Numerical Investigation on Channel Vortex in a Francis Turbine

When a Francis hydraulic turbine operates under different working heads at small flow condition, the fluid in the flow passage will generate vortex shedding near the blade leading-edge and form the channel vortex in the blade passages due to the mismatch between the outlet angle of guide vane and the inlet angle of runner blade. The severity of channel vortex will trigger high-frequency vibration or generate unit resonant vibration, affecting the operational stability of the turbines. In this paper some typical operation points were chosen out for the steady simulation of a model turbine according to a unit hill-chart. The computational domain was chosen as the whole flow passage from the inlet of the volute to the outlet of the draft tube. Based on RNG k–ε turbulence model, the internal flows was simulated, and the occurrence of vortex between the turbine runner blades was discussed. The numerical results show that the vortex motion near the development-line (IVDL) is stronger than that near the channel vortex inception-line (IVIL) in channel vortex zone marked in the hill-chart. The velocity triangle is used to explain the reasons that channel vortex occur in the suction side at high working head while in the pressure side at the low working head, and two different forms and formation mechanism of the channel vortex were analyzed.