Intrinsic aeroacoustic instabilities in the crosstalk apertures of can-annular combustors

Abstract

This paper presents an experimental and numerical study of aeroacoustic instabilities at the interface between neighbouring combustion chambers in modern heavy-duty gas turbines. A simplified laboratory-scale geometry of the gap separating the outlet of these chambers, just upstream of the turbine inlet in can-annular combustor architectures, is considered. It consists of two channels with anechoic and choked conditions on the upstream and downstream sides, respectively. Right before the choked-flow vanes which represent the turbine inlet, a small aperture leads to an aeroacoustic crosstalk between the channels. The dimensions and flow conditions are defined such that relevant Mach, Strouhal and Helmholtz numbers of gas turbines are reproduced. The alignment of the vanes with respect to the crosstalk aperture is varied. An intense whistling is observed for some conditions. The oscillation frequency depends on the aperture area and scales with the Strouhal number based on the aperture length. The upstream anechoic condition in each channel implies that no longitudinal acoustic mode participates to the mechanism of this whistling, which is in agreement with the Strouhal scaling of this intrinsic aeroacoustic instability. It is shown that the geometry of the upstream edge of the aperture is an essential element in the occurrence and intensity of the whistling. Compressible Large Eddy Simulations of the configuration have been performed and remarkably reproduce the whistling phenomenon. A detailed investigation of the numerical results revealed the region of sound production from the shear layer oscillations in the crosstalk aperture. This work contributes to the understanding of aeroacoustic instabilities at the crosstalk apertures of can-annular combustors. It will help designing combustor–turbine interfaces to suppress them, which is important since the vibrations they induce may be as damaging as those from thermoacoustic instabilities.