Analyse et prédiction de la baisse de rendement des turbines Francis par cavitation à bulles

The setting level of an hydraulic machine, specially for low head machines, is decided with respect to the possible alteration of the efficiency due to the cavity development. This alteration can easily be noticed by plotting the evolution of the efficiency η as a function of the Thoma number σ leading to the so-called η – σ cavitation curves. Observation of the cavity extent in the flow passage of the runner allows to associate the drop of efficiency with a particular type of cavity development. However, depending on the type of cavities this drop cannot be very easily explained. Obviously, for a leading edge attached cavity corresponding to high head operating points, the presence of the vapour phase on the blade suction side limits the pressure at the vapour pressure value which causes the flow alteration. In the case of travelling bubble cavitation, corresponding to the outlet cavitation at the nominal head, this limitation of the pressure field due to the presence of the vapour was never characterized. In order to protect the efficiency at the best operating point against the travelling bubbles effects, the IEC norms suggests some measures relating to turbines standards cavitation tests. These measures are based on the systematic control of the test water quality in term of nucleation, which is the main factor of the inception of this type of cavitation. A prediction method of the efficiency drop due to the outlet travelling bubble cavitation is then a necessity from the point of view of the possibility to modify the runner geometry in the first state of the project. The main object of this work is then the understanding of the physical phenomenon acting on the Francis turbine efficiency alteration with a development of travelling bubble cavitation in order to elaborate a prediction method. The first aspect of this thesis work is the characterisation of the bubbles influence with some experiments carried out in the hydraulic machines tests rigs and the high-speed cavitation tunnel. The second part consists in the development of a cavitating pressure field prediction method. The main work is finally intented to analyse the actual standards cavitation tests methods.