ENGINEERING AND CFD
Simulation of Cavitation Phenomena in Francis Turbines
Principal Investigator:
Jonas Wack
Affiliation:
Institute of Fluid Mechanics and Hydraulic Machinery, University of Stuttgart (Germany)
Local Project ID:
HYPERBOL
HPC Platform used:
Hazel Hen of HLRS
Date published:
The integration of renewable energies like wind energy or photovoltaics is a challenging task as their power output can vary within short periods. Hydro power can help to preserve electrical grid stability because these power plants can be easily regulated. However, this comes across with a significantly higher share of operating turbines at off-design conditions. For this operation away from the design point, strong swirling flows occur in the turbine that can cause high pressure fluctuations. These can significantly reduce the lifetime of the turbine. Furthermore, at off-design conditions cavitation may occur, which 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. Cavitation can cause severe damage in the turbine and affects the power output.
In this project, researchers from the Institute of Fluid Mechanics and Hydraulic Machinery at the University of Stuttgart focus on the two-phase numerical simulation of a Francis turbine at off-design conditions. The special focus lies on a full load operating point that is facing a self-induced instability. The goal of this project is to explain the physical mechanism behind this instability as it is not yet fully understood.
Figure 1: Cavitation on the suction side of the runner blades and cavitating vortex rope in the draft tube cone for full load conditions at three instants of an instability cycle.
Copyright: IHS, University of Stuttgart (Germany)Within an instability cycle it comes to an interaction of different cavitation regions that results in a strong variation of the cavitation volume (see Figure 1) and causes severe pressure pulsations. With the new insights from this project, it is possible to identify measures that can avoid the occurrence of the full load instability. Consequently, this allows extending the operating range of a turbine, which increases the flexibility of a power plant. Furthermore, it has a positive effect on turbine lifetime.
The use of two-phase simulations is crucial for this project as single-phase simulations cannot capture the phenomenon of the full load instability. This results in a significantly increased computational effort as the governing equations that have to be solved are more complex.
To accurately predict the occurring flow phenomena fine computational grids and appropriate turbulence models have to be used. Furthermore, it is necessary to simulate a lot of runner revolutions to capture the instability phenomenon. Together with the need to perform simulations at several different pressure levels, this results in a tremendous computational effort that requires the use of supercomputers like the Hazel Hen at HLRS in Stuttgart.
Scientific Contact:
Jonas Wack
Institute of Fluid Mechanics and Hydraulic Machinery
University of Stuttgart
Pfaffenwaldring 10, D-70569 Stuttgart (Germany)
e-mail: jonas.wack [@] ihs.uni-stuttgart.de
HLRS project ID: HYPERBOL
March 2019