Flow Field Investigation in Draft Tube of Francis Turbine at Off-Design Operation Using a Vortex Identification Algorithm

At off-design operations, flow instabilities such as vortex breakdown, reverse flows, and stagnant regions are observed in Francis turbines. The present work shows the numerical flow field investigations of a Francis turbine at two different part loads (PL) by employing a vortex identification algorithm. The analysis has been performed at various locations in the draft tube by extracting the velocity fields at different time steps of the simulation. The first operating point involves a fully developed rotating vortex rope (RVR) in the draft tube, which precesses at a frequency of 0.28 times of the runner rotation. The present algorithm is able to identify the regions along with the eccentric local rotation center. The second operating regime shows characteristics of deep part load with central solid body rotation in the draft tube flow field. The results show highly swirling flows with very low axial velocity. The flow is confined primarily near the walls. The analysis shows that the extent of stagnation region at deep part load is more and no inner shear layer is present as compared to the part-load operation. The spatial harmonic decomposition (SHD) of the pressure data is also performed to evaluate the synchronous and asynchronous components of pressure pulsations.