Mechanisms of Combustion Instability in Premixed Low NOx Combustors: CFD Simulation

The identification of the mechanism able to sustain thermo-acoustic oscillations have emerged as a critical issue for the development and running of premixed low NOx combustors but it is still one of the most difficult task. In this paper the use of two different formulations of the Rayleigh criterion is illustrated in application to a CFD model of a low NOx combustor. It is shown that the general formulation of the Rayleigh criterion furnishes the necessary condition for sustained stable oscillations, but it does not allow for a distinctive identification of the exciting mechanism. A more specific formulation in terms of time-lag can help to identify system-type exciting mechanisms but cannot exclude other possible exciting mechanisms. The possibility of the occurrence of a thermo-kinetic mechanism, still able to excite the transfer of combustion energy into acoustic energy, but unable to satisfy the time-lag formulation of the Rayleigh criterion, is demonstrated.