Session: 23-08 Last Stage Blades and General Design Aspects of Steam Turbines

Paper Number: 80368

80368 - Prediction of Rotating Instabilities in Low Pressure Steam Turbines Operating at Low Load 

Due to the increasing need for operational flexibility nowadays, the low-pressure (LP) steam turbine might face widely variable operation conditions up to extreme low load flow situations. Hereby, rotating instabilities (RIS), well known from compressor aerodynamics, can initiate non-synchronous blade excitations which can lead to increased vibrations of the last stage moving blade (LSMB). This paper describes a numerical method able to be used in an applicable way for blade designing.

To judge the onset and the shape of RIS a numerical approach based on a transient 3D URANS flow computation of the single passage L-1 stage, a full row L-0 stage and an axis-rotational diffuser was established. Mesh size, solver settings and other parameters were optimized to reduce the computational effort without losing prediction accuracy.

The dominant harmonic compound of a spatial Fourier transform of all monitored LSMB blade forces over the blade row’s circumference reveals the stall cell count and excitation amplitude for each time step. The transient change of the corresponding phase lag provides the stall cell speed; and multiplied with the stall cell count the excitation frequency. Their distribution over decreasing flow rates finally displays the likely onset of RIS.

The approach is capable to run the analyses of several load conditions within a couple of days and was successfully validated using measured blade vibrations and unsteady pressure probes in multistage test turbines. Furthermore, it is shown that the approach can be used to find LP blade designs where the effects of RIS can be significantly suppressed.

Presenting Author: Oliver Pütz Siemens Energy

Presenting Author Biography: Name: Oliver Puetz<br/>Date of Birth: 06 May 1967<br/>University: Ruhr University, Bochum, Germany<br/>Ph D Thesis: Experimental and numerical investigations of the flow fields of a 4-stage axial compressor of industrial design with considerations of rotor stator interactions in the inlet stage<br/>Current position: Senior research engineer (16 years at Siemens Energy)

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