EXPERIMENTAL INVESTIGATION OF FORMATION AND MITIGATION OF VORTEX ROPE IN AN ELBOW DRAFT TUBE

Abstract

Continual and intermittent energy sources are exploited to meet the constantly increasing global electricity demand. Hydropower contributes to about one-sixth of the world's energy needs. The inclusion of power generated by intermittent sources, like solar and wind energy, in the power supply results in grid instability issues. Moreover, the continuously varying energy demand requires the power supply to be highly dynamic. These fluctuating energy demands are usually met using hydropower plants because of their capability to change their power output swiftly. As a result, the hydraulic machines are frequently subjected to off-design operations and quick set-point changes. The flow field of the turbine is severely affected due to these off-design operations. Francis turbines, in particular, are the worst affected because of their fixed-pitch blades. Specifically, the draft tube (DT) displays an abrupt increase in hydraulic losses with deviation from the best efficiency (BEP) condition. The flow in the DT is swirling and involves strong adverse pressure gradients, flow separation, and secondary flows. After a particular condition, this decelerating swirling flow becomes unstable, leading to flow instabilities such as the rotating vortex rope (RVR). The development of RVR at part load (PL) operation is a source of pressure fluctuations in DTs and power swings, which may lead to runner failure under extreme conditions. The fatigue damage rate at PL may be 100 times higher than that of the BEP. The low-frequency pressure pulsations, pulsative pressure recovery, power swings, noise and vibrations are linked to the precessing nature of the RVR. These pulsations can be even more detrimental if the pulsating frequency is near the natural frequency of turbine components, especially the rotor. The sense of rotation of this vortical structure at PL is the same as that of the runner, with a precessing frequency of around 20-40% of the runner's rotational frequency. The decrease in efficiency of a reaction turbine at off-design conditions is mainly related to a poor pressure recovery in the DT owing to the presence of RVR.