A FURTHER NOTE ON ACTIVE CONTROL OF COMBUSTION INSTABILITIES BASED ONHYSTERESIS

Hysteretic behaviour, as found in the dump combustor facility at GALCI T, allows nonlinear active control of the instability, demonstrated re cently by Knoop et al. (1996). As in that work, pulses of secondary fu el, based on a simple on/off control law, have been successfully used to drive the transition between the two modes present in the hystereti c region, thereby reducing the amplitude of the pressure oscillations with minimal use of fuel. In order to clarify the origin of the phenom enon, high speed shadowgraph images of the flowfield during the transi tion between 'unstable' and stable burning have been taken, showing di stinctive features that may help in modeling the observed behaviour. A preliminary parametric study (type of injector, duration of pulses, t ype of secondary flow) has also been conducted, showing that the trans ition can be obtained over a broad range of conditions.