The draft tube flow is a two-sided challenge for the operation of a hydraulic turbine. On one side, it is an important component for the performance of low to medium head turbines, where it can provide up to 40% of the extracted energy from the flow. On the other side, being a diffuser with a complex vorticity distribution at the inlet, vortex breakdown instability can occur at part load and generate a corkscrewed precessing vortex that can be associated with cavitation. The cavitating vortex rope, may generate undesired power output fluctuation and/or structural vibration. Therefore, draft tubes are much studied components but hard to tackle both numerically and experimentally. Within the framework of the AxialT project, the flow in the draft tube of a propeller turbine model operating at part load was studied using a combination of two-phase Particle Image Velocimetry (PIV) measurements and Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. The paper main focus is on the experimental methodology and results. It explains how Particle Image Velocimetry measurements were implemented, validated and post-treated to provide flow measurements in the draft tube cone at part load in the cavitating and non-cavitating regimes. It also describes various image processing techniques used to extract the velocity field around the cavitating vortex rope and to estimate the location of the water-vapour interface of the cavitating region. In the spirit of feeding experimental data to numerical simulations, an analysis of measured velocity profiles just under the runner is presented. Comparison between PIV measurements and preliminary URANS simulations is also illustrated.
In this paper, the rotor-stator interaction in an axial hydraulic turbine is studied with the help of a 2D test case and validation with experimental data. The computational method is presented in the first part of the paper along with the results from the shedding flow behind a square cylinder to investigate a numerical interface between non-matching meshes. The turbulent kinetic energy budget and the centerline velocity past the interface are analyzed and compared with literature. In the second part of the paper, knowledge gained from the 2D test case is applied to 3D simulation of a hydraulic turbine model. Potential interactions are studied using FFT of the time signal on different positions upstream and downstream of a sliding mesh interface. The wake dissipation is investigated for several meridional positions downstream of the turbine guide vanes. The numerical flow field is compared against 2D-LDV experimental measurements at the runner outlet. Numerical results are in good agreement with experimental data.
Part load operation of hydro turbines with fixed pitch blades causes complex instable cavitation flow in the diffuser cone. Application of PIV systems provides the opportunity to investigate the flow velocity and turbulent fields in the case of development of cavitation vortex, the so-called turbine rope, at the outlet of a Francis turbine runner. The synchronization of the PIV flow survey with the rope precession allows to apply phase averaging techniques in order to extract both the periodic velocity components and the rope layout. The influence of the turbine setting level on the volume of the cavity rope and its center is investigated, providing a physical insight on the hydrodynamic complex phenomena involved in the development of the cavitation rope at Francis turbine operating regimes.
For certain geometries of elbow draft tubes of a hydraulic turbine, a drop in the pressure recovery coefficient is observed for a small variation of the flow rate. In order to determine the possible causes of this characteristics shape, the flow field analysis for 4 nearby operating points have been investigated. For velocity and turbulence fields investigation in the outlet section of the studied draft tube, LDV measurements were performed in a transversal section and the 3D-PIV system was qualified for global velocity measurements in longitudinal sections, with an accuracy of less than 3%. By correlating the LDV and PIV results, the quantification of the flow rate through each channel, related to the operating points, and the description of the secondary flow in the outlet zone are possible.
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