High-Fidelity Simulation and Validation of Transient Operation of Model Francis Turbine

The contemporary global energy mix and its evolution mandate off-design and transient operation of hydraulic turbines to stabilize the variable renewable energy-related power fluctuation in the electric grid. This work numerically investigates a transient operational sequence from high load to best efficiency point in a model high head Francis turbine. The dynamic meshing technique in OpenFOAM v2012 software is used to morph the mesh around guide vanes as they close to reach the best efficiency point. Shear stress transport-Scale adaptive simulations based approach is implemented to achieve turbulence closure. In view of the mesh and flow complexity, the simulation is run in parallel mode on a high-performance computing cluster, decomposed over 320 computing cores, consuming a wall time of 9 days. Axial, radial and tangential velocity fields at high load are determined. Evolution of axial velocity over time is tracked to achieve a complete flow characterization across the transition to best efficiency point operation. High load operation is a typical off-design condition characterized by the occurrence of vortex breakdown phenomenon in the draft tube cone. An enlarged vortex core with a centrally stagnated flow wrapped by a well-sped outflow is seen. An intermittent flow reversal, more pronounced at upstream locations, is also present along the axis of conical diffuser. As load rejection procedure commences, the disorganized velocity field presenting a wake-like axial velocity profile at high load condition gradually gives way to a non-defective jet-like flow at best efficiency point.