Experimental and numerical investigation of axial turbine models

Abstract: Hydropower is a versatile renewable source of power generation able to change rapidly operating conditions. Hydropower plants may today work over a larger operating range than designed for due to the introduction of renewable sources of energy and the deregulation of the electricity market. Such operating conditions may involve large stresses and losses due to complex unsteady and transient flow phenomena, which have to be taken into account under design or refurbishment phase. The use of computational fluid dynamics (CFD) in the design and refurbishment process is becoming increasingly popular due to its flexibility, detailed flow description and cost-effectiveness comparing to model testing used since a century in the development of turbines. However, issues have still to be resolved due to the combined flow physics involved in hydropower machines such as partly separated flow at curved surfaces, vortices, unsteadiness, swirl flow, strong adverse pressure gradients, convoluted geometry as well as numerical artefacts. Therefore, experimental data in such complicated systems are required to validate numerical simulations and develop more accurate models.The first part of this thesis is a numerical investigation of the three-dimensional flow of the axial Hölleforsen model spiral casing and distributor, where the influence of the penstock on the flow is analysed using different turbulence models and inlet boundary conditions. Comparisons with experimental results indicate the importance of the penstock to perform accurate simulation in the present case. Therefore, detailed inlet boundary conditions are necessary to simulate accurately the spiral casing flows if the penstock is not included in the simulation.The second part of the thesis focuses on an experimental investigation of an axial hydropower turbine model known as Porjus U9. The measurements are part of a project aiming to investigate experimentally the flow in different regions of the machine to build a data bank in order to validate numerical simulations and study scale-up efficiency between model and prototype, since the corresponding prototype is available for similar experiments. The investigation was performed at 3 different working points: part load, best efficiency point and high load. The inlet flow of the spiral casing as well as some sections in the spiral casing and draft tube are investigated with a two components laser Doppler anemometer (LDA). To improve the signal quality and measurement accuracy refractive index matching optical box was mounted on the circular pipe of the spiral casing inlet. LDA result of the mean velocities and corresponding RMS are presented to investigate the flow before the runner and at the inlet of the spiral casing, since the flow is influenced by the existence of a bend before the inlet. The results of the draft tube measurements are also presented. Good quality data are obtained for initial boundary conditions at the inlet of the casing and drafttube cone to perform numerical simulations.