Mitigation of self-excited oscillations at full load: CFD analysis of air admission and effects of runner design

In full and over load operating points some Francis turbines experience strong self-excited pressure and power oscillations, which restrict the range of operation and maximum output of the turbine. Previously the authors proposed a 1D-3D two-phase model and numerical method for investigation of this phenomenon. In the present paper this model is further extended and applied to investigation of countermeasures, used for prevention of high load oscillations. First, the third phase non-condensable air is introduced into the model in order to investigate the effect of air admission. Then, several modifications of runner cone are examined, showing negligible effect on the amplitude and frequency of full load oscillations. Next, different modifications of runner blade shape are considered, giving different axial and circumferential velocity profiles downstream the runner. It is shown that variation of blade shape significantly affects the onset and intensity of self-excited oscillations. The obtained results indicate that proper runner design is able to eliminate high load instability without the need of air admission and reduction in turbine efficiency.