Feedback Control of a Dump Combustor with Fuel Modulation

Closed-loop control tests were performed to suppress the combustion instability of a 0.7-MW dump combustor and to extend its flammability limits. The pressure oscillations originating from the unstable combustion were measured at the dump plane and at the exhaust nozzle. The signals were used as a reference to lock on and to produce an acoustic signal that modulated the fuel flow at a predetermined phase shift relative to them. At a certain range of phase shift angles, the amplitude of the combustion oscillations was reduced to 50% of its unforced level. The control system was most effective when the reference signal was picked up at the dump. The amount of reduction was proportional to the acoustic forcing level, but leveled off for high forcing amplitudes. The effectiveness of the control system was reduced as the mass flow rate of the air was increased. The combustion instability became bimetal with multiple unstable frequencies, and a more sophisticated lock-on and phase-shift system is required to suppress effectively oscillations with more than a single dominant frequency. However, even for the high flow rates, the amplitude of the instability was reduced by nearly 40%.