Numerical simulation of unsteady turbulent flow through a Francis turbine

This paper introduces the 3D numerical simulation of unsteady turbulent flow in the entire flow passage of a Francis turbine with computational fluid dynamics (CFD) technology. The boundary conditions have been implemented based on the 3D averaged Navier-Stokes equations. The governing equations are discreted on space by the finite volume method and on time step by the finite difference method. The 3D unsteady turbulent flow in an entire Francis turbine model is calculated successfully using the CFX-TASCflow software and RNG k-ε turbulence model. Transient flow fields are simulated in the spiral case, the distributor, the runner, and the draft tube for the optimum operating condition. Meanwhile, the velocity and pressure at any points in the flow fields can be obtained so as to provide the great value on the performance prediction. By means of the numerical simulation in a flow field, it is verified that every component in a Francis turbine model is designed reasonably. The basis for the further researches on hydraulic turbines is also built.     

Computer Simulation of Turbulent Flow through a Hydraulic Turbine Draft Tube

Based on the Naviev-Stokes equations and the standard κ-ε turbulence model, this paper presents the derivation of the governing equations for the turbulent flow field in a draft tube. The mathematical model for the turbulent flow through a draft tube is set up when the boundary conditions, including the inlet boundary conditions, the outlet boundary conditions and the wall boundary conditions, have been implemented. The governing equations are formulated in a discrete form on a staggered grid system by the finite volume method. The second-order central difference approximation and hybrid scheme are used for discretization. The computation and analysis on internal flow through a draft tube have been carried out by using the simplee algorithm and cfx-tasc flow software so as to obtain the simulated flow fields. The calculation results at the design operating condition for the draft tube are presented in this paper. Thereby, an effective method for simulating the internal flow field in a draft tube has been explored.     

Polyamide-6 Polymerization in the VK Tube Reactor with Baffle Structure

The melt flow mechanism of polyamide-6 (PA6) was simulated with the mathematical flow model and tested by Particle Image Velotrimitry (PIV) . The complete mathematical model for PA6 polymerization while flowing through the VK tube reactor was established according to PA6 hydrolytic polymerization kinetics. The characteristic data such as residence time, caprolactam conversion and the degree of polymerization (D. P.), at every stage of polymerization reaction and at every point of the VK tube were presented for the melt flowing through ten alternate conical baffles in VK tube.     

Dynamic Stresses in a Francis Turbine Runner Based on Fluid-Structure Interaction Analysis

Fatigue and cracks have occurred in many large hydraulic turbines after they were put into production. The cracks are thought to be due to dynamic stresses in the runner caused by hydraulic forces. Computational fluid dynamics （CFD） simulations that included the spiral case, stay vane, guide vane, runner vane, and draft tube were run at various operating points to analyze the pressure distribution on the runner surface and the stress characteristics in the runner due to the fluid-structure interactions （FSI）. The dynamic stresses in the Francis turbine runner at the most dangerous operating point were then analyzed. The results show that the dynamic stresses caused by the hydraulic forces during off-design operating points are one of the main reasons for the fatigue and cracks in the runner blade. The results can be used to optimize the runner and to analyze other critical components in the hydraulic turbine.     

Mixed Model for Silt-Laden Solid-Liquid Two-Phase Flows

The kinetic theory of molecular gases was used to derive the governing equations for dense solid-liquid two-phase flows from a microscopic flow characteristics viewpoint by multiplying the Boltzmann equation for each phase by property parameters and integrating over the velocity space. The particle collision term was derived from microscopic terms by comparison with dilute two-phase flow but with consideration of the collisions between particles for dense two-phase flow conditions and by assuming that the particle-phase velocity distribution obeys the Maxwell equations. Appropriate terms from the dilute two-phase governing equations were combined with the dense particle collision term to develop the governing equations for dense solid-liquid turbulent flows. The SIMPLEC algorithm and a staggered grid system were used to solve the discretized two-phase governing equations with a Reynolds averaged turbulence model. Dense solid-liquid turbulent two-phase flows were simulated for flow in a duct. The simulation results agree well with experimental data.     

Analytical determination of flow resistance characteristic for combination channel inside hydraulic manifold block

Computational fluid dynamics （CFD） was used in conjunction with BP neural network to study the flow resistance characteristic of the combination-channel inside hydraulic manifold block （HMB）. The input parameters of the combination-channel model were confirmed to have effect on the pressure-drop by the numerical method, and a BP neural network model was accordingly constructed to predict the channel pressure-drops. The flow resistance characteristic curves of various channels were achieved, and a performance parameter was given to evaluate the through-flow characteristic of the channel according to the curves. The predictions are in agreement with the numerical computation, indicating that the method can be utilized to accurately determine the flow characteristic of the combination channel with high efficiency.     

Numerical Investigation of the Impact of Different Configurations and Aspect Ratios on Dense Gas Dispersion in Urban Street Canyons

The dispersion of chlorine gas in urban street canyons was numerically simulated using the fire dynamics simulator, a code developed by the National Institute of Standards and Technology of USA, which uses large eddy simulation coupled with the Smagorinsky sub-grid scale model. The unsteady flow fields were computed by solving the filtered incompressible Navier-Stokes equations under low Mach number ap-proximation by the finite difference method. The studies analyzed the influence of different street canyon configurations and aspect ratios on the flow and chlorine gas dispersion. The geometric configuration and aspect ratio both affect the vortices and the local concentration distributions in street canyons.     

Building a high-resolution regional climate model for the Heihe River Basin and simulating precipitation over this region

Based on the Regional Integrated Environmental Model System （RIEMS 2.0） developed by START TEA-COM RRC and Department of Atmospheric Science of Nanjing University, a dataset of observation and remote sensing over the Heihe River Basin （HRB） was used to recalibrate the model＇s parameters, including topography elevation, land cover type, saturated soil water po- tential, saturated soil hydraulic conductivity, field moisture capacity, wilting point moisture, soil porosity, and parameter b of soil hydraulic conductivity, to build a high-resolution regional climate model for the HRB. Continuous simulation for the period from January 1 to December 31, 2000 was conducted using the regional climate model, with NCEP-FNL reanalysis data as the driving fields. The study focused on the ability of the model to simulate precipitation in the HRB. The analysis results showed the following： （1） For precipitation, the model could reproduce well the spatial pattern and seasonal cycle in different regions of the HRB, simulated precipitation was overestimated in the upper reaches of the HRB and underestimated in the lower reaches of the HRB, and seasonal precipitation biases were mostly in the range of -39.9%-9.6% of observations, which is mostly consistent with the IPCC report （2001）. （2） The model could reproduce reasonably temporal evolution of pentad precipitation in different regions. Correlation coefficients between the simulated and observed pentad precipitation were 0.8123, 0.5064, and 0.7033 in the upper, middle, and lower reaches of the HRB, respectively. They reached a significance level of 1%. （3） Dynamical downscaling of a high-resolution regional climate model was used to overcome the deficiency of long-term, high-temporal/spatial-resolution meteorological dataset in the HRB, and to drive directly the integrated model that coupled ecological-hydrological and socioeco- nomic processes in the HRB.     

The bubble leakage mechanism for vertical upflows by the phase separation concept

Recently,our research group proposed the phase separation condenser tube,in which a mesh cylinder was inserted to form the flow structure of‘‘gas near the tube wall and liquid in the tube core’’,significantly enhance the condensation heat transfer.But the bubble leakage towards the core region may worsen the heat transfer enhancement.In order to prevent the bubble leakage,the critical criterion was proposed based on the Young–Laplace equation,considering the inertia force,viscous force and pulsating flow.It was found that the critical criterion depends on the dimensionless parameter G*,the We number and a coefficient C.The numerical model was developed in terms of the volume of fluid method to predict the two-phase laminar flow in the phase separation condenser tube.The results show that the bubble leakage takes place at the bubble tip,which is agreed with the experimental observations.The critical curve distinguishing the non-bubble-breaking and bubble-breaking was obtained by comparing the bubble dynamics at different G*and We.The coefficient C was determined.The critical criterion for the bubble leakage is given as G We0:22We0:99349 G4:7 103 We0:00651t196:39We0:006514cosa DHW,providing the design and operation guidance for the phase separation condenser tube.     

Unsaturated flow and solute transport in a porous column using spherical ore particles

This paper dealt with the development of a two-dimensional (2D) mathematical model for column leaching and confirmed the important simulation parameters through experiment. The unsaturated state of the variably saturated flow column and the solute transport of copper ions were studied during leaching. The fluid flow problem was handled using the Richards equation on the premise of an ambient pressure column air, where the van Genuchten formulas were applied to define the nonlinear relationships of pressure head with the retention and permeability properties. The ore column permeability test gave a varied hydraulic conductivity, which was analyzed in the model. In the solute transport problem, the copper ion concentration was solved using the advection-diffusion-reaction equation whose reaction term was determined by the joint analysis of experimental copper leaching rate and the shrinking core model. Particle- and column-scale leaching tests were carried out to illustrate the difference and connection of copper extraction in both processes. This fluid flow and solute transport coupled model was determined through the finite element method using the numerical simulation software, COMSOL Multiphysics.     

小流量下尾水管锥管内流的PIV测试试验研究

采用PIV激光内流测试设备对一个改进的HL220转轮模型试验所使用的尾水管锥管流场进行了测试，并对所测试各截面上的流场分布进行了详细的分析，研究了小流量工况下锥管径向面和于午面上的流场分布规律．结果表明：在小流量工况下，尾水管锥管内的瞬态流场呈现出明显的非定常特征，锥管中心处存在明显的回流，管壁处存在涡流；不仅小流量工况下的流场与最优工况下流场存在明显区别，小流量工况下瞬态流场与时均化流场的分布规律之间也存在一定的差异．PIV测试中采集收集的实验数据，有助于混流式水轮机小流量下尾水管非稳定性分析的内在机理研究，为CFD分析提供对比依据．     

筒阀紧急动水关闭特性的数值模拟

为了防止水轮机飞选现象的发生,运用动网格和滑移网格技术,对动水关闭状态下的筒阀运动进行了三维非定常数值模拟和水力特性的研究．结果表明,筒阀的表面压力呈不均匀分布,引起的倾覆力矩最大值达2．94×10^4N·m．轴向力在筒阀关闭90％时达到峰值,并根据其变化的平稳程度提出了优化的筒阀关闭方式．此外,对筒阀表面的压力脉动进行了频谱分析,表明压力脉动的形成是尾水管涡带振动向上传递的结果．上述水力特性的分析不仅可以预测混流炎水轮机的工作状态．而且可以为筒阀运动的执行机构——多液压缸同步运动系统提供理论依据．     

水轮机尾水管涡带诱发的转轮横向激振力计算

针对引起水轮机机组振动最主要的振源——尾水管涡带,建立了一种计算水轮机转轮轴系横向激振力的可行性方法。通过对尾水管涡带对转轮流场影响方式和前人试验、数值结果的调查研究,建立了转轮出口延伸处涡带对转轮内流体的扰动简化模型,将这种涡带扰动流场作用于转轮三维紊流流场计算模型上,最终计算得到了不平衡横向激振力,可以作为轴系分析的已知外力,为水轮机轴系动力分析提供必要的数据。     

导叶开口对混流水轮机尾水管压力脉动的影响

将水压力脉动测试、流场测试以及初生空化观测等实测结果进行对比分析,研究水轮机活动导叶开口的变化对尾水管水压力脉动的影响,为进一步研究并建立尾水管水压力脉动数学模型提供依据     

长输水管道尾部保水堰的水力特性试验研究

在长距离输水管路出口设置溢流堰可以有效控制水击和防止管路脱空。为了分析该结构在非恒定流情况下的工作特性,设计水力模型试验对突然断水情况的水流特性进行观察测试,通过测试结果建立优化设计的方法和准则。试验表明:长管道水体的惯性力作用可能引起衔接池的水位跌落和波动,从而导致管路进气和水流拍打管口,通过增加溢流堰的高度或扩大衔接池的面积可以避免这一现象。     

应用三维空间导叶的混流式水轮机流动

为减小混流式水轮机转轮内的叶道涡,设计了两种三维空间导叶,推导了流量调节方程,采用了全三维全流道的湍流计算方法,完成了从蜗壳进口到尾水管出口,包含所有流道在内的数值计算;并进行了模型对比试验。结果显示:应用三维空间导叶对水轮机能量性能没有太大的影响,但是增大了活动导叶上部的水流出口角,从而减小了转轮叶片进口处的冲击,尤其是在高水头、小开度情况下,上冠处的冲击和脱流比二维常规导叶有明显改善,降低了叶道涡发生的可能性,提高了水轮机的运行稳定性。     

两种非均匀进口速度下尾水管内流CFD模拟分析

对两种不同负荷条件时给定尾水管非均匀进口速度条件，进行了内流计算流体动力(CFD)分析，采用定常流动计算方法对同一问题进行了对比，给出了两种工况条件下，各主要特征截面上的参数变化特征和详细细节，计算分析结果显示A工况速度条件的流动特征明显优于B工况，解决了CFD非均匀进口速度条件下的给定问题．     

贯流风机气动噪声数值预估

通过精细求解二维非定常Reynolds平均的Navier-Stokes方程,数值模拟了贯流风机内部的复杂流场。随后从流场的数值结果中提取出叶片、涡墙和后墙的脉动压力作为声源,进行声场计算。以声学中的Ffowcs Williams-Hawk-ings(FW-H)方程作为出发方程,数值求解贯流风机的噪声场。计算结果表明在贯流风机中,后墙的压力脉动与涡墙的压力脉动是主要的噪声源。该文的数值预估不仅在贯流风机的总体气动性能上与实验测试结果吻合,同时气动噪声场的预估结果也与实验测试结果吻合良好。     

贯流式水轮机尾水管设计与试验研究

从工程实际需要出发，研究了贯流式水轮机水平扩散型尾水管的水力性能，提出了这种尾水管的设计方法，从理论上分析了其水力损失。然后对这种管和常用的圆方型尾水管进行了模型水力性能对比试验，用五孔测流探针测量了模型尾水管的流速分布。对试验结果进行了分析和探讨，论证了水平扩散型尾水管用于实际工程的可能性。     

尾水管水力振动的监测与故障判断

从模型及真机试验结果出发，叙述混流式水轮机一般的水力故障形式，研究了尾水管的水力振动特点及振动随运行工况而变化的情况，介绍了以压力脉动频率及幅值作为特征参量对尾水管进行振动监测及故障判断的方法。