Analysis of Flow Features in a Propeller Hydroturbine
Open AccessHydropower is a high-performance electrical power generation method and an important source of "green" energy today. It has been in commercial operation for at least a century and North America's fluvial resources are practically fully tamed with dams. In this context, the North American market is specialized in developing solutions to boost existing old hydroelectric power plants with cutting-edge turbine technologies or in adding modern power plants to existing flood control dams which never had their hydroelectric potential developed.This scenario is what put low-head turbines in high demand since they are the most common solution for existing non-powered dams. This work presents the study of flow features in a propeller turbine model, which is a fixed-blade axial flow design, typically more affordable than an adjustable blade Kaplan but on the other hand limited to a narrower operating range. The model is analyzed with modern CFD techniques using existing scale model laboratory measurements as validation. Emphasis has been put on the investigation of turbulent effects at the discharge region of the runner. This research employs the CFD software Ansys CFX using the shear stress transport (SST) turbulence model. Six numerical modeling strategies with increasing in complexity are presented. Their results are compared side-by-side with the utilization of several post-processing techniques. Similar post-processing is also performed with existing experimental data so that the effectiveness of each numerical modeling strategy can be properly assessed. Three common operating conditions were considered in this study: Best efficiency (optimum), maximum power (overload), and partial load. It was observed that the flow at the best efficiency condition is better predicted by the numerical simulations, whereas maximum power and partial load show stronger deviations attributed to flow phenomena such as the central vortex rope. Relevant coherent structures like vortices were observed in both numerical and experimental results. Since axial turbines do not have a rotating outer shroud, a gap exists between the blade and the stationary channel. This feature is often neglected in numerical simulations, but it usually triggers the generation of vortices that cannot be simulated if a proper gap geometry is not included in the numerical grid. This study highlights the importance of modeling such a feature and compares vortices originating it the gap region for different sizes of the gap. It was confirmed that the larger the simulated gap, the larger the velocities inside the vortices, although the trajectories remain rather unaltered. In the past, scientific attention was mainly on medium and high head radial flow Francis turbines. Their designs have been perfected throughout the years. However, little attention has been given to propeller turbines which are low-head axial flow turbines with fixed blade. This work aims to provide a better understanding of the flow behavior inside this type of turbine, and to confirm the hypothesis that modern CFD techniques can successfully predict important flow features in a propeller turbine runner and can be used reliably in the design and assessment of such machine.
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