Numerical Simulation of Hydraulic Turbine During Transient Operation Using OpenFOAM

Power generation from intermittent renewable energy resources (e.g. wind, solar) requires regulation of the electric grid. Although most hydraulic turbines are designed to work in their best efficiency points, nowadays they are being used more often under varying operating conditions to stabilize the electric grid. Unstable and varying conditions of fluid flow in hydraulic turbines during transient operation cause significant pressure fluctuations and load variations that could negatively affect the turbine lifetime. Therefore, the development of high-fidelity numerical tools for hydraulic turbine flow during transient operation, i.e. changing from one condition to another or during start-up and shut-down, is of great importance for the lifetime prediction of the machines.

In the present work, we are investigating the capabilities of the OpenFOAM open-source CFD tool to predict such phenomena. The transient operation of hydraulic turbines most of the time involves changing the guide vane angles while the runner is rotating, which must thus also be allowed by the employed numerical techniques.

The high-head Francis-99 turbine is used as a test case, due to the availability of the geometry and rich experimental data. The turbulence resolving computations are performed using the SAS turbulence model. In the present test case, the guide vanes angles are changing while the runner is rotating. Hence, mesh morphing and solid body rotation techniques are employed simultaneously for the handling of dynamic mesh. First, the numerical results are validated against the experimental data and compared with each other in terms of accuracy and usability. The numerical results are also used for analyzing different flow behaviors, such as vortex rope formation.