竖直圆管内气泡运动的大涡模拟研究

基于两流体模型框架,使用雷诺平均N-S方程(RANS)和大涡模拟(LES)两种湍流模型对竖直圆管内的绝热离散气 液两相流动进行数值模拟研究。计算结果表明,采用恰当的相间相互作用模型,两种模型的时均模拟结果同实验均符合较好。气泡的壁面聚集现象被准确预测,速度场预测也较为准确。与基于RANS的SST湍流模型相比,采用WALE亚网格应力的大涡模拟得到的结果同实验符合得更好,且大涡模拟可给出流动的瞬态细节。     

The Effect of Mixing-vane Arrangements in a Subchannel Turbulent Flow

Large eddy simulation (LES) of developed turbulent flows in a rod bundle was carried out for four spacer designs. The mixing-vanes attached at the spacer were inclined at 30° or 20° they were arranged to promote the swirling or convective flow. These arrangements are possible elements to compose an actual rod bundle. Our LES technique with a consistent higher-order immersed boundary method and a one-equation dynamic sub-grid scale model contributed to an efficient treatment of the complex wall configurations of rods and spacers. The computational results reasonably reproduced experimental results for the drag coefficient and the decay rate of swirling flow. The profiles of the axial velocities and the turbulence intensities indicated reasonable trend for the turbulent flow in the rod bundle. The effect of mixing-vane arrangement on the lateral flows was successfully clarified: the cross flow took the longer way on the rod surface than the swirling flow and then was more significantly influenced by momentum diffusion at the no-slip wall. Therefore, the largely inclined mixing-vanes promoted the cross flow only in the neighborhood of the spacer; the swirling flow inside a subchannel could reach farther downstream than the cross flow.     

等速入口三喷口流动的温度振荡数值模拟

由于来自核反应堆堆芯不同通道冷却剂的温度存在差异,它们在上腔室混合后将产生复杂的温度振荡现象,可能干扰堆芯出口的温度测量,也可能造成周围固体结构的热疲劳破坏。本文基于FLUENT软件,采用大涡模拟湍流模型(LES)对三维平行三喷口流动进行数值模拟,并与实验结果进行了比较。结果表明：随着入口速度的增加,流体混合越来越剧烈,同时温度振荡的最大振幅逐渐减小,而相应的频率则逐渐增加。     

基于喷嘴临界流实验对现有物理模型的评价

以水为工质、直径1.41mm的喷嘴为实验段,在稳态条件下进行临界流实验,覆盖的参数范围为：进口压力,0.4～22MPa;进口温度,40～371℃;进口含汽率,－3.5～0.98。结果分析表明：在欠热度高于一定值下流动未达到临界条件;在低欠热度和低含汽率区域两相间存在热力不平衡性,在较低压力下,它对临界流率的影响十分显著。将实验数据与均匀平衡模型、Moody模型、Henry-Fauske模型、Burnell模型以及Bernoulli公式的计算结果进行比较,显示了各模型在不同条件下的适应性。     

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。     

Large eddy simulation of thermal striping in the upper plenum of the PGSFR

A computational study of thermal striping in the upper plenum of the prototype generation-IV sodium-cooled fast reactor (PGSFR) being developed at Korea Atomic Energy Research Institute is presented. First, previous experimental and numerical studies on the thermal striping are briefly discussed. Both Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) approaches are employed for the simulation of thermal striping in the upper plenum of the PGSFR. For the RANS approach, the conventional k ? ? turbulence model is employed and the LES is performed using the wall-adapting local eddy viscosity model. From the RANS results, the time-averaged velocity components and temperature field in the complicated upper plenum of the PGSFR are calculated. In the LES results, the time history of temperature fluctuation at several locations of upper internal structure (UIS) and intermediate heat exchanger (IHX) are additionally stored. Comparisons of the predicted time-averaged velocity and temperature between the two methods show that the prediction by the LES shows faster thermal mixing than that by the k ? ? turbulence model. From the computed results of the temporal variation of temperature, it was possible to find the amplitude and frequency of the temperature fluctuation at the several locations of the UIS and IHXs. It was found that the location where the thermal stress is largest in the upper plenum of the PGSFR is the ?-shape region of the first grid plate.     

Turbulence-induced Heat Transfer in PBMR Core Using LES and RANS

This paper introduces the results of numerical simulations on flow fields and relevant heat transfer in the pebble bed reactor (PBR) core. In the core, since the coolant passes a highly complicated random flow path with a high Reynolds number, an appropriate treatment of the turbulence is required. A set of simple experiments for the flow over a circular cylinder with heat transfer was conducted to finally select the large eddy simulation (LES) and k-ω model among the considering Reynolds-averaged Navier-Stokes (RANS) models for PBR application. Using these models, the PBR cores, whose geometries were simplified to the body-centered cubical (BCC) and face-centered cubical (FCC) structures, were simulated. A larger pressure drop, a more random flow field, a higher vorticity magnitude and a higher temperature at the local hot spots on the pebble surface were found in the results of the LES than in those of RANS for both geometries. In cases of the LES, the flow structures were resolved up to the grid scales. Irregular distributions of the flow and local heat transfer were found in the BCC core, while relatively regular distributions for the FCC core. The turbulent nature of the coolant flow in the pebble core evidently affected the fuel surface temperature distribution.     

入口速度比对同轴喷射流三维温度振荡影响的实验研究

实验测量同轴喷射流模型三维温度场和二维速度矢量场,分析不同入口速度比对三维温度振荡的影响。实验结果表明：R<1时,冷热流在底部完成了混合行为,平均温度不呈轴对称分布,而R≥1时,呈轴对称分布,但流体瞬态温度不呈轴对称分布。随着R的增加,冷流随高度增加温度上升的梯度减小,而热流温度下降的梯度则增大;R>1时,流场温度振荡强度小于R≤1的,其强温度振荡发生在冷热流之间和热流与环境流之间的区域。不同R下,温度振荡主要频率均分布在7Hz以内。     

Computational study of conjugate heat transfer in T-junctions

In this work we focus on the numerical prediction of temperature fluctuations induced in solid materials through turbulent mixing processes. As test case we use the mixing of two streams of different temperature in a T-junction. Due to the turbulent mixing of the two streams temperature fluctuations occur which are also transferred to the solid walls in contact with the fluid. Such fluctuations in the solid material may lead to thermal fatigue and are therefore relevant for the lifetime management of components used in nuclear power plants (NPP).We investigate the mixing in T-junctions made of different materials and having different pipe wall thicknesses. The temperature difference between the streams in the main and side branch is set to 75 °C and the mass flow rate in the main pipe is three times larger than in the side branch. In a first step we perform a set of simulations by using different formulations of the large-eddy simulation (LES) subgrid scale model, i.e. classical Smagorinsky model and dynamic procedure, to identify the influence of the modeled subgrid scales on the simulation results. The comparison between available experimental data and the numerical results reveals a good agreement when using the dynamic procedure. In a second step we address the temperature fluctuations in the solid wall subject to the wall thickness. The influence of the wall thickness is represented as a damping effect on the temperature fluctuations in radial direction in the pipe material. This study shows the capability of LES to predict thermal fluctuations in turbulent mixing.     

低压自然循环复合流量脉动实验研究

在低压流动沸腾不稳定性实验中,研究了自然循环流动在不同入口过冷度下的演化过程。对实验中的流动沸腾不稳定性入口流量信号进行快速傅里叶变换,基于振幅和频率特性区分了3种流动脉动模式：小幅流量脉动、复合流量脉动和逆流。分析了加热功率和入口过冷度对自然循环不稳定性的影响。根据加热段出口水温变化得到了出口的流型变化,当流量波动振幅较小时加热段出口流体始终是饱和状态,而当流量波动振幅较大时,加热段出口为单相液体和两相混合物交替通过。给出了这3种流量脉动的边界图,分析了热流密度和入口过冷度对流量脉动模式的影响。结果表明：出口含气率大于0时发生流动不稳定性,热流密度达到间歇干涸型临界热流密度时发生逆流。     

用于同轴喷射流引起的温度振荡数值模拟的湍流模型有效性研究

本文采用Fluent软件对同轴喷射流冷热流体搅混引起的三维温度振荡现象进行数值模拟。模拟中，分别选取几种代表性的湍流模型，如大涡模拟(LES)、雷诺应力模型(RSM)及标准k-ε模型。并将模拟结果与实验结果进行了对比，发现：LES是最适合预测冷热流体搅混特性的模型；LES可准确预测温度振荡在半径、高度和方位角方向的平均温度，而雷诺平均法(RANS)的结果则延长了流体的混合过程；LES可预测流体的瞬态温度振荡，RANS则无法预测。     

Two-phase flow instability in a parallel multichannel system

The two-phase flow instabilities observed in through parallel multichannel can be classified into three types, of which only one is intrinsic to parallel multichannel systems. The intrinsic instabilities observed in parallel multichannel system have been studied experimentally. The stable boundary of the flow in such a parallel-channel system are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results show that parallel multichannel system possess a characteristic oscillation that is quite independent of the magnitude and duration of the initial disturbance, and the stable boundary is influenced by the characteristic frequency of the system as well as by the exit quality when this is low, and upon raising the exit quality and reducing the characteristic frequency, the system increases its instability, and entrance throttling effectively contributes to stabilization of the system.     

基于CFD方法的非能动余热排出系统数值模拟

本文应用FLUENT软件对APl000的非能动余热排出热交换器和换料水箱进行了数值模拟，分析了不同c型传热管数量和冷却剂入口温度对热交换器换热性能和换料水箱内热分层、自然循环现象的影响。分析表明，总体通流面积不变，随着传热管数量增加，热交换器出口温度变小，水箱水温整体提升，热分层现象显著，自然循环趋势明显；质量流量不变，随着冷却剂入口温度的增加，入口流速增加，热交换器出口温度变大，但降温幅度也变大，水箱平均水温升高，热分层范围扩大，自然循环流速加快。     

Tritium Separation by CO2 Laser Multiphoton Dissociation of Trifluoromethane

Integral system tests with the Cylindrical Core Test Facility (CCTF) were performed to investigate the effect of the initial clad temperature on the reflood phenomena in a PWRLOCA. The initial peak clad temperatures in these three tests were 871, 968 and 1,047 K, respectively. The feedback of the system on the core inlet mass flow rate was estimated to be little influenced by the variation of the initial clad temperature except for the first 20 s in the transient. The observed temperature rise from the reflood initiation was lower with the higher initial clad temperature. This qualitatively agreed with the results of the small scale forced feed reflood experiments. However, the magnitude of the temperature rise in CCTF was significantly low due to the high initial core inlet mass flow rate. Also observed were the multi-dimensional thermal behaviors for the three cases in the CCTF wide core. The analysis codes REFLA and TRAC reasonably predicted the effect of the initial clad temperature on the core thermo-hydraulics under the simulated core inlet flow conditions. However, the calculated temperature rise of the maximum powered rod based on the one-dimensional core analysis was higher than that of the average powered rod, which contradicts the tendency observed in CCTF tests.     

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.     

Application of large-eddy simulation to pressurized thermal shock problem: A grid resolution study

Life time extension assessment is a very important issue for the nuclear community. A serious threat to the life time extension of a Reactor Pressure Vessel (RPV) is an occurrence of the Pressurized Thermal Shock (PTS) during an Emergency Core Coolant (ECC) injection in a loss-of-coolant accident. Traditional system codes fail to predict the complex three-dimensional flow phenomena resulting from such ECC injection. Improved results have been obtained using Computational Fluid Dynamics (CFD) analysis based on the Reynolds-Averaged Navier-Stokes (RANS) equations. However, it has been also shown that transient RANS approaches are less capable to predict the complex flow features in the downcomer of the RPV. More advanced CFD methods like Large-Eddy Simulation (LES) are required for modeling of these flow phenomena. This paper addresses the feasibility of the application of LES for single-phase PTS. Furthermore, the required grid resolution for such LES analyses is identified by evaluation of the solution on different mesh sizes. A buoyancy-driven PTS experiment has been considered here. This experiment has been performed in the 1:5 linear scale Rossendorf Coolant Mixing Model (ROCOM) facility. In the applied numerical model, the incompressible Navier-Stokes equations are solved in the LES formulation, with an additional transport equation for a scalar, which is responsible for driving the embedded buoyancy term in the momentum equations. Instantaneous mixing characteristics are investigated based on evaluation of the scalar concentration. The analysis presented in this paper indicates that the application of LES is feasible nowadays for single-phase PTS. It is demonstrated that the mixing in the downcomer is quite sensitive to small turbulent disturbances at the ECC inlet, i.e., two simulations performed with slightly different fluctuations at the inlet result in substantially different flow in the downcomer. This complicates the analysis of the data from simulations and suggests that evaluation of the results should be performed in the frequency-amplitude domain instead of the classically employed temporal data analysis.     

基于OpenFOAM的过冷流动沸腾数值模拟

过冷流动沸腾现象被广泛应用于工业生产和动力系统中,对该现象的准确预测是两相流CFD模拟的重要研究方向。本文详细阐述了该模拟过程中的欧拉两流体模型及相关辅助模型,基于开源CFD平台OpenFOAM,模拟了4.5 MPa下竖直圆管内的过冷流动沸腾,得到了截面空泡份额、液相平均温度及壁面温度沿轴向的分布。计算结果与实验值符合良好,说明了模型的有效性和程序的正确性。本文可为在OpenFOAM中添加新的模型及开发新的求解器以模拟过冷流动沸腾问题提供参考。     

超临界水并联通道流动不稳定性理论研究

根据并联通道的结构特点,建立了合理的数学物理模型,采用半隐式差分和交错网格技术对超临界水并联通道中的流动传热进行了数值模拟。运用小扰动法验证了超临界水密度波型流动不稳定的发生,并计算了流量、入口温度、入口压力对其流动不稳定性发生边界的影响。并联通道系统的稳定性随入口压力和入口流量的增大而增强,随入口温度的增大而减弱。     

两相流模型在直流蒸汽发生器热工水力分析中的比较研究

根据组成气液两相流基本场方程数量所反映的流动与传热特性的不同,两相流方程分为三方程、四方程、五方程和六方程模型,结合流动压降模型、传热模型、两相相互作用模型以及流动工质的状态参数和结构材料热物性等辅助关系式,可很好地对蒸汽产生系统进行设计和研究分析。本文分析了不同数量的两相流场方程的特点和局限性,结合直管式直流蒸汽发生器实验装置,分别选取最佳估算程序中4种不同的两相流场方程计算模型进行流动传热计算分析,重点比较了强制流动的单相过冷水被加热至单相过热蒸汽过程中的压力与传热特性,从而给出不同场方程的两相流模型在分析具有较大相变过程中的差异性,验证了RELAP5程序和RETRAN-3D程序计算分析直流蒸汽发生器的能力。结果表明,RELAP5程序的六方程模型更适合模拟直流蒸汽发生器。     

基于遗传BP神经网络的超临界水自然循环稳态流量研究

利用遗传BP神经网络建立超临界水自然循环稳态流量预测模型,采用平均影响值(MIV)的概念进行参数敏感度分析。研究结果表明,遗传BP网络可以很好的预测超临界水自然循环稳态流量值,误差落在了±10%范围内。在所选的参数范围内,入口温度增大,稳态流量减小,提高试验段高度或减小加热段长度、出入口阻力系数可以使自然循环流量增加,其重要度排序为入口温度、试验段高度、入口阻力系数、出口阻力系数、加热段长度,且入口阻力系数、出口阻力系数、加热段长度影响基本对等。