Simulation of the structural behavior of a Francis runner in Deep Part Load Operation

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Hydropower plants can play an important role to stabilize the electrical grid when the share of volatile renewable energies increases. This goes hand in hand with a more flexible operation of the turbines and longer operation in off-design operating points. The increased pressure fluctuations that occur at off-design conditions may reduce the lifetime of the turbines. Consequently, this should be considered within the design process to develop efficient but also robust turbines. To derive suitable simulation setups that can be used in the design process, it is crucial to have a deep understanding of specific flow phenomena. As pressure fluctuations are of main importance in terms of fatigue, it is important that simulations can capture them well. Within this study, a deep part load operating point of a Francis turbine at model scale is investigated. By using three different meshes the influence of mesh refinement on the predicted pressure fluctuations is analyzed. Furthermore, by means of single- and two-phase simulations it is investigated how the pressure fluctuations are affected by the occurrence of cavitation. The results show only minor differences between the different meshes, whereas significant differences are observed between a single- and two-phase treatment.

Section: Methodsmentioning

confidence: 99%

“…The goal of this study is to answer the question how mesh refinement and the occurrence of cavitation affect the pressure fluctuations in the runner of a Francis turbine. This is an important task as CFD results are used as input for structural mechanical investigations to predict the lifetime of turbines [8,9].…”

Section: Introductionmentioning

confidence: 99%

Numerical investigation of the impact of cavitation on the pressure fluctuations in a Francis turbine at deep part load conditions

Wack

Grübel

Conrad

et al. 2022

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“…In the works of [3,4], the structural behaviour of Francis model runners was investigated. Herein, a combination of numerical and experimental techniques was applied.…”

Section: Introductionmentioning

confidence: 99%

Towards accurate vibration prediction of pump-turbine runners in deep partial load operation

Khalfaoui

Zorn

Riedelbauch

2022

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Within the XFLEX HYDRO project, operating power plants in low load conditions is being investigated as one of the possibilities to enhance the flexibility of hydropower. Therefore, the structural dynamic behaviour of a pump-turbine runner is numerically analysed in deep partial load (DPL) operating points. Based on highly resolved unsteady CFD simulations performed prior to this work, the pressure fluctuations are extracted and mapped onto the structure for subsequent Finite Element (FE) analyses. Herein, an efficient hybrid methodology combining purely structural and coupled Fluid-Structure-Interaction (FSI) modelling is applied. The obtained simulation results are compared to experimental measurements. Herein, the influence of several factors such as the CFD mesh size, the turbulence model, the simulated time window width, and the structural simulation type is studied. As a result, the numerical outcome is validated with good accuracy. The importance of highly resolved CFD simulations with appropriate turbulence modelling is highlighted. Besides, the necessity of coupled acoustic-structural analyses to accurately resolve the dynamic response to excitation by rotor stator interaction (RSI) pressure shapes is demonstrated despite the chaotically dominated nature of the structural vibrations. The results of this work can be used to extend the operating range of the pump-turbine runner and, most importantly, to calibrate numerical models to predict the structural vibrations accurately and efficiently in DPL and finally ensure safe operation of these components.

“…Therefore, special care must be taken of understanding the flow phenomena and predicting the structural vibrations. In [3][4][5], the focus was laid on the structural and flow behaviour of Francis runners in deep partial load. Herein, both numerical and experimental techniques are applied.…”

Section: Introductionmentioning

confidence: 99%

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

Khalfaoui

Tismer

Riedelbauch

2022

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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.