Hydraulic turbines are increasingly used for power grid regulation as intermittent energy resources, such as wind and solar power, gain prominence. The power output from these renewable sources fluctuates over short and long periods depending on weather conditions. Therefore, hydraulic turbines often operate away from their design point to mitigate grid imbalances, such as low-load operation, which presents challenges. The guide vanes control the flow, and their opening angle is limited during low-load conditions to restrict the flow rate and reduce power output, creating a high swirl flow condition. During part load (PL) operation, the residual swirl entering the draft tube can initiate a rotating-vortex-rope (RVR) that wraps around a stagnant region, introducing severe pressure pulsations. Some turbines are expected to provide a spinning reserve, enabling rapid responses to grid power shortages. One method to achieve a spinning reserve is to allow the turbine to operate under speed-no-load (SNL) conditions, where the turbine rotates at synchronous speed without generating electrical power. Since the turbine does not extract any power, the energy must be dissipated through the flow field. The resulting chaotic flow field features sometimes rotating vortices attached to the head cover in the vaneless space, extending into the draft tube. The axial flow primarily occurs in a thin region near the outer wall, while a recirculating region exists at the center of the draft tube, potentially extending upstream of the runner. Similar to the RVR, the rotating vortices induce harmful pressure pulsations throughout the turbine, jeopardizing safe operation and shortening the turbine's lifespan due to an increased risk of material fatigue.

This thesis aims to study the flow under low-load operating conditions for a Kaplan model turbine, specifically the Porjus U9 model, using computational fluid dynamics (CFD), and to explore a mitigation strategy that reduces pressure pulsations during these low-load operating conditions. The idea is to limit swirl and, consequently, the pressure pulsations caused by the flow structures by employing asynchronous guide vanes. This involves adjusting some guide vanes to a larger opening angle, while keeping the others closed and maintaining the same power output. Unlike other mitigation techniques, no additional installations are needed aside from an option to control some of the guide vanes asynchronously. CFD is combined with machine learning to explore various guide vane configurations efficiently.

Results indicate that vortices in the vaneless space during SNL operation can be mitigated, significantly reducing the associated pressure pulsations by opening one consecutive section of guide vanes. However, the jet like flow from the opened guide vane section generates a significant radial force on the runner due to the asymmetric flow field and pressure pulsations on the runner blades, oscillating at the runner's rotational frequency. Both issues can be addressed by opening two sections of guide vanes on opposite sides of the runner axis while maintaining most of the mitigation effect. Furthermore, the flow field is predictable, and most stochastic pressure pulsations are reduced, positively impacting the turbine's lifespan. One section with open guide vanes during PL operation can decrease the amplitude of the pressure pulsations related to the RVR rotating mode (RM) and plunging mode (PM) in the draft tube. Conversely, the stochastic pressure variations increase, and the runner is subjected to an asymmetric radial force and pressure pulsations on the blades that oscillate at the runner's rotational frequency, much like for SNL operation. Additionally, efficiency decreases, and the torque on the runnervaries more. The mitigation effect diminishes when opening two guide vane sections, as the RVR reduces in size, but is not completely mitigated.

The asynchronous guide vanes primarily affect the flow upstream of the runner, making the technique suitable for SNL operation, as the vortices originate upstream of the runner. The mitigation strategy is less effective during PL because the RVR originates in the draft tube. While the pressure pulsations related to the RVR can be reduced, significant stochastic variations persist. Furthermore, since the flow rate is higher during PL than SNL, the runner is subjected to higher-amplitude pressure pulsations and a more excessive radial force. Ultimately, implementing asynchronous guide vanes balances increased life expectancy and the cost of turbine operation. Experimental investigations are necessary to validate the above findings and clarify the actual effect on material fatigue and other parts of the turbine not included in the simulations.