Aeroelastic investigation of turbine blade assemblies: cluster systems and mistuned rows

Abstract

In modern turbomachinery design, one of the main objectives of aviation industry is the continuous research for higher performance with lighter engines. This trend leads to a reduction in the number of blades, which become increasingly thin and loaded, with a consequent increase in the occurrence of aeroelastic phenomena, compromising the structural integrity. This paper aims to present a numerical flutter assessment of two different types of blade assembly: a turbine cluster system typical of stator segments and an intentionally mistuned row representing an up-to-date low pressure turbine rotor. The numerical results obtained by a time-accurate CFD solver with vibrating blades will be compared with experimental data measured in the context of the EU project FUTURE. The first part of the paper will describe the study of a stator turbine cascade assembly, whose blades are mounted in packets and vibrate as a cluster mode. The comparison between numerical and experimental data showed an excellent agreement and further validated the aeroelastic solver. Then, the attention will be focused on the flutter analysis of an intentionally mistuned turbine rotor bladerow in comparison with a traditional row consisting of identical blades: this highlights how this type of assembly may stabilize the bladerow. The results of the numerical blade stability analysis show a flutter instability for the first bending mode which becomes stable, once the modal mistuning is introduced by adding masses at the tip of alternate blades. This numerically predicted flutter stabilization was confirmed by the experimental campaigns.

Experimental and Numerical Study of Controlled Flutter Testing in a Linear Turbine Blade Cascade

“…The 3D aerodynamic solver TRAF was extended to analyze unsteady flows around vibrating rows by using a re-meshing strategy, thus implementing an aeroelastic non-linear method able to deal with tuned rows, cluster systems and mistuned disks and linear cascades. The validation of the flutter procedure in subsonic and transonic tuned turbine environments can be found in [2,3]. To perform a flut- ter analysis, time-sinusoidal blade vibrations coming from modal analysis or from vibration measurements are assigned to the unsteady calculations as input data.…”

“…Fourier transforms are used to assign quantities on periodicity boundaries with appropriate time shifts, limiting the memory storage requirements. It is also possible to compute blade cluster system with different blade geometries applying an arbitrary phase shift between adjacent blades within annular and linear packet [3].…”

“…Two different numerical methods were applied for the flutter assessment of blade cascade: a commercial code AN-SYS CFX and an in-house code TRAF. Numerical methods for flutter assessment have been firstly validated in low pressure environment for the aeronautical industry [2,3]. More recently, the code validation has been extended to compressor blades and steam turbine rows [4,5] so that the actual design process now includes an extensive use of these methods to achieve a free flutter design [6].…”

Experimental and Numerical Assessment of a Last Stage St Blade Damping at Low Load Operation

“…Many documents in literature are available [1][2][3][4][5] showing that the aerodynamic damping prediction is possible and reliable enough when the steady flow field on the blade is as designed.…”

“…They are usually classified in time-linearized, harmonic balance and non-linear approach both in frequency and time domain [4,6,7]. The validation of all these methods firstly started in the branch of low pressure turbines for aviation [3,8] and has been gradually extended to compressor blades and steam turbine rows [9]. For the analysis of steam turbine last rotor operating at strong part load conditions, non-linear approaches turns out to be recommended as this method is able to deal with non-linear phenomena as shock waves and wide flow separations.…”