Interaction of vortex shedding and transverse high-frequency pressure oscillations in a tubular combustion chamber

Intense research on the thermoacoustic stability of premixed gas turbine combustors in the past two decades has led to an improved understanding of instabilities of longitudinal modes in the sub-kHz range and predictive tools for thermoacoustic stability analysis have also been developed. Circumferential modes in annular combustors have been studied in the past as well, even though to a much lower extent due to the high experimental effort. Combined experimental-numerical methods for the low-frequency regime (i.e. acoustically compact flames) are widely used. However, such experimental and numerical approaches with predictive capability have to be developed to also address the high-frequency (HF) regime. An experimental study of HF thermoacoustic coupling is presented in this paper. A fully premixed swirl-stabilized flame at atmospheric condition in a cylindrical combustion chamber is investigated. The test rig is equipped with several dynamic pressure transducers to identify and reconstruct the acoustic field in the combustion chamber. Planar information about the flame front location is obtained from Mie-scattering and the flow field is measured with particle image velocimetry (PIV). In the tests the first transverse mode of the combustion chamber exhibits instability for a particular operating condition, which leads to sustained limit-cycle pulsations. Mie-scattering images reveal periodic vortex shedding at the outlet of the burner. PIV results provide quantitative information on the strength of these coherent shear layer vortices.