The increasing variability of the energy market has created a less favourable context for hydropower plants. The operating conditions of the hydraulic turbines are impacted by the fast power regulation required to compensate for the implementation of renewable energy resources such as wind, wave, or solarpower. The turbines are frequently working in off-design conditions and therefore, their efficiency and life span are reduced. Under damaging operating regimes such as part load and speed-no-load operation, unsteady flow structures, asymmetric flow and high-pressure fluctuations develop.

Low-head power plants are usually equipped with Kaplan turbines, i.e., double-regulated axial hydraulic machines. The guide vanes and the runner blades can be adjusted separately allowing Kaplan turbines to operate at high efficiency over a wider range of flow rates compared to single-regulated turbines. However, recurrent transient and off-cam operation is accounted for the premature wear, fatigue, and failure of Kaplan turbines.

Experimental and numerical studies are carried out to understand, prevent and mitigate the negative effects of transient operation on hydraulic turbines. Numerical simulations serve as a practical and cost-efficient supplement to model testing and can provide detailed flow information that is difficult to obtain otherwise. The experimental and numerical investigations carried out on small-scale turbine models are convenient and accessible but limited. Studies concerning full-scale large turbines are, on the other hand, challenging considering the production losses, large scales, high Reynolds numbers, and significant computational demands.

This thesis presents a numerical analysis of the flow developed inside a Kaplan turbine model and prototype, working as a propeller turbine, under different operation conditions. The objective was to explore the means of creating numerical models that could be used in the industry to test, diagnose and optimize the exploitation of axial turbines. The test case was the Porjus U9 Kaplan model and prototype. All the numerical simulations were validated against experimental data. Different operating regimes of the turbine model and prototype were modelled.

The model turbine was investigated numerically at the best efficiency point and during the transient operation from the best efficiency point to part load. The influence of different turbulence models and inlet boundary conditions on the accuracy of the numerical simulations was assessed. Additionally, a time step sensitivity analysis showed that the main parameters of the turbine model were reasonably predicted with large time step values, 61° and 121° of the runner rotation, considerably reducing the simulation time and computational costs. The formation of the rotating vortex rope was captured during the guide vane closure. The frequency of the pressure fluctuations monitored on the runner blade was accurately predicted compared to the experimental values.

The operation of the Porjus U9 prototype at the best efficiency point, part load and speed-no-load was investigated numerically. The sensitivity of the numerical models to the runner blade clearance size, the epoxy layer added to the runner blade in the experimental campaign to fix the pressure sensors in their position and the runner blade angle was explored. Similar to the model simulations, the rotating vortex rope was visible at part load in the prototype simulations. The frequency of the pressure pulsations was accurately predicted while the amplitude was poorly estimated regardless of the operating point and scale of the turbine.