混流式水轮机主轴中心孔补水对尾水管性能的影响

混流式水轮机在低负荷工况下运行时,尾水管内出现旋转的偏心涡带,会引起强烈的压力脉动和振动,严重威胁厂房的安全。为了使机组稳定运行,该文提出了一种通过从上冠泄水锥引入高压补水的方法来降低尾水管的不稳定性。该文首先采用商业软件CFX16.0,对某电站混流式水轮机在低负荷工况下进行了可靠而准确的全三维非定常数值模拟,结果表明在该工况下尾水管内部存在明显的偏心涡带,并伴随着振幅较大的压力脉动,这与试验结果相吻合。其次,对该工况下不同补水流量进行了数值模拟计算,研究表明:尾水管内补高压水可以有效降低尾水管内部的流动损失,且随着补水量的增加而越小,但过大的补水量会引起叶片正背面压力的降低,影响水轮机的空化性能,故补水量的大小必须综合考虑;主轴中心孔高压补水可以增加转轮出口的轴向速度,从而改变涡带内速度场的分布,可有效消除回流现象,当补水流量过小时,抑制回流作用不明显;当补水量为进口流量1%时,尾水管内部压力脉动振幅变化不大,改善效果不明显;当补水量为进口流量3%时,尾水管内部涡带由双螺旋变成单螺旋,锥管段压力脉动振幅不减反增,不稳定性有所加剧;当补水量为进口流量5%时,尾水管内部压力脉动振幅从18.4%降低至1.63%,同时改变了压力脉动的主频,使其远离转轮主频,避免发生共振,提高了机组的稳定性。

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.