Instabilities of diffusion flames near extinction

Abstract

The linear spatio-temporal stability of a diffusion flame, represented by a simplified one-dimensional model, located in a mixing layer is investigated. The analysis focuses on recently discovered `heat release' or combustion modes reported for flames near the extinction limit, i.e. for low Damköhler number. Numerical simulations of the two-dimensional linearized impulse response are performed to uncover the convective versus absolute nature of these combustion modes. To complement these two-dimensional simulations, the convective–absolute transitions of these modes are confirmed with spatio-temporal linear stability calculations. The effects of initial reactant temperature, flow shear Reynolds number, as well as low fuel Lewis number, are explored. In addition to the Kelvin–Helmholtz mode, the generalized model predicts a variety of instabilities near the extinction state, such as travelling and stationary cellular modes, zero wavenumber instabilities or `pulsations', and coupled hydrodynamic-combustion modes. The results elucidate the fundamental destabilizing mechanisms for these near-extinction flames and their relationship to previous work.

Additional information

Notes on contributors

Ananias G Tomboulides

Present address: Aristotle University Thessaloniki, 50100 Kozani, Greece.