Investigating the effects of edge flames in liftoff in non-premixed turbulent combustion

Abstract

To help to understand and to model liftoff in non-premixed turbulent combustion, we have studied effects of unsteadiness at the edge of a diffusion flame stabilized by a triple flame. A model problem involving squeezing of the triple flame between two moving vortices has been solved using direct numerical simulation. Numbers representative of propagation and extinction properties of edge flames were chosen for this particular configuration. By varying those key numbers, the response of a triple flame submitted to vorticity and unsteady micromixing was quantified. The outcome is a diagram delineating conditions for upstream and downstream movement of a diffusion flame in such an environment. It is found that edges of diffusion flames can take advantage of vorticity and micromixing to propagate faster than laminar triple flames. This propagation was also observed for conditions at which the trailing diffusion flame is quenched. Moreover, local quenching of this trailing diffusion flame generates new ends of reaction zones, which may help to sustain combustion if they relax to triple flames. The properties of the relative velocity of the edge of the reaction zone are compared with the asymptotic solution obtained for a steady triple flame. This helps to discriminate between edge flames able to progress within a zone of intense unsteady mixing from those moving downstream with the flow: in particular, it is observed that a triple flame submitted to strong unsteady micromixing effects may lose its propagation properties and evolve into a “hot spot.”