3-D Time Domain Unsteady Computation of Rotating Instability in Steam Turbine Last Stage

The rotating instability phenomenon in a last stage of steam turbines at low mass flow conditions has been previously identified experimentally. Recently, the rotating instability has also been numerically studied in a whole annulus domain on 2D blade sections. In the present work, 3D simulations of unsteady flows are carried out on two model steam turbines over a range of mass flow conditions. The pressure-ratio volume-flow characteristics in rotor row tip region under different flow conditions are well captured in the computations in comparison with the experiment. The effect of blade scaling is examined to identify the influence of changing blade counts for a circumferential domain reduction, showing relatively small effects on the overall performance characteristics. The present 3D unsteady solutions on a reduced multi-passage domain have been able to predict a rotating instability in the rotor blade tip region, in accord with the corresponding experiment. Further Fourier analysis is carried out to examine the frequency pattern and spatial modal features. The 3D flow behavior is highlighted by comparison between the 3D and 2D calculations. The present results seem to suggest that the rotating instability onset in the rotor tip region is largely independent of the large scale flow separation in the downstream diffusor.© 2012 ASME