Stability Operating Conditions of a Francis Turbine

Introduction

Hydroelectric power plants are known for their ability to cover variations of the consumtion in electrical power networks. In order to follow this changing demand, hydraulic machines are subject to off-design operation. In that case, the swirling flow leaving the runner of a Francis turbine may act under given conditions as a hydroacoustic excitation source for the whole hydraulic system. Two types of excitation may occur and result respectiveley in system resonance and instability. Thefirst excitation appears at part load operation behaves like an external forcing function exciting the system at given frequencies leading to a resonance phenomena. The second excitation, may occur at high load operating conditions, and behaves as an internal energy source which leads to the self excitation of the system.

Doctoral student : Sébastien Alligné

Objectives

The aim of the research project is to be able to predict and to simulate the resonance and instability of a Francis turbine connected to a hydraulic system. CFD computations of the vortex rope and hydroacoustic simulations of the connected hydraulic system with the SIMSEN software, are the means available to reach this purpose. The vortex rope excitation parameters must be identified in the CFD computations and injected in the hydroacoustic simulation to predict the occurence of resonance or instability. Both CFD and hydroacoustic simulations are performed concurrently with a coupling method which allows to inject rope parameters in real time into the hydroacoustic model, while the hydroacoustic computation provides realistic boundary conditions for the CFD computation.