Effects of vortex rope formation in draft tube of reversible pump-turbine - A review

Reversible pump-turbines need to operate often at off-design operating conditions to stabilize the electric grid. These operating conditions cause hydraulic instabilities due to the development of vortex rope in the draft tube. The research and design communities of reversible pump-turbines are paying attention to this hydraulic instability being important in ensuring the stability and safety of the functioning of a pumped hydro storage plant. The objective of the study is to synthesise the existing research and to enhance the understanding of the mechanism of vortex rope formation, its associated pressure fluctuations and its mitigation strategies in reversible pump-turbines under different operating conditions. Such understanding is important for enhancing the efficiency, reliability, and lifespan of reversible pump-turbines by improving the design and optimisation of future reversible pump-turbine systems. The study identifies that the intensity of the vortex rope is highly sensitive to the residual angular momentum of water leaving the runner. The flow decelerates with an adverse pressure gradient, resulting in a flow reversal; thus, a stagnation zone is formed, which leads to vortex breakdown and the development of vortex rope in the draft tube. The rotating vortex rope causes pressure fluctuations in the draft tube which can lead to vibrations and structural fatigue failure of the unit. Further, the vortex rope in the draft tube brings the power swings in the pumped hydro storage plants. The study recommends different mitigation techniques to overcome the adverse effects of vortex rope formation and its associated pressure fluctuations. The implementation of geometrical and fluid mitigation techniques is essential to avoid conditions that favour the development of vortex rope and to reduce the pressure fluctuations in the draft tube.