PREDICTION OF HIGH-FREQUENCY BLADE VIBRATION AMPLITUDES IN A RADIAL INFLOW TURBINE WITH NOZZLE GUIDE VANES

Impeller blades in radial inflow turbines are not only exposed to high thermal loads and centrifugal forces. Additional dynamic stresses occur by the aerodynamic excitation of a variety of blade and disc modes and can lead to damages by fatigue. This is a critical consideration for engines with nozzle guide vanes in particular, where excitation is caused by the interaction between guide vanes and rotor blades. This leads to high excitation frequencies, which are within the range of eigenfrequencies of the stiff impeller. Previous experimental analyses provide vibration amplitude data for resonances in a radial inflow turbine equipped with three nozzle rings with varying vane numbers. The experimental data is used for validation of numerical investigations. The numerical work presented involves the simulation of the transient flow field of the entire turbine as a first step. Aerodynamic excitation forces on the blades are derived from the results for various resonance conditions. The influence of the operating condition and the vane number is pointed out. Higher speed and lower vane number increase the amplitudes of the blade force. In a second step, the transient and spatially resolved pressure distribution is used as a boundary condition in an FE model. The damping ratio is an essential parameter in order to calculate the forced response of the structure, and it is determined from the experimental data. The damping behavior is characterized and compared to ratios derived from additional experimental studies using laser vibrometry at the non-rotating turbine wheel under ambient conditions. A disparity in the damping ratios is recovered, depending on the eigenmodes and the boundary conditions. The forced response of the structure is computed using the individual damping ratios for four resonance conditions. Harmonic analyses are conducted, applying the pressure forces from CFD. The calculated amplitudes are validated with data from strain gauge measurements under operating condition. The prediction of the vibration amplitudes shows acceptable agreement to the test data with a tendency towards lower values.