Flow Characterization of an Industrial Size Francis Turbine Operating at Ultra-Low Load – The Effect of Water Injection

Abstract

Large Eddy Simulations (LES) are carried out for a Francis turbine operating at an ultra-low load with and without injection. The flow rate of the turbine is 40% of the design value. The injection aims to improve turbine operation for the already unstable base case away from the design flow rate. Tangential water injection was introduced through the draft tube wall in the same and opposite runner rotation direction. The injection angle was varied (15°, 30°, 45° and 60°). Two water injection rates were applied at 4% and 8 % of the optimal design flowrate. While injection with the 4% rate and 30° in the opposite runner rotation direction helped reduce pressure fluctuations downstream of the injection inlets; no injection configuration could completely mitigate the power and pressure fluctuations. The injection was found to increase the amplitude of pressure fluctuations close to the injection inlets by 2 to 20 times the magnitude of fluctuations without injection. There was a slight reduction in mean power production (4–10% loss) by injection. The high amplitude fluctuations were observed in power signals with and without the injection.