Cross-frequency coupling during thermoacoustic oscillations in a pressurized aeronautical gas turbine model combustor

Abstract

This paper demonstrates cross-frequency coupling between pressure, heat release rate, fuel spray and velocity oscillations in a model aeronautical gas turbine combustor operating at a pressure of approximately 10 atm. Heat release rate was characterized by 10 kHz chemiluminescence (CL) imaging of several species. Stereoscopic particle image velocimetry and laser Mie scattering from the fuel droplets were used to measure the gas velocity and spray dynamics, respectively, at 5 kHz. The pressure fluctuations were dominated by oscillations at a frequency f0, whereas the spray, CL and velocity oscillated at approximately 2f0. All oscillations were non-stationary, exhibiting temporal changes in frequency and amplitude. Comparing the time evolution of the dominant frequencies and amplitudes indicated a behaviour consistent with mutually coupled and unsynchronized self-oscillators; the observed dynamics are consistent with variations in reactive coupling strength. Specifically, increases in the frequency of the ca. 2f0 velocity oscillations away from the 1:2 harmonic ratio (increased frequency detuning) were correlated with decreases in the power of the f0 pressure oscillations. The corresponding nonreacting flow had a natural hydrodynamic mode at a frequency slightly greater than 2f0. Hence, the data are consistent with the f0 acoustic mode ‘pulling’ the hydrodynamic frequency towards the 1:2 harmonic ratio.