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

Abstract

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 non-synchronous 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.

Numerical Investigation of Potential Flow Induced Vibrations of Steam Turbine Last Stage Rotor at Low Load Operation - Part 2: Rotating Instabilities Detection

“…In order to explain this question, an extensive numerical study, presented in a two-part publication, has been carried out to investigate two different mechanisms potentially accountable for flow induced vibrations. The first part of this work [21], focused on the bladefluttering, has allowed to exclude this aero-mechanical mechanism as a potential source of vibration in low load conditions, in agreement with what has recently been shown experimentally by Bessone et al [22] and numerically by Pinelli et al [23]. The second part of the present work is instead focused on the purely aerodynamic instabilities by carrying out unsteady simulations on a full annulus last stage coupled with a real geometry axial diffuser of a steam turbine manufactured by Baker Hughes for Concentrated Solar Power (CSP) system applications.…”

Aeroelastic investigation of an open 3D steam turbine test case and blade shape optimization with improved flutter behavior

“…In this context, different EU projects (e.g. FLEXTURBINE) have increased the knowledge in terms of physical understanding and simulation technology thus leading to a deeper insight on flutter phenomena of low pressure turbine rotor (Bessone et al (2020); Pinelli et al (2020)).…”