In the present energy scenario, hydropower accounts for about one-sixth of energy generation worldwide. Apart from being a clean, renewable, and highly efficient energy source, modern hydraulic turbines are supposed to provide grid stability. They are subjected to less efficient operating conditions far from their design point or best efficiency point (BEP). This off-design operation may lead to flow instabilities such as vortices, stagnant regions, or flow reversals in the draft tube.Francis turbines are singly regulated turbines and have fixed pitch blades. They are worst affected by changes in operating point. At part-load (PL) operation, a spiral rotating vortex rope (RVR) develops in the draft tube cone due to swirling flow at the runner exit. This RVR rotates in the direction of runner rotation and has a precession frequency in the range of 0.2-0.4 times the rotational frequency of the runner. The formation of RVR is accompanied by severe pressure pulsations, power swings, fatigue damage, and torque fluctuations in the turbine unit. These detrimental effects may lead to runner failure in extreme cases.Over the years, researchers proposed different methodologies to either mitigate the vortex rope or to reduce its adverse effects for broadening the turbine operating range. Air and water injection in the draft tube are two such methods. The main advantage of these methods is that they may be varied with the variation of turbine operating point. Air injection in the draft tube is one of the most widely used and effective methods to reduce the pressure fluctuations in the draft tube. It leads to the damping of low-frequency pressure pulsations and affects the water flow topology by softening the central core of the RVR. The water jet injection was reported to increase the momentum in the central region and to suppress the pressure fluctuations associated with the RVR. However, water injection was only studied on simplified test rigs, and its implementation in prototypes requires further investigations.The present work reports the numerical simulations performed to compares the effect of axial air and water injections on the flow field in a Francis turbine unit at PL operation. A small turbine test rig is under erection at IIT Roorkee, India, with the aim to study the flow field characteristics during RVR formation and air/water injection methods to mitigate it. The work presents the simulation results of this turbine unit at the part-load operation at which a fully developed RVR is estimated. The simulations have been performed using commercial software ANSYS CFX. The results compare the effect of fluid injections on the pressure pulsations in the draft tube and their influence on the turbine operation.

Comparison of Axial Water and Air Injections in the Draft Tube of a Francis Turbine for RVR Mitigation

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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