Effects of flow strain on triple flame propagation

The primary objective of this study is to determine the effect of strain ra te and scalar dissipation rate on the instantaneous local displacement spee d at the triple flame edge. This is accomplished by performing direct numer ical simulations of a hydrogen-air triple flame subjected to an unsteady st rain field induced by a pair of counter-rotating vortices. It is observed t hat the triple flames maintain a positive displacement speed when the vorte x strength is weak, such that they penetrate into the channel between the v ortices. For the stronger vortex cases, the intense compressive strain fiel d induced by the vortex pair yields a negative displacement speed and parti al quenching of the leading edge of the flame in an extreme case. The displ acement speed variations are analyzed in terms of curvature, and effective Karlovitz and Damkohler numbers. It is found that the triple flame tip spee d is predominantly governed by the curvature-induced compressive strain rat her than by scalar dissipation rate. As a result, the displacement speed me asured at the triple flame tip exhibits a strong correlation with flame str etch and curvature, and not with scalar dissipation rate. The correlation w ith flame stretch is similar to results found in earlier studies of turbule nt premixed flames, suggesting that the propagation aspects of triple flame s are the same as for a premixed flame. The trailing diffusion flame essent ially has minimal impact on the propagation of the leading edge. A secondar y observation is that for real chemical systems, ambiguity in the definitio n of the "leading edge" can lead to significant differences in the propagat ion response to strain. For instance, the displacement speed measured at th e maximum heat release location rather than at the leading edge remains pos itive throughout the entire duration of interaction. Ibis suggests that car e should be taken in identifying the triple flame speed subjected to a larg e strain field. (C) 2001 by The Combustion Institute.