Liquid-fueled active instability suppression

Active instability suppression using periodic liquid-fuel injection was demonstrated in a dump combustor. The controller fuel, which made up 12%–30% of the total heat release, was pulsed directly into the combustion chamber, and the injection timing was adjusted with respect to the combustor pressure signal. Because the injection timing determined the degree of interaction between pulsed fuel sprays and periodic large-scale flow features, it significantly affected the spatial distribution of fuel droplets inside the combustion chamber. Simple closed-loop control of the pulsed injection timing was applied to two different cases that developed natural instabilities. In the first case, the instability frequency was unchanged at the onset of the closed-loop control, and this fact allowed up to 15 dB reduction in the sound pressure level. A detailed investigation showed that the pressure oscillation amplitude reached the minimum value when the start of the pulsed fuel injection was synchronized with the inlet vortex shedding process. In the second case, the same controller was applied to a higher output combustor, where the injection timing affected not only the oscillation amplitude but also the instability frequency. For the high output case, the controller was able to suppress the oscillations initially, but it could not maintain the suppressed amplitude, resulting in unsteady modulation of the oscillation amplitude and frequency. The intermittent loss of control was linked to the frequency-dependent phase shift, associated with an electronic band-pass filter. The present results open up the possibility of utilizing direct pulsed liquid-fuel injection for active combustion control in propulsion devices, but they also show the limitation of a simple phase-delay approach in completely suppressing the natural oscillations under certain conditions.