Abstract: Vortex ropes and pressure pulsations in a hydropower unit－under regulating operation of frequency, peak load, or new energy fluctuation－severely restrict regulation capability of the unit and have attracted much attention. In this work, a RANS-LES method is used to simulate the vortex ropes and pressure pulsations in the draft tube of a Francis turbine under the mode of variable-speed operation. The results show that the strength of vortex ropes is decreased obviously near the vortex elimination speed (VES) but increased when the speed deviates from VES. From VES to the optimal speed, turbine efficiency is increased, though the number of vortex ropes and the amplitude of pressure pulsations are increased. At the optimal speed, the turbine achieves its highest efficiency, 7% higher than that in the conventional fixed-speed operation mode.

Key words: Francis turbine, variable-speed operation, vortex rope in the draft tube, pressure pulsation, operation stability