1 The 4 th International Symposium on Fluid Machinery and Fluid Engineering November 24-27, 2008, Beijing, China NO. 4ISFMFE- IL18 DECELERATED SWIRLING FLOW CONTROL IN THE DISCHARGE CONE OF FRANCIS TURBINES Romeo SUSAN-RESIGA* 1 and Sebastian MUNTEAN 2 *1 Dept. of Hydraulic Machinery and National Center for Engineering of Systems with Complex Fluids “Politehnica” University of Timisoara Bvd. Mihai Viteazu, No. 1, Timisoara, 300222, Romania Tel: +40-256-403692 / FAX: +40-256-403692 E-mail: resiga@mh.mec.upt.ro (Corresponding Author) 2 Hydrodynamics and Cavitation Laboratory, Center for Advanced Research in Engineering Science Romanian Academy – Timisoara Branch Bvd. Mihai Viteazu, No. 24, Timisoara, 300223, Romania Tel: +40-256-491816 / FAX: +40-256-491816 E-mail: seby@acad-tim.tm.edu.ro 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. Keywords: Francis turbine, vortex rope, swirling flow control, axial control jet, flow feedback. NOMENCLATURE p - static pressure 0 p - total pressure Q - volumetric flow rate r - radial coordinate z V , V θ - axial and circumferential velocity components z - axial coordinate Greek symbols α - absolute flow angle β - relative flow angle ρ - density ψ - Stoke’s streamfunction ω - runner angular velocity Superscripts (1) – survey section downstream guide vanes (2) – survey section downstream free runner INTRODUCTION Modern Francis turbines tend to be operated over an extended range of regimes quite far from the best efficiency point because of the variable demand on the energy market which require a great flexibility in operating hydraulic turbines. Therefore, such turbines with fixed pitch runner have a high level of residual swirl at the draft tube inlet as a result of the mismatch between the swirl generated by the guide vanes and the angular momentum extracted by the runner. When decelerating this swirling flow in the draft tube cone the flow becomes unstable, leading to a helical vortex