混流式水轮机减压管和密封间隙流场数值模拟
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摘要
水轮机中的减压管和转轮上冠密封间隙对水力发电机组稳定性的影响,已经引起研究学者和电站运行人员的普遍关注。本文采用计算流体动力学软件CFX,基于N-S方程和RNGκ-ε湍流模型,通过对含减压管和转轮上冠密封间隙的某水电站原型水轮机进行三维定常全流道数值计算,主要研究成果有:
     分析了含减压管和转轮上冠密封间隙的混流式水轮机三维全流道数值模拟的建模过程,讨论了减压管和上冠密封间隙计算域的网格划分方法,通过分块网格技术和延伸计算域的方法解决了小区域网格和大区域网格的参数传递问题。
     针对减压管对混流式水轮机流场的扰动作用,在上冠密封间隙值为1.5mm时进行数值模拟计算,分析了减压管对混流式水轮机转轮上冠密封间隙、转轮叶片前、上冠和顶盖之间的压力腔、尾水管流场的影响,以及不同数目的减压管对转轮上冠密封间隙的泄漏量、机组轴向水推力的影响。计算结果表明减压管的存在有利于降低推力轴承的荷载,但需要考虑在转轮上冠密封间隙以及转轮上冠和顶盖之间的压力腔中产生的压力扰动区的影响。
     在四个减压管开启,40%、60%、80%、100%额定出力的工况下进行原型水轮机全流道数值模拟计算,分析了不同转轮上冠密封间隙值对混流式水轮机转轮上冠密封间隙、顶盖与上冠之间的压力腔流场的影响,以及不同上冠密封间隙值对转轮上冠密封间隙的泄漏量、机组轴向水推力的影响。并对所计算的原型水轮机,根据不同上冠密封间隙值的流场分布规律,提出了转轮上冠密封间隙值的稳定区和非稳定区概念。
     所以在设计时需要对是否设减压管以及减压管的数目、转轮上冠密封间隙值的大小进行综合考虑,既要保证机组所受轴向水推力不能过高,又要防止转轮上冠密封间隙中激发的振动,以保证机组安全稳定的运行。
The vibration of the hydropower unit caused by the relief pipes of the turbine has attracted scholars' and station staff's attention. In this paper, the three-dimensional steady turbulent flow is simulated through the whole flow passage of a power plant including the relief pipes and the runner's sealing clearance using Computational Fluid Dynamics software-CFX based on N-S equation and RNG k -εturbulence model. The main research findings are as follows:
     The modeling process of the turbulence numerical simulation of the three-dimensional whole flow passage including the relief pipes and the runner crown's sealing clearance are analyzed. The mesh generation method of the calculation domain in the relief pipes and runner crown's sealing clearance are discussed. The transmission parameters problem of the small regional grid to large regional grid is solved through multiblock grid techniques and computational domain extension.
     For the Francis turbine flow field disturbance of the relief pipes, the influence of the relief pipes on the flow field of sealing clearance, the flow field before the runner blades, the flow field of the chamber between the head cover and the crown, the flow field of draft tube are analyzed, in the crown's sealing clearance value of 1.5 mm. The influence of the relief pipes' number on the leakage of runner crown's sealing clearance and the axial hydraulic thrust of the unit are also analyzed. The results show that the existence of relief pipes is beneficial to reducing the load of the thrust bearing, however, the disturbance of the runner crown's sealing clearance and the chamber between the head cover and the crown must be considered.
     The influence of the crown's sealing clearance value on the flow field of sealing clearance and the flow field of the chamber between the head cover and the crown are analyzed with four relief pipes under 40% rated output, 60% rated output, 80% rated output and 100% rated output working conditions. The influence of the crown's sealing clearance value on the leakage of runner crown's sealing clearance and the axial hydraulic thrust of the unit are also discussed. The concept of stable region and unstable region are presented according to the distribution of sealing clearance flow field for the simulation case.
     Therefore, the design of the relief pipe and crown's sealing clearance value should be considered in a comprehensive way to ensure that the unit avoids higher axial thrust, at the same time to prevent the vibration excitated by the runner crown's sealing clearance. Consequently the unit can be operated safely and stably.
引文
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