Instabilities of diffusion flames near extinction

• Published in: Combustion theory and modelling Vol. 7; no. 4; pp. 603 - 633
• Main Authors: Papas, Paul, Rais, Redha M, Monkewitz, Peter A, Tomboulides, Ananias G
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 01-12-2003
• ISSN: 1364-7830; 1741-3559
• DOI: 10.1088/1364-7830/7/4/301

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.