超(超)临界多级套筒调节阀空化抑制模拟研究

针对超(超)临界火电机组疏水系统中存在的套筒式多级降压调节阀高温水高压降空化引起汽蚀破坏的问题,基于汽液传递理论与多级降压理论,引入Z-G-B空化模型与混合两相流模型,建立阀内流场高温水空化计算模型.研究了套筒层数与级间间隙对空化流动的影响,获得了不同套筒模型条件下的阀内流场压力、速度及汽液两相体积分布等流场信息.分析结果表明:多级套筒使阀内最低压力由1.5 MPa上升至4.0MPa,空化最大气体体积分数由0.99降低至0.18.增加套筒级数可有效实现逐级降压、限制流速的目的,还可抑制空化的发生及发展,同时增加级间间隙也有利于减轻空化.     

空化热力学效应对相间质量传输过程的影响

为了分析空化热力学效应对相间质量传输过程的影响,应用二次开发技术在商业软件中引入修正的Merkle空化模型及液氮物性参数随流场温度的变化,并在能量方程的源项中考虑了空化的影响,对绕对称回转体液氮空化流动进行了计算.通过将计算结果与实验数据进行对比,对计算方法进行了验证.基于数值计算结果分析了空化热力学效应对质量传输过程的影响.相间质量传输过程包括蒸发和凝结过程.在蒸发过程中,低温流体空化的发展不仅与当地空化数σ有关,且依赖于流场的密度比R(T).随温度升高,液体/蒸汽的密度比梯度(dR/dT)增加,导致蒸发量减小;随温度升高,饱和蒸汽压变化梯度升高,导致蒸发量减小.凝结过程中,随温度升高,(1-φl)值减小,凝结量减小;随温度升高,max(0,Cp+σ)值增大,凝结量增加.     

螺旋桨空化崩溃性能图谱的多相流模拟

为了提高螺旋桨空化性能多相流模拟的精度,采用改进数值模型对NSRDC4381螺旋桨的片空化形态和空化崩溃性能图谱进行了模拟与校验．数值模型由混合密度中显示包含非凝结性气核（NCG）质量分数和体积分数的混合物多相流模型,考虑湍流脉动影响的相变临界压力、NCG质量分数取7．8×10^-4、NCG体积分数取1．0×10^-6、气泡平均初始半径取1．5μm的改进Sauer空化模型,以及剪切应力输运（SST）湍流模型组成．结果表明：预报空化崩溃性能图谱与实验值符合很好;非设计进速系数下模拟空化形态与采用Sauer空化模型的结果一致,较实验值略偏小;中度和重度空化下,推力和力矩系数的预报精度明显高于采用Kunz空化模型的结果,严重空化时精度提升约16％;即使在高负载且严重空化的非设计工况下,预报推力和力矩系数误差分别为4．95％和2．28％,也在工程可控范围内,证明了所采用数值模型的有效性．该模型可进一步用于螺旋桨空化初生的数值判定．     

三种空化模型下挤压油膜阻尼器空化流场特性对比

为准确预测挤压油膜阻尼器空化流场特性,基于ANSYS-Fluent软件建立非同心型挤压油膜阻尼器三维非定常空化流场求解模型.分别采用Zwart-Gerber-Belamri(Z-G-B)、Schnerr-Sauer(S-S)及Singhal三种空化模型计算得到挤压油膜阻尼器各测点的压力结果,并将其与实验数据进行对比,比较了三种空化模型下阻尼器内的油膜压力与气穴分布、流场中的气穴比以及油膜力特性.结果表明,三种空化模型中S-S模型的数值结果与试验数据最为吻合,适用于描述挤压油膜阻尼器空化流场特性,为挤压油膜阻尼器空化流场模拟提供了可靠的模型和方法.     

基于当地流动特征的Schnerr-Sauer空化模型改进研究

为消除空化模型中人为确定的经验参数对数值模拟结果的影响,引入当地流动特征对考虑非冷凝气体的Schnerr-Sauer空化模型进行修正.采用Mixture多相流模型及Realizable k-ε湍流模型,对二维NACA0009 MOD水翼的定常空化进行了数值模拟,空化数范围为0.75~0.90.将数值模拟结果与已有实验结果进行对比表明:对于考虑了非冷凝气体的Schnerr-Sauer空化模型,在引入当地流动特征修正前,非冷凝气体数密度n的取值对数值模拟结果影响较大,非冷凝气体直径d_nuc的取值对模拟结果的影响较小;在引入剪切应变率修正后,空化模型中的参数n的取值对数值模拟的结果不再有显著影响.同时,该修正方法提高了考虑非冷凝气体的Schnerr-Sauer空化模型的准确性,准确地预测了空穴造成压力梯度突变的位置.      

螺旋桨叶截面空化模拟数值模型的改进与评估

考虑了多相流空化黏性模拟中多相流模型、空化模型、湍流模型和相变临界压力选择等因素,提出了在混合密度中同时包含非凝结性气核质量分数和体积分数以及在相变临界压力中考虑湍流脉动影响的改进Sauer空化模型,分析了直接引入混合密度的SST模型、湍流黏度取μt=ρmk/(w)的修正SST模型和分别引入密度函数和滤波函数的修正k-ε模型4种湍流模型对螺旋桨叶截面NACA66 (mod)片空化模拟的影响,并应用改进后的数值模型对E779A螺旋桨不同空化程度下的片空化进行了模拟与校验.结果表明,NCG质量分数取7.8×10-4和体积分数取1.0×10-6及气泡初始平均半径取1.5μm时相对最优.对二维水翼轻度和中度片空化模拟时,后3种湍流模型修正方式精度相当,且优于第一种方式.对水翼云空化模拟时,两种修正k-ε模型相对最优.联合改进Sauer空化模型和修正SST湍流模型对螺旋桨轻度、中度和重度空化下的空化范围模拟均与实验值吻合很好,表明了改进数值模型的有效性,并可直接用于低噪声桨的设计.     

云状空化流动数值模拟的空化模型评价

为深入了解空化模型对云状空化流动计算的影响,分别采用Kubota,Singhal,Merkle,Kunz四种不同的空化模型对绕水翼的云状空化流动进行了数值模拟,获得了空穴形态和升、阻力等流场动力随时间的变化特性. 通过与实验结果的对比表明,这四种空化模型均可以准确地捕捉云状空化区域的空穴形态和空泡脱落的非定常细节;不同空化模型对计算所得的空穴形态和长度、绕翼型速度矢量场、升阻力均有影响, 但对流场动力特性的主要频谱分布影响不大.     

螺旋桨初生空化湍流的多相流数值模拟

摘要：
同时采用修正剪切应力输运(SST)湍流模型和Baseline雷诺应力模型（RSM）求取了E779A螺旋桨在无空化状态和初生空化状态下的梢涡运动轨迹,分析了涡核最小压力系数、湍动能、轴向速度分量和涡核半径沿运动轨迹的变化,并从模拟得到的梢涡卷曲起始和梢涡涡束的角度阐述了梢涡形成机理.空化模型采用改进Sauer模型,考虑了非凝结性气核质量分数、体积分数和气泡初始半径以及湍流脉动的影响,并针对轻度、中度和重度空化面积进行了可信性校验.当空化数σ>初生空化数σi时,叶梢截面压力系数分布相对不再改变的判定准则来确定.涡核中心位于螺旋线垂向截面上最小压力点,涡核边界由湍流涡频率峰值决定.数值模拟结果表明,RSM模拟梢涡路径较修正SST湍流模型稍长、局部梢涡空化范围略大、叶梢最小压力系数和轴向速度分量要小,涡核湍动能分布更为合理.但两者模拟得到的涡核运动轨迹几乎重合,并且初生空化状态下的涡核运动轨迹、最小压力系数和轴向速度分布均与各自无空化状态下非常接近,表明了初生空化状态判定的正确性和改进数值模型对梢涡运动轨迹模拟的适用性.     

滤波器湍流模型在超空化流动计算中的应用评价

对滤波器湍流模型(FBM)在超空化流动计算中的应用进行分析和评价.在CFX软件中,分别采用FBM模型和标准κ-ε模型对Hydronautics翼形的超空化流动进行模拟,结合水洞实验结果从空泡形态和流场结构两方面对两种模型的计算结果作对比分析.结果表明,FBM模型对空泡形态和空泡尺度的模拟更符合实际.FBM模型可以捕捉超空化流动中较大尺度的涡团结构,并能更清晰地模拟出空化区尾部涡团交替脱落的非定常细节,同时.FBM模型对超空化流动中反向射流现象的模拟更敏感.     

离心泵内部流场的三维空化湍流的数值计算

采用Singhal等人发展的一种三维混合流体完整空化湍流模型,基于RNG的k-ε湍流模型和SIMPLEC算法,对离心泵内部的全流道三维空化流动进行定常和非定常数值模拟.根据空化流动模拟计算的结果,得到定常和非定常情况下空化余量和扬程的曲线,与实验结果进行对比,说明计算结果能够真实反映离心泵内部空化流动的性能.进而有效地预测叶轮内部流道空化发生的部位和空化流动的发展情况,分析空化余量的降低对离心泵空化性能的影响.研究结果表明,所采用的空化模型和数值计算方法能够用于离心泵在设计工况下的空化性能的计算.     

Modeling cavitation flow of cryogenic fluids with thermodynamic phase-change theory

Cavitation is the formation of vapor bubbles within a liquid where the flow dynamics causes the local static pressure to drop below the vapor pressure. The so-called full cavitation model (FCM) developed by Singhal has been widely used in numerical modeling of the cavitation flow for thermosensible and non-thermosensible fluids. Within the FCM, the bubble size is taken to be equivalent to the maximum possible value to forego the calculation of bubble number density. We developed a new cavitation model by re-calculating the bubble radius in FCM to account for the effects of local pressure. The new model was obtained by combining the thermodynamic phase-change theory and the Young-Laplace equation with the assumption of thermodynamic equilibrium during the cavitation process. The cavitation calculations were performed based on the mathematical framework of the homogeneous equilibrium flow model and the transport-equation-based model for vapor phase mass fraction. The model was validated by modeling the cavitating flow of liquid nitrogen and liquid hydrogen through NASA hydrofoil and Ogive with consideration of the phase-change thermal effects. The temperature and pressure distributions with the new model are found to agree well with data from existing experimental studies, as well as the simulations with the FCM.     

考虑当地流动特征的Zwart-Gerbera-Belamri空化模型改进及应用

为减小Zwart-Gerbera-Belamri空化模型中经验参数取值对数值模拟结果的影响,基于当地流动特征修正了Zwart-Gerbera-Belamri空化模型.联立Realizable k-ε湍流模型,分别采用修正前后的Zwart-Gerbera-Belamri空化模型对二维NACA0009 MOD水翼空化流场进行了数值模拟,空化数范围为0.75~0.90.并采用修正前后的Zwart-Gerbera-Belamri空化模型对二维Clark-Y水翼的初生空化、片状空化、云空化进行非定常数值模拟,空化数范围为3.00~0.55.将数值模拟结果与已有实验结果进行对比,结果表明:引入当地流动特征可以有效减小Zwart-Gerbera-Belamri空化模型中的气泡半径及气核体积分数的取值对数值模拟结果的影响,且修正后的空化模型能够捕捉不同空化数下绕Clark-Y翼型的时均升阻力系数的变化趋势.      

泵喷推进器非定常空化性能数值模拟分析

摘要： 基于均质多相流的RANS(Reynolds Averaged Navier Stokes)方程,采用分块网格技术生成高质量结构化网格,同时结合剪切应力SST（Shear Stress Transport）k－ω湍流模型和Z G B(Zwart Gerber Belamri）空化模型以对NACA66翼型进行了定常流空化性能数值模拟,数值计算结果与实验值吻合良好,验证了数值计算法的实用性与可靠性.在此基础上,利用URANS(Unsteady Reynolds Averaged Navier Stokes）方程和滑移网格技术,对装有泵喷推进器的无人水下航行器（Unmanned Underwater Vehicle,UUV）进行了非定常流空化性能数值模拟与分析.计算结果表明,数值模拟可以较为精确地预测泵喷推进器空化现象的起始与发展、形状与位置等特性;空化现象发生时,泵喷推进器效率出现明显的降低且达到20%以上;UUV表面与定转子叶片的压力分布较为合理,与空化现象吻合;转子叶顶区域压力面与吸力面之间的压差引起了顶隙梢涡与顶隙空化,造成了泵喷推进器效率进一步的损耗.     

A modified density based cavitation model for time dependent turbulent cavitating flow computations

The predictive capability of transport equation-based cavitation models including the Kubota cavitation model (Model-1) and interfacial dynamics cavitation model (Model-2), is evaluated for the attached turbulent cavitating flows. In this study, the test problem is the unsteady cloud cavitating flows around a Clark-Y hydrofoil. Based on the evaluations of existing models, we identified the differences between these two vaporization and condensation processes in the affected region, and provided a modified density based cavitation model (Model-3). The numerical results of the cavity shapes, velocity distributions and dynamics of the cavity oscillations were compared to existing experimental data. Compared with the other cavitation models, a significant improvement for the numerical results of unsteady cavitating flows has been obtained with the new model. Our study provides the information for further modeling development.     

Numerical study of unsteady cavitation on 2D NACA0015 hydrofoil using free/open source software

The free/open source software as OpenFOAM, Salome meshing and python language under Debian Linux system are evaluated to model unsteady cavitating turbulent flow around a NACA0015 hydrofoil. Based on the cavitation model proposed by Kunz and large eddy simu- lation （LES） method, we denote the benefits of free software and open source tools as an alternative to proprietary software of computational fluid mechanics, and provided a modified cavitation model to improve numerical accuracy. The simulation results of typical break-off cycle for cavitation shedding are compared to available experimental data, and validated using image processing to find percentage of similarities. The sheet cavity frequency of 7.752 Hz was obtained based on cavitation evolution and pressure fluctuations. The study gives relevant information for CFD software development in the future.     

栅中水翼的空化流体动力特性

采用数值模拟的方法对空化条件下Clark-y水翼和栅中水翼的水动力特性进行了研究.计算中,空化模型采用基于Rayleigh-Plesset方程的Kubota模型,湍流模型则采用基于滤波器的混合湍流模型.通过实验结果对计算结果的可靠性进行了验证.结果表明,受上下翼型的影响,栅中水翼吸力面的低压区和压力面的高压区相对水翼情况有所收缩,造成平均升力系数变小;与水翼相比,栅中水翼的升阻力变化周期更长,波动范围更大.     

Validation of full cavitation model in cryogenic fluids

Numerical simulation of cavitation in cryogenic fluids is important in improving the stable operation of the propulsion system in liquid-fuel rocket. It also represents a broader class of problems where the fluid is operating close to its critical point and the thermal effects of cavitation are pronounced. The present article focuses on simulating cryogenic cavitation by implementing the “full cavitation model”, coupled with energy equation, in conjunction with iterative update of the real fluid properties at local temperatures. Steady state computations are then conducted on hydrofoil and ogive in liquid nitrogen and hydrogen respectively, based on which we explore the mechanism of cavitation with thermal effects. Comprehensive comparisons between the simulation results and experimental data as well as previous computations by other researchers validate the full cavitation model in cryogenic fluids. The sensitivity of cavity length to cavitation number is also examined. Supported by the National Natural Science Foundation of China (Grant No. 50706042)