Hydraulic Disturbance Method to Reduce the Pressure Fluctuation in Francis Turbine Draft Tube

For a Francis turbine, when operating at partial flow rate the fixed-pitch runner shows a strong swirl at the runner outlet which induces a helical vortex (so-called vortex rope) in the draft tube. The precessing vortex rope causes severe pressure fluctuation which effects the steady and secure operating of the turbine. Three-dimensional unsteady turbulent flow simulation with RNG k-ε turbulence model of complete flow passage of a model Francis turbine at partial discharge is performed. To verify the simulation, the model turbine is tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China. An ideal result of the simulation is obtained. The simulation predicts the shape of the helical vortex successfully in the draft tube cone, and the predicted values of the pressure fluctuation frequencies and amplitudes agree well with the test data. The hydraulic disturbance method is introduced, i.e., injecting water with velocity circulation from the runner cone to reduce the pressure fluctuation in Francis turbine draft tube. The injected water with velocity circulation destroys the forming mechanism of vortex rope and eliminates the pressure fluctuation accordingly. The flow in the turbine with water injection is simulated, and it is indicated that with appropriate flow rate and velocity circulation water injection the pressure fluctuation in the draft tube is reduced effectively.