rather than at the leading edge remains positive throughout the entire duration of interaction. This suggests that care should be taken in identifying the triple flame speed subjected to a large strain field. © 2001 by The Combustion Institute INTRODUCTION Triple flames, or more broadly known as edge flames, have recently attracted strong research interest because of their practical relevance to turbulent diffusion flame stabilization, flame spread, and autoignition. An earlier study [1] demonstrated a formal mathematical analysis of the effects of strain rate on the propagation speed of a triple flame, thereby first identifying the existence of positive and negative speed, which was later termed ignition and extinction fronts, respectively. More recent studies have focused on the effects of various parameters, such as heat release [2] or Lewis number [3] effects on flame propagation. Spatial and tem- poral instability issues have also been consid- ered [4, 5]. A recent study of direct numerical simulation [6] (see also [7]) confirmed the exis- tence of negative edge flame speed in response to unsteady strain induced by a pair of vortices. All of these studies, however, adopted a one- step, two-species chemistry model to simplify the analysis. Consideration of detailed chemis- try has been addressed in recent direct numer- ical simulations of methanol-air [8], methane- air [9], and hydrogen-air [10] systems. In a recent study [10], the structure and propagation characteristics of hydrogen-air tri- ple flames were investigated. Consideration of full hydrogen chemistry revealed the detailed structure of major and minor species concentra- tion and reaction rates within the triple flame. It was shown that, because of its important role in chain branching and in the three-body recombi- nation heat release reaction, the reaction rate of H atom was found to be the best indicator to demarcate the three branches of the flame structure. For the freely propagating triple flames, the enhancement of the stabilization speed, defined as the flame speed with respect to the far-upstream flow, was found to be pri- * Corresponding author. E-mail: hgim@umich.edu COMBUSTION AND FLAME 126:1384 –1392 (2001) 0010-2180/01/$–see front matter © 2001 by The Combustion Institute PII S0010-2180(01)00261-9 Published by Elsevier Science Inc.