Unsteady Simulation for Francis Turbine During Load Rejection Events

This paper presents an automated tool chain for simulating Francis turbine behavior during the transient processes induced by a load rejection event. The proposed methodology combines a commercial CFD solver and a user function and scripts to address the simulation challenges caused by the wicket gate motion and runner speed variation during emergency shutdown. Mesh deformation and re-meshing techniques are used to simulate the large displacement of the wicket gates. The runner speed variation is computed using an angular momentum equation implemented in a user defined function. The proposed methodology was developed and validated by performing 2D unsteady simulations on a high head model Francis turbine used in the Francis-99 workshop, followed by a 3D unsteady simulations on a medium head Francis turbine. These simulations allow computing the evolution of engineering quantities such as turbine angular speed, flow physics and unsteady load on blades during the process. The validation of CFD results with experiments showed 9% discrepancy in the prediction of runaway speed. The investigation of flow physics reveals the presence of complex flow structures such as reversed flow (pumping flow) near the draft tube cone center and a downward tangential flow near the cone wall of the draft tube. Pressure fluctuations are captured when the Francis turbine operating point moves through conditions of zero and negative torque. The proposed methodology is fast and simple to present a qualitative analysis of the flow physics and the turbine behavior during load rejection.