Numerical Analysis of Water Jet Injection in the Draft Tube of a Francis Turbine at Part Load Operations

Abstract

The off-design operation of Francis turbines results in the onset of flow instabilities. These instabilities lead to severe pressure pulsations, power swings, fatigue damage, and torque fluctuations in the turbine unit. Axial water jet injection in the draft tube is a relatively recent method proposed to reduce the detrimental effects of flow instabilities on turbine performance. However, its efficacy at different operating points needs to be ascertained before implementing in actual prototype turbines. This work reports the findings of numerical investigations performed with water injection at three different part-load conditions. These operating points represent distinct flow regimes in the draft tube. The effect of water injection on the velocity and pressure fields in the draft tube is investigated. The results indicate that the water jet strongly influences the turbine performance at part-loads involving a precessing vortex rope. However, little influence of water jet is observed at deep part-load operation. The interaction of the jet with the draft tube bend is also investigated. The results show that the amount of water jet needs to be cautiously controlled as higher water jet injection impacting the bend may deteriorate the performance. The influence of water jet injection on the pressure recovery, power output, and efficiency of the turbine unit is also reported.