Experimental Study of Thermo-Acoustic Instability Triggering in a Staged Liquid Fuel Combustor Using High-Speed OH-PLIF

A staged injector developed by JAXA and fueled with kerosene is studied in a highpressure combustion experiment. With a stable pilot fuel flow rate, the fuel flow rate in the main stage is progressively increased. A high-speed OH-planar laser-induced fluorescence (PLIF) system is used to record the flame motion at 10,000 fps. In the beginning of the recording, the flame behavior is dominated by relatively low-frequency rotation due to the swirling motion of the flow. These rotational motions then coexist with a thermoacoustic instability around 475 Hz which increases the amplitude of the pressure fluctuations inside the chamber. Dynamic mode decomposition (DMD) analyses indicate that this instability is associated with a widening of the flame occurring when the pressure fluctuations are the highest, giving the instability a positive feedback. The instability frequency then abruptly switches to 500 Hz, while the mode shape remains the same. This frequency change is studied using time-frequency analysis to highlight a change in the feedback mechanism characterized by a modification of the time delay between pressure and heat release fluctuations.