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Abstract
The unstable flow caused by the vortex rope in the Francis turbine draft tube has impacted the safe operation of hydraulic turbines. The Francis turbine of Ban Coc Hydropower Station was taken as the research object to conduct an in-depth study of the flow characteristics in the draft tube. Based on the SST k-ε turbulence model, numerical simulation of the full-flow channel was performed under different working conditions to analyze the flow characteristics in the Francis turbine draft tube. The results show that when the guide vane is opened at the design opening, the pressure is symmetrical distributed, and there is no low-pressure zone at the draft tube inlet; when the guide vane opening is the largest for the flow rate Qmax, the low-pressure zone increases and has a columnar shape vortex rope; when the guide vane opening is partially opened (Qmin), a spiral distribution of the low-pressure zone appears, that is, a spiral cavitation vortex rope is formed in this area. This phenomenon will cause greater vibration in the hydraulic turbines.