Influence of water admission through main shaft central hole on performance of Francis turbine draft tube

Abstract: Low load operation of hydropower station refers to an abnormal working condition in which a hydraulic turbine operates under small guide vane opening. If the turbine operates under low load condition for a long time, it will affect not only the performance of the turbine, but also the stability of the whole power station and even the power system. Practice shows that, when operating under low load condition, the rotating eccentric vortex causes strong pressure fluctuation and vibration to appear in the draft tube, and threatens the safety of the plant. Generally, injecting air to draft tube is one of the most widely used methods to improve pressure fluctuation at present, but it may cause some acoustic effects. This paper proposes a method of injecting high pressure water from the runner cone to reduce the instability in the draft tube of a Francis turbine. The method does not need to modify the runner's geometry, or add any equipment in the draft tube. In order to verify the feasibility of this method, three-dimensional unsteady numerical simulations of a Francis turbine under low load conditions in a power plant were carried out accurately. The results indicated that there obviously existed an eccentric vortex in the draft tube under part load conditions in the investigation, accompanied by large amplitude pressure fluctuation. The CFD (computational fluid dynamics) results on turbine performance in hydraulic efficiency were observed to be in good agreement with the experiment results. According to the above results, the CFD numerical simulation was further applied to verify the technical effect of high pressure water supply in draft tube. The main research results could be summarized as follows. Firstly, the method of injecting high pressure water to draft tube could effectively reduce the energy loss of the flow in the draft tube, and the loss reduced with the increase of the amount of water admission. However, excessive water admission would not only decrease the turbine efficiency due to the increase of the jet efficiency loss, but also cause pressure reduction on runner blades, which can affect the cavitation performance of the Francis turbine. Therefore, the amount of water admission must be considered synthetically. For the current situation, the water admission should not exceed 5% of the inlet flow rate. Secondly, previous studies showed that the pressure fluctuation caused by the spiral vortex was related to the sharp decrease of the axial velocity of the runner. Therefore, injecting high pressure water to draft tube could increase the axial velocity at the outlet of the runner, and the increase of the axial velocity could change the distribution of velocity field in the vortex rope, which could effectively eliminate the backflow phenomenon, but the inhibition of backflow was not obvious when the amount of water admission was too small. Thirdly, when the water admission was 1% of the inlet flow rate (0.488 m3/s) in this condition, there was no obvious improvement because the fluctuation amplitude changed slightly. When the water admission was increased to 3% of the inlet flow rate, the vortex shape in the draft tube changed from double-helix to single helix, and the amplitude of pressure fluctuation in the draft tube increased rather than decreased, and thus the instability of the flow in the draft tube increased. When increasing the water admission further to 5% of the inlet flow rate, the amplitude of pressure fluctuation in the draft tube decreased sharply from 18.4% to 1.63%. At the same time, the main frequency of the pressure fluctuation was also changed, which was helpful for avoiding the resonance and improving the stability of the unit. In short, it is feasible and effective to inject high pressure water from the runner cone to the draft tube of the Francis turbine, which can improve the flow field in the draft tube and reduce the instability of the draft tube.