Numerical Investigation of Potential Flow Induced Vibrations of Steam Turbine Last Stage Rotor at Low Load Operation – Part 1: Sensitivity to Flutter Occurrence

The present paper is part of a two-part publication that aims to numerically investigate the occurrence of flow induced vibrations in a steam turbine last stage rotor at low load operations. Experimental and numerical investigations of low load operating conditions are raising in interest for the design of steam turbine blades due to the required effort to achieve the objectives of the carbon free scenario leading to a higher flexibility for steam power plants. In fact, the coupling with renewables and the need for peaking power plants to stabilize the energy market require reliable machines able to safety work at low load conditions. In this part 1 paper, the last rotor of a low-pressure steam turbine module installed in concentrated solar plants has been numerically evaluated to assess the flutter stability at different low load conditions, while the companion part 2 deals with the investigation of unsteady aerodynamic instabilities which might arise in the last stage rotor.

In the present work, aerodynamic and flutter simulations were performed at different load conditions, from the design point to very low mass flows to achieve a real sensitivity of flutter behaviour to the mass flow rate. By means of steady state multi-stage results, aerodynamic performance and flow structures have been evaluated: the decreasing mass flow and blade load reveal the formation of wide separation areas both on the last rotor pressure side and in the exhaust duct, bringing the stage up to ventilating conditions. The flutter analyses have been performed with an uncoupled non-linear method in order to estimate the aerodynamic damping for a selection of mode-shape families. From aerodamping results it has been possible to observe the trend in the flutter response at different load conditions and it is evident how the shock wave system near the tip play a major role in energy exchange between flow field and vibrating profile. The results show an overall flutter stability of the blade up to very low load conditions.  This means that for this type of rotor profiles in their relative applications flutter occurrence are not a main issue, and the further vibration causes like rotating instabilities have to be investigated as demonstrated in the companion part 2 paper.