混流式水轮机尾水管涡带引起的水压脉动预测及控制

Hydraulic instability of a Francis hydro turbine is often occurred when operating at partial load,which mainly caused by the pressure surge originated from the vortex rope inside the draft tube. We consider that the generation of the vortex rope have relations with the geometries of the whole passage of a hydro turbine and operating conditions, and the pressure surge caused by the vortex rope mainly depend upon the inside flow of the runner and primarily related the swirl flow at draft tube inlet. This research is to seek for the analytical model of the vortex rope inside the draft tube with the Helical Vortex Theory and to further investigate the relationship between the geometries of flow passage and the vortex rope，and try to establish the predicted model of relationship between the flow inside the runner and pressure surge caused by the vortex ropes inside the draft tube. The reliability of the predicted method will be validated by 3D flow measurement of the draft tube with advanced instruments such as PIV and high-speed digital image system. 3D non-stationary flow inside whole turbine flow passage can be numerically simulated with a hybrid RANS-LES turbulance model,then,the amplitude, frequency and phase of pressure surge caused by the vortex rope at partial load can be predicted by means of these flow simulations based on the predicted model and methodology, so that the evaluation of hydraulic stability with traditional model test can be gradually replaced by numerical test. Subsequently, we will seek for an optimization way that take the pressure surge as one of object function to effectively control the pressure surge caused by the vortex rope and enlarge the operating zone of the vortex-rope free during the hydraulic design. These researches have great importance to safety operation of a Francis hydro turbine and further exploit the abundant water resource in our country.

混流式水轮机在部分负荷下运行的水力不稳定性问题尤为突出，尾水管涡带引起的强水压脉动是最主要的起因。申请人认为尾水管涡带产生主要与流道的几何参数及运行工况有关，其尾水管涡带引起的水压脉动特性主要取决于转轮出口的流场。本立项拟用螺旋涡理论来探索尾水管涡带解析模型，深入研究流道几何形状和参数与尾水管涡带形成的关系，试图建立转轮流场与尾水管涡带引起的压力脉动特性的预测模型和方法。用PIV和数字高速摄影等对尾水管流场测量分析来验证预测方法的可靠性。采用混合RANS-LES湍流模型模拟全流道流场，基于转轮出口的流场和建立的预测模型来预测部分负荷下尾水管涡带引起的压力脉动特性，以数值试验逐步取代模型试验。在此基础上研究在水力设计中如何通过水压脉动目标优化转轮和尾水管设计来有效控制尾水管涡带引起的水压脉动及扩大无涡区的方法。其成果对混流式机组的安全运行、及开发我国丰富的水力资源都有重大的意义。

大量研究已证实尾水管涡带产生的低频压力脉动是引起水轮机水力不稳定的主要起因之一，因此如何在设计阶段就能准确地预测尾水管涡带产生的工况及控制压力脉动成为目前水力机械学科最为关注的问题之一。本项目通过对尾水管涡带的近似解析、数值模拟和实验验证，深入研究预测尾水管涡带及引起压力脉动特性的方法和技术。基于螺旋涡理论推导出了尾水管中涡带的近似解析计算模型，引用实验数据验证了该模型在一定范围内的可用性。针对混流式水轮机过流部件几何和内流动特点，探索出较为适合尾水管涡带及产生压力脉动分析要求的全流道三维定常和非定常流场计算方法。为验证可靠性，对30个工况进行模拟和效率预测，预测与试验结果相对偏差均小于0.5%。基于数值模拟结果，采用等压面方法可视化尾水管中涡带。引入旋流强度数来描述涡带强度与流速的关系，根据旋流强度数和可视化涡带等来综合预测出尾水管涡带出现工况和无涡区，对比对应工况试验结果验证了预测方法的可行性，探索出较为准确地预测尾水管涡带的方法。提取三维非定常模拟结果中尾水管监测点的压力数据进行时域和频域分析，并区分其他因素引起的压力脉动，探索出能更为准确地预测尾水管涡带引起的低频压力脉动特性方法，预测与实测值偏差均小于5%。针对采用LDV测量尾水管流速的方法及技术进行研究，使测量误差均小于1.6%，接近最优工况区小于0.6%。对模型水轮机在一系列工况下进行测试，获得尾水管中的时均和瞬态流速分布，同时用高速摄影记录涡带图像。分析各测量工况下尾水管涡带的形态及与速度脉动的关系，并从内流场进一步验证了流场模拟及预测方法的准确性。在预测方法基础上，初步探索出控制及降低尾水管涡带引起的压力脉动的多目标优化设计理论与方法。本项目的研究方法和成果为在设计阶段进行压力脉动预测与控制等奠定了基础，为在工程中减少模型试验提供了可行的方法，对保证研发的水轮机的水力稳定性和电站安全稳定运行等有重要意义。