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THE EFFECT OF VORTEX FLOW IN A HORIZONTAL FRANCIS TURBINE DRAFT TUBE

The effect of vortex flow in off-design conditions on the performance of the draft tube in a horizontal Francis turbine has been investigated numerically. DTo validate the numerical approach, having experimen­tal data in the draft tube is the immediate objective of this research. Thus, a three-hole pressure probe has been designed and mounted insicfe tlie draft tube for measur­ing the axiaf and tangential components of the velocity field. The specific speed of the turbine in the test rig was such that the radial component of the velocity at the inlet of the draft tube could be neglected, which jus­tifies implementation of the applied probe, especially at the inlet of the draft tube. The velocity field is measured by traversing the probe at two sections of the inside of the draft tube. The velocity field data at the inlet, of the draft tube are used as the boundary conditions of the 3D numerical analysis. It is well known that a high intensity vortex causes con­siderable degrees of anisotropy in stress and dissipation tensors, leading to a highly anisotropic eddy viscosity. Thus, all conventional eddy-viscosity-based models will no longer be valid under these conditions and Reynolds stresses in Navier-Stokes equations are modeled by using the RSM formulation. In this study, an emphasis is also placed on the influence of the inlet condition. Numerical results are in fairly good agreement with experimental data. Grid indepen­dency is carefully checked in the numerical approach. Several operating points, with different flow rates, under the constant head and rotational speed of the turbine, are investigated. For each case, by applying a numeri­cal simulation at a constant mass flow rate, the pressure recovery factors are calculated in two ways, with and without the circumferential velocity component at the inlet of the draft tube. These are necessary tools to find the influence of the vortex on the draft tube efficiency. It is found that the vortex flow has an adverse effect on the performance of the draft tube and may, conse­quently, decrease turbine production.