NUMERICAL ANALISYS OF A SWIRLING FLOW GENERATOR AT LOWER RUNNER SPEEDS

When the hydraulic turbines (especially Francis turbines) operate at partial discharge, the decelerated swirling flow downstream the runner becomes highly unstable, with the development of a spiral vortex breakdown, also known as pressing vortex rope in the engineering literature. The flow unsteadiness from the draft tube cone results in severe pressure fluctuations that hinder the turbine operation. In order to investigate the flow unsteadiness, an experimental test rig was developed in our laboratory,[1]. The test rig is used to determine the parameters of the swirling flow with vortex rope and different methods to control the vortex rope in order to mitigate the pressure fluctuations, [2]. The main component of the test rig is the swirl apparatus with two parts: the swirl generator and the test section similar with the draft tube cone from a real turbine. The swirl generator from our test rig has two blade rows. The upstream non-rotating blades (guide vanes) produce a free-vortex tangential component, while keeping the axial velocity practically constant. The second row of rotating blades (free runner) is used to create a specific energy deficit near the hub with a corresponding excess near the shroud. The runner spins at the runaway speed, acting as a turbine near the hub and as a pump near the shroud, with vanishing overall torque, [3]. The paper focuses on numerical evaluation of energetic and hydrodynamic behaviour of the flow from the outlet of free runner at lower runner speeds. The energetic behaviour of the free runner consists in the analysis of the moment and power, while from the hydrodynamic behaviour is analysed the swirl configuration. By reducing the speed of the free runner is obtained a swirling flow in the draft tube cone, similar with a real runner operated at different discharge and head. This new approach involves to operate a swirl generator in order to obtain the same configuration of the swirling flow in the draft tube inlet as a real runner. First, 3D steady turbulent flow is performed in the experimental swirl apparatus configuration. The mixing interface method is used in order to couple the non-rotating domains with free runner computational domain. In our computation only one guide vane and one free runner interblade channel is selected. Second, from the computation of the free runner at different speeds, is analysed the hydrodynamic behaviour of velocity profiles (circumferential and meridian distribution) in order to evaluate the swirl configuration and correlate with the velocity profiles from a real runner at the outlet. REFERENCES Susan-Resiga R., Muntean S. , Bosioc A. , Stuparu A. , Milos T. , Baya A. , Bernad S., Anton L.E., Swirling Flow Apparatus And Test Rig For Flow Control In Hydraulic Turbines Discharge Cone , 2 nd Iahr International Meeting Of The Workgroup On Cavitation And Dynamic Problems In Hydraulic Machinery And Systems, Timisoara, October 2007, Scientific Bulletin of the Politehnica University of Timisoara, Romania Transactions on Mechanics, Tom 52(66), Fascicola 6, ISSN 1224-6077, pp. 203-217. Susan Resiga, R., Muntean, S., Hasmatuchi, V., Anton, I., Avellan, F., „Analysis and prevention of vortex breakdown in the simplified discharge cone of a Francis turbine”, Journal of Fluids Engineering, 132, p. 051102, 2010 Muntean S., Bosioc A., Stanciu R., Tanasa C., Susan-Resiga R., “3D Numerical Analysis of a Swirling Flow Generator”, Proceedings of the 4 th International Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, pp. 115-122, ISBN 9788670837409, 2011;