The effect of variable fuel staging transients on self-excited instabilities in a multiple-nozzle combustor

Combustion instability in gas turbine engines is often mitigated using fuel staging. Fuel staging, sometimes referred to as fuel splitting, is a strategy by which fuel is unevenly distributed between different nozzles of a multiple-nozzle combustor. These fuel splits are conducted in a transient manner in real engines, and the effects of these transients on instability are not well characterized. This work fills this gap by systematically studying the effects of transient fuel staging on self-excited combustion instability by varying the amount of staging fuel (staging amplitude), timescale in which the fuel is added (transient duration), and whether staging fuel is added or subtracted (transient direction). In this work, three staging amplitudes, five transient durations, and both transient directions are considered. The transient timescales are broadly divided into “short” duration transients, which have fuel delivery timescales shorter than the characteristic instability decay or onset timescales, and “long” duration transients, which have fuel delivery timescales longer than the characteristic instability decay or onset timescales. For short duration transients, we find the instability decay timescale depends on staging amplitude but does not depend on transient duration. For long duration transients, we find the instability decay timescale does not strongly depend on staging amplitude. The instability onset timescale is found to be longer and more variable between runs than the instability decay timescale for a given fuel delivery timescale. The onset timescale is also longer in duration and more variable than the decay timescale at a given fuel delivery timescale, implying that the instability rise process is overall more variable and slower than the instability decay process. Analysis of combustor damping rates show a strong dependence of damping rate on staging amplitude but no strong dependence on transient duration or direction. Instantaneous phase difference images between p′ and q˙′ are used to differentiate regions in the combustor that have constructive versus destructive interference between heat release rate oscillations and pressure fluctuations. The phase images show that p′ and q˙′ become in-phase early in the transient for the onset transients.