Numerical and Experimental Assessment of Flutter Stability in a Turbine Linear Cascade

Aerodynamic and structural optimal design of Last Stage Blades (LSBs) of steam turbine is a challenging aspect for designers. The extension of operational flexibility jeopardizes the structural integrity of these slender blades as the risk of asynchronous blade vibrations induced by flow becomes more and more relevant and may lead to undesired HCF (High Cycle Fatigue) failures. An entire work package of the Flexturbine project (EC Horizon 2020, n° 653941) is dedicated to design and test flutter-resistant blades for a wide range of steam turbine operating conditions. All the experimental and numerical studies in the project enable a deeper understanding of flutter phenomenon with the aim of deducing design rules for a next generation of flutter-resistant steam turbine blades. As a part of this project, a subsonic test rig installed at the University of West Bohemia (UWB) is employed for a controlled flutter testing of LSB tip profiles in air-flow. The test rig is a linear cascade with eight turbine blade profiles made of carbon fiber: the four inner blades are flexibly mounted, each with two degrees of freedom (i.e. bending and torsion motions). Unsteady aerodynamic forces and moments induced by blade cascade oscillations are measured in order to evaluate the work exchange between the blade and the flow. This kind of detailed measurement is not possible in an actual steam turbine and the data will be used for a validation of numerical methods. The numerical flutter assessment related to the blade torsion mode of this linear cascade has been carried out with the TRAF code, an in-house CFD solver developed at the University of Florence suitable for aerodynamic, aeromechanical and aeroacoustic analyses in turbomachinery environment. Despite some approximations adopted for rig geometry discretization, the flutter results are in excellent agreement with the experimental data confirming the applicability of this method and further validating the code in steam turbine environment. Numerical results are presented and discussed in detail: aerodynamic work (AEW), unsteady loads and unsteady pressure harmonic of the vibrating profiles are shown in order to gain a deeper understanding of the unsteady interactions within the vibrating linear cascade, responsible for flutter occurrence.