Aspects of vortex breakdown phenomenon in hydraulic turbines

Operational flexibility of hydraulic turbines is impeded by the vortex breakdown phenomenon. The phenome-non, a consequence of instability in vortical flows in general, manifests as a rotating structure called vortex rope located in the central zone of a draft tube diffuser in case of hydraulic turbines. Occurrence of vortex breakdown is ascribed to an instability prone swirl in the flow exiting the runner at off-design operating conditions, the part load and high load. The intended point of operation by design, the best efficiency point, however, yields a nearly swirl-free flow at the runner outlet and as such is devoid of vortex breakdown. Formation of the rotating vortex rope causes low frequency pressure fluctuation which may transmit to the entire hydraulic circuit. Power swings may occur if the frequency of pressure fluctuation resonates with the system natural frequency. Due to chaotic flow, losses in the diffuser increase and pressure recovery is compromised leading to a drop in efficiency. This work deconstructs and links the important literature to build a holistic understanding of the vortex breakdown phenomenon and related effects in hydraulic turbines.