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Hydrokinetic Turbines at High Blockage Ratio
Malte, Philip C.
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This study investigates the possibility of using hydrokinetic turbines for both power generation and flow control, with the potential to replace some of the sluice gates in constructed channels. Theoretical and numerical approaches are used to model the horizontal axis hydrokinetic turbines (HAHTs) in open channel flows where blockage ratio is high. The theoretical method uses one-dimensional control volume analysis to predict maximum power that an ideal rotor can extract from the flow as a function of the axial induction factor and the blockage ratio. This method is then compared to the three-dimensional actuator disc model (ADM) developed in the commercial computational fluid dynamic (CFD) code ANSYS Fluent 14.0. This model uses a porous media to represent the HAHTs and Reynolds-Average Navier-Stokes (RANS) equations along with the volume of fluid (VoF) model to solve for the flow field and track the free surface of the water. Finally, the HAHTs are modeled with a more advanced approach, this being the virtual blade model (VBM) in Fluent, which uses blade element theory to consider geometry of the blade. The effects of tip-speed-ratio (TSR) and pitch angle on turbine power extraction and total flow power loss are computed. At high blockage ratios, due to large changes in free surface elevation, surface must be tracked using the VoF model. The VBM is thought to be the best representation of the HAHT at high blockage ratios compared to 1D theory and ADM. Both one-dimension theory and ADM overpredicted the extracted power from the turbines by about 43 percent compared to the VBM. In addition, 1D theory underpredicted the extracted power from the total flow by 30 percent relative to the VBM; while this difference was about 10 percent with the ADM.