Experimental Studies of Active Combustion Control

摘要

Lean burning combustors are needed to conform to low emission requirements but they are prone to instability. Combustion instability is a serious and complex phenomenon which affects the performance of propulsion systems including solid and liquid propellant rocket motors, ramjets and turbo-jet thrust augmentors. It primarily occurs due to the complex coupling of the acoustic resonances with the fluctuating heat release in the combustion chamber which manifests itself as large amplitude pressure oscillations. These pressure driven oscillations may cause intense vibrations, structural damage and high burn rates. Due to its ability to inflict serious damage on the performance parameters of the propulsion system, it is mandatory to somehow reduce the magnitude of these oscillations during the design stage. In the past, the control and suppression has been addressed by introducing passive schemes. The most fundamental notion of active control of combustion instability is the Rijke tube phenomenon. Simple demonstrations using Rijke tube have shown that due to varying positions of heat source within the tube, loud noise (called screech, buzz, etc) is emitted. This is augmented by Rayleigh's criterion which says that self-excitation of a combustion system occurs if the fluctuations in heat release from the combustion process are in phase with the pressure fluctuations. It primarily focuses how the damping of thermo-acoustic instability can be achieved. A common approach is to employ sources which extract energy from perturbations and release heat out of phase with pressure fluctuations. Since combustion instability is largely system dependent, the theoretical foundation and the employment of a particular active control scheme varies from one application to the other. The present paper describes the various types of active measures employed to control instabilities and presents a mathematical model for the same.