Modal dynamics of self-excited instabilities in a can-annular lean-premixed combustor

Abstract

this observation demonstrates experimentally the existence of a chiral state in can-annular modal dynamics. The present results, for the first time, reveal a variety of phenomena involved in the response of a can-annular combustion system to higher frequency acoustic perturbations. Although can-to-can acoustic interactions have a prominent role in the development of modal dynamics and pattern formations in can-annular gas turbine combustion systems, the impact of unconventional pairing caused by the spatial asymmetry of a network with an odd number of combustors remains unclear. To address this fundamental question, we compare key aspects of thermoacoustic instabilities in four-coupled (N = 4) and five-coupled (N = 5) can-annular combustor configurations for uniform and non-uniform equivalence ratios. Using integrated analyses of experimental data and FEM-based Helmholtz simulations, we show that the modal dynamics of the even-N case are described by well-defined rotationally symmetric interaction patterns. Due to the presence of an unpaired combustor, however, the eigenmodes of the odd-N can-annular system are governed by regular two-fold degeneration, making the system more vulnerable to simultaneous excitation of multiple eigenmodes. This leads to the predominance of superposition modes, and in stark contrast to the behavior of the even-N system, their phase dynamics cannot be unequivocally determined to be either in-phase or out-of-phase interactions between networked combustors. Despite the formation of remarkably different inter-combustor interactions in the two cases, however, the entire set of instability data collapses into three distinct frequency regimes, suggesting the role of flame-acoustic interactions occurring within each combustor, namely intra-combustor interactions.

The prediction of self-excited combustion instabilities in a can-annular gas turbine combustion system is a significant challenge, mainly because the instabilities originate from the acoustic interactions between adjacent combustors via a cross-talk region upstream of the first stage turbine nozzles. Detailed characterization of these instabilities requires a thorough understanding of engine-level dynamics. Until now, a comprehensive experimental examination of such a can-annular configuration had not been conducted. Here we present new experiments using four lean fully-premixed swirl-stabilized combustors connected via a full-annular cross-talk section. We demonstrate that the global fluctuations at limit cycles are either in-phase interactions (push-push modes) or one of two different forms of out-of-phase interactions (push-pull modes), subject to even and alternating equivalence ratio combinations. Under certain even conditions, the can-annular system undergoes in-phase synchronous modulations (Type I), giving rise to the formation of pressure antinodes at the inlets of the four combustors and in the cross-talk region. By contrast, out-of-phase interactions are sustained in the form of either an alternating pattern in four-coupled combustors (Type II) or a push-pull interaction in two opposite combustors only (Type III). The latter is dictated by strong out-of-phase fluctuations between two of the combustors and a pressure node-like condition – thermoacoustically decoupled from the global fluctuations – over the entire region of the other two combustors, experimentally demonstrating the existence of mode localization in can-annular thermoacoustic instabilities. We show that the mode clustering phenomenon is responsible for the excitation of closely-spaced multiple eigenmodes in the can-annular acoustic environment, and as a consequence the system can feature a mixed state with several distinct types of interaction patterns. By analyzing a large amount of experimental data acquired systematically for coupled two-combustor and four-can-annular configurations, we demonstrate that longitudinal-mode instabilities in a can-annular combustion system will preferentially emerge in the form of out-of-phase interactions. The impact of rotational asymmetry on can-annular modal dynamics is still a largely open subject

here we use the same experimental setup to perform multiple independent analyses of the effect of broken rotational symmetry on large-scale pattern formations and modal dynamics of multiple eigenmodes. We demonstrate that the presence of rotational asymmetry can lead to fundamentally different dynamic states, including pairwise push-pull modes, spinning azimuthal instabilities, superposition modes, and strong mode localization, which are totally absent under rotationally symmetric conditions. Of particular importance is the experimental observation of spinning azimuthal instabilities in the annular cross-talk section, which are acoustically separate from flame-dynamics-driven standing wave motion in the flame tube sections. This leads to the coexistence of traveling and standing waves in the can-annular system. In conjunction with FEM-based Helmholtz simulations, we determine the origin of the azimuthal modes to be the simultaneous excitation of two different pairwise push-pull interaction modes at the same frequency – a phenomenon known as degenerate eigenmodes. Previous experimental and numerical studies have focused on identifying the formation of large-scale interaction patterns and the modal dynamics of multiple eigenmodes. Since those investigations were primarily concerned with low-frequency interactions between adjacent combustors, there are currently no experimental observations that enable decisive discrimination between low- and high-frequency can-annular combustion instabilities. Here, we use pure hydrogen-air flame ensembles to trigger higher acoustic modes in four-coupled lean-premixed combustors, ultimately to understand the potential influence of self-excited instabilities on the spatiotemporal evolution of a can-annular system. The use of lean-premixed hydrogen-air flames enables measurements of previously unidentified phenomena, particularly in association with the excitation of high acoustic modes up to approximately 1.3 kHz. We demonstrate that self-excited standing azimuthal modes can be excited in the annular cross-talk section, particularly when the phase dynamics of the upstream flame tube sections are defined by alternating anti-phase oscillations (Type II). In this case, the temporal evolution of the can-annular system is governed by twofold degeneracy, incorporating an alternating push-pull mode in the longitudinal direction and a standing azimuthal mode in the circumferential direction at the same frequency. Based on experimental observations and Helmholtz simulations, we also show that a mixed state of synchronization and desynchronization can arise simultaneously as a result of symmetry breaking. The coexistence of coherent and incoherent motions is observed to be controlled by interactions between two closely spaced, but slightly misaligned, localized in-phase modes