Effects of flow strain on triple flame propagation

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 Damköhler 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.