Using bionic tubercles to control swirling flow instabilities of a hydraulic turbine during the load rejection process

With the development of renewable energy sources, operations of hydroelectric units under off-designed conditions and frequently transition processes have gradually become normalized, which is accompanied by threats from swirling vortex ropes, high amplitude pressure fluctuations, and strong hydraulic vibrations. This study introduces bionic tubercles into the Francis turbine's draft tube and guide vane design to suppress pressure fluctuation and stabilize swirling vortex rope during the load rejection process. Results reveal that the bionic Francis turbine (BFT) reaches its maximum speed of 686.4 rpm at 3.804s, marking a 6.08 % reduction from the prototype Francis turbine's maximum speed during load rejection. Moreover, it significantly diminishes the principal frequency amplitude of low-frequency pressure fluctuations caused by swirling flow instability from the draft tube cone, achieving a reduction by one to two magnitude orders. It is interesting that the scale of the circumferential vortex in the vaneless region and blade vortices within the runner channels is verified to decrease. More importantly, it is attributed that the bionic tubercles effectively disrupt and disintegrate the walled circumferential vortex structures in the draft tube.