Acoustics of can-annular combustors: Experimental characterisation and modelling of a lab-scale multi-can setup with adjustable geometry

Abstract

In can-annular combustors, a narrow can-turbine junction leads to an acoustic coupling between the cans, yielding unique features that greatly impact the (thermo)acoustic response of these geometries. To investigate these features, we designed and assembled a modular lab-scale can-annular system equipped with 12 cans connected downstream by a short annulus, creating an annular gap that models the can-turbine junction. The acoustic response of the setup is measured experimentally for various gap heights on a broad range of frequencies. We observe that eigenfrequencies form clusters, confirming recent theory. To aid the analysis of the experimental results, a low-order model is proposed, which has analogies with methods recently developed for studying azimuthal modes in annular combustors using azimuthal averaging. Our model accounts for the fact that the azimuthal structure of the modeshapes is not purely sinusoidal, and for the influence of the can’s first transverse mode. We observe that the eigenfrequencies obtained experimentally are well predicted by the low-order model. The experimental and low-order model results are then compared to 3D Helmholtz calculations, which allow us to visualise the 3D acoustic modeshapes, assess the validity of the assumptions used in the low-order model, and test the low-order model beyond the parameter range achievable in our experiments. Last, we discuss how the behaviour of the low-frequency clusters can be predicted qualitatively without calculations. Our experiments are the first to characterise the presence of clusters of eigenvalues in the acoustic response of a can-annular setup as a function of the can-turbine junction size.