Impeller blades in radial inflow turbines are exposed to unsteady aerodynamic forces. They cause excitation of blade vibrations and the resulting dynamic loads can lead to damages by fatigue. Turbines in turbochargers are operated in a wide range of rotational speeds, leading to a broad spectrum of excitation frequencies.
So, resonance cannot be avoided during operation and a multitude of modes are excited. Excitation must notably be attended to when nozzle guide vanes are installed. They interact with the impeller blades, whereon the presented analysis is focused. Measurements with varying vane numbers already revealed differences in vibrational amplitudes for the excited mode shapes.
For this observation, evidence is found in the present study by numerical analyses at nine resonance speeds. The vibrational behavior of the impeller at resonance is characterized utilizing the finite element method, and calculations of the unsteady flow field were carried out. As a result three flow phenomena could be identified causing unsteady aerodynamic forces. The force amplitudes are derived for different operating speeds and vane numbers. Considerably higher unsteady loads were calculated for lower vane numbers. The spatial resolution of stimulating pressure forces is visualized at excitation frequency. Comparing the excitation function with the local deformation of the blade indicates the excitability of discrete modes.