论文标题:基于计算流体动力学的水轮机及水电站尾水系统数值研究
Numerical Simulation of Hydraulic Turbine and Hydropower Station Tailrace System Based on the Computational Fluid Dynamics
论文作者
论文导师 杨建东,论文学位 博士,论文专业 水利水电工程
论文单位 武汉大学,点击次数 121,论文页数 103页File Size20752k
2005-05-01论文网 http://www.lw23.com/lunwen_91168517/
optimization design; turbulent flow simulation; efficiency prediction; rotor-stator interaction; pressure pulsation; tailrace system; layout pattern; volume-of-fluid (VOF) method; rigid-lid assumption
随着计算流体动力学(CFD)的进步和计算机硬件性能的不断提高,三维紊流的数值模拟越来越精确,在水电站的设计、优化中已得到了广泛应用,数值模拟技术已经成为水电站开发、设计及优化的有力工具。 水电站水力发电系统主要包括引水系统、发电系统和尾水系统等系统。随着单机容量、电站装机容量的不断增大,水力发电系统的高效性、稳定性日益受到重视,而要保证机组的高效、稳定运行,各系统内部流动就必须要有良好的流态及尽可能小的水力损失。而通过数值模拟,可获得各系统内部稳态、非稳态的流速、压力分布,从而可实现系统的优化设计,并为机组的高效、稳定运行提供可靠的依据。 作为水电站关键设备之一的水轮机,对每个电站都需根据具体条件和要求进行设计制造,以确保良好的能量特性、空化特性和稳定性。而这需要在水轮机水力设计时做精心研究。近几年,水轮机数值模拟的一个突出进步是对水轮机所有过流部件的整体解析及转动部件(转轮)和非转动部件(导叶、尾水管)的耦合流动计算。整体解析是指对水轮机的整体(包括多个过流部件)进行流动计算。水轮机由蜗壳、固定导叶、活动导叶、转轮和尾水管等多个过流部件组成,这些部件在力学上具有很强的相互依赖关系。因此,不管是模型试验,还是数值模拟都应建立在对水轮机所有过流部件的整体模拟基础上,以获得真实、准确的数据。耦合计算是考虑了转动部件和非转动部件的动静干涉。与水轮机单部件的流动模拟相比,水轮机所有过流部件的整体耦合计算的边界条件更容易给定,在动静部件间不会产生不准确的边界条件,只需指定进口和出口的边界条件即可,计算结果与实际情况更接近,因而能更准确地预测水轮机的特性,为水轮机的优化设计提供更坚实的基础。 本文针对水布垭混流式模型和原型水轮机进行了三维定常计算。三维定常紊流计算采用多部件、动静耦合的计算方法,计算中针对模型和原型水轮机分别采用了两种物理模型,相应的流动计算分别称为“水轮机整体计算”和“水电站全流道计算”。水轮机整体计算的计算区域从蜗壳进口到尾水管出口,包括了蜗壳、固定导叶、活动导叶、转轮和尾水管等过流部件;水电站全流道计算的计算区域从压力引水管道进水口到尾水渠(管)出口,包含的过流部件与实际水电站的输水系统完全一样。与水轮机单部件计算和水轮机整体计算相比,水电站全流道计算是与电站实际运行情况最接近的一种计算方式,因而能获得原型水轮机更准确的流动、能量特性。本文根据水轮机整体计算和水电站全流道计算的数值计算结果预测了模型和原型水轮机的效率,预测结果与模型能量试
Three dimension turbulent flow simulations are becoming more and more accurate with both computational methods and hardware performances rapid development. In the field of waterpower station optimization design, Computational Fluid Dynamics (CFD) is routinely used today in research and development as well as in design.Recently one of the most important achievements in CFD is simultaneous calculation of the flow in rotating and non-rotating parts. Since very often there are strong interactions between the components (especially between guide vanes, runner and draft tube), it is inevitable to introduce this interaction into the simulation for accurate results. Using coupled analysis of the flow through the whole turbine we can take into account the stator-rotor-draft tube interaction and avoid inaccurate boundary conditions between turbine components.In the paper, the three-dimensional steady turbulent flow in the Shuibuya model and prototype Francis turbines were simulated. At first the flow through the whole turbine was calculated simultaneously. The computational domain included spiral case, stay vanes, guide vanes, runner and draft tube. There were two stage-sliding interfaces between rotating and non-rotating parts. Then the flow through the whole flow passage of waterpower station-from penstock inlet to draft tube or tail channel outlet, was analyzed simultaneously. Thesimulation is based on Navier-Stokes equations, the standard k-ε turbulence model andthe SIMPLEC algorithm, which is applied for the solution of the discrete governing equations. The distribution of velocity and pressure through the flow passage of the Francis turbine was attained. The energy and cavitation property of the model turbine was then predicted. Calculated turbine efficiency was compared with the measured one.Different problems in hydraulic machinery arise from unsteady flow phenomena. In order to get information on this phenomena or solutions to the problems an unsteady flow analysis is necessary. Two major groups of unsteady problems can be distinguished. The first group is flows with an externally forced unsteadiness. This can be caused by unsteady boundary conditions or by changing of the geometry with time. Examples are the closure of a valve, the change of the flow domain in a piston pump, or the rotor-stator interactions. The second group is flows with self-excited unsteadiness, which are e.g. turbulent motion, vortex shedding (Karman vortex street) or unsteady vortex behavior (e. g. vortex rope in a draft tube). Here the unsteadiness is obtained without any change of the boundary conditions or ofthe geometry. There can also occur a combination of both groups (e. g. flow induced vibrations, change of geometry caused by vortex shedding). All these phenomena can take place in a turbine or pump and require different solution procedures. The oscillations in hydraulic machines are becoming more and more important with the increasing of unit power and size, and the oscillation must be analyzed by investigating the unsteady flow field in the hydraulic machine.In this paper different numerical schemes are discussed for the rotor-stator interactions and moving grid. The rotor-stator coupling by application of sliding mesh is shown on the Shuibuya complete Francis model turbine. The PKO (pressure-implicit with splitting of operators) method was used for the pressure velocity coupling. The realized k-e model was used for the turbulence. The instantaneous velocity and pressure distribution through the whole Francis model turbine was attained. Three measuring points were placed at the spiral case inlet center, in front of runner and on the draft tube cone, approximately 0.3 runner diameters upstream of the runner outlet. The pressure pulsations versus time at different positions and its" amplitude spectrums versus frequency were analyzed by FFT (fast Fourier transform) to determine the pressure oscillation frequencies at different points. Although Unsteady simulations have in common a quite large requirement of computational resources, especially for rotor-stator interactions the complete turbine has to be considered and all flow channels in the stator as well as in the rotor have to be included, which leads to many grid nodes and an enormous computational effort, the frequencies and amplitudes of integral quantities (e. g. forces) can be predicted with sufficient accuracy for most of the problems, and on the other side flow phenomena can be predicted and potential of influencing then can be assessed.Large underground stations with long conduits usually construct surge tanks at the downstream of a turbine to accommodate flow fluctuations. In order to obtain an ideal design for the water diversion systems, physical and numerical simulation is necessary. In Longtan waterpower station, three turbine-generator units share one tailrace surge chamber, and there are two different type tailrace systems. One is fork pipe subsequent to the tailrace surge chamber; another is fork pipe under the tailrace surge chamber. In the paper, Flow regime and head loss of two different tailrace systems in hydropower station were investigated.Numerical simulation was based on the RNS equations, closed with the standard k-e turbulence model. Volume-of-fluid (VOF) method and rigid-lid assumption were used to describe the free surface. Computational result shows that the layout pattern of fork tube behind throttled surge chamber is more efficient than the fork tube under surge chamber.
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