Edge-flames

Abstract

Edge-flames arise in non-premixed combustion, and include the familiar triple, or tribrachial flames. They can exist for all Damköhler numbers for which the upper and lower branches of the S-shaped response of the underlying diffusion flame simultaneously exist, and have negative propagation speeds (corresponding to failure waves) when the Damköler number is close to the quenching value, positive speeds (corresponding to ignition waves) when the Damköler number is close to the ignition value. A previously described one-dimensional model of edge-flames is here applied to a number of new situations. These include: a description of unbounded edge-flames, for unit Lewis numbers, over the entire range of Damköler numbers; a description of unbounded edge-flames when one of the Lewis numbers differs from unity, for which it is shown that propagating edge-flames of stationary structure may not exist; and an analysis of an edge-flame near a wall, without flow between the wall and the flame. In the case of unbounded edge-flames, a simple formula for the edge speed is derived that may be of value in the computation of turbulent combustion fields in the laminar flamelet regime.