ALLEVIATION OF ROTATING PRESSURE OSCILLATIONS IN THE LAST LP TURBINE STAGE DURING LOW-LOAD CONDITIONS

In Steam Turbines, under low flow conditions, the flow structure on;the long last stage blades is complex. The rotor blades create outward radial flow. Recirculations are setup near the tip in the gap between the fixed and moving blades, and near the hub downstream of the moving blade. The blade carries negative loading and encounters gross flow separations. In this environment, fluctuations in pressure are detected rotating at about half of the rotor speed. Some similarities exist with rotating stall, as found in compressors. In the validation of a new blade design, checks are therefore included to ensure that the rotating excitation does not pass over a natural frequency of the blading. In turn, this can reduce the available design space. A less restrictive approach is to consider alleviation techniques. A promising candidate is a scheme where steam jets are directed into the flow, onto the LSB, from the outer boundary. Jets have been introduced and tested on a 113th scale multistage steam turbine. The test turbine is both aerodynamically and mechanically representative of a full size machine. The blowing scheme was shown to reduce and then practically eliminate the rotating pressure pattern. 3D CFD computations reveal the major influence of the jets. The solution is elegant because it does not lead to loss of efficiency or design space.