Numerical and experimental investigation of the wake region of vertical axis water turbine

Vertical axis water turbines are generally used to generate electricity from current energy. Darrieus type turbines, one of the most widely used of these turbines, have been used to generate electricity from wind energy for many years, and their use as water flow turbines has been increasing in recent years. Since the density of water is very high compared to that of air, the energy of the water flow obtained per unit area is much higher than that of air. Therefore, it has the potential to extract more power. Darrieus turbines are one of the best options to be used as hydrokinetic turbines, especially in open water channel flows, due to their ease of construction and simple design. Within the scope of the thesis, studies in which experimental and CFD methods are used, with different thicknesses; NACA 0012, NACA 0018 and with different camber; NACA 0024 and NACA 2418 and NACA 4418 profiles were used to examine the hydrodynamic performance of a three-bladed H-type Darrieus turbine. In the study, the experimental performance results of the H-type Darrieus hydrokinetic turbine using the NACA 0018 profile in the literature were verified by using the 2-D and 3-D CFD method using the SST k-ω turbulence model. Performance and flow structures in the wake region were compared with the validated 2-D HAD method for different thickness and camber blades configurations. In the 2-D analysis with the tip speed ratio λ=0.9-1.4, the maximum power coefficient was obtained from the turbine configuration using the NACA 4418 profile at the tip speed ratio of λ=1.2. The flow structures in the wake region of the turbines were compared at the operating point where the maximum power coefficient was obtained for each turbine, and it was determined that the velocity recovery was the fastest in the turbine where the NACA 4418 blade profile was used. In the application of a hydrokinetic turbine farm, both the performance and the velocity recovery in the wake region must be fast in order to increase the amount of energy transferred from the unit area. In this context, based on the results of the 2-D analysis, the prototype of the turbine using the NACA 4418 profile was manufactured and adapted to our existing experimental setup. The turbine in which NACA 4418 profile is used has been analyzed using 3-D LES turbulence model in the tip speed ratio range of λ = 0.5-1.4 and performance experiments in the open water channel have been performed by dynamic torque measurement. In the system where the free stream velocity is 0.3m/s, the maximum power coefficient for the NACA 4418 profile was obtained at the tip speed ratio of λ = 1.0 in both experimental and LES results. At the tip speed ratio of λ = 1.0, where the maximum power coefficient was obtained, the flow structure in the wake region of turbine was examined with the results obtained from the Particle Image Velocimetry (PIV) method and the LES turbulence model to compare the time-average velocity fields, vorticity patterns and turbulence kinetic energy. A turbine with the same geometric characteristics was placed downstream of the turbine whose flow structure in the wake region was examined. The effect of the streamwise distance between the two turbines, the tip velocity ratio and the co-/counter rotation of the two turbines on the performance of the second turbine was investigated by using experimental torque measurement and the analysis using the LES turbulence model. In the results obtained, as the distance between the two turbines increased, the performance of the second turbine increased. The maximum power coefficient for the second turbine was generally obtained in the range of λ = 1.0-1.1 of the tip speed ratio for all downstream locations. Due to the asymmetric flow structure in the wake of the first turbine, the power coefficient of the second turbine obtained when the turbines rotate in counter-rotating with respect to each other is higher than the power coefficient obtained in the co-rotating arrangement. In both experimental and numerical analysis results, while the second turbine was positioned at x/D=11 in the downstream, it reached approximately 72 % of the performance of the upstream turbine in the co-rotation, while this value increased and was obtained around 78 % in the counter-rotation.