混流式转轮上冠型线变化对水轮机水力性能的影响

为了获得水电站水轮机技术改造中混流式转轮上冠型线变化时水轮机水力性能的变化规律,本文以某混流式水轮机模型转轮为研究对象,在原转轮上冠型线的基础上,提出了四种改型的上冠型线方案,采用CFD方法对不同上冠型线的转轮内部流动特性、能量特性、空化性能及过流能力进行了数值模拟和对比分析。结果表明:上冠型线的适当上抬可以增大转轮的过流能力,并且改善转轮在大流量工况下的能量特性和空化性能;但过高的上冠型线对小流量工况下的叶道涡的位置和强度有负面影响;与原转轮相比,方案4的效率略有提高;小流量工况时,上冠型线越高,叶道涡的位置越靠近叶片头部,且涡的强度越大,从而使转轮效率降低;大流量工况时,上冠型线上抬可能导致叶片进口靠上冠部分出现回流,进而导致压力分布不均匀;上冠型线的改变对最优工况下转轮效率和内部流动特性影响较小;可以为水轮机技术改造中转轮的改型设计与性能预估提供必要的参考。

Influence of Francis runner crown profile on hydraulic performance of water turbines

Modifying runner crown profile is a common method of turbine retrofit design, and for optimization and technical reform of a water turbine, exploring the influence of its crown profile on hydraulic performance is very significant. For this purpose, a Francis model runner was selected in this work, and four schemes were examined to modify its crown profile. Then, a CFD method was used to simulate and compare different crown profile schemes in terms of internal flow behaviors, energy characteristics, cavitation performance, and discharge capacity of this runner in three working conditions: small flow, optimal operation, and large flow. The results show that an appropriate elevation of the crown increases the discharge capacity through raising the lowest pressure on the runner blades, and that in large flow cases, it also improves energy characteristics and cavitation performance. However, an excessive crown height, in small flow cases, would produce negative impacts on the location and intensity of those vortices in the blade channels. Compared with the original runner design, scheme 4 can increase efficiency by 0.33% and 0.67% in large and small flow cases respectively. In small flows, a higher crown moves the blade channel vortices toward the blade inlet edge and intensifies their strength, thus resulting in a greater hydraulic loss and lower efficiency of the runner. In large flows, elevating crown profile may lead to a backflow zone near the crown and at the runner blade inlet, and consequently to an uneven distribution of pressure on the blades. Our study also reveals that runner efficiency and its internal flow at optimal operation generally vary little under different elevation of the crown, while the internal flow is very sensitive to even a subtle change in the flow parameters of a practical water turbine remodeling project. Thus, in estimating the change in energy characteristics of a new runner, we cannot rely on any intuitive judgment but rather, we must be very careful and perform multiple checking in making choice based on CFD analysis. This is the way to ensure the hydraulic performance of the new runner, the only way to reduce to the lowest the risk in technical improvement of a Francis turbine. The conclusions above would be helpful for runner remodeling design and performance prediction of Francis turbines for hydropower stations.