Improving the overload performance of a Francis turbine by means of a passive vortex control device

The increasing demand for flexible operation of hydropower plants forces Francis turbines to operate frequently under off-design conditions, where draft-tube flow instabilities can severely limit performance and structural integrity. In particular, overload operation is associated with strong residual swirl at the runner outlet, leading to vortex-induced pressure pulsations and reduced pressure recovery. Despite its growing relevance, effective mitigation strategies specifically targeting overload conditions remain limited. This study proposes a novel passive vortex-control concept based on a bladed runner cone, inspired by boss-cap fins commonly used in marine propellers. The device is conceived as a minimally invasive retrofit solution aimed at reducing the tangential velocity component at the draft-tube inlet during high-discharge operation. Unsteady numerical simulations are performed on a medium–specific-speed Francis turbine over its entire operating range, from deep part load to overload conditions. The results show that the proposed device effectively suppresses overload vortex structures and significantly reduces draft-tube pressure pulsations, while improving diffuser pressure recovery. At the highest investigated discharge, an increase in hydraulic efficiency of up to approximately two percentage points is achieved, with only minor efficiency penalties at nominal and part-load conditions.