Sammendrag
The calculation of the flow through a draft tube was performed with Fluent 5.0. The grid was generated with the grid generator GAMBIT. The mesh is a combination of a structured and unstructured grid, with high resolution at the boundary layers and regions where large velocity gradients occurs. The grid was refined to avoid cell size dependence using the adaption function in Fluent. The Reynolds Stress turbulence model was used. The choice of the model was based on the work by Dahlhaug (1997), where different turbulence models performed different compared with experiments over a range of swirl numbers. The boundary condition at the inlet was the measured mean velocity and an estimated turbulent kinetic energy. A uniform pressure distribution was used at the outlet. Calculation is provided for the best operation point T on the propeller curve given by Turbine 99 (1999). The calculation shows large velocity gradients in the draft tube cone and small areas in the elbow. At the outlet, regions of boundary layer separation were located at the upper part of the draft tube diffuser.
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