For annular combustors, the interactions between different modal structures (longitudinal, azimuthal spinning, standing, and slanted) remain a challenge in ongoing studies of thermoacoustic instabilities. The present work develops an annular combustor named TurboCombo, similar to the multiple-injector annular combustor named MICCA, but with a convergent turbine section connected to the combustor outlet. A Helmholtz solver provides a proper estimate of the modal frequencies and reveals the spatial structures for both two rigs. The experimental results present the consistencies of frequency domains between the acoustic pressure signals and the hydroxyl (OH^*) radicals emission signal. The high-speed flame images are processed by the dynamic mode decomposition method, statistically showing longitudinal, azimuthal spinning, or standing nature in different cases. The stability maps of the two combustors show slight differences under atmospheric conditions. In the TurboCombo rig, there are two types of interaction behaviors between the 1/4 wave longitudinal mode of the chamber and the 1st order azimuthal mode of the plenum. Unlike the prevalent “dual-mode coexistence” characteristics, an interesting behavior of “intermittent switching” is revealed, which manifests as conspicuous two-way jumps between the two modes with different frequencies. Such intermittent switching occurs randomly between two stable limit cycles in a long-time observation without any external excitation, but is driven by the turbulence-induced stochastic process. Moreover, there still exist some weak azimuthal oscillations while the longitudinal mode is dominated. The standing or spinning bias of the azimuthal mode also features intermittent transition, corresponding to the occasions dominated by longitudinal mode or azimuthal mode, respectively.

Combustion instability is one significant issue in the development of aero-engines and gas turbines. In practical combustors, the exit boundary of the combustion chamber is complex and the mechanisms of thermoacoustic coupling become more complicated. In order to study the effect of outlet boundary on combustion instabilities, the premixed swirling combustor was built and several different damping rings were adopted to obtain the outlet contraction of combustion chamber. Acoustic impedances corresponding to different configurations were measured in cold flow, and reactive experiments were conducted in different conditions. The results show that combustion instabilities were tightly related to both the equivalence ratio and exit boundary. The quarter wave mode would be triggered at a certain interval of equivalence ratio and suppressed in different levels while applying various damping rings. The outlet contraction left an effect on both the frequency and amplitude of thermoacoustic mode. It affected the resonant frequency by influencing the gas temperature and modifying the acoustic boundary. Oscillation amplitudes were weakened due to the change in acoustic energy generated from unstable heat release. Moreover, the helical mode corresponding to the precessing vortex core was detected, which was also affected by the outlet boundary. These results are rarely reported in previous work and may provide useful ideas in the prediction and control of combustion instabilities.