COUPLING BETWEEN VORTICITY AND PRESSURE OSCILLATIONS IN COMBUSTION INSTABILITY

The operation of premixed dump combustors is hindered by large-amplitu de, low-frequency oscillations leading to flame flashback. The genesis of this instability is the subject of this article. A physical model that accounts for the relevant components of flow-combustion interacti ons in this system is formulated assuming that the combustor is an aco ustically compact trough, the flow is two dimensional, the flame is a thin front, the inlet section is charged by a constant-pressure reserv oir, and the exit pressure is forced. Numerical solutions at high Reyn olds number are obtained using the vortex method. The nonreacting now exhibits coherent oscillations that scale with the trough depth and in let velocity to a Strouhal number of O(0.1). Simulations of the reacti ng flow show that vorticity-flame-acoustic coupling is the driving mec hanism for the observed instability. In this work, the exit pressure i s modulated near the frequency of the natural mode, and the amplitude of oscillation is observed to grow with increasing heat release due to phase-matched coupling between flow and heat release oscillations, in accordance with the Rayleigh criterion. This amplification ultimately leads to now reversal and the propagation of the flame into the inlet channel.