EXPERIMENTAL FLUTTER INVESTIGATIONS OF AN ANNULAR COMPRESSOR CASCADE: INFLUENCE OF REDUCED FREQUENCY ON STABILITY

Joachim Belz and Holger Hennings

DLR – Institute ofAeroelasticity

Bunsenstrasse 10

D-37073 Gottingen, Germany

joachim. belz@dlr. de

holger. hennings@dlr. de

Abstract Due to the trend of increasing power and reducing weight, the fan and compres­sor bladings of turbomachinery might be more sensitive to flitter, which must strictly be avoided already in the design process. In order to increase our under­standing of the flitter phenomena for fan and compressor cascades, aeroelastic investigations are essential.

This paper presents the achievements and results of experimental flitter in­vestigations with a compressor cascade in the test facility of non-rotating annular cascades at EPFL. Flow conditions such as those that occur in rotating cascades are simulated by generating a spiral fbw in the upstream. The construction of the cascade which takes into account the structural properties necessary to per­form flutter experiments is described. For the simulation of elastic torsional vibrations of a two-dimensional blade section, the cascade consists of 20 blades (NACA3506 profile) mounted on elastic spring suspensions which allows for torsional motion about the midchord.

In order to investigate the inflience of the reduced frequency on the global stability of the cascade and its local contibutions, experiments were performed for two different reduced frequencies. At the higher reduced frequency the cas­cade remains aerodynamically stable, however, at the lower reduced frequency and transonic flow conditions, some of the interblade phase angles appear to be aerodynamically unstable.

Keywords: Flutter, Reduced Frequency, Experiments, Annular Cascade

77

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 77-91. © 2006 Springer. Printed in the Netherlands.

Introduction

The demand for a decrease in engine weight and a reduction in fuel con­sumption has, among other things, led to engines that contain a decreased num­ber of compressor stages and slender fan or compressor blades. This results in both more fhxible blades and in higher pressure ratios at each stage with a higher fbw velocity around the blades. The variations of these parameters in – flience the aeroelastic stability of the blade assembly and can lead to flitter, i. e. self-excited blade vibrations due to an interaction with the motion-induced unsteady aerodynamic forces. For this reason, aeroelastic investigations are es­sential to provide detailed knowledge about flitter phenomena, especially for compressor cascades in transonic fl»w. Experimental data of unsteady aerody­namic and flitter tests are required for the validation of theoretical results as well.

In the past years, the increasing number of theoretical investigations has been accompanied by several experiments on vibrating cascades. Szechenyi et al. (1980), Carta (1982), Buffum et al. (1998), and Lepicovski et al. (2002) have obtained unsteady aerodynamic data by harmonic torsional oscillations of one blade or all blades of their linear compressor cascades. Carta partic­ularly showed the influence of the interblade phase angle on stability at low Mach numbers, whereas Szechenyi and Buffum concentrated their investiga­tions on the infhence of large incidence angles on stability. Korbacher (1996) investigated the bending motion of the blades of a compressor cascade in an annular wind tunnel. A cascade with the same geometry was used by Hennings and Belz (1999, 2000) to examine the aerodynamic damping by forced pitch­ing motions of the blades with respect to shock movements. The results were compared with theoretical investigations by Carstens and Schmitt (1999) as well as Kahl and Hennings (2000), who took into account leakage fl>w effects.

The experimental investigations presented here were performed in the wind tunnel for annular cascades at the Ecole Polytechnique Federale de Lausanne (EPFL). This wind tunnel has been used by several other researchers for the measuring of unsteady pressure distributions due to blade vibrations for aero­dynamic stability investigations of compressor and turbine cascades or up­stream generated aerodynamics gusts for forced response investigations. The following investigations performed are cited as examples: Korbacher and Bolcs (1996), Korbacher (1996), Nowinski and Panovsky (1998)), Rottmeier (2003).

The aim of the investigation presented here was to investigate the influence of reduced frequencies on the cascade’s aerodynamic and aeroelastic behavior at transonic flow. Two reduced frequencies were chosen in such a manner, that in one case self-excited cascade vibration (flutter) occured and in the other case the cascade remained aerodynamically stable. In order to realize cascade flutter the structural damping had to be minimized by redesigning the elastic

spring suspensions. The unsteady pressure distribution, the infhence of the shock impulse at transonic fOw conditions, and the local and global stability measured for the two reduced frequencies are compared.