The modern power generation system requires steam turbines operating at flexible operating points, and flow instabilities readily occur in the low-pressure (LP) last stage under low-load conditions, which may cause failure of the last stage moving blades. Some studies have shown that within this operating range, a shift of the operating point may lead to flow instabilities. Numerical simulation has gradually developed into a popular method for such researches, but it is expensive for a complex model, which has to be balanced between efficiency and accuracy. This work is divided into three parts: Firstly, one of the low-load conditions is selected to provide both URANS model and the Scale-Adaptive Simulation (SAS) model. The results of the two models are compared to evaluate specific flow phenomena; Secondly, through calculations of different low-load conditions, the flow structure and propagation characteristics of instabilities in the last stage are obtained; Finally, flow analysis is applied to explain the formation mechanism of flow instabilities in LP steam turbines. The results show that, the introduction of SAS model increases the randomness of flow over time, but does not fundamentally change the flow instabilities. Flow instabilities take different forms at different flow rate, from rotating instability to rotating stall. The formation of flow instabilities is related to the radial flow in the cascade passages.

现代发电系统要求蒸汽涡轮机在灵活的工作点运行，并且在低负荷条件下低压（LP）末级中很容易发生流量不稳定性，这可能会导致末级动叶片失效。一些研究表明，在此工作范围内，工作点的偏移可能会导致流量不稳定。数值模拟已逐渐发展成为此类研究的一种流行方法，但是对于复杂的模型而言，它是昂贵的，因为它必须在效率和精度之间取得平衡。这项工作分为三个部分：首先，选择一种低负载条件以提供URANS模型和Scale-Adaptive Simulation（SAS）模型。比较两个模型的结果以评估特定的流动现象。其次，通过计算不同的低负荷工况，得到末级失稳的流动结构和传播特性。最后，应用流动分析来解释低压汽轮机中流动不稳定性的形成机理。结果表明，SAS模型的引入增加了流量随时间的随机性，但并没有从根本上改变流量的不稳定性。从旋转不稳定性到旋转失速，流动不稳定性在不同的流量下具有不同的形式。流动不稳定性的形成与级联通道中的径向流动有关。结果表明，SAS模型的引入增加了流量随时间的随机性，但并没有从根本上改变流量的不稳定性。从旋转不稳定性到旋转失速，流动不稳定性在不同的流量下具有不同的形式。流动不稳定性的形成与级联通道中的径向流动有关。结果表明，SAS模型的引入增加了流量随时间的随机性，但并没有从根本上改变流量的不稳定性。从旋转不稳定性到旋转失速，流动不稳定性在不同的流量下具有不同的形式。流动不稳定性的形成与级联通道中的径向流动有关。