The performance of hydropower turbine in shallow water can be affected by the presence of free surface. Therefore, it is of great interest to investigating the influence of free surface on hydropower turbine performance through computational simulations. For a better understanding of flow field around hydropower turbine operating in shallow water, it is important to analyze the flow over a single hydrofoil beneath free surface first. Therefore, as the first part of this thesis, the Computational Fluid Dynamics (CFD) methodology was used for numerical simulation of 2D unsteady incompressible viscous ﬂow over a hydrofoil under the free surface. The computation was based on finite volume discretization incorporated with the interface capturing volume of fluid method (VOF) to solve the flow field. The SST k-ω turbulence model was used to capture the turbulent flow in the field. A comparison of the present numerical results with experimental data and previous numerical results was presented to show how accurate to use turbulence model to simulate the result. A comprehensive simulation of quantities like wave proﬁles and forces was performed for various angles of attack ranging from -15 to 15 degree, and h⁄c from 0.2 to 0.9 resulting in low Froude numbers ranging from 0.1 to 0.9. It was found that the presence of the free surface reduced the lift coefficient by 33.24% in the case of Froude number of 0.3 and increased the drag coefficient by 79.01%. As the second part of this thesis, the numerical simulations of flow over a 3-blade vertical axis hydropower turbine were performed. A good agreement between the current simulations and previous works was observed through validation process. Then, a comprehensive simulation was performed for submerged depths ranging from h/R = 1.2 to 2, and tip speed ration from λ = 1 to 3 in the case of fix-pitch blades. Variation in submerged depth brought substantial changes in the flow and vortex pattern. The results revealed that the presence of the free surface decreased the power coefficient by 19.05% for the closest submerged depth of h/R = 1.2 at optimal tip speed ratio of λ = 2.5. The wave breaking also occurred when the submerged depth was smaller than h/R = 2. In order to understand the speeds limit for hydropower turbine, free-to-spin cases were investigated by six DOFs method. The top speed had a 4.24% drop by comparing the largest and the smallest submerged depths. Variable pitch improved the power coefficient by 28.10% when the free surface was far from the hydropower turbine. However, the power coefficient improvement became significantly small when the hydropower turbine was place close to the free surface.
|Commitee:||Sardahi, Yousef, Hua, Xia|
|School Location:||United States -- West Virginia|
|Source:||MAI 81/8(E), Masters Abstracts International|
|Keywords:||CFD, Free surface, Hydropower turbine, Tip speed ratio|
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