  The effects of external forcing on a turbulent, liquid-fuelled,
swirl-stabilized gas turbine combustor operating at a pressure of approximately
1 MPa are explored experimentally. In particular, the dynamics and coupling
between the hydrodynamics, heat release rate and acoustics are compared for
various forcing amplitudes at a fixed forcing frequency $f_f$. The
hydrodynamics were characterized via laser Mie scattering from droplets in the
fuel spray, while the heat release rate was qualitatively measured using
chemiluminescence (CL) emissions in the 312 $\pm$ 12.5 nm wavelength range,
both at 10 kHz. The dynamics at the frequencies of interest were extracted
using spectral proper orthogonal decomposition (SPOD). In the unforced case,
the spray and CL oscillations exhibited similar dynamics, dominated by
oscillations at frequency $f_0$, whereas the pressure fluctuations were
predominantly at $f_P$. As the forcing amplitude was increased from zero, the
spray and CL exhibited changes in their power spectra characteristic of the
suppression route to synchronization. The pressure fluctuations, however, were
observed to follow the phase-locking route to synchronization. In contrast with
expectations from synchronization theory, the amplitude of the pressure
fluctuations increased significantly not only after lock-on, but also as the
frequency detuning with $f_f$ decreased. It is shown that this increase in
amplitude is not due to intermittency in the frequency of the pressure
oscillations. The simultaneous occurrence of phase-locking and suppression
illustrates the rich variety of dynamics that can occur in practical combustor
systems. In addition, the amplification of the pressure oscillations based on
the frequency detuning with the forcing suggests that classical reasoning based
on the Rayleigh Index may not be sufficient to understand the high amplitude
behaviour of multimodal systems.
