COUPLING BETWEEN VORTICITY AND PRESSURE OSCILLATIONS IN COMBUSTION INSTABILITY

The operation of premixed dump combustors is hindered by large-amplitude, 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 interactions in this system is formulated assuming that the combustor is an acoustically compact trough, the flow is two dimensional, the flame is a thin front, the inlet section is charged by a constant-pressure reservoir, and the exit pressure is forced. Numerical solutions at high Reynolds number are obtained using the vortex method. The nonreacting now exhibits coherent oscillations that scale with the trough depth and inlet velocity to a Strouhal number of O(0.1). Simulations of the reacting flow show that vorticity-flame-acoustic coupling is the driving mechanism for the observed instability. In this work, the exit pressure is 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.