Self-sustained acoustic-wave interactions with counterflow flames

摘要

The interaction of acoustic waves with a planar counterflow flame is investigated numerically employing a detailed kinetic model and one-step global kinetic models. The mathematical formulation of quasi-one-dimensional fully unsteady laminar counterflow flames is presented and the governing equations are integrated numerically based on a MacCormack predictor-corrector scheme with second-order accuracy in space. Navier-Stokes characteristic boundary conditions are implemented to accurately represent perfect and partial reflection of acoustic waves at the boundaries. For well-resolved simulations, the occurrence of self-excited flame-acoustics instabilities is analysed in both non-premixed and premixed flames for a range of flow strain rates and flame locations, and employing two finite-rate kinetic models. Unlike the detailed kinetic model, one-step global models with large activation energy and overall reaction order greater than unity promote the amplification of acoustic pressure fluctuations in counterflow non-premixed flames. In contrast, premixed counterflow flames exhibit flame-acoustics instabilities with both kinetic models. While previous unsteady counterflow studies required external perturbations, the resonant unsteady phenomena predicted in this study are self-sustained under favourable boundary conditions. Detailed analyses of the characteristic time scales associated with convection, diffusion, chemistry and acoustics are presented to provide a better understanding of the exact coupling mechanisms.