Swirling flow in a hydraulic turbine discharge cone at different speeds and discharge conditions

•Swirl number limitations is revealed when shifting toward non optimal operating regimes.•Boundaries of unstable PVC and vortex rings formation lead in range S = 0.4–0.5.•Maxima of frequency and amplitude of wall pressure surge do not coincide in operating map.•PVC patterns and velocity distribution are differed at S values 0.42 and −0.42. An experimental investigation of the cavitating turbulent flow in a simplified hydraulic turbine was carried out to examine flow features in a wide range of speed and discharge conditions. A swirling device that allows reproducing the speed distribution behind the runner of a real turbine was manufactured using a rapid 3D prototyping technology. Laser Doppler velocimetry was used to measure both axial and tangential velocity components at the runner outlet for 96 operating regimes. The limitations of the swirl number used for swirling flow characterization through swirl free speed and discharge conditions were investigated. Particular attention was paid to the instability of the precessing vortex rope in the transition regime at a low swirl number. The boundaries of the regime with an aperiodic pressure surge in terms of the swirl number were defined, contributing to insight into the stability of swirling flow in hydraulic turbines.