Influence of a Rib in the Diffuser of a Low-Pressure Steam Turbine on Aerodynamic Excitation at Part Load Operation

Damian M Vogt,Fabian F Müller,Jens Aschenbruck,Philipp Doll,Stefan Schippling

doi:10.1115/gt2023-104100

Damian M Vogt, Fabian F Müller + Show 3 more

https://doi.org/10.1115/gt2023-104100

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Abstract In recent years, the requirement profile for steam turbines used for power generation has changed significantly. In addition to highly efficient, eco-friendly and continuously available electricity generation, further demands have become more and more important: flexible operation in an extended operating range, compensation for short term fluctuations caused by renewable energies, and improved efficiency at part load. Due to the more flexible operation, steam turbines operate frequently at part load. At such operating conditions, excitation phenomena can occur in the flow field, potentially leading to forced vibrations of the last stage rotor blades. A possible phenomenon in this context are rotating instabilities (RI). RI are vortex structures moving circumferentially at a certain frequency in the axial gap between guide vane and rotor blade triggered by flow separations in the blade path. In the first part of the paper, excitation phenomena at part load operation are experimentally investigated in a model steam turbine. The outer casing of the turbine is equipped with unsteady pressure sensors at various circumferential positions upstream and downstream of the last stage rotor blades to identify operating conditions at part load where RI occur. The radial location and extent of the RI are determined by means of unsteady pressure probe traverses in the axial gap between guide vane and rotor blade. In the second part of the paper, the influence of a supporting rib in the close vicinity to the last stage blade row on the location and intensity of the RI is investigated both quantitatively and qualitatively. The occurrence and appearance of RI at various operating conditions is assessed from test data for both the setup without and with supporting rib. Based on the evaluation of this test data, it is shown that the pressure excitation caused by RI is significantly reduced by the rib.

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