Large eddy simulation of unsteady flow in vortex diode

Vortex diode, as an important component in power fluidics, has been widely used in nuclear reprocessing engineering for about two decades. The fluidic system with vortex diodes is always working under a condition with a pulsant pneumatic power input. However, no detailed analysis of the unsteady flow inside the vortex diodes is available. Therefore, we carried out large eddy simulation (LES) to investigate the transient start-up process on the foundation of the experiment by Jacob et al. The numerical results were compared with the experimental data. It is shown that the special flow-time profiles in the start-up process are well predicted. From a further analysis on the internal flow, it is indicated that the change of flow rate with time is relevant to the varying of internal flow pattern and the swirling level. The above-mentioned methods and results of the vortex diode are of great guiding significance to predict the transient performance of the fluidic transfer system.     

ADS无窗靶件自由界面模拟实验和数值研究

冷却剂自由界面形态的形成和控制是加速器驱动的次临界系统(ADS)无窗靶件设计的关键技术之一。采用水介质对无窗靶件模型的自由界面特征进行了实验和数值研究。实验中采用激光诱导荧光的示踪方法实现了流场的可视化,得到Re=30000～50000范围内的自由界面和可视化流场。在高Re工况下,流场中出现大尺度的非稳定涡结构,随着Re的降低,流场中涡结构的紊乱程度增加。分别采用大涡模型（LES）和两方程动能-特征耗散率模型（kω-SST）对无窗靶件实验工况进行了数值分析,计算结果表明,LES能较好地模拟实验中所得的流场现象和界面特征。     

裂变气体分离器气泡分离轨迹的数值模拟

钍基熔盐堆是我国重点开发的第四代核反应堆之一,其裂变反应产生的中子俘获截面大的Kr、Xe等裂变气体以微气泡的形式存在于熔盐冷却剂中,对裂变气体的分离是提高熔盐堆中子经济性、实现燃料深燃耗的重要环节。为定量地获得分离器内气泡的分离行为,采用数值模拟和理论建模相结合的方法,得到旋流场的流场分布特征;通过建立旋流场中气泡运动控制方程,分析并计算不同旋流度和气泡直径下的分离长度。与实验数据对比发现,数值模拟结果和实验数据吻合良好,表明数值模拟方法可以用于气液分离器的优化设计。     

涡流式分离器气液两相分离数值模拟研究

对于气液两相分离,传统分离器或体积过大,或旋流强度低,因此考虑提出一种新式的涡流式分离器。利用涡流二极管逆向流动形成强度较高旋流的特点,在旋流腔上方加入一根支管,从切向入口进入的两相流由于密度差和旋流的作用,气相会聚集在中心由于浮力作用从上支管流出分离器,液相会分布在四周由于重力作用从下支管流出分离器,从而实现两相分离。采用数值模拟的方式分别对不同旋流腔尺寸以及出口形状的分离器进行计算,模拟结果表明,在进口流量为0.5 t/h、入口含气率1%～5%工况下,控制底流口压力和入口相同,溢流口与入口压差在80～90 kPa范围内,分离器对粒径在50～100μm的气泡分离效率可以达到90%以上。     

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 3: turbulent flow and plume dispersion in building arrays

We have developed a LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergencies against accidental or intentional release of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume over a flat terrain was successfully simulated. In Part 2, a new scheme to generate a spatially developing turbulent boundary layer flow was proposed. Then, the large-eddy simulation (LES) model for turbulent flow and plume dispersion around an isolated building was validated. In this study, we extend the LES model to turbulent flows and plume dispersion in various building arrays that represent typical urban surface geometries. Concerning the characteristics of flow and dispersion in building arrays, the flow patterns associated with obstacle densities and the distribution patterns of mean and root-mean-square (r.m.s.) concentrations agree well with those of the wind tunnel experiments. It is shown that the LES model successfully simulates the unsteady behaviors of turbulent flows and plume dispersion in urban-type surface geometries.     

基于超声多普勒测速法对二维涡流场初步测量研究

为研究第四代先进反应堆燃料组件、散裂靶窗及换热器等复杂部件内的涡流场，利用超声多普勒测速（UDV）法在四直叶轮搅动装置上开展了涡流流场的测量实验。通过径向和轴向流场的矢量合成，获得了轴向截面上二维涡流流场分布。结果表明：轴向截面上流速随雷诺数的增大而增大；在叶轮上、下两侧区域分别存在较大的漩涡，上漩涡为逆时针方向，下漩涡为顺时针方向；截面上流速分布存在较大梯度，且湍流强度随雷诺数的增大逐渐增大，在与叶轮等高处流速很大，而在叶轮上、下两侧区域流速较小，这表征了涡流场的变化特性，说明UDV测量技术能用于复杂流场测量。     

Distribution of void fraction for gas-liquid slug flow in an inclined pipe

In order to investigate the effect of inclination angle on the spatial distribution of phases,experiments on gas-liquid two-phase slug flow in an inclined pipe were carried out by using the optical probe and an EKTAPRO 1000 high speed motion analyzer.It has been demonstrated that the inclination angle and the mixture velocity are important parameters to influence the distribution of void fraction for upward slug flow in the inclined pipe.At high mixture velocity,the gas phase profile is axial symmetry in the cross-section of the pipe.This is similar to that for vertical slug flow.In contrast.most of the gas phase is located near the upper pipe wall at low mixture velocity.By measuring the axial variation of void fraction along the liquid slug.it can be concluded that there is a high void fraction wake region with length of 3-4D in the front of liquid slug.In the fully developed zone of liquid slug.the peak value of the void fraction is near the upper wall.     

Analysis of Unsteady Turbulent Triple Jet Flow with Temperature Difference

Suitable analysis methodology is required to obtain detailed information about magnitude and frequency of temperature variation of flow field for the study of thermal stripping phenomena. The large eddy simulation (LES) is applied to analyze unsteady turbulent triple jet water flow which can be a direct cause of thermal stripping. Current analyses are performed with different sub-grid scale models, number of grids, time increments, and inlet temperature intensities to find the effects of these on prediction. Predicted results of the LES are compared with experimental results. The LES successfully produces a time history of turbulence variables, which can be used to evaluate magnitude and frequency of instantaneous temperature. The LES tends to predict higher levels of root mean square temperatures compared to those of an experiment, indicating very active mixing effect among triple jets. The LES is found to be able to provide reliable frequency information about temperature fluctuation. The different sub-grid scale models show no significant difference in prediction ability and other variations of the LES prediction show no significant difference in prediction either. However, cases using the fine grid and the small time increment are slightly better than others. Further study is desired with different levels of inlet temperature intensities and separate sub-grid scale models for temperature field.     

Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part II: URANS CFD Simulation

In Part II, we described the unsteady flow simulation and proposed a modification of a traditional turbulence flow model. Computational fluid dynamics (CFD) simulations of an isothermal, fully periodic flow across a tube bundle using unsteady Reynolds averaged Navier-Stokes (URANS) equations, with turbulence models such as the Reynolds stress model (RSM) were investigated at a Reynolds number of 1.8 × 10⁴, based on the tube diameter and inlet velocity. As noted in Part I, CFD simulation and experimental results were compared at five positions along (x; y) coordinates. The steady RANS simulation showed that four diverse turbulence models were efficient for predicting the Reynolds stresses, and generally, SRANS results were marginal to poor, using a consistent evaluation terminology. In the URANS simulation, we modeled the turbulent flow field in a manner similar to the approach used for large eddy simulation (LES). The time-dependent URANS results showed that the simulation reproduces the dynamic stability as characterized by transverse oscillatory flow structures in the near-wake region. In particular, the inclusion of terms accounting for the time scales associated with the production range and dissipation rate of turbulence generates unsteady statistics of the mean and fluctuation flow. In spite of this, the model implemented produces better agreement with a benchmark data set and is thus recommended.     

无窗散裂靶自由界面流动水力特性研究

自由界面形成和控制是加速器驱动的次临界系统(ADS)无窗靶研究的重要内容。本文通过水模拟实验,结合360°全尺度模拟计算,验证了自由界面的特征主要受入口速度和出口压力控制,得到了自由界面长度随入口速度增大而二次递减、随出口压力增大而线性递减的结论。对流场的结构和特征进行了研究,说明了漩涡区域对自由界面流动的影响。采用VOF界面追踪方法、大涡湍流模型(LES)以及PISO算法进行瞬态模拟计算,计算得到的自由界面流动和流场结果均与实验结果符合较好,表明计算模型和方法可用于液态重金属靶的流体力学计算。     

Numerical simulation of flows in tight-lattice fuel bundles

The flow in tight rod bundles is characterized by long-term, large-scale coherent patterns in the stream-wise direction.

In the present work, the issue of simulating these structures through unsteady CFD simulations employing periodic boundary conditions in the stream-wise direction, will be addressed. The validity of the approach is assessed through the comparison of a large eddy simulation (LES) for similar flow conditions inside a simplified geometry and experimental data.

A powerful statistical tool (proper orthonormal decomposition) is used to analyze the time varying solution. The flow field has been decomposed into a series of normal modes, identifying the structures responsible for the flow transfer between sub-channels. Additional insights on the physics of these coherent structures are obtained.

An unsteady Reynolds averaged Navier–Stokes simulation (URANS) of the flow in a rod bundle has then been carried out. The comparison between numerical results and experimental results [Krauss, T., Meyer, L., 1998. Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle. Nucl. Eng. Design 180, 185–206] proves that accuracy can be achieved for averaged statistics such as stream-wise velocity, turbulent intensity and wall shear stresses.     

双钩波形板汽水分离器疏水钩优化研究

为提高双钩波形板汽水分离器的分离性能，采用计算流体力学方法建立波形板内的两相流动模型，并对不同结构疏水钩的波形板汽水分离器进行研究。通过数值计算得到了波形板内的速度云图和液滴运动轨迹，并分析了不同进口速度下疏水钩结构对压降和分离效率的影响。结果表明：大部分液滴在前两级通道被分离，进口速度为0.922 m/s时其质量份额可超过50%；疏水钩通过影响流场的局部流速和湍流强度进而影响压降和分离效率，疏水钩对液滴的直接拦截作用有利于提高分离效率。综合考虑分离效率和压力损失获得了综合性能良好的双钩波形板汽水分离器结构型式。     

单相4×4棒束流动试验的CFD方法验证

为研究计算流体力学（CFD）方法预测棒束通道内流场分布的准确性,基于网格敏感性分析所确定的网格方案,采用标准k-ε模型（SKE）、可实现k-ε模型（RKE）、标准k-ω模型（SKW）和剪切应力传输模型（SST模型）对单相棒束流动进行模拟,并将横向速度与轴向速度与试验结果进行量化比较。结果表明:4种湍流模型均能较好地预测棒束通道内的流场分布,其中SKE与RKE的在横向速度预测上相对偏差较小,为19.6%;对于近格架区域的横向流场分析,SKE模拟较优,反之RKE模拟较优;对于轴向速度的预测,SKE的相对偏差最小为4.9%;4种湍流模型均低估均方根（RMS）速度,但能够预测棒束通道内RMS速度的分布规律,近格架区域采用RKE,反之SST较优。本文的计算结果可为单相棒束流动CFD分析的最佳实践导则建立提供参考。      

堵塞条件下紧密栅湍流交混特性研究

开展堵塞工况下紧密栅内流体子通道间隙湍流交混研究,对事故工况下燃料组件热工水力行为的预测具有重要意义。本文采用CFD方法对紧密栅内堵塞工况的流体流动现象进行了模拟,模拟结果与相关文献结果吻合较好。进一步对比分析了不同堵塞工况下,堵塞段及堵塞下游的速度场、涡结构以及湍流交混系数分布。所得不同堵塞工况下的横向与轴向湍流交混系数变化规律,可为子通道分析程序的参数设置提供参考。     

CFD simulation of bubble column

Three-dimensional simulations of gas-liquid flow in the bubble column using the Euler-Euler approach is presented. The attempt is made to assess the performance and applicability of different turbulence models namely, k-?, k-? RNG, k-ω, Reynolds stress model (RSM) and large eddy simulation (LES) using a commercial code (ANSYS-CFX). For this purpose, the predictions are compared against the experimental data of Kulkarni et al. (2007). Performance of the turbulence models is assessed on basis of comparison of axial liquid velocity, fractional gas hold-up, turbulent kinetic energy and turbulent eddy dissipation rate. All the non-drag (turbulent dispersion, virtual mass and lift force) and drag force were incorporated in the model. The low-Reynolds number treatment of the k-ω yields a better qualitative prediction than the k-? model. The RSM predictions are comparable with LES results and seemed to give better prediction near the sparger, where the flow is more anisotropic and gives a clue why RANS approaches fails to predict the flow in this region. However, the large eddy simulations showed good agreement with the experimental data, but requires higher computational time than RSM.     

LES and URANS simulations of turbulent flow in 4 rod bundles in ocean environment

The flow and heat transfer of turbulent flow in typical 4 rod bundles in rolling motion is investigated with LES and URANS. The effect of rolling motion consists of two parts, the axial additional force which causes velocity oscillation and the radial additional force. The effect of rolling motion on the flowing similarity is considerable. The effect of radial additional force on the flow should not be neglected. In ocean environment, the effect of radial additional force on the flow should not be neglected. The average parameters are determined by the drive force and axial additional force, but the parameter profiles in the cross section are mainly determined by the radial additional force.     

Study on Two-phase Flow in Fluid Channel of Lead-based Fast Reactor Based on OpenFOAM

In order to study the thermal-hydraulic characteristics of two-phase flow caused by the special thermal properties of lead/lead-bismuth in lead-based fast reactors, the influence of bubble in fluid channel on the heat transfer capacity and safety of the core was simulated. In this paper the open source CFD calculation software OpenFOAM was adopted, and the numerical simulation was applied based on VOF method to construct a common triangular channel model in lead-based fast reactor. By simulating the two-phase flow of the coolant channel, it is found that as the flow rate increases, the outlet temperature of the coolant decreases. In the flow process of the gas-liquid two-phase flow in the channel, it can be found that the gas phase basically flows inside the channel. In the simulation of the fuel assembly, the corner channel is an area with a large amount of bubbles, which will cause the local heat transfer to deteriorate and cause the fuel assembly to burn out.     

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 4: turbulent flows and plume dispersion in an actual urban area

We have developed a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergency situations resulting from accidental or deliberate releases of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume dispersing over a flat terrain was successfully simulated. In Parts 2 and 3, LESs of turbulent flows and plume dispersion around an isolated building and in building arrays with different obstacle densities were performed, which showed the basic performance comparable to wind tunnel experimental technique. In this study, we apply the LES model to turbulent flows and plume dispersion in an actual urban area. Although some of the turbulence and dispersion characteristics are quantitatively different from the wind tunnel experimental data, the distribution patterns are generally similar to those of the experiments. It is concluded that our LES model simulates reasonably the unsteady behavior of turbulent flows and plume dispersion even for complex heterogeneous urban areas.     

基于OpenFOAM的铅基快堆流体通道内两相流研究

为研究铅基快堆中铅/铅铋的特殊热物性导致的在两相流情况下的热工水力特性,模拟流体通道中空泡存在对堆芯的输热能力以及安全性的影响,本文采用开源的CFD计算软件OpenFOAM,应用基于VOF方法的数值模拟,构建了铅基快堆中常见的三角形通道模型,通过与子通道程序的验证和单相条件下实验的校核,检验了所用代码的准确性,并对堆内冷却剂通道的两相流进行了模拟。模拟结果表明：随着两相流流速的增大,冷却剂出口温度降低。气液两相流在内通道流动过程中,气相基本在通道内部流动。随着轴向高度的升高,气泡会在内通道的中心区域聚合;燃料组件的角通道是气泡含量多的区域,会造成局部传热恶化,导致组件烧毁。     

AP1000核主泵排气过渡工况下瞬态流动特性研究

为了研究核主泵在排气过渡工况下的气液两相流瞬态流动特性,基于非均相流模型,采用CFX软件对核主泵排气过渡工况进行瞬态数值模拟,通过分析叶轮、导叶流道内的压力脉动、涡量变化及速度分布,得到了排气过渡过程的流动变化规律。研究结果表明：气液两相工况下,叶轮各流道内气相、液相的不均匀分布及两相之间的滑移作用,导致叶轮径向力产生大幅度波动;核主泵采用的扭曲型径向导叶,在进口含气率较高的工况下,其流道内易产生气泡堆积现象,使过流面积减小,产生较大的能量损失;核主泵类球形蜗壳的对称性结构,使左侧类隔舌部位出现低流速区,堵塞了部分出口流道,这也是核主泵排气过渡工况运行不稳定的重要原因。