Dynamics and mechanisms of pressure, heat release rate, and fuel spray coupling during intermittent thermoacoustic oscillations in a model aeronautical combustor at elevated pressure

Dynamics of thermoacoustics oscillations occurring in a liquid fueled aeronautical gas turbine model combustor burning Jet A fuel were investigated experimentally at a pressure of approximately 10 bar. Data was acquired using 5  kHz repetition-rate stereoscopic particle image velocimetry (S-PIV) for both gas phase and fuel droplet velocities, 10 kHz repetition-rate OH* chemiluminescence (CL), and a variety of pressure transducers. Methods for addressing challenges in the application of PIV at these conditions are presented. Analysis of the pressure and CL data showed two coexisting thermoacoustic modes at Strouhal numbers of St ≈ 0.3 and 0.8, both of which exhibited intermittent changes in the oscillation amplitudes. The spatial distribution of the transient pressure-heat release rate coupling, i.e., Rayleigh index, demonstrated repeated dynamics during intermittent oscillations. Specifically, different combustor regions added and/or removed energy from the oscillations at different times. For the tested experimental conditions, the gas phase velocity did not feature any detectable coherent oscillations. However, the fuel droplet velocities in the immediate vicinity of the combustor dump plane exhibited oscillations with a similar intermittent spectral signature as the pressure and CL, indicating coupling through oscillations in the fuel. To further investigate the fuel coupling, the laser scattering signal from the fuel droplets was evaluated. Coherent oscillations in the fuel droplet scattering persisted over the entire length of the fuel spray, which is consistent with an oscillating fuel supply being convected by a non-oscillating air supply for the conditions studied here. The amplitude of the total fuel droplet scattering oscillations was linearly correlated with that of the pressure oscillations. As the pressure amplitude increased, the droplet and pressure oscillation cycles became more coherently out-of-phase, which is hypothesized to lead to the observed intermittent behavior.