Numerical Prediction of Cavitation Free Zone Operation for Francis Turbine

The Francis Hydro Turbine Performance is normally presented in the form of Hill Curves with Unit Head, Unit Flow and Unit Power etc. The Hill Curves give hydraulic performance at variety of flow, head and speed. The cavitation performance of the turbine is marked by curves additionally put on the Hill Curves. Though the turbine could be operated over the entire range specified on the Hill Curves, the site reports have indicated that the operation of turbine is restricted to what is called safe operating zone on Hill Curves. Beyond this region the machine is expected to operate under cavitation, the cavitation occurring at different location on blade, depending on which zone of Hill Curve Diagram the machine is operating. A model test data was available in the form of Hill Curves indicating safe zone of operation. Efforts were made to capture the safe operating zone for this Francis Turbine using numerical approach. A well validated commercial software was used to capture cavitation occurring. The paper describes the approach used and presents results in the form of cavitation, occurring or not occurring, by choosing two operating zones on either side of the safe operating area. The results were highly encouraging. The safe operating zone was clearly identified. The paper also presents results in the form of overall performance and volume fraction distribution etc in both the zones. The paper describes the approach used in numerical experiments and the results thereof.