Hydrodynamics of Francis turbine operation at deep part load condition

The electrical energy production from New and Renewable Energy (NRE) sources have become increasingly important in the past decades. However, intermittent electrical generation from NRE sources due to their stochastic nature often prevents stable power output from existing power grids. To enable a smooth integration of NRE sources, flexible operations at hydraulic power plants are key to providing the capability of primary and secondary grid control to balance the energy production. The rapid growth of the NRE sources nonetheless requires hydraulic machines to function in an extended operating range, especially down to extremely low discharge conditions called deep part load operation. Such off-design conditions provoke various types of cavitation flows, posing a threat to stable operations at hydraulic units. Inter-blade cavitation vortices are a typical example of cavitation phenomena observed at deep part load operation. However, its dynamic characteristics are insufficiently understood today. The main objective of the present research is to unveil the flow characteristics at deep part load conditions and the physical mechanisms responsible for the inter-blade vortex development. The experimental is carried out with a physical reduced scale model of a Francis turbine. The characteristics of the flow in the draft tube is first investigated by wall pressure fluctuation and velocity surveys by PIV (Particle Image Velocimetry) measurements. For investigations of inter-blade cavitation vortices, the present study introduces a novel visualization technique using an instrumented guide vane, providing unprecedented images of cavitation development inside the runner blade channel. The binary image processing technique enables the successful evaluation of inter-blade cavitation vortices in the images. The analyses demonstrate that the probability of the inter-blade cavitation development is significantly high close to the runner hub. Furthermore, the mean vortex line is calculated. Additionally, the impact of the inter-blade vortex on the pressure field is investigated by on-board instrumentation on the runner blade. It reveals that the presence of an inter-blade vortex induces stochastic pressure oscillations on the blade. Moreover, the survey of the wall pressure difference between pressure and suction sides of the blade suggests the development of a backflow region near the hub, which is closely related to inter-blade vortex development. In an effort to better understand the flow structure in the draft tube and inside the blade channel, numerical simulations by an unsteady RANS approach are performed. Flow analysis in the draft tube and the simulated inter-blade vortices are in good agreement with the experimental results. Furthermore, the simulated flow inside the blade channel confirms the development of a backflow region on the hub near the runner outlet. The skin-friction analyses evidence that the backflow region as well as inter-blade vortex development are provoked by flow separation on the hub, which is caused by the misaligned flow condition inside the blade channel. The investigations are furthermore extended to identify the influence of inter-blade vortices on the specific energy dissipation in the runner. The quantitative evolution of the specific energy loss using the specific rothalpy reveals that inter-blade vortices cause the energy loss through the blade channel.