Experimental Study of the Unsteady Blade Forces in an Oscillating Annular Compressor Cascade

Abstract

The benefits of minimizing the weight of an aircraft are substantial, due to which all aircraft components are designed so as to perform acceptably with minimal weight. As a result, modern compressor, fan and turbine blades are increasingly being designed with thinner airfoil profiles, while also being subjected to high loading, so as to improve the thrust-to-weight ratio of the engine. These conditions make the blades extremely likely to result in large-amplitude vibrations. A class of critical vibrations are caused due to aeroelastic instabilities within the turbomachinery blade rows. Forced response or the self-excited flutter can lead to high-cycle fatigue which can cause a catastrophic failure of the blades. The absence of a reliable prediction methodology for the occurrence of these instabilities signify the importance of unsteady aerodynamic studies in turbomachine cascades. The present experiments are conducted on a newly commissioned annular cascade tunnel. The test section consisting of 14 compressor blades is studied under subsonic conditions. The profiles of the blades have a constant span design, and the cascade parameters are chosen as EPFL’s Second Standard Test Configuration at the mid-span location. A set of guide vanes upstream of this blade row set the required incidence to the cascade by imparting a circumferential component to the velocity. For the present unsteady studies, selected blades are subjected to controlled vibrations while the unsteady response on a reference stationery blade is measured. Two blades are connected to individual servo motors through a mechanism so as to execute controlled, low-amplitude, torsional (pitching) oscillations about its mid-chord axis. A reference blade is mounted on a load cell to enable measurement of both the axial and transverse forces and moments. In order to simulate the effect of the inter-blade phase angle prevalent in a rotating turbomachine blade row, the phase angle between the vibrating blades is varied to all admissible values. The fundamental parameters for evaluating the stability are the phase difference between blade position and the forces responses. This is estimated from the Fourier transform of the displacement and load signals. The parameters are evaluated for a range of reduced frequencies and inlet velocities to evaluate the stability of the cascade at all specified flow conditions.