Hydropower is a renewable, reliable and highly efficient source of energy. Hydropower has the ability to run as a base load and to adjust load rapidly. This makes hydropower suitable for coupling with other renewable energy sources to stabilize frequency fluctuations. This ability has been used increasingly over the last decade due to the deregulation of the electricity markets and the introduction of other renewable energy sources, such as wind power. These changes have involved a substantial increase in the load variations and frequent start-stops. Such operating conditions may lead to unnecessary stresses and losses in turbines. Throughout the world, hydropower is the largest renewable source of energy. Currently, there is a need for refurbishment of old hydropower plants because most of them have reached the end of their design period. Efficiency may be improved by upgrading these older turbines. During the design or refurbishment phase of turbines, model testing and computational fluid dynamics (CFD) are the main tools available to predict, test and verify the performance as well as to investigate the flow characteristics.The use of CFD in the design and refurbishment process is becoming increasingly popular due to its flexibility, detailed flow description and cost effectiveness compared to model testing, which has been used over the last century of turbines development. However, issues still must be resolved due to the combined flow physics involved in hydropower machines, such as flow turbulence, separation, vortices, unsteadiness, swirl flow, strong adverse pressure gradients, convoluted geometry and numerical artifacts. Therefore, experimental data in such complicated systems are required to validate the numerical simulations and develop more accurate models.This thesis presents an experimental and numerical investigation performed on a reaction type axial water turbine. The investigation was performed on a model known as the Porjus U9. It is a geometrically similar model of the prototype turbine produced on a 1:3.1 scale. The main objectives were to characterize the flow phenomena in this modern Kaplan turbine model, to build a data bank for the validation of the CFD tools and to study the scale-up between the model and prototype, because the corresponding prototype is available for similar experiments. The investigation was performed at three different operating points: part load, best efficiency point (BEP) and high load. The technique used to investigate the flow was laser Doppler anemometry. The investigation was performed with time- and phase-averaged velocity measurements in several sections of the turbine where different periodic and non-periodic flow phenomena were captured. Some engineering quantities were also calculated to describe the turbine characteristics, such as the pressure recovery factor and the swirl number. At off-design operations, vortex breakdown was present.The numerical analysis of the model is also presented, where several RANS turbulence models were tested. The aim was to evaluate the capability of the turbulence models to predict the flow physics in the water turbines at the BEP. Validation was made with the experimental results in order to extend the range of confidence in the CFD results.