MOC-CFD coupled model of load rejection in hydropower station

A modelling study investigates the consequences of transient flow conditions due to a turbine load rejection. The case study considers a large hydropower station with a long penstock. A three-dimensional (3D) Computational Fluid Dynamics (CFD) model is used to represent the spiral casing, guide vanes, runner, and draft tube. A one-dimensional (1D) Method of Characteristics (MOC) solver simulates water hammer in the penstock. The two models are coupled, to simulate a full load rejection. The results are compared with reference to field measurements and a pure 1D solver, combining the penstock and a turbine model based on machine and conveyance characteristics. A comparison of the high level data (head, flow, torque and rotational speed) reveals the two models reproduce the field data reasonably well. The exception being rotational speed toward the zero torque region, where both models underestimate speed. The model predicts high cycle pressure fluctuations on the turbine blade, which would produce serious mechanical loading. The source of the fluctuations is determined to be unstable vortices within the runner.