Low-Frequency Phenomena in Swirling Flow

The presence of swirl is often the cause of separations and pressure fluctuations in the draft tube of hydraulic reaction turbines, in particular Francis turbines. At the design point, water turbines generally operate with little swirl entering the draft tube and no flow separations, but at off-design, at both high and low load, the flow leaving the turbine has a large swirling component. The present chapter describes a number of physical mechanisms that work to produce the pulsation. Their occurrence and impact depend mainly on the actual flow rate of the turbine, but also on the local pressure level, on the exit velocity field of the particular runner, the shape of the draft tube, and the dynamic response of the whole hydraulic circuit. In medium- and large-sized turbines, the frequency of these ‘draft tube surges,’ which are often approximately periodic, is of the order of 1 Hz, and therefore they may also produce significant electrical power swings. Given a sufficiently strong vortex flow, comparable phenomena may also occur in other locations, for example in the runner channels, or in penstock manifolds.