中文摘要：

混流式水泵水轮机普遍存在S特性区，严重影响蓄能机组的安全稳定运行，其产生的内在原因和解决方法是目前研究重点。本文以模型水泵水轮机为研究对象，对小导叶开度下的水轮机及反水泵运行的多个工况进行了整体流道的三维数值计算，探讨小导叶开度下S特性区机组内部的三维流动特性。选取了性能曲线在S形区域的不同运行工况点进行内流特性分析，发现在飞逸附近工况导叶和转轮内主要表现为漩涡流；在制动工况活动导叶和转轮间的无叶区内表现为明显的“水环”状流态，仅有少部分水流能流入转轮；反水泵工况的流动则更加复杂，整个流道中均有大量的漩涡流存在。综合S特性区的三维流动特性发现，当机组运行在小流量制动工况时，无叶区内的回流会堵塞通道，造成转轮不能在更高的转速下维持该小流量状态的运行，表现为在特性曲线飞逸点附近开始发生S形的弯折。

英文摘要：

S-shaped curve is a widespread characteristic of pump-turbines, which has significant impact on unit operation. It is important to analyze underlying causes and control methods. This study has conducted numerical simulations of the whole passage of a model pump turbine and examined the characteristics of 3-D flow in the S-curve region for small guide vane opening, focusing on four different operating points to analyze the behaviour of internal flow in the unit. We found out that a large number of vortices appeared in the vane and runner sections at the points near runaway. The vaneless region between the guide vanes and runner was featured with an obvious water ring, where only a small amount of water was able to flow into the runner at the turbine braking point. In the reverse mode, the flow was very complicated. A comprehensive analysis of the 3-D flow reveals that in the S-curve region, a backflow in the vaneless region will block up all the channels if the unit operates in the condition of small discharge. This makes the runner unable to keep operating in the small discharge at a higher speed, and hence the Sshaped bend is created around the runaway point on the characteristic curve.