混流式水轮机在含沙水流下的空化特性研究

【作者】 王杰；

【导师】 焦磊；

【作者基本信息】 浙江大学 ， 船舶与海洋工程， 2016， 硕士

【摘要】 中国大部分河流中都含有大量泥沙,运行在这些河流上的混流式水轮很容易受到泥沙的冲击磨损,尤其是叶片靠缝隙处的边缘磨损更为严重。同时河流中的泥沙也使得更加容易发生空化现象,随之产生的空蚀与泥沙磨损联合作用于水轮机叶片,进一步导致了更严重的磨蚀,极大地降低水轮机的效率与水电机组的稳定性。因此,为提高水轮机的抗空化、磨损能力并改善其优化设计方法,亟待探究空化、泥沙磨损以及两者联合作用机理。然而影响空化、磨损及两者联合作用行为的因素十分复杂,既有速度、压力等流场参数,又有沙粒和材料特性等,且目前该方面的研究大多聚焦在实验研究,缺少相关的数值仿真研究。基于以上问题,本文基于数值模拟方法对含沙水流中混流式水轮机在清水和含沙情况下的内部流动进行了数值模拟,并进一步分析了这两种不同工况下水轮机的空化性能,具体包括以下步骤：1)采用UG三维实体建模软件对混流式水轮机的蜗壳、固定导叶、活动导叶、转轮和尾水管的过流通道进行实体建模,并采用四面体、六面体非结构化网格进行划分；2)对混流式水轮机的清水空化进行了研究,采用了SST k-湍流模型,结合基于均值空化流理论的输运空化模型(Zwert-Gerber-Belamri空化模型),对不同进口流量、空化系数和导叶开度工况下的空化模拟计算结果进行了比较分析；3)对水轮机含沙空化的数值模拟来研究空化与磨损的联合作用,采用欧拉模型和空化模型来计算三相的作用,获得了不同泥沙粒径和浓度下泥沙磨损与空化的相互影响关系以及出力变化情况。结果表明,在清水情况下,随着进口流量的增大,转轮背面靠近下环出口处的空化区域先增大后减小,转轮区的叶道涡先减小后增大,但是没有引起叶道涡空化。随着空化系数的降低,转轮背面的空化区域逐渐增大。随着导叶开度的减小,叶片背面的空化区域逐渐减小,并且在小开度下出现了叶道涡空化。在低于最优效率点流量时,尾水管进口处出现螺旋涡带,在高于最优效率点流量时,尾水管进口处出现泡状涡带。在空化工况下,水轮机的效率比非空化工况下的低。在含沙情况下,随着泥沙粒径及含沙浓度的增加,泥沙从叶片背面进口处向正面出口处大量聚集。在空化对泥沙磨损的影响上,空化计算时转轮区的泥沙浓度会大于等于非空化计算的泥沙浓度；在泥沙磨损对空化的影响上,随着泥沙粒径及含沙浓度的增加,转轮中的空泡含量出现先增大后减小的趋势；在对尾水管涡带的影响上,随着泥沙粒径的增加,水轮机的稳定性会下降。在泥沙磨损对效率的影响上,随着泥沙浓度的增加,转轮的出力变小。以上对水轮机内部流动特性的研究结果对提高混流式水轮机的效率和稳定性具有一定的理论参考意义和实际应用价值。更多还原

【Abstract】 Most rivers in China contain a lot of sediment, and mixed-flow water turbine running in those rivers is subjected to the abrasion from sediment, especially for the edges close to the gaps on the blades.At the same time, the sediment in rivers also make it easy to produce cavitation,which that generate cavitation erosion.Cavitation erosion Combined with abrasion generated by sediment has more serious impact on the blades of turbine, greatly reduce the efficiency and stability of turbine.In order to improve the ability to resist cavitation and abrasion of turbine, it’s very important to explore the mechanism of cavitation and abrasion, as well as their combined effect on turbine. However, the factors affecting on the work of cavitation, abrasion and their coefficient is complicated, not only the flow field parameters like velocity and pressure, but also material characteristics like size and density of silt.Further more, researches focused on this domain were most carried out on experimental study, lack of related numerical simulation.Based on the above problems, this article simulated the internal flows of mixed-flow water turbine in rising and silt water these two different situations,then analyzed their cavitation performance,respectively. The main research content is summarized as follows:1) established the models of casing, guide vane, stay vane, draft tube of the mixed-flow water turbine by UG modeling software, then used tetrahedron, hexahedron non-structural mesh methods to mesh above parts of turbine.2) explored the cavitation characteristic of mixed-flow water turbine in the situation of rinsing. Simulated cavitation of different working conditions like inlet flow, cavitation coefficient, opening angle of guide vane using SST k-ω turbulence model and transport cavitation model (Zwert-Gerber-Belamri cavitation model) which based on the mean cavitation flow theory.3) explored the combined effect generated by cavitation and abrasion of mixed-flow water turbine in the situation of sand water. Obtained the mechanism how abrasion produced by silt of different size and density and cavitation effect together on the turbine using Euler model and cavitation model. In the results of rinsing situation, with the increasing of mass flow in the inlet, the cavitation region at the back of the runner near the shroud outlet first increased and then decreased, the posterior lobe channel vortex runner zone decreased and then increased, but did not cause leaf channel vortex cavitation. With the reduce of cavitation coefficient, runner cavitation region increased gradually. With the decrease of guide vane opening, the cavitation area of the blade gradually reduced and the leaf channel vortex cavitation emerged when in the small opening. When the flow less than the optimum efficiency point flow, the spiral vortex occurs at the draft tube inlet and when the flow above the optimum efficiency point flow, the bubbly vortex appears at the draft tube inlet. The turbine efficiency in the cavitation condition is lower than non-cavitation conditions. In the results of sand water situation, sediment area at the back of the runner blade inlet numerous extended to the front of the runner blade outlet when the sediment grain size and concentration of sedimen increased. On the influence of cavitation to the sediment wear, sediment concentration zone in the cavitation condition would be greater than the calculated non-cavitation condition; on the influence of sand to the cavitation, when the particle sediment and concentrations of sediment increased, it would promote runner cavitation occur firstly, then inhibit runner cavitation; on the influence of sand to draft tube vortex, the spiral vortex occur when the particle sediment increased; On the influence of sand to efficiency, the torque of the runner became smaller with the increase of sediment concentration. The results of the above flow characteristics within the turbine to improve the efficiency and stability of Francis turbine has a certain theoretical reference significance and practical value.更多还原