Forced Response Excitation due to Stagger Angle Variation in a Multi-Stage Axial Turbine

Abstract

Blade repair is often economically more attractive than the replacement of damaged blades by spare parts. Such regenerated turbine blades, however, can introduce non-uniform flow conditions which lead to additional forced response excitation of blades. A forced response excitation due to a typical geometric variation, introduced through current repair methods applied in an upstream stage, is investigated using a fluid-structure interaction (FSI) model previously experimentally validated in a five-stage axial turbine. In this study, geometrical variations are applied to the stator vane of the fourth stage of the five-stage axial turbine. The reference configuration, without variations, is compared with experimental data. The focus of the analysis is the determination of the aerodynamic excitation in a multi-stage setup. For both configurations, with and without variations, the stage loading coefficient of the last turbine stage remains constant. In contrast, the aerodynamic work acting on the last rotor blade increases by a factor of 4 dependent on the operating point. The vibration amplitude of the downstream blade is determined using a unidirectional fluid-structure interaction approach. The impact of the variations on the vibration amplitude decreases by a factor of 10 with increasing number of blade rows between the modified vane row and the analyzed blade row. However, the geometric variations induce vibration amplitudes 4 times higher than the reference case. Based on the methodology used, a linear correlation between the excitation of the blade by the aerodynamic work and the vibration amplitude is shown to exist.