Flutter Mechanisms in Low Pressure Turbine Blades

Abstract

The work described in this paper is part of a comprehensive research effort aimed at eliminating the occurrence of low pressure turbine blade flutter in aircraft engines. The results of fundamental unsteady aerodynamic experiments conducted in an annular cascade are studied in order to improve the overall understanding of the flutter mechanism and to identify the key flutter parameters. In addition to the standard traveling wave tests, several other unique experiments are described. The influence coefficient technique is experimentally verified for this class of blades. The beneficial stabilizing effect of mistuning is also directly demonstrated. Finally, the key design parameters for flutter in low pressure turbine blades are identified. In addition to the experimental effort, correlating analyses utilizing linearized Euler methods demonstrate that these computational techniques are adequate to predict turbine flutter. NOMENCLATURE INTRODUCTIONThis paper summarizes the results of a series of experiments conducted in an annular cascade facility to investigate torsional flutter in low pressure turbine (LPT) blades. The development of improved design guidelines for LPT flutter is an active research area because there have been several recent occurrences of instability in this class of blades. Because of these concerns, a research project was initiated with the goal of eliminating flutter in LPT blades.To date, several forced vibration experiments similar to those described in this paper have been conducted in both linear and annular cascades (Bölcs and Schläfli, 1984, Buffum andFleeter, 1990). In general, this work has been focused on a characterization of the aerodynamic stability of the cascade based on measurements of the blade surface unsteady pressures. For the current LPT cascade, this topic was the general focus of a previous paper (Panovsky, Nowinski, and Bölcs, 1997). In addition, comparisons were presented between the measured unsteady pressures and predictions from computational fluid dynamics (CFD) codes.The current paper represents an extension of this work. The primary objective of the tests presented here is to investigate the influence of the reduced frequency, inlet flow incidence, location of the torsion axis, and other key parameters on the cascade unsteady response. In addition, an enhanced understanding of the unsteady behavior of the cascade is obtained based on unsteady pressure measurements made along the test section outer wall. Tests were also performed to check the applicability of the influence coefficient technique to these cases, as well as to study the effects of cascade mistuning. Computational predictions are included to compare with the experimental measurements. EXPERIMENTAL FACILITY AND DATA ANALYSISIn this section, a brief description of the experimental facility, test article, and data analysis technique is provided. A more complete presentation can be found in Panovsky, Nowinski, and Bölcs (1997). The experimental measurements were conducted in the non-rotating annul...

Computation of Mistuning Effects on Cascade Flutter

“…These ndings agree qualitatively with the experiment data by Nowinski and Panovsky, 6 although our computations and analysis were performed independently, for a different cascade and without the knowledge of the experimental results.…”

“…Except for Nowinski and Panovsky, 6 all of these authors performed the analysis in the frequency domain by solving an eigenvalue problem of the structural utter equations with areodynamic loads as input in the form of in uence coef cients. In this paper we propose to study mistuning in the time domain by directly solving the unsteady ow of a cascade under mistuned oscillations.…”

“…A recent experimental and numerical investigation on an annular turbine cascade was done by Nowinski and Panovsky. 6 The blades were oscillated in a harmonic torsional mode. Three vibrational modes of the blades were tested: the traveling wave mode, the single blade mode, and the alternating blade mode.…”

Physics of Vibrating Airfoils at Low Reduced Frequency

“…The unsteady aerodynamics of low-pressure turbine rotor blades and stator vanes associated to the airfoil vibration is nowadays routinely analyzed within the design loop of the aeroengine companies and it has been the subject of dedicated experiments [3,4,5]. The standard application of all these efforts is the derivation of the aerodynamic stability of the rotor blades and the quantification of the aerodynamic damping, which is the result of the application of the unsteady pressures on the airfoil displacements.…”

Investigation of Mistuning Effects on Cascade Flutter Using a Coupled Method

“…An experimental and numerical investigation on an annular turbine cascade was done by Nowinski and Panovsky [6]. The blades were oscillated in a harmonic torsional mode.…”