Experimental Investigation of Shock-Induced Blade Oscillation at an Elastically Suspended Turbine Cascade in Transonic Flow

Abstract

Experimental investigations on shock-induced flutter in a linear transonic turbine cascade are presented. To examine the relation between trailing edge shock oscillations on adjacent blades in transonic flow and observed turbine blade vibrations, an elastic suspension system has been developed so that only aerodynamic coupling occurs in the system. The experimental investigations have been performed on a linear test rig with superheated steam as working fluid. The test facility enables Mach and Reynolds numbers to be varied independently. The investigated cascade consists of seven blades which are taken from the tip section of a transonic low pressure steam turbine blade. Each blade is attached by an elastic spring system which allows the respective blade to vibrate in a mode equal to the real blade’s first bending mode. By varying the individual spring stiffness it is possible to either get a tuned or mistuned cascade. The examinations mainly deal with the oscillatory behavior of the blades with respect to a variation in the isentropic outlet Mach number. In addition, the complex shock-boundary-layer interaction on the blades’ suction sides is described. An important result is that the maximum blade oscillation amplitude can be related to a specific outlet Mach number. At this Mach number all seven blades are vibrating with exactly the same frequency. This phenomenon is observed at both the tuned and the mistuned cascades. Spectrum analysis shows that one of the major shock oscillation frequencies corresponds to the flutter frequency. In addition to this frequency the spectrum analysis of the blade oscillation shows the dominant frequencies of the shock oscillation which are not natural blade frequencies. The experimental results show that oscillating shocks in a linear cascade give high potential for aeroelastic excitation of transonic blades under certain flow conditions. Blade oscillations and shock characteristics are discussed in detail.