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Numerical investigation of the structural behaviour of a headwater bifurcation in hydraulic short circuit

Authors: K Khalfaoui, A Tismer and S Riedelbauch

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Within the XFLEX HYDRO project, hydraulic short circuit operation (HSC) is being investigated as one of the possibilities to enhance the flexibility of pumped storage powerplants. Therefore, the structural behaviour of a headwater piping bifurcation of a pumped-storage power plant is investigated. Based on unsteady CFD simulations, time history mechanical simulations are carried out to assess the dynamic integrity of the structure in pump (PP), turbine (TT) and hydraulic short circuit operation. Since the plant at hand was initially designed for PP and TT operation, simulating these load conditions delivers a base of assessment for the admissibility of HSC loading. Herein, the focus of the assessment lays on the welded joints. First, a modal analysis of the rock-supported structure is conducted to identify the required numerical analysis method to be employed for the time-history structural simulations. The CFD pressure fluctuations are then applied to the structure within time history FE analyses. The dynamic structural simulations deliver low stress fluctuations which go hand in hand with the low pressure fluctuations previously determined using CFD. Besides, the structure exhibits comparable stress oscillations for all three loading conditions. The latter stress oscillations lie below the fatigue limit of the welds. The study herewith presents promising findings towards safe HSC operation ensuring more flexibility of the power plant at hand.

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