离心泵内部空化流动的定常数值模拟及性能预测

为了研究离心泵内部的空化流动,利用fluent软件中的空蚀模型及混合流体两相流模型,对离心泵的三维湍流空蚀流场进行定常数值模拟,根据模拟计算结果显示的液相和空泡相流动特征,预测了离心泵在设计工况下运行时流道内空化发生的位置和程度;通过分析空蚀发生过程中叶片上的压力分布,揭示出离心泵流道内部流场的内在特性,最后对泵的性能进行了预测,说明数值模拟可以为离心泵在特定工况下运行时的空化性能预测提供依据。     

高功率密度液力变矩器空化特性研究

高功率密度液力变矩器由于其内部流速高、局部压力低而易出现空化现象,导致其液力性能恶化。针对液力变矩器内空化现象进行试验及数值研究,通过对不同转速、不同速比及不同补偿油压力下液力变矩器性能测试,获得空化随工况及供油条件变化规律。构建基于Rayleigh-Plesset的全流道瞬态空化仿真模型对不同工况下液力变矩器内部两相空化流动进行预测,利用应力混合涡模拟湍流模型精确捕捉涡流状态,实现对有/无空化下液力变矩器内部流场及液力特性的计算。结果表明,液力变矩器在高泵轮转速、低速比及低补偿压力下容易发生空化,空化程度随着速比的下降而升高,在起动工况时达到最大。在空化工况下,液力变矩器导轮流道内产生大量空泡,空泡阻碍油液流动,导致循环流量降低,进而使液力变矩器传递功率的能力下降,起动工况下能容系数降低高达31%。全流道瞬态空化模型能够实现液力变矩器空化特性的精确预测,对变矩比、能容系数及效率的最大预测误差由无空化的30%降低至5%。     

水泵水轮机泵工况下近设计点驼峰现象的流动机理研究*

中高比转速水泵水轮机泵工况下近设计点驼峰严重制约了其稳定运行范围,是制约抽水蓄能电站安全与经济运行的关键问题之一。作为流动问题的宏观表现,驼峰现象必然与水泵水轮机内部的非定常流动存在密切关系。为此,基于试验和数值模拟,对水泵水轮机泵工况下可调导叶流道内的非定常流动进行研究,探究模型机泵工况下的压力脉动特性、非定流动机理及与性能曲线稳定性之间的关系。结果表明：在设计和小于设计工况下,可调导叶流道内均存在两种显著的周期性压力脉动。模型机泵工况下的流量扬程曲线在0.45~0.75倍设计流量区间内出现驼峰,频域分析清晰地揭示了以上两种压力脉动是对该驼峰存在重要影响的流动参量。     

混流式水轮机转轮优化设计及CFD分析

为了解决某小型水电站的混流式水轮机部分工况压力脉动大,空化、磨损严重的问题,对原水轮机进行了深入研究,并在原水轮机转轮的基础上研制出了新型的、适合该电站运行工况的水轮机转轮,对水轮机转轮进行了更换改造。对新型转轮和原转轮进行三维建模,分别与其余过流部件组装成水轮机全流道。采用ICEM进行网格划分,利用CFX软件,基于RNGκ-ε湍流模型,对原始全流道和改造后的全流道进行了数值模拟计算,通过计算结果分析转轮改造后水轮机内部流动以及外特性的变化,预估改造后水轮机的实际运行效果。     

水轮机参数化网格生成及整体性能数值模拟

为了对水轮机整体性能做出预测,采用参数化方法生成了水轮机全流道的结构网格,并在动静界面的数据交换上采用周向平均插值的方法进行了数值模拟,最后通过程序和CFX进行了数值模拟计算,并把结果与实际测得的数据进行比较,验证了模型和计算的准确性。研究结果表明,采用这种模型既能模拟水轮机内部流场,又能对整体性能做出较精确的预测。     

基于内部流场数值模拟和性能预测的水电站改造

通过CFD技术对映秀湾水电站某混流式水轮机改造方案的内部流动特征进行了全流道模拟,并对改造后的转轮性能进行预测.确定了以雷诺时均方程和标准k-ε方程湍流模型为数学模型,并确定了相应流动控制方程和全流道的边界条件,采用全三维黏性流动计算分析技术来模拟转轮内部流场.数值试验与模型试验结果对比结果表明：CFD技术的模拟结果与模型试验测试比较一致,能满足工程要求.     

基于开缝引射流的离心泵空化流动研究

为确定不同形式的开缝对离心泵的影响,采用引射流技术对其进行了六种开缝设计,采用RNG k-ε湍流模型和改进的Kubota空化模型对其空化流动进行了数值求解。结果表明:在大流量工况下运行时,离心泵外特性突降的原因在于叶片发生空化后附着空化改变了叶片型线,堵住了整个流道,造成了叶片做功能力逐步丧失;选择合理的开缝宽度和角度,可以使叶轮流道内的流动更加均匀,湍流减小,提高泵的效率,还可以改善离心泵的空化性能,延缓空化的发生,降低泵的必需空化余量。     

空化对尾水管区域驼峰特性影响研究

为了研究水泵水轮机空化对驼峰特性的影响,采用SST k-ω湍流模型和Z-wart空化模型对全流道进行了三维定常数值模拟计算,并分析了不同工况点下尾水管在驼峰区域的水力性能和内部流动状态。研究结果表明,不同工况点下,流量大小会改变尾水管区域液流的流动方向,从而产生偏心涡带使尾水管区域出现不稳定性,造成机组振动和噪声;单相计算结果比空化计算结果更早受到剪切流的影响。来流与壁面射流相互作用产生漩涡,出现回流现象。在速度梯度变化方面,空化计算结果的速度值要比单相的值高,能量损失有所增加。     

轴伸式贯流泵装置全流场三维湍流数值模拟

为探讨轴伸式贯流泵装置双向运行时的内部流动结构并进行性能预测,应用三维湍流Navier-Stokes、Realizable 两方程湍流模型、壁面函数法和滑移网格技术,进行泵装置双向运行时的全流场三维湍流数值模拟研究。计算所采用的模型贯流泵具有特定的S形叶片及正向运行时的后置弯曲导叶。研究结果揭示了贯流泵装置正、反向运行时的全流道速度等值线、静压等值线、出水流道断面矢量及出水流道的流线形状特征,探讨在泵装置实际安装条件下泵与流道的相互影响,进行泵装置性能的数值模拟预测,并与试验结果进行比较分析。数值模拟结果表明,泵段的出口流态及导叶对流道的流动结构有很大的影响,扩散形流道对流道进口流态反映敏感,无导叶的直形泵装置出口扩散流道内的流动为轴向旋涡、环形旋涡、轴向流动的叠加,数值模拟计算预测泵装置性能的方法是有效、可行的,能够满足工程需要。     

不同设计参数对离心泵内部不对称空化的影响

为研究不同设计参数对离心泵内部不对称空化现象的影响,以一台单级单吸离心泵为研究对象,保持泵体和叶轮其他参数不变,分别改变叶轮的叶片数、包角和冲角并建立相应模型,采用ANSYS-CFX软件对各模型进行计算并对结果进行分析。研究结果表明:在一定范围内,叶片数越多,越不利于各流道内的对称性流动,会造成不对称空化现象越来越严重;加大叶片包角可以使叶轮各流道内流动更均匀,缓解离心泵内的不对称空化现象;选择合理的冲角不仅可以提高泵的抗空化性能,同时也可以减轻离心泵内的不对称空化现象。     

NUMERICAL ANALYSIS ON THREE- DIMENSIONAL FLOW FIELD OF TURBINE IN TORQUE CONVERTER

Three-dimensional flow field of turbine in torque converter is simulated by numerical calculation in order to improve the performance of torque converter. Calculation model of a torque converter is presented based on the mixing-plane technology. In the calculation of flow field,the 3D N-S equations are separated by finite-volume method and solved by semi-implicit method for pressure-linked equations(SIMPLE). Based on flow field calculation,the flow field of turbine is simulated. The velocity and pressure in the flow field of turbine are analyzed. The external performance of the torque converter is also calculated. Results of flow simulation show that there are secondary flow,off flow and velocity gradient in turbine passage. The validity of numerical simulation is verified by comparing the results of numerical simulation with experiment data.     

水平轴海流机空化流动模拟

海流机是海流能利用的一种新形式,其空化性能的好坏直接影响到海流发电机组的安全和稳定运行。基于两相混合流模型对一模型水平轴海流机空化流动进行研究,得到不同空化数下的空泡形态及其性能曲线。数值模拟结果表明,采用数值计算方法能够有效地预测海流机的空化现象,计算得到模型海流机的初生空化数,空泡形态与试验结果相类似;海流机空化通常发生在近叶尖吸力面上,并随空化数的降低沿叶片展向和弦向变长;空化数相对较高时,空化出现在叶片弦向中间区域,随空化数降低向叶片尾缘发展;空化初生时,海流机性能没有明显改变;只有当空化数降低到一定程度时,海流机推力升高,输出功率下降,海流机性能恶化,实际运行中应当避免在此工况下运行。     

Numerical analysis of flow in a Francis turbine on an equal critical cavitation coefficient line

Numerical simulation and model test were applied to study the cavitation flow in Francis turbines. The SST k-ω turbulence model and the mixture model were used to simulate the cavitating flow in the Francis turbine. An equal critical cavitation coefficient line was calculated and the flow in the Francis turbine was analyzed. Simulation results show reasonable agreement with the experimental data. It is confirmed that these cavitation model and numerical method is a useful way to study the two-phase cavitation flow in Francis turbines. On the equal critical cavitation coefficient line, the energy loss in the turbine may be caused by the rotating of vortex rope in the draft tube or flow separation in the runner. The study of equal critical cavitation coefficient line can provide a basic guidance for industry practice.     

Comparison of cavitation prediction for a centrifugal pump with or without volute casing

Cavitation may not only cause head and efficiency breakdown of hydraulic machines but also generate other unfavorable phenomena such as noise and vibration. Therefore, the accurate prediction of cavitation development is important for various pump applications. In this paper, two numerical models, namely, models A and B, are applied to simulate the turbulent cavitating flows inside a centrifugal pump to investigate the effect of calculation domain on the prediction accuracy of cavitation performance for hydraulic machines. Model A has a calculation domain with volute casing, whereas model B has a single blade-to-blade flow passage without volute casing. Steady simulations of cavitating flow in the pump have been conducted based on the shear stress transport k-ω turbulence model and the homogeneous cavitation model. Both models A and B predicted that the pump performance decreases with decreasing cavitation number. Experimental results show that model B can predict better the critical cavitation number at the best efficiency point compared with model A, which is the full flow passage model. Internal flow investigations indicate that an asymmetrical feature of cavitating flow exists when the calculation domain with volute casing is applied. The asymmetrical cavitation development in different blade-to-blade flow passages for model A results in an over-estimation of the decrease in pump performance because of the interaction between the impeller blade and the tongue of the volute casing. A simple calculation domain without volute casing is preferred for steady cavitation prediction in pumps rather than the full flow passage with volute casing because the former has better convergence, less resource requirements, and lower time consumption.     

Numerical investigations on cavitating flows with thermodynamic effects in a diffuser-type centrifugal pump

A cavitation model with thermodynamic effects for cavitating flows in a diffuser-type centrifugal pump is developed based on the bubble two-phase flow model. The proposed cavitation model includes mass, momentum, and energy transportations according to the thermodynamic mechanism of cavitation. Numerical simulations are conducted inside the entire passage of the centrifugal pump by using the proposed cavitation model and the renormalization group-based k-? turbulent model coupled with the energy transportation equation. By using the commercial computational fluid dynamics software FLUENT 6.3, we have shown that the predicted performance characteristics of the pump, as well as the pressure, vapor, and density distributions in the impeller, agree well with that calculated by the full cavitation model. Simulation results show that cavitation initially occurs slightly behind the inlet of the blade suction surface, i.e., the area with maximum vapor concentration and minimum pressure. The predicted temperature field shows that the reduction in temperature restrains the growth of cavitating bubbles. Therefore, the thermodynamic effect should be treated as a necessary factor in cavitation models. Comparison results validate the efficiency and accuracy of the numerical technique in simulating cavitation flows in centrifugal pumps.     

Scale effects on propeller cavitating hydrodynamic and hydroacoustic performances with non-uniform inflow

Considering the lack of theoretical models and ingredients necessary to explain the scaling of the results of propeller cavitation inception and cavitating hydroacoustics from model tests to full scale currently, and the insufficient reflection of the nuclei effects on cavitation in the numerical methods, the cavitating hydrodynamics and cavitation low frequency noise spectrum of three geometrically similar 7-bladed highly skewed propellers with non-uniform inflow are addressed. In this process, a numerical bridge from the multiphase viscous simulation of propeller cavitation hydrodynamics to its hydro-acoustics is built, and the scale effects on performances and the applicability of exist scaling law are analyzed. The effects of non-condensable gas(NCG) on cavitation inception are involved explicitly in the improved Sauer’s cavitation model, and the cavity volume acceleration related to its characteristic length is used to produce the noise spectrum. Results show that, with the same cavitation number, the cavity extension on propeller blades increases with diameter associated with an earlier shift of the beginning point of thrust decline induced by cavitation, while the three decline slopes of thrust breakdown curves are found to be nearly the same. The power of the scaling law based on local Reynolds number around 0.9R section is determined as 0.11. As for the smallest propeller, the predominant tonal noise is located at blade passing frequency(BPF), whereas 2BPF for the middle and both 2BPF and 3BPF for the largest, which shows the cavitating line spectrum is fully related to the interaction between non-uniform inflow and fluctuated cavity volume. The predicted spectrum level exceedance from the middle to the large propeller is 6.65 dB at BPF and 5.94 dB at 2BPF. Since it just differs less than 2 dB to the increment obtained by empirical scaling law, it is inferred that the scale effects on them are acceptable with a sufficient model scale, and so do the scaling law. The numerical implementation of cavitating hydrodynamics and hydro-acoustics prediction of propeller in big scale in wake has been completed.     

双流道转轮水力透平机的全流道数值模拟

依据给定的参数,对水力透平机进行模型设计。采用FLUENT软件对所设计的水力透平机进行了全流道三维定常湍流数值模拟,计算以雷诺时均N-S方程为基础,基于贴体坐标和交错网格划分,对控制方程进行变换和离散。采用SIMPLEC算法实现速度、压力变量的分离求解。定常湍流计算采用了标准k-ε湍流模型。模拟计算获得了水力透平各过流部件内部的流动机理,该数值计算方法和结果对水力透平机的水力优化设计具有重要意义。     

Assessment of a turbulence model for numerical predictions of sheet-cavitating flows in centrifugal pumps?

Various approaches have been developed for numerical predictions of unsteady cavitating turbulent flows. To verify the influence of a turbulence model on the simulation of unsteady attached sheet-cavitating flows in centrifugal pumps, two modified RNG k-? models (DCM and FBM) are implemented in ANSYS-CFX 13.0 by second development technology, so as to compare three widespread turbulence models in the same platform. The simulation has been executed and compared to experimental results for three different flow coefficients. For four operating conditions, qualitative comparisons are carried out between experimental and numerical cavitation patterns, which are visualized by a high-speed camera and depicted as isosurfaces of vapor volume fraction α _v = 0.1, respectively. The comparison results indicate that, for the development of the sheet attached cavities on the suction side of the impeller blades, the numerical results with different turbulence models are very close to each other and overestimate the experiment ones slightly. However, compared to the cavitation performance experimental curves, the numerical results have obvious difference: the prediction precision with the FBM is higher than the other two turbulence models. In addition, the loading distributions around the blade section at midspan are analyzed in detail. The research results suggest that, for numerical prediction of cavitating flows in centrifugal pumps, the turbulence model has little influence on the development of cavitation bubbles, but the advanced turbulence model can significantly improve the prediction precision of head coefficients and critical cavitation numbers.