Abstract: The flow features are presented on different openings in Francis turbine by unsteady numerical simulation. The unsteady flow results are as follows：when the opening is 12 mm, pressure fluctuation at runner outlet near hub is not obvious and pressure fluctuation amplitudes near midstream and shroud are bigger. Helical vortex is situated on conical tube on opening 12 mm. It will disappear at the elbow tube. Pressure fluctuation of low frequency in draft tube is mainly located at conical and elbow tube. The frequency of helical vortex is low and is about 0.233 times that of runner frequency. And pressure fluctuation amplitudes in conical tube is higher. Frequencies of two points on the same measurement section are almost equivalent. But when the opening is 24 mm, there is no back flow region at runner outlet near hub and pressure fluctuation amplitude is smaller. And the ones near midstream and shroud are greatly higher. The frequency is several times that of product of runner frequency and blade number. The columnar vortex formed in conical tube will become small helical vortex in the elbow tube. The result is that the long vortex rope forms the cutting effect like drilling rig on the empennage. It will have serious effect on the tubine efficiency and stability. The length of vortex rope provides the theoretical basis for studying the depth of Francis turbine draft tube and optimization design.

Key words: Draft tube vortex rope, Francis turbine, Pressure fluctuation, Unsteady flow