Cécile Münch

RANS computations of tip vortex cavitation

The present study is related to the development of the tip vortex cavitation in Kaplan turbines. The investigation is carried out on a simplified test case consisting of a NACA0009 blade with a gap between the blade tip and the side wall. Computations with and without cavitation are performed using a RANS modelling and a transport equation for the liquid volume fraction. Compared with experimental data, the RANS computations turn out to be able to capture accurately the development of the tip vortex. The simulations have also highlighted the influence of cavitation on the tip vortex trajectory.

U-RANS Simulations and PIV Measurements of a Self-excited Cavitation Vortex Rope in a Francis Turbine

In the course of the massive penetration of alternative renewable energies, the stabilization of the electrical power network significantly relies on the off-design operation of turbines and pump-turbines in hydropower plants. The occurrence of cavitation is, however, a common phenomenon at such operating conditions, often leading to critical flow instabilities, which undercut the grid stabilizing capacity of the power plant. In order to predict and extend the stable operating range of hydraulic machines, a better understanding of the cavitating flows and mainly of the transition between stable and unstable flow regimes is required. In the case of Francis turbines operating at full load, an axisymmetric cavitating vortex rope develops at the outlet runner in the draft tube. The cavity may enter self-oscillation, with violent periodic pressure pulsations propagating throughout the entire hydraulic system. The flow fluctuations lead to dangerous electrical power swings and mechanical vibrations through a fluid-structure coupling across the runner, imposing an inconvenient and costly restriction of the operating range. The paper deals with a numerical and experimental investigation of the transition from a stable to an unstable operating point on a reduced scale model of a Francis turbine at full load. Unsteady homogeneous two-phase RANS simulations are carried out using the ANSYS CFX solver. Cavitation is modelled using the Zwart’s model that required solving an additional transport equation for the void fraction. Turbulence is solved using the SST k-ω model. Simulations are compared with the experimental measurements and some insights are provided for a first comprehensive analysis of the transition between the stable and unstable states.

URANS Models for the Simulation of Full Load Pressure Surge in Francis Turbines Validated by Particle Image Velocimetry

Due to the penetration of alternative renewable energies, the stabilization of the electrical power network relies on the off-design operation of turbines and pump-turbines in hydro-power plants. The occurrence of cavitation is however a common phenomenon at such operating conditions, often leading to critical flow instabilities which undercut the grid stabilizing capacity of the power plant. In order to predict and extend the stable operating range of hydraulic machines, a better understanding of the cavitating flows and mainly of the transition between stable and unstable flow regimes is required. In the case of Francis turbines operating at full load, an axisymmetric cavitation vortex rope develops at the runner outlet. The cavity may enter self-oscillation, with violent periodic pressure pulsations. The flow fluctuations lead to dangerous electrical power swings and mechanical vibrations, dictating an inconvenient and costly restriction of the operating range. The present paper reports an extensive numerical and experimental investigation on a reduced scale model of a Francis turbine at full load. For a given operating point, three pressure levels in the draft tube are considered, two of them featuring a stable flow configuration and one of them displaying a self-excited oscillation of the cavitation vortex rope. The velocity field is measured by two-dimensional (2D) particle image velocimetry (PIV) and systematically compared to the results of a simulation based on a homogeneous unsteady Reynolds-averaged Navier–Stokes (URANS) model. The validation of the numerical approach enables a first comprehensive analysis of the flow transition as well as an attempt to explain the onset mechanism.

Rans Computations of a Cavitating Tip Vortex

The Swiss national research project Hydronet 2 gathers a consortium of industrial and academic partners supported by the Competence Center of Energy and Mobility (CCEM) and Swiss Electric Research (SER) in order to improve the hydropower plants. One of the research topic focuses on the cavitating tip vortex. Such a vortex takes place in axial turbines as Kaplan turbines used for producing hydroelectricity. This phenomenon drives a lot of drawbacks such as erosion, unsteady flow rate and a decrease of the turbine efficiency. To better understand the behaviour of the tip vortex, computations of a simple test case are performed. The test case consists in a NACA profile mounted in a channel with a gap between the NACA tip and the lateral wall. The computations are carried out with the OpenFOAM solver both in one-phase and two-phase flows. The turbulent motion is modelled with a RANS approach. For the two-phase flow computations, the phase change between liquid and vapour is achieved with the model proposed by Kunz. The results will be compared in cavitating and non-cavitating cases with the experimental data provided by the EPFL Laboratory for Hydraulic Machines. The comparisons deal with global picture of the flow, the trajectory of the tip vortex and the velocity field downstream the NACA profile.