Research on cavity vortex of draft tube under high load condition of Francis turbine

In order to investigate the distribution of the internal flow field in a Francis turbine under high-load conditions and to analyze the flow instability issues caused by cavitation vortex zones, a numerical model for two-phase flow in a Francis turbine was established based on the Finite Volume Method (FVM). Using the SST k-ω turbulence model and the Z-G-B cavitation model, the distribution of cavitation phenomena in the runner and draft tube under various high-load conditions was studied, and the flow effects of blade cavitation and draft-tube cavity cavitation under different cavitation numbers were revealed. It was found that cavitation of varying degrees occurs in both the runner and draft tube when the turbine operates under high-load conditions, with the cavitation vortex rope in the straight conical section of the draft tube presenting a spindle-like shape. Moreover, the cavity cavitation in the draft tube exhibits periodic variation in vapor volume fraction, which may lead to low-frequency fluctuations in the turbine′s external performance parameters and pressure pulsations, indirectly resulting in low-frequency power oscillations. The findings can provide a reference for suppressing cavitation in Francis turbines under high-load operation.