Abstract To investigate the effect of runner cone length and shaft center hole water feeding on flow characteristics of draft tube, the SST turbulence model is used to simulate the three-dimensional unsteady flow in a Francis turbine. Under the rated condition, two cases, i.e. the original runner cone and a longer runner cone are simulated. Under the part-load condition, however, three cases are involved, namely, the original runner cone, the longer runner cone, and the longer cone with shaft center hole water feeding. The streamlines, pressure variations in various cross-sections in the tube, vortex rope and eddy viscosity of flow in these cases are analyzed. The results showed that under the rated working condition the lengthened runner cone can intensify the stability of the flow in the draft tube and improve the turbine efficiency slightly. Under the part-load condition, the lengthened runner cone can reduce the size of vortex rope in the tube but increase the eccentricity of the rope a bit. After a 2% rated flow water feeding is applied to the draft tube, the flow characteristics are improved significantly, the eddy viscosity of the flow in the straight diffusion section is lowered. Both the eccentricity and the rope size are decreased, but also the amplitude of low-frequency pressure pulsation caused by the eccentric vortex rope is reduced considerably.
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