Flames in vortices & tulip-flame inversion

Abstract

This article summarises two areas of research regarding the propagation of flames in flows which involve significant fluid-dynamical motion [1]–[3]. The major difference between the two is that in the first study the fluid motion is present before the arrival of any flame and remains unaffected by the flame [1, 2] while, in the second study it is the flame that is responsible for all of the fluid dynamical effects [3]. It is currently very difficult to study flame-motion in which the medium is both highly disturbed before the arrival of a flame and is further influenced by the passage of the flame.

The first study highlights the role of stretched Navier-Stokes vortices (bearing some resemblance to vortices observed in direct numerical simulations of turbulence) in distorting a flame and influencing its passage through a fluid in non-uniform motion [2]. The vortices used for this purpose are taken from a recently discovered class of exact steady Navier-Stokes solutions [1].

In the second study, the role of vorticity generated at a flame-front is considered in arriving at an approximate evolution equation (which is second-order in time) for a flame's movement. A more general approach is outlined than that used in [3]. Unlike first-order equations which either do not take into account vorticity-production or which neglect its stabilising effect, this equation successfully mimics the process by which an overstretched tulip flame inverts itself. The inversion of such hydrodynamically sculptured flames has been observed to play a major role in some combustion-driven acoustic instabilities [4].