Turbine Aerodynamic Low-Frequency Oscillation and Noise Reduction Using Partial Shrouds

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Turbine Aerodynamic Low-Frequency Oscillation and Noise Reduction Using Partial Shrouds

Rebholz, Patrick S

The design of highly efficient low-pressure gas turbines in compliance with strict emission regulations for pollutants and noise, as well as long component lifetime, requires the detailed knowledge of potential excitations in the flow. The analysis of fast response measurement data from a 1.5-stage low-pressure axial turbine in the frequency domain shows multiple relevant excitation and noise sources in the flowfield, in addition to the primary rotor–stator interaction. Especially nonsynchronous low-frequency modes at 10% of the blade-passing frequency, occurring in the tip shroud exit cavity, are found to communicate with the main flow and are amplified through the downstream stator row by a flow instability in the interaction area of the passage vortex and boundary layer. Full-annular unsteady computational fluid dynamics simulations are carried out in addition to the experiments to probe the origin of the cavity modes. They predict the rotor–stator interaction well, but they struggle to resolve low-frequency oscillations in the cavities. Out of the four tested configurations, a shroud trailing-edge cutback is the most efficient option to prevent the formation of the cavity modes. The tradeoff for a low-frequency fluctuation reduction of 12 dB using a trailing-edge partial shroud is a total–total stage efficiency drop of 0.7%.