A review of active control of combustion instabilities

Combustion instabilities in modern high-performance propulsion systems are often manifested as large amplitude pressure oscillations and can result in serious performance degradation. These pressure oscillations are often coupled with oscillations in heat release as well as oscillations in the combustor through-flows. The coupling between various oscillatory phenomena has often been traced to an acoustic resonance of a system component which acts as a host for the coupled oscillations. Other phenomena which can lead to unstable oscillations have been identified as well, these include naturally occurring hydrodynamic instabilities and convectively coupled oscillations. Due to their potential harm to system performance, it is often necessary to find ways to reduce the magnitude of these oscillations in the course of developing a new combustion system. Historically, the control and suppression of combustion instabilities has been achieved through hardware design changes. These modifications have included changes in the fuel delivery system, including feedlines and pumps, changes in the fuel injection distribution pattern, and modifications to the combustor or the combustor liner geometry. In general, these design modifications have been made in an attempt to change the resonant behavior of the combustion system so as to avoid the resonantly coupled oscillations which lead to combustion instability. In recent years, much attention has been focused on the control and suppression of combustion instabilities by actively and continuously perturbing certain combustion parameters in order to interrupt the growth and persistence of resonant oscillations. The strategies used in this field of active control vary greatly in nature, both with respect to the theoretical basis for the control system and the system hardware which is employed. The purpose of this paper is to present a discussion of different methods which can be used to suppress combustion instabilities using active control, as well as to give a review of the work which has recently been performed in this area of combustion research.