Active Flow Control of Hydrodynamic Instabilities in Francis Turbines based on Linear Stability Theory

• Applicant: Professor Dr.-Ing. Kilian Oberleithner
• Subject Area: Fluid Mechanics ; Hydraulic and Turbo Engines and Piston Engines
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429772199

To guarantee a stable electricity grid in the future, flexible energy sources are needed to balance the intermittent contribution from weather-dependent renewables such as solar or wind power. Hydropower plants are a well suited energy source for balancing intermittent energy contributions due to their flexible operation capabilities. Most hydropower plants are running Francis turbines, which need to be operated at part load to provide the required energy balance. Under these operating conditions a strong swirling flow is generated in the draft tube that gives rise to a helical vortex structure known as the precessing vortex core (PVC). Recent developments in linear stability theory allow to identify this structure as a globally unstable mode that is triggered by a hydrodynamic feed-back process in the draft tube of the turbine. In this project we use global linear stability theory and related adjoint methods to develop flow control solutions that are tailored to this instability. Key goal is to develop a control solution that suppress the PVC with minimal energy input. The main innovation of this approach in comparison to previous control attempts is that the methodology is based on a rigorous theoretical framework that allows to derive an optimal flow control solution. This control will be validated first on a turbine mock-up using air as a working fluid and in the final state at a hydro turbine facility. The developed control methods range from open- to closed-loop periodic and constant forcing as well as shape modifications. The project is structured in a baseline study, controller development phase and an optimization phase. To characterize the flow dynamics at natural and controlled conditions, experimental measurements in air and water as well as numerical simulations are conducted in conjunction with novel empirical data reduction strategies. The project benefits from the strong background in linear stability theory and flow control of the group at the TU Berlin and the excellent experimental facilities and experience of the group at the Institute of Thermophysics within the field of Francis turbine flows.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Foundation for Basic Research, until 3/2022
• Co-Investigator: Professor Dr.-Ing. Christian Oliver Paschereit
• Cooperation Partners: Professor Sergey Alekseenko, Ph.D., until 3/2022; Vladimir Dulin, Ph.D., until 3/2022; Dr. Ivan Litvinov, until 3/2022; Rustam Mullyadzhanov, Ph.D., until 3/2022; Sergey Shtork, Ph.D., until 3/2022