Decelerated Swirling Flow Control in the Discharge Cone of Francis Turbines

Abstract

The decelerated swirling flow in the draft tube cone of Francis turbines is a complex hydrodynamic phenomenon, particularly when the turbine is operated at partial discharge. In this case, the self-induced instability of an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field, with precessing helical vortex (also called vortex rope) and associated severe pressure fluctuations. The paper presents the development of a swirling flow apparatus designed to generate the same flow conditions as in a Francis turbine at partial discharge, with corresponding helical vortex breakdown in a conical diffuser. This experimental setup allows the investigation of a novel flow control method aimed at mitigating the precessing vortex rope by injecting a water jet along the cone axis. Earlier investigations considered a high speed jet, with relatively small discharge, for stabilizing the flow. However, further parametric studies revealed that a jet with a discharge of up to 10% the turbine discharge and velocity close to the average value at the turbine throat is more effective for mitigating the quasi-stagnant central region associated with the vortex rope. It is shown in this paper that such a control jet can be produced by using a flow feedback method, where a fraction of the discharge is collected from downstream the cone wall and injected upstream along the axis without any additional energy input.