基于MOOSE平台的高阶全隐式核反应堆一回路系统分析

基于多物理场耦合平台MOOSE开发了模块化系统安全分析程序ZEBRA,并采用高阶全隐式离散格式建立了核反应堆一回路系统模型,对核反应堆系统中子扩散、二维固体导热和一维流体进行耦合计算。针对单管流动传热问题,对ZEBRA程序进行了耦合验证,对比了稳态工况下一阶、二阶空间离散格式和瞬态工况下Implicit-Euler、Crank-Nicolson、BDF2 这3种时间离散格式的求解精度,并对压水堆回路系统稳态和降功率瞬态工况进行了模拟分析。结果表明,高阶空间离散格式具有较高的求解精度,BDF2时间离散格式与理论解符合最好;压水堆回路系统温度、速度、压力分布合理,稳态、瞬态计算结果与RELAP5程序计算结果符合良好。      

基于CFD方法的核动力系统热工安全特性研究进展

西安交通大学核反应堆热工水力团队（XJTU-NuTheL）长期致力于计算流体动力学（CFD）方法的核动力系统高精度热工水力模型开发及应用方面的研究。近些年,团队在单相CFD工程应用、两相CFD模型开发、大涡模拟（LES）及直接数值模拟（DNS）高性能并行计算、跨尺度多物理场耦合等方面取得了系列研究成果。主要包括：构建了核反应堆压力容器、蒸汽发生器、非能动余热排出系统换热器等核动力系统关键设备的三维多孔介质热工水力计算模型,建立了复杂物理现象及运动瞬变工况下的两相CFD数学物理模型,开发了CFD程序与核反应堆系统程序、堆芯子通道程序之间的跨尺度耦合以及与中子物理、力学程序之间的多物理场耦合分析平台。本文将重点阐述XJTU-NuTheL基于CFD方法在核反应堆热工水力研究方面的最新成果及进展,并提出CFD方法在核反应堆工程领域应用的主要挑战及发展方向,旨在促进CFD方法更好地服务于核动力系统设计与运行安全分析。     

对流传热问题的粒子-网格混合方法数值模拟

移动粒子半隐式（MPS）方法是一种完全基于拉格朗日体系的粒子法,应用于不可压缩流体。MPS方法在描述存在大变形和相变的问题时具有很大的优势。然而当MPS方法中的Laplace模型应用于求解能量方程时会存在求解不精确的情况。本文在MPS方法的基础上,提出了一种粒子 网格混合求解方法。通过MPS方法求解动量守恒方程,耦合有限体积法（FVM）求解能量守恒方程,以达到对对流传热问题的精确求解。对一维导热问题进行了方法验证,证明了粒子 网格混合方法求解导热的有效性,并有较高的求解精度。利用粒子-网格混合方法模拟了方腔内的自然对流,结果表明,基于MPS方法和FVM的耦合方法可用于求解对流传热问题。     

双弹性管流固耦合振动的数值模拟

为研究反应堆结构中诸如燃料棒、蒸汽发生器和其他换热器等管束类结构的流固耦合振动问题,利用有限体积法离散大涡模拟的流体控制方程及有限元方法离散结构动力学方程,结合动网格技术,建立了三维流体诱发弹性管束振动的数值模型,实现了计算结构动力学与计算流体力学之间的双向耦合。得到横流作用下单管的振动响应,并与已有的实验数据比较,证明了本文模型的合理性;对横流作用下的两串列管、两并列管的流固耦合振动进行了数值模拟,着重研究了节径比为1.2、1.6、2、3、4的两弹性管在不同流速作用下的动力学响应及流场特性;得到串列管、并列管的临界间距与临界流速。     

基于流固共轭传热的两相流动数值模拟及燃料子通道CHF预测研究

为对过冷沸腾两相流动进行准确模拟,并探索临界热流密度（CHF）预测方法,本文基于共轭传热和两相CFD分析的方法,通过流固界面耦合,建立流固共轭传热两相流动耦合求解的数值模型。首先通过典型燃料棒栅元过冷沸腾两相流动的模拟,验证数值模型的正确性。随后对燃料子通道内两相流动进行模拟,并在两相流动模拟的基础上,通过准瞬态的方法,建立与CHF试验过程非常近似的CHF预测方法,将加热壁面的温度飞升作为CHF判定的标准,实现对燃料组件子通道CHF的数值预测。研究表明,本文建立的数值模拟方法,可为燃料组件或其他换热系统的CHF预测奠定基础,为燃料组件的设计提供新的辅助手段。     

基于CFD方法的核动力系统热工安全特性研究进展

西安交通大学核反应堆热工水力团队(XJTU-NuTheL)长期致力于计算流体动力学(CFD)方法的核动力系统高精度热工水力模型开发及应用方面的研究。近些年,团队在单相CFD工程应用、两相CFD模型开发、大涡模拟(LES)及直接数值模拟(DNS)高性能并行计算、跨尺度多物理场耦合等方面取得了系列研究成果。主要包括:构建了核反应堆压力容器、蒸汽发生器、非能动余热排出系统换热器等核动力系统关键设备的三维多孔介质热工水力计算模型,建立了复杂物理现象及运动瞬变工况下的两相CFD数学物理模型,开发了CFD程序与核反应堆系统程序、堆芯子通道程序之间的跨尺度耦合以及与中子物理、力学程序之间的多物理场耦合分析平台。本文将重点阐述XJTU-NuTheL基于CFD方法在核反应堆热工水力研究方面的最新成果及进展,并提出CFD方法在核反应堆工程领域应用的主要挑战及发展方向,旨在促进CFD方法更好地服务于核动力系统设计与运行安全分析。     

失水事故引起反应堆堆内结构动响应分析的声学有限元方法

失水事故引起的反应堆堆内结构动响应是反应堆结构动力学的重要问题。现有的分析计算方法主要是基于计算流体动力学(Computation Fluid Dynamics,CFD)的单向/双向流固耦合分析方法,数值预测精度较高但计算成本过高。本文基于以流体和固体位移和流体压力为基本未知量的势流体声学有限元理论,采用商用有限元软件ADINA和流固界面共节点网格技术,对德国HDR(Heiss Dampf Reaktor)实验堆V32破口失水事故进行了数值模拟。声学有限元-结构耦合计算结果与文献中的实验测试结果和基于CFD的双向流固耦合结果吻合良好,但比CFD方法更易于实现且具有较高的计算效率。研究结果为反应堆失水事故下堆内结构动响应提供了一种简单而高效的分析方法。     

液态金属内单个气泡上升行为的MPS法数值模拟

液态金属冷却核反应堆采用气泡泵的概念设计来提升堆芯自然循环能力。液态金属内气液两相流动特征将直接影响核反应系统一回路的自然循环能力及堆芯安全。本研究通过采用移动粒子半隐式（MPS）方法,对液态金属中单个上升气泡的气泡动力学行为进行数值模拟。分析了铅铋合金中3种初始直径不同的单个氮气泡在上升过程中的气泡形状和速度的变化趋势;对比了初始直径相同的单个氮气泡在液钾、液钠、铅铋合金、钾钠合金和锂铅合金5种液态金属中的上升行为;同时将模拟得到的气泡形状与Grace经验关系图进行了对比,验证了MPS方法数值模拟结果的正确性。     

对流传热问题的粒子-网格混合方法数值模拟

移动粒子半隐式(MPS)方法是一种完全基于拉格朗日体系的粒子法,应用于不可压缩流体。MPS方法在描述存在大变形和相变的问题时具有很大的优势。然而当MPS方法中的Laplace模型应用于求解能量方程时会存在求解不精确的情况。本文在MPS方法的基础上,提出了一种粒子-网格混合求解方法。通过MPS方法求解动量守恒方程,耦合有限体积法(FVM)求解能量守恒方程,以达到对对流传热问题的精确求解。对一维导热问题进行了方法验证,证明了粒子-网格混合方法求解导热的有效性,并有较高的求解精度。利用粒子-网格混合方法模拟了方腔内的自然对流,结果表明,基于MPS方法和FVM的耦合方法可用于求解对流传热问题。     

平行喷口流动引起温度振荡流固热耦合数值模拟研究

本文基于FLUENT软件,采用大涡模拟(LES)方法对平行喷口冷热流体搅混产生的流固热耦合现象进行了数值模拟。首先将模拟值与实验值进行了对比,验证了模拟方法的正确性。其次,针对多种入口工况进行计算和分析,得出:某一时刻,流体温度处于波谷时,热量从固体传递给流体,下一时刻,当流体温度处于波峰时,热量则从流体传递给固体,故而形成了流固间周期性的热传递现象;随着固体厚度的增加,温度振荡的振幅呈非线性减小,但频率不变;随着入口速度的增加,流体和固体温度振荡的主频逐渐增大,当速度从0.5 m/s变化到1.5 m/s时,主频率可从2 Hz升高到11 Hz。     

Numerical simulations of gas–liquid–particle three-phase flows using a hybrid method

For the analysis of debris behavior in core disruptive accidents of liquid metal fast reactors, a hybrid computational tool was developed using the discrete element method (DEM) for calculation of solid particle dynamics and a multi-fluid model of a reactor safety analysis code, SIMMER-III, to reasonably simulate transient behavior of three-phase flows of gas–liquid–particle mixtures. A coupling numerical algorithm was developed to combine the DEM and fluid-dynamic calculations, which are based on an explicit and a semi-implicit method, respectively. The developed method was validated based on experiments of water–particle dam break and fluidized bed in systems of gas–liquid–particle flows. Reasonable agreements between the simulation results and experimental data demonstrate the validity of the present method for complicated three-phase flows with large amounts of solid particles.     

流动泡核沸腾中汽泡行为的MPS数值模拟

采用移动粒子半隐式（MPS）方法分别对加热面水平、竖直和45°倾斜放置时流动泡核沸腾中1个汽泡的成长至脱离过程进行了二维数值模拟,并将数值模拟结果与实验结果进行了对比。结果表明：汽泡的形状与成长速率的数值模拟结果与实验结果相符合,数值模拟成功地预测了实验中观察到的汽泡脱离前的滑移现象。     

Simulation of a Single Bubble Rising with Hybrid Particle-Mesh Method

A computational study of the dynamics on a gas bubble rising in a viscous liquid by a hybrid particle-mesh method is presented. The hybrid particle-mesh method has been developed for the simulation of a two-phase flow. One phase is represented by moving particles and the other phase is defined on a stationary mesh. The flow field is discretized by conservative finite volume approximation on the stationary mesh, and the interface is automatically captured by the distribution of particles moving through the stationary mesh. The moving particles are calculated by Moving Particle Semi-implicit (MPS) method. The effect of surface tension is evaluated by the continuum surface force model. In this study, we simulate the motion of a gas bubble rising in a viscous liquid. The buoyancy driven motion of the bubble in a wide range of flow regimes is simulated successfully by the present method. The deforming interface of the bubble is captured effectively by the moving particles although significant density and viscosity differences exist. By comparing the simulation results with experimental ones, we approve the possibility of applying the hybrid method to the two-phase flow with particles representing the gas phase and mesh representing the liquid phase.     

采用多边形壁面的MPS粒子分裂算法研究

反应堆严重事故模拟中涉及大量含有自由界面或组分界面的多相流动,不具有拓扑结构的粒子法在这类流动的模拟中有其独特的优势。本文在移动粒子半隐式（MPS）法的基础上开发了适用于多边形壁面的粒子分裂模型。针对多粒径模拟及多边形壁面的特殊性,对粒子有效半径、数密度、梯度算子模型、拉普拉斯算子模型和表面粒子识别模型等进行改进,并对分裂过程进行简要阐述。将改进后的MPS法运用于无挡板及有挡板的溃坝实验模拟计算。计算结果表明,采用多边形壁面的粒子分裂模型对复杂自由液面捕捉清晰,且计算时间仅为传统MPS法的18.75%,但监测点的压力波动较大。改进后的模型在保证准确性和精度的同时提高了计算效率,为进一步计算三维相变传热奠定基础。     

基于UDF的二维管束流弹失稳模拟研究

针对现有流弹失稳模拟研究中的流固耦合模型存在计算精度较低、计算成本巨大的问题,建立了一种可以预测管束临界流速的二维单向流固耦合模型,该模型基于商用ANSYS Fluent软件,通过SST k-ω湍流模型进行流场计算,再由自编译的用户自定义函数（UDF）提取管子所受的流体力,并利用4阶Runge-Kutta法求解结构动力学方程实现单向流固耦合计算。利用该模型对节径比为1.5的转角三角形排布管束进行了流固耦合计算,得到了中心管的临界流速、振幅时程曲线及振幅频谱图,并通过水洞实验进行了验证。结果表明,本模型以较低的计算成本准确地预测了临界流速,同时也获取了管子真实的振动特征,模拟计算的中心管振幅时程曲线及振幅频谱均与实验相近。此外,模拟计算获取的阻力和升力系数数据表明,随着流速增大,阻力和升力系数时程曲线经历了从紊乱到规律的变化,换算流速达到2.44时,阻力和升力系数主频包含管子在静水中固有频率的成分。      

Study on Solid-Liquid Two-Phase Flow on PWR Sump Clogging Issue

It has been a concern that sump screen clogging would occur in pressurized water reactors (PWRs) in the case of a loss-of-coolant accident (LOCA), because two-phase jet flow would strip off thermal insulation from the piping and wash down the broken and fragmented debris to sump screens. It is necessary for the evaluation of the effectiveness of sump screens to estimate the amount of transported debris from a break position to sumps. In general, conservative logic trees have been used to determine debris transport rates. Realistic debris transport evaluation is useful for considering measures and rational decision making in licensing. The purpose of this study is to develop a debris transport evaluation model and to apply the model to this issue. We developed a solid-liquid multiphase model that is capable of simulating debris transport, settling, and resuspension. The model is able to treat solid particles of different sizes, which are smaller than uniform-sized liquid particles. This approach contributes to reducing the calculation cost in a large-scale simulation. The model and a turbulence model were implemented into a code based on the moving particle semi-implicit (MPS) method. Several open-channel hydraulic experiments with fibrous debris were conducted. The code named SANSUI 2.0 was validated by the comparison of the analytical results with experiments. This method was applied to the debris transport analysis of a full-scale PWR containment vessel floor, and the debris transport behavior was evaluated.     

Numerical study on sedimentation behavior of solid particles used as simulant fuel debris

This paper presents numerical simulations using the discrete element method (DEM) to model sedimentation behavior of solid debris particles, which is significant for estimates of the coolability of debris beds. A series of experiments of gravity driven discharge of solid particles into a quiescent water pool was used to validate the DEM simulation method. We evaluated the effects of three crucial factors: particle density, particle diameter, and nozzle diameter on three key quantitative parameters: particle dispersion angle, particle fall time in the pool, and the height of the deposited particle bed to express the particle sedimentation behavior. The three crucial factors play a significant role in the particle sedimentation behavior. We compared the experimental and simulated results of the particle dispersion angle and particle fall time in the pool, and the height and shape of the deposited particle bed. The general trend of the simulation results indicates a reasonable agreement with the experimental observations. The simulations exhibit the potential applicability of the DEM-based simulation technique for the prediction of particle sedimentation behavior.     

A moving subgrid model for simulation of reflood heat transfer

In the quench front and froth region the thermal-hydraulic parameters experience a sharp axial variation. The heat transfer regime changes from single-phase liquid, to nucleate boiling, to transition boiling and finally to film boiling in a small axial distance.One of the major limitations of all the current best-estimate codes is that a relatively coarse mesh is used to solve the complex fluid flow and heat transfer problem in proximity of the quench front during reflood. The use of a fine axial mesh for the entire core becomes prohibitive because of the large computational costs involved. Moreover, as the mesh size decreases, the standard numerical methods based on a semi-implicit scheme, tend to become unstable.A subgrid model was developed to resolve the complex thermal-hydraulic problem at the quench front and froth region. This model is a Fine Hydraulic Moving Grid (FHMG) that overlies a coarse Eulerian mesh in the proximity of the quench front and froth region. The fine mesh moves in the core and follows the quench front as it advances in the core while the rods cool and quench. The FHMG software package was developed and implemented into the COBRA-TF computer code. This paper presents the model and discusses preliminary results obtained with the COBRA-TF/FHMG computer code.     

Simulation of drop deposition process in annular mist flow using three-dimensional particle method

The three-dimensional moving particle semi-implicit (MPS) method is employed to simulate the deposition process of single droplet on the liquid film. The model accounts for the presence of inertial, gravitation, viscous and surface tension and is validated by comparison with experimental results. The parameters of liquid droplets and film are calculated by a one-dimensional mixture model in which correlations and methods on void fraction, entrainment fraction and droplet velocity and size distribution are employed. The simulation results are analyzed to study the effect of splash on the deposition and re-entrainment processes in annular-mist flow. It is found that splash plays an important role in the deposition and re-entrainment processes in high quality conditions of BWR.