Active Combustion Control in a Coaxial Dump Combustor

Suppression of combustion instability in a dump combustor was demonstrated with open-loop and closed-loop active control systems. Pressure oscillations and CH-emission sensors were used to monitor the combustion process. Acoustic drivers were used to modulate the fuel and modify the shear layer at the dump. The open-loop active-control system applied high frequency acoustic forcing to break down the coherence of the large-scale vortical structures, thus reducing the periodic heat release which excites the instability. The closed-loop controller used the CH-emission signal or the pressure fluctuations, after filtration, to lock the acoustic excitation at various relative phase angles. The pressure locking was more effective in suppressing the oscillations at a relative phase angle range of 250-330 ° . The reduced oscillations were observed by both CH and pressure sensors. The CH locking was significantly less effective relative to the pressure locking, but had a similar range of phase angles in which combustion oscillations were suppressed. Hot wire measurements in the cold flow showed that the level of fuel modulation was relatively low (22%) at the range of unstable combustion oscillations; however, combined with the effect on the shear layer at the dump, it was sufficient to effectively modify the combustion and to partially suppress its instability.