Aeromechanical Characterization of a Last Stage Steam Blade at Low Load Operation: Part 1 — Experimental Measurements and Data Processing

This paper is the first of a two-part publication that aims to experimentally evaluate, simulate and compare the aerodynamic and mechanical damping for a last stage steam turbine rotor blade at part load operation. Resulting strong off-design partial load regimes expose the last stage moving blade (LSMB) to the possible onset of aero-elastic instabilities, such as stalled and un-stalled flutter. This interaction can lead to asynchronous blade vibrations and then the risk of blade failures for high cycle fatigue. In this framework, it is necessary to develop and validate new tools for extending operating ranges, controlling nonsynchronous phenomenon and supporting the design of new flutter resistant LSMB. To this end, a 3-stage downscaled steam turbine with a snubbered LSMB was designed by Ansaldo Energia and tested in the T10MW test facility of Doosan Skoda Power R&D Department within the FlexTurbine European project. The turbine was operated in a wet steam environment at very low volume flow conditions simulating different part load regimes. The steady flow field throughout the LSMB was characterized and the occurrence of flutter was investigated by inducing the blade resonance through an AC magnet excitation and measuring the overall damping. The results presented in this paper indicate that the blade always operates over the flutter stability margin validating this new blade design. In the second part of this work, the mechanical and aerodynamic contribution to the damping will be separated in order to validate the aerodynamic damping prediction of an upgraded CFD tool, already adopted in the design phase of the blade at design point.