Numerical investigation of the flow behavior into a Francis runner during load rejection

The main objective of the work presented in this paper is to investigate numerically the flow behavior inside a Francis hydro-turbine during the transient event of load rejection. First, a theoretical description of the flow during the event is presented in order to predict the global flow characteristics to be anticipated since no velocity profiles are available for this transient event. The issue of choosing the proper boundary conditions to obtain the absolute pressure and the correct flow characteristics within the runner when using a typical truncated geometry is then discussed. Finally, by using a hypothesis of ''quasi-stationarity'' and a validated methodology, global flow characteristics within the turbine are highlighted near the no-load operating condition and the unsteady vortical motions within the runner are assessed