Mutual synchronization of lean-premixed model gas turbine combustors due to cross-talk interactions

Abstract

In a can-annular gas turbine combustion system, low-frequency self-excited combustion instabilities can arise from coupled interactions between adjacent can combustors, rather than from individual combustors. These interactions—in either a push–push or a push–pull mode –are caused by the presence of a cross-talk area upstream of the first stage turbine stator vanes. The mechanisms governing the selection and growth of the interaction modes are not well understood, particularly for systems with multiple oscillators (combustors). In this research, experimental observations of mutual synchronization between two coupled-combustors, each containing a lean-premixed swirl-stabilized turbulent flame, subjected to symmetric and asymmetric inlet boundary conditions are described. Our results reveal that when two combustors oscillating at different natural frequencies are connected via a cross-talk area, they can become mutually synchronized with each other, exhibiting global oscillations at a common frequency. The presence of cross-talk communication can excite strong oscillations in the coupled dual-combustor system, even when each combustor is individually stable in isolation. By contrast, for certain asymmetric inlet boundary conditions, coupling the two combustors together stops them from oscillating completely, even though each combustor in isolation oscillates in a large-amplitude limit cycle. In nonlinear dynamics, such mode suppression is known as amplitude death and is a classical feature of coupled self-excited oscillators, with potentially wider applications for passive control of combustion instabilities. A large set of experimental data are analyzed in non-dimensional domains, and it is demonstrated that the coupled mode is not defined by the phase difference between the heat release rate oscillations of the two combustors, but rather by the phase difference between their acoustic pressure fluctuations. Mode selection is found to be strongly correlated to the degree of asymmetry in the magnitude of the flames’ heat release rate oscillations in adjacent combustors. As well as providing the first experimental evidence of amplitude death in a combustion system, this study reveals a previously unrecognized role of combustor-to-combustor interactions in provoking self-excited combustion instabilities in multiple-combustor gas turbine systems. The influence of non-identical flame transfer functions (FTFs) between adjacent combustors on the development of self-excited thermoacoustic oscillations was investigated experimentally. To create different FTFs, five different swirl nozzles are used, one with high swirl (HS) and four with low swirl (LS), all with different porosities. It was found that, compared with the LS FTFs, the HS FTF exhibits a smaller and flatter gain as well as a smaller phase difference. This behavior is attributed to differences in the flame structure and the stabilization mechanisms, namely an inner shear layer-stabilized diverging front in the HS case versus a detached reaction zone in the presence of a central jet with an outer swirl flow in the LS cases. Using two tunable lean-premixed combustors connected via a cross-talk section, it is shown that ( i ) symmetric FTF combinations (HS + HS or LS + LS) produce in-phase interactions, leading to push-push modes, but that ( ii ) asymmetric FTF combinations (HS + LS) produce out-of-phase interactions, leading to push-pull modes. Phase-resolved visualization of the asymmetric cases reveals that the inner shear layer-stabilized HS flame exhibits large angle fluctuations, whereas the aerodynamically stabilized LS flame is characterized by the periodic emergence of a bow-shaped front and an oval structure. For all the conditions tested, asymmetry in the FTFs leads to either ( i ) a completely stable state with negligible amplitude or ( ii ) a mildly unstable state with an amplitude lower than that of the equivalent symmetric cases. The influence of non-identical FTFs was also tested in a full-annular cross-talk environment with four combustors in a variety of nozzle arrangements consisting of two injectors (HS and LS04), i.e. (HS, HS, HS, HS), (LS, HS, HS, HS), (LS, LS, HS, HS), (LS, HS, LS, HS), (LS, LS, LS, HS), and (LS, LS, LS, LS). Self-excited dynamics of four-couple combustors exhibited instability modes with a variety of phase patterns that are governed by either in-phase interaction leading to push-push mode, or out-of-phase interaction leading to alternating/2-can/pairwise push-pull modes. In particular, the pairwise arrangement of the injectors led to the excitations of alternating and pairwise push-pull modes, while the alternating arrangement promoted the excitations of 2-can push-pull modes.