Aeromechanical Characterization of a Last Stage Steam Blade at Low Load Operation: Part 2 — Computational Modelling and Comparison

Abstract

This paper is part of a two-part publication that aims to experimentally and numerically evaluate the aerodynamic and mechanical damping of a last stage ST blade at low load operation. A three-stage downscaled steam turbine with a snubbered last stage moving blade LSMB has been tested in the T10MW test facility of Doosan Skoda Power R&D Department in the context of the FLEXTURBINE European project (Flexible Fossil Power Plants for the Future Energy Market through new and advanced Turbine Technologies). Aerodynamic and flutter simulations of different low load conditions have been performed. The acquired data are used to validate the unsteady CFD approach for the prediction of the aerodynamic damping in terms of logarithmic decrement. Numerical results have been achieved through an upgraded version of the URANS CFD solver, selecting appropriate and robust numerical setups for the simulation of very low load conditions, such as increased condenser pressure at the exhaust hood outlet. The numerical methods for blade aerodamping estimation are based on the computation of the unsteady pressure response caused by the row vibration. They are usually classified in time-linearized, harmonic balance and non-linear approaches both in frequency and time domain. The validation of all these methods historically started in the field of aeronautical low-pressure turbines and has been gradually extended to compressor blades and steam turbine rows. For the analysis of a steam turbine last rotor blade operating at strong part load conditions, non-linear methods are recommended as these approaches are able to deal with strong nonlinear phenomena such as shock waves and massive flow separations inside the domain. Experimental data have been used to separate the contributions of mechanical and aerodynamic damping, extrapolating to zero mass flow the total measured damping. Finally, the comparisons between the aerodynamic damping coming from measurements and CFD results have been reported in order to highlight the capability to properly predict the last stage blade flutter stability at low load conditions. Such comparisons confirms the flutter free design of the new snubbered LSMB blade.

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.…”

Validation of a Modal Work Approach for Forced Response Analysis of Bladed Disks

“…After the forcing decomposition, a second in-house tool was implemented to compute the modal work done by the rotating force acting on the corresponding bladed disk mode shape in order to derive the scaling factor to compute actual oscillating displacements and stresses. The method was conceived for its integration in a CFD environment based on the URANS solver TRAF (developed at the University of Florence) capable of solving the force evaluation for the stator-rotor interaction [23,24] and also performing flutter simulations to compute the aerodynamic damping to be added to forced response analyses together with the mechanical contribution [25].…”

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

“…The current trend of designing more flexible steam turbine is worsening the vibration issues due to flutter and forced response phenomena, especially on the last-stage long blades (Rice et al (2008); Pinelli et al (2020)). Asynchronous vibrations caused by flutter occurrence are one of the main concerns when designing new-generation low pressure steam turbine rotors.…”

“…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)).…”

“…Nowadays flutter assessment of turbine blade in subsonic condition is a common practice during the turbomachinery design phase, and the available numerical tools (linearized and non linear in time or frequency domain) are now mature and reliable (Kielb et al (2003); Lottini et al (2019); Biagiotti et al (2018)). However there is still a lack of confidence when such solvers are applied to unsteady flows with high non linear phenomena (Doi and Alonso (2002); Barreca et al (2018)) like shock boundary layer interaction or massive recirculating areas that are present in the low pressure steam turbine blade (Rice et al (2008); Pinelli et al (2020)). The analysis of these components in such severe operating conditions usually leads to a result dispersion among different computational methods also depending on the level of simulation accuracy.…”