Determination of Hydroacoustic Draft Tube Parameters by High Speed Visualization during Model Testing of a Francis Turbine

Francis turbines can experience critical instabilities at high load operating points, limiting their maximum power output. The swirling flow developed in the draft tube produces a cavitating axisymmetric volume, acting as an internal energy source leading to a self-excited surge phenomenon. The pulsation of the vortex rope corresponds to one of the eigenfrequencies of the hydraulic system. Efforts to accurately characterize, simulate and predict this phenomenon have been undertaken by several researchers, using a 1-D hydroacoustic model of the full load vortex rope. The key physical parameters are the mass flow gain factor, standing for the excitation mass source of the hydraulic system, the cavitation compliance factor, representing the wave speed and the thermodynamic damping, modeling the energy dissipation between the liquid and the gas. These parameters need to be determined either numerically or experimentally. The aim of the present investigation is to determine the mass flow gain factor and the cavitation compliance using experimental data obtained during a measurement campaign on a reduced scale Francis turbine model and to compare the results to existing CFD data.