Thermo-diffusive instabilities in axisymmetric, non-premixed jet flames

A general linear stability model, including hydrodynamic effect; has been developed for non-premixed axisymmetric jet flames. This stability model can be employed to-investigate several types of instabilities which form in reacting jet flows, including (i) the Kelvin-Helmholtz modes which typically dominate at, relatively high Damkohkler numbers, and (ii) the thermo-diffusive modes which occur for relatively low Damkohler numbers near the extinction limit-the focus of the current study. The linear stability model developed for the reacting jet allows for variation of several important hydrodynamic and combustion parameters, including the velocity and density ratios of the fuel-to-oxidizer streams, the jet Reynolds number Re, and the ratio of the jet radius to the momentum thickness of the mixing layer, Damkohler number, and the initial mixture strength parameter. The jet flume was modelled using a one-step reaction with finite activation energy, and different Lewis numbers, ratios between-the thermal and mass diffusivities, could be specified for the reactants. For jet flames at low Damkohler numbers near the extinction limit, investigations concentrated on the various absolutely unstable thermo-diffusive modes that form. Parameters were chosen that were representative of previous experimental jet name studies. Consistent with previous experimental work, computations reveal that an axisymmetric pulsation is the dominate instability at relatively large reactant Lewis numbers, and cellular modes (single-cell or multi-cell flames) are dominate at relatively low Lewis numbers, The propensity for the cellular instabilities was shown to increase with decreasing reactant Lewis number, initial mixture strength and Damkohler number near the extinction limit, For relatively high initial mixture strength, computations also show that an azimuthal wavenumber m = 1 mode can be dominate at intermediate fuel Lewis number values near the extinction limit. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.