Swirling instability suppression through a honeycomb-like grid in a giant hydraulic turbine

With the large-scale integration of renewable energy into power grids, large hydraulic turbines serving as critical grid regulators must expand their operating ranges and enhance regulation flexibility. This operational requirement forces the units to operate under off-design conditions for extended periods, consequently inducing swirling flow instability such as spiral vortex ropes and low-frequency high-amplitude pressure pulsations in draft tubes, which significantly compromise unit operational safety and grid regulation reliability. This paper investigates the use of a honeycomb-like grid in the draft tube to evaluate its effectiveness in suppressing pressure pulsations induced by vortex ropes. Results indicate that a honeycomb-like grid effectively breaks down large-scale vortices into small-scale turbulent structures while dissipating kinetic energy. Furthermore, it promotes significantly enhances pressure field uniformity, and suppresses eccentric motion of the swirling flows. The incorporation of the honeycomb-like grid in the draft tube configuration yields significant suppression of pressure pulsations in large hydraulic turbines. Maximum dominant frequency amplitude reductions of 97.4 % at the draft tube center (observed at 93 % of monitoring points), 89.7 % along the wall (87 % of points), and 42.5 % in the vaneless region, while simultaneously eliminating broadband spectral components. Thus, the use of the honeycomb-like grid can broaden the stable operating range of hydraulic turbines.