A Numerical Investigation of Cavitation-Induced Instabilities and its Mitigation Inside a Draft Tube of a Francis Turbine using an Intrusive Structure

Cavitation phenomenon not only affects the part load (PL) operation of the Francis turbine but also the stable energy production at the best efficiency point (BEP). The draft tube’s (DT) peripheral fins help with low-pressure recovery, reducing the cavitation risk. This ongoing study examines the effects of fins and goes further into the internal flow dynamics of the turbine while taking different Thoma numbers into account. The study makes use of numerical simulations with both structured and unstructured grids, modelling turbulent flow with cavitation using the Rayleigh-Plesset and Shear Stress Transport (SST) models. According to preliminary results, fins significantly reduce the creation of low-pressure zones and restore 54% of the pressure at PL. Additionally, it reduces the volume of the vortex ropes by 63.36% and the corresponding swirl strength by 94%. Furthermore, the fins at the runner exit have a good effect on axial velocity, which lessens swirl intensity and attenuates pressure pulses by 60% at PL. On the other hand, the influence of these fins is less noticeable at the BEP, indicating that there is no negative effect of fins at this operating point.