New instability modes of a diffusion flame near extinction

Abstract

The linear temporal stability of a diffusion flame, modeled with a simplified one-dimensional approach, in a two-dimensional mixing layer is investigated. In addition to the well-known “shear” or Kelvin–Helmholtz mode, which is also present in nonreacting flows, the current analysis reveals new “heat release” or combustion modes for flames near the extinction limit, i.e., when the Damköhler number is low. For conditions near extinction, the shear mode is found to be damped and only the combustion modes are unstable. The fundamental destabilizing mechanism of these combustion modes is elucidated.