Investigation of role of fins in a Francis turbine model's cavitation-induced instabilities under design and off-design conditions

Abstract

Harnessing hydraulic energy for power generation using Francis turbines presents formidable downstream swirl instabilities, which can lead to pressure surges at part load (PL) conditions. Cavitation-induced issues affect not just the PL operations but also the power generation at the best efficiency point (BEP). To combat these challenges, fins strategically placed on the draft tube (DT) periphery aid in low-pressure recovery, which suppresses the hurdles for stable energy extraction. This ongoing study delves deep into the internal flow dynamics of the turbine, considering various Thoma numbers, and evaluating the impact of fins. The investigation employs numerical simulations utilizing structured and unstructured grids, incorporating the Shear Stress Transport (SST) model and the Rayleigh-Plesset model to model turbulent flow in a two-phase mixture. Preliminary findings reveal that at PL, the presence of fins substantially reduces low-pressure zone formation, minimizing vortex ropes and associated instabilities. The fins positively influence axial velocity, reducing swirl intensity and mitigating pressure pulsations by 60 % at PL. Conversely, at the BEP, the impact of fins appears less pronounced, with zero detrimental effect. In conclusion, with no negative effect at BEP, the incorporation of fins enhances the turbine's operational range and ensures smoother and more stable energy production at PL.