ENGINEERING AND CFD

In the last decades, hydraulic machines have experienced a continual extension of the operating range in order to integrate other renewable energy sources into the electrical grid. When operated at off-design conditions, the turbine experiences cavitation which may reduce the power output and can cause severe damage in the machine. Cavitation simulations are suitable to give a better understanding of the physical processes acting at off-design conditions. The goal of this project is to give an insight in the capabilities of two-phase simulations for hydraulic machines and determine the range for safe operation.

Due to the integration of other renewable energies like photovoltaics or wind energy, hydro power plants play a key role in terms of electrical grid stability as the energy generation can be easily regulated. This comes across with operating the turbine at conditions away from the best efficiency point, and the consequences are strong swirling flows and phenomena like cavitation. Cavitation is the process from the evaporation of water due to the fact that the pressure is falling below the vapor pressure to the re-condensation in regions of high pressure. Typically, it is affecting the power output of the turbine and can lead to severe damage when occurring close to walls like the runner blades.

In this project, researchers from the Institute of Fluid Mechanics and Hydraulic Machinery at the University of Stuttgart focus on the numerical simulation of the cavitating flow in hydraulic turbines at various operating points. The aim is an accurate prediction of the occurring cavitation phenomena at different flow conditions and the determination of the acting pressure fluctuations.

Two-phase simulations are very challenging. Compared to single-phase simulations, where only the liquid phase is considered, the governing equations are more complex. For advanced modeling approaches, this results in an increased computational effort which is up to three times higher compared to single-phase simulations. Additionally, vortex induced cavitation phenomena like the inter-blade vortices in the runner at deep part load conditions or the vortex rope in the draft tube forming at full load conditions (see Figure 1) are very challenging for an accurate prediction of the flow phenomena. On the one hand advanced turbulence models need to be applied, and on the other hand highly refined computational grids are necessary to resolve the pressure minimum in the vortex core which is relevant for a correct prediction of the cavitation volume. Furthermore, for the simulation of water turbines at least 40 runner revolutions need to be simulated. All in all, this leads to a huge computational effort that requires the use of supercomputers like HPC system Hazel Hen at HLRS in Stuttgart.

The simulation results show that depending on the pressure level in deep part load, part load and full load conditions severe pressure fluctuations can be present. These results are now used as input for FEM-analyses (finite element method) to predict the stresses on the runner. With the structural analysis, critical operating conditions can be observed and a safe operation of the hydropower plant can be determined.

Scientific Contact:

Jonas Wack 
Institut für Strömungsmechanik und Hydraulische Strömungsmaschinen (IHS)
Pfaffenwaldring 10, D-70550 Stuttgart/Germany
e-mail: jonas.wack [@] ihs.uni-stuttgart.de