Analysis of Two-Phase Cavitating Flow with Two-Fluid Model Using Integrated Boltzmann Equations

In the present work, both computational and experimental methods are employed to study the two-phase flow occurring in a model pump sump. The two-fluid model of the two-phase flow has been applied to the simulation of the three-dimensional cavitating flow. The governing equations of the two-phase cavitating flow are derived from the kinetic theory based on the Boltzmann equation. The isotropic RNG$k-\epsilon-k_{ca}$ turbulence model of two-phase flows in the form of cavity number instead of the form of cavity phase volume fraction is developed. The RNG $k-\epsilon-k_{ca}$ turbulence model, that is the RNG$k-\epsilon$ turbulence model for the liquid phase combined with the $k_{ca}$model for the cavity phase, is employed to close the governing turbulent equations of the two-phase flow. The computation of the cavitating flow through a model pump sump has been carried out with this model in three-dimensional spaces. The calculated results have been compared with the data of the PIV experiment. Good qualitative agreement has been achieved which exhibits the reliability of the numerical simulation model.     

数值模拟两相汽蚀流动的新模型和算法

提出了数值模拟两相汽蚀流动现象的新汽蚀模型和算法.提出的汽蚀模型和算法耦合了考虑紊流粘性效应的Reynolds-Averaged Navier-Stokes方程求解方法,可以自动模拟空泡起始点、空泡长度和汽蚀空泡形状.在流场计算和界面修正的迭代计算过程中,跟踪并得到液相/气相界面.数值模拟了圆锥形圆柱体和半球形圆柱体在不同汽蚀系数下的汽蚀流动现象,得到了与实验值完全吻合的数值模拟结果,并且与已发表的数值研究结果进行了比较.计算结果表明提出的汽蚀模型和算法能够有效地模拟汽蚀流动中的气泡界面和气泡长度,汽蚀模型和算法的正确性和实用性得到了相应的验证.     

Numerical Prediction of Two-Phase Flow through a Tube Bundle Based on Reduced-Order Model and a Void Fraction Correlation

Predicting the void fraction of a two-phase flow outside of tubes is essential to evaluate the thermohydraulic behaviour in steam generators. Indeed, it determines two-phase mixture properties and affects two-phase mixture velocity, which enable evaluating the pressure drop of the system. The two-fluid model for the numerical simulation of two-phase flows requires interaction laws between phases which are not known and/or reliable for a flow within a tube bundle. Therefore, the mixture model, for which it is easier to implement suitable correlations for tube bundles, is used. Indeed, by expressing the relative velocity as a function of slip, the void fraction model of Feenstra et al. and Hibiki et al. developed for upward cross-flow through horizontal tube bundles is introduced and compared. With the method suggested in this paper, the physical phenomena that occur in tube bundles are taken into consideration. Moreover, the tube bundle is modelled using a porous media approach where the Darcy–Forchheimer term is usually defined by correlations found in the literature. However, for some tube bundle geometries, these correlations are not available. The second goal of the paper is to quickly compute, in quasi-real-time, this term by a non-intrusive parametric reduced model based on Proper Orthogonal Decomposition. This method, named Bi-CITSGM (Bi-Calibrated Interpolation on the Tangent Subspace of the Grassmann Manifold), consists in interpolating the spatial and temporal bases by ITSGM (Interpolation on the Tangent Subspace of the Grassmann Manifold) in order to define the solution for a new parameter. The two developed methods are validated based on the experimental results obtained by Dowlati et al. for a two-phase cross-flow through a horizontal tube bundle.     

低温气液两相流数值计算

低温工质两相流动在很多场合都存在,研究深冷剂在两相流动时参数的分布有着很重要的作用.文中采用一维均相流模型对液氮循环流动过程进行稳态数值模拟,得到了循环流动过程中液氮含汽率、压力和温度沿管程方向的分布情况,并对由于不同回路条件引起的不同结果进行了分析.     

变燃速发射药两相流内弹道模型及数值模拟

 为了更好地描述变燃速发射药的膛内燃烧及内弹道过程,对中心开孔式双层结构的变燃速发射药建立了一维两相流内弹道数学模型,在Φ30 mm火炮装填条件下采用二阶精度的Lax-Wendroff差分格式进行计算,得到了数值解。计算结果表明：膛内压力和炮口初速度变化曲线的计算值与实测值符合较好,说明此两相流内弹道模型可以准确地反映变燃速发射药的内弹道过程;变燃速发射药可以有效地减少膛内压力波的产生,对于安全射击具有重要意义。通过对气、固相速度及膛内压力波等计算参数进行分析,可以对变燃速发射药的内弹道过程研究起指导作用。     

A Numerical Method on Eulerian Grids for Two-Phase Compressible Flow

We develop a numerical method to simulate a two-phase compressible flow with sharp phase interface on Eulerian grids. The scheme makes use of a level set to depict the phase interface numerically. The overall scheme is basically a finite volume scheme. By approximately solving a two-phase Riemann problem on the phase interface, the normal phase interface velocity and the pressure are obtained, which is used to update the phase interface and calculate the numerical flux between the flows of two different phases. We adopt an aggregation algorithm to build cell patches around the phase interface to remove the numerical instability due to the breakdown of the CFL constraint by the cell fragments given by the phase interface depicted using the level set function. The proposed scheme can handle problems with tangential sliping on the phase interface, topological change of the phase interface and extreme contrast in material parameters in a natural way. Though the perfect conservation of the mass, momentum and energy in global is not achieved, it can be quantitatively identified in what extent the global conservation is spoiled. Some numerical examples are presented to validate the numerical method developed.     

Particle Mass Loading Effect on a Two-Phase Turbulent Downward Jet Flow

The particle mass loading effect on the flow structure of a two-phase turbulent jet flow was studied. A particle mass loading ratio ranging from 0 to 3.6 was used as the control parameter. The polystyrene solid particles used had nominal diameters of 210 and 780 μm. The flow Reynolds number, which was based on the pipe nozzle diameter and the fluid-phase centerline mean velocity, was 2 × 10⁴ in the current test. A two-color laser Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed to measure the mean velocity distributions for the particle and fluid phases, and the turbulent intensities and Reynolds stresses of the flow. The two-phase jet flow field was measured from the initial pipe exit to 90 D downstream. Another one-component He? Ne laser LDA system was also applied to obtain the energy spectra and temporal correlations of the two-phase jet flow.     

混合通量差分格式的比较

系统研究了几种混合通量差分格式的构造方法和耗散模型,分别对低速平板绕流、二维跨音速喷管流动和高超音速钝头体无粘绕流进行了数值模拟,结合先进的EASM湍流模型对格式的粘性分辨率和激波稳定性进行了细致的比较分析.结果表明混合通量差分格式兼顾了FDS和FVS格式的优点,具有较高的间断分辨率和数值稳定性.     

On the Computations of Gas-Solid Mixture Two-Phase Flow

This paper presents high-resolution computations of a two-phase gas-solid mixture using a well-defined mathematical model. The HLL Riemann solver is applied to solve the Riemann problem for the model equations. This solution is then employed in the construction of upwind Godunov methods to solve the general initial-boundary value problem for the two-phase gas-solid mixture. Several representative test cases have been carried out and numerical solutions are provided in comparison with existing numerical results. To demonstrate the robustness, effectiveness and capability of these methods, the model results are compared with reference solutions. In addition to that, these results are compared with the results of other simulations carried out for the same set of test cases using other numerical methods available in the literature. The diverse comparisons demonstrate that both the model equations and the numerical methods are clear in mathematical and physical concepts for two-phase fluid flow problems.     

气固两相流中流型和电荷分布的可视化研究

提出了一种非侵入式同时测量气固两相流型和电荷分布的可视化测试方法。数值仿真结果表明,ECT提供的介电常数分布信息可用于修正实际流型下的EST电荷灵敏场,降低了电荷分布反演的不确定性,EST的图像重建误差有所减小,有效的提高了电荷分布反演的精度。将该测试方法分别应用于粉体重力输送系统和高压密相气力输送系统,结果表明重力输送试验中颗粒浓度大的区域电荷密度值较大,而在气力输送试验中,颗粒浓度大的区域,电荷密度值反而较小,且随着管内煤粉颗粒浓度的增加,粉体总的电荷强度值减小。     

Schemes for Hybrid Bidirectional Controlled Quantum Communication via Multi-qubit Entangled States

We present two schemes for hybrid bidirectional controlled quantum communication (HBCQC) via six- and nine-qubit entangled states as the quantum channel, respectively. In these schemes, two distant parties, Alice and Bob are not only senders but also receivers, and Alice wants to teleport an unknown single-qubit state to Bob, at the same time, Bob wishes to help Alice remotely prepares an arbitrary single- and two- qubit state, respectively. It is shown that, only if the two senders and the controller collaborate with each other, the HBCQC can be completed successfully. We demonstrate, in our both schemes, the total success probability of the HBCQC can reach 1, that is, the schemes are deterministic.     

泵驱动两相系统流动稳定性的时域理论研究

蒸汽发生器中可能出现的两相流不稳定现象会导致二回路流量、压力和温度的脉动,不仅干扰控制系统,还可能引起传热管热疲劳损坏.首先,采用集总参数法和小扰动线性化方法,建立了由泵驱动的两相系统的时域理论模型.然后对比分析了泵驱动边界条件和恒定压降边界条件,以及泵特性曲线的斜率对系统稳定性的影响.结果 表明,随着泵特性曲线斜率绝...     

四阶紧致差分格式对静力模式中垂直网格计算频散性的影响

为了说明四阶紧致差分格式在大气和海洋数值模式中的潜在价值,提出一种通用方法,推导静力线性斜压适应方程组在微分和差分情况下的频散关系,水平尺度分100 km,10 km和1 km三种情况,从频率、水平群速和垂直群速方面,对采用二阶中央差和四阶紧致差分格式情况下,非跳点网格(N网格)、Lorenz网格(L网格)、Charney-Phillips网格(CP网格)、Lorenz时间跳点网格(LTS网格)和Charney-Phillips时间跳点网格(CPTS网格)的计算特性进行比较,发现采用高精度的四阶紧致差分格式总体上可以明显减少上述三种水平尺度波动在N网格、CP网格、L网格和CPTS网格上的频率、水平群速和垂直群速误差,但对LTS网格,采用四阶紧致差分格式,会使得计算水平群速和垂直群速误差变大.     

作用力求取方法对多相伪势模型的影响

基于伪势模型将有效密度和势函数结合计算粒子间相互作用力,提出一种改进描述气液相变的格子Boltzmann模型.对不同状态方程控制的单组份流体相变过程进行模拟发现,与单纯由有效密度或势函数计算粒子间相互作用力的方法相比,该模型的结果与Maxwell构建更吻合,且最大伪速度更小,扩大了温度的计算范围,提高了模型的稳定性.基于Laplace定律对界面现象模拟结果进行讨论,模拟结果与Laplace定律吻合.     

Large Deviations for a Stochastic Model of Heat Flow

We investigate a one-dimensional chain of 2N harmonic oscillators in which neighboring sites have their energies redistributed randomly. The sites −N and N are in contact with thermal reservoirs at different temperature τ₋ and τ₊. Kipnis et al. (J. Statist. Phys., 27:65–74 (1982).) proved that this model satisfies Fourier’s law and that in the hydrodynamical scaling limit, when N → ∞, the stationary state has a linear energy density profile , u ∈[−1,1]. We derive the large deviation function S(θ(u)) for the probability of finding, in the stationary state, a profile θ(u) different from . The function S(θ) has striking similarities to, but also large differences from, the corresponding one of the symmetric exclusion process. Like the latter it is nonlocal and satisfies a variational equation. Unlike the latter it is not convex and the Gaussian normal fluctuations are enhanced rather than suppressed compared to the local equilibrium state. We also briefly discuss more general models and find the features common in these two and other models whose S(θ) is known.     

基于非接触电导的气液两相流参数检测系统

研究了电容耦合式非接触电导检测技术(C~4D)应用于气液两相流测量的可行性,建立了一套基于非接触电导的毫米级气液两相流参数检测系统。利用所建立的系统,在内径为5 mm,9 mm的水平管上进行了气液两相流实验,获得了不同流型下的电导测量信号,并对段塞流、波状流、泡状流和环状流四种典型流型下的电导信号进行了分析与比较。研究结果表明,所建立的基于非接触电导的气液两相流参数检测系统是有效的,C~4D技术应用于气液两相流参数测量是可行的。不同流型下所获的电导信号特征均表现出明显差异,有望用于流型辨识。     

二维非均匀多孔介质中不可压两相驱替的有限分析算法

为提高油藏数值模拟算法的计算效率,在求解单向稳态渗流的有限分析算法基础上,构建二维非均匀多孔介质中不可压两相渗流的有限分析算法.算法中,网格界面上的平均渗透率不是简单地取为相邻网格渗透率的调和平均值,而是通过奇点邻域解析解积分求得.相比于传统的数值算法,有限分析算法随着网格的加密,能够很快地收敛(仅需将原始网格细分至2×2或3×3),并且其计算精度和收敛性不依赖于介质的非均匀强度,从而计算效率得到提高.     

垂直圆管氮气-磁性液体两相流电磁感应现象

基于磁性液体与非磁性气泡磁导率的差异,理论分析与实验研究了氮气-磁性液体两相流的电磁感应现象。以颗粒体积浓度为3.83%的水基Fe₃O₄磁性液体为工质,同时采用物性与其相近的去离子水进行流型的可视化对比研究。研究结果表明,当管内为泡状流、弹状流、搅拌流时均能产生毫伏级的感应电压,且感应电压的峰峰值随着气体流量的增加而增大,另外根据感应电压随时间变化的波形可以辨识出管内的流型。     

Generalized Hybrid Nanofluid Model with the Application of Fully Developed Mixed Convection Flow in a Vertical Microchannel *

The fully developed mixed convection hybrid nanofluid flow in a vertical microchannel is examined in detail. The simplified hybrid model that omits the nonlinear terms due to the interaction of different nanoparticle volumetric fractions is derived and compared with the existing one. The generalized model describing hybrid nanofluid suspended with multiple kinds of solid particles is suggested. The argument that the corresponding nanofluid solutions obtained by the homogenous model can be recovered from the results of the regular problems through simple arithmetic operations is checked. Solutions in similarity form for this flow problem are formulated by means of a set of similarity variables. The effects of various parameters on important physical quantities are analyzed and discussed.