非能动安全壳冷却系统CFD冷凝和蒸发模型研究

利用计算流体动力学(CFD)程序平台CASTEM,开发非能动安全壳冷却系统的冷凝、蒸发模型.冷凝模型中分别采用Buleev和k-ε两种湍流模型,蒸发模型采用了GAiello(2009)发展的液膜蒸发模型.试验验证分析表明,计算值与试验值吻合较好,所建立的冷凝、蒸发模型能较好地模拟非能动安全壳冷却系统在稳态下的传热传质特性.     

非能动安全壳冷却系统传热传质模型研究

安全壳分析程序CONTAIN采用的滞止液膜模型(SFM)对非能动安全壳冷却系统冷凝质量流量的预测低于试验值。因此,根据扩散层模型(DLM)与SFM在计算冷凝质量流量是等效的,通过对比利用质量分数形式菲克定律发展的改进扩散层模型(GDLM)与DLM的差异,对不可凝气体存在下的CONTAIN冷凝和蒸发模型加以改进。利用冷凝板试验和热板蒸发试验对改进后的CONTAIN程序进行验证,结果表明改进后模型计算的传热系数和传质系数比原模型更接近试验值。     

金属化有机膜制备方法的研究

计算了真空蒸发制备薄膜时造成衬底温度升高的热量－蒸发源的辐射热和沉积时的冷凝热。总结了制备金属化有机膜时防止衬底软化的各种方法，包括提高蒸发距离，降低沉积率，减少蒸发源尺寸和采用挡板等。     

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。      

一种适用于钠冷快堆绕丝棒束组件热工水力特性分析的多孔介质模型

为准确预测钠冷快堆堆芯三维热工水力参数分布同时降低计算资源需求,基于表征体元概念建立了针对绕丝棒束组件的三维多孔介质模型,根据组件几何结构特点将冷却剂与固体壁面间的相互作用力分解为分布式阻力,引入包含湍流搅混传热、流体导热和燃料棒导热的有效传热系数模型刻画组件的径向传热。采用日本东芝公司核能工程实验室37棒液态钠冷却绕丝棒束组件实验进行模拟计算,数值计算结果与实验结果对比发现,基于论文提出的多孔介质模型可以在多种工况下较好地复现实验结果。因此,本研究提出的多孔介质模型可用于钠冷快堆绕丝棒束组件三维热工水力参数分布预测。     

竖直通道内下降液膜蒸发换热的数值研究

下降液膜蒸发换热是CAP1400型反应堆非能动安全壳采用的重要换热机理,准确计算下降液膜蒸发换热量对非能动安全壳换热性能的评价有至关重要的影响。本文利用ANSYS FLUENT软件二次开发,实现了两种下降液膜蒸发换热模型的构建,并将两种模型计算结果与实验结果进行了对比分析。计算结果表明：两种模型均可较为准确地计算壁面下降液膜的蒸发换热系数;模型1的计算结果更加可靠,但在靠近壁面处需非常精细的网格;模型2在壁面处可使用较粗网格,但计算结果对对流换热系数的依赖较大。     

基于SIMPLEC算法的高温热管启动特性数值模拟

为研究高温液态金属热管的瞬态特性,开发了高温热管瞬态分析程序,程序可模拟高温液态金属热管自凝固状态启动过程中涉及到的复杂的相变、蒸气流动及气液交界面的传质传热特性。热管管壁与吸液芯区域采用纯导热模型,考虑了吸液芯内部工质的熔化和凝固以及气液界面的蒸发和冷凝。蒸气模型采用一维的尘气模型(DGM),该模型可精确描述热管启动过程中不同阶段的蒸气的运动特性。热管启动过程中蒸气为高度可压缩,因此蒸气速度与压力的求解采用可压缩流动的SIMPLEC算法进行计算。最后程序模拟了钠热管的启动过程,并与试验数据进行了比较。结果表明,程序模拟结果与试验数据符合较好。     

竖直波形板折角处液膜破裂研究

通过对曲线坐标系下2维边界层的纳维-斯托克斯（N-S）方程与连续性方程的推导，并结合定义无量纲量的方法，建立了液膜破裂模型，得出了一种波形板折角处的液膜临界破膜速度的理论计算方法；开展了液膜破裂实验研究，利用平面激光诱导荧光技术与图像处理方法分析了临界破膜速度与液膜厚度的关系。实验与模型计算结果均表明:波形板折角处的临界破膜速度与液膜厚度呈负相关关系；当液膜厚度相同时，波形板临界破膜结构参数越大，其临界破膜速度亦越大，利于气水分离效率的提高。      

低流速过冷沸腾净蒸汽产生点计算模型

已有的过冷沸腾净蒸汽产生点计算方法以高质量流速为主.对于低流速工况,到目前还未见普遍认同的净蒸汽产生点计算模型。本文通过加热壁面汽相蒸发与汽液界面冷凝传热的分析，建立于适用于低流速强迫循环及自然循环工况净蒸汽产生点的蒸发冷凝模型，在较宽广的工况范围内与已有实验数据进行了比较.研究表明,低流速工况下．该模型可以得出满意的预测结果，比目前广泛应用的Salla-Zuber模型以及Levy模型具有更高的预测精度.     

棒束通道防腐蚀特性数值研究

为获得先进反应堆中燃料组件通道表面防腐蚀层的生成情况，以对反应堆运行策略分析提供支撑，本文提出了一套棒束通道中氧输运分析计算模型，结合计算流体动力学方法，对燃料组件典型19棒束通道内的防腐蚀层生成情况进行了分析。获得了棒束通道内的流场、温度场以及两种入口氧浓度、三种运行时间下的棒束氧浓度分布情况以及棒束表面防腐蚀层的生成情况。研究结果表明，棒束通道中防腐蚀层的生成主要与温度、初始氧浓度以及运行时间有关，对于现有模型来说，定距条与棒接触点附近是防腐蚀层难以形成的主要区域，需要重点关注。本文的计算方法及结果将对反应堆运行策略评价提供支撑。     

Molecular Approach to Sodium Vapour Condensation,Rewetting and Vaporization in LMFBR Bundle under Hypothetical Accident Conditions

In the frame of safety analysis of Liquid Metal Fast Breeder Reactors (LMFBRs) under hypothetical Unprotected Loss-of-Flow (ULOF) conditions, two phase flow of sodium is simulated in a reactor core. Traditional approaches used in safety analysis codes to simulate sodium vapour condensation and vaporization rely upon application of macroscopic semi-empirical correlations for heat transfer and vapour condensation or evaporation rates. As an alternative to this macroscopic approach, we developed a microscopic methodology based upon the application of the basic laws of the kinetic theory for the determination of the evaporation and condensation rates of vapour in a reactor bundle. This microscopic approach is based upon a Monte Carlo simulation of the molecular trajectories, collision rates between vapour molecules and of the molecules with the surfaces of the claddings of the pins of a reactor bundle. The pins surfaces are treated in the Monte Carlo simulation as diffusely reflecting surfaces. Scattering of sodium particles is simulated with the “hard sphere” collision model. The “step splitting” technique is applied, which consists in separating the collisions dynamic calculation from collisionsless paths of the molecules. Vapour particles are assumed to condense on the surfaces of the pins when, after diffuse reflection, their velocity would be less than one third of the most probable velocity corresponding to the wall temperature. Rewetting of dried out regions of the cladding surfaces is simulated with a dynamic film model which computes the velocity distribution of the liquid across the film thickness and then the mean liquid film velocity. Evaporation of sodium molecules from the film yields a source of molecules which re-enter into the Monte Carlo calculation of the molecular dynamic approach. The coupling of the micro- and macroscopic models has been applied to the numerical simulation of an out-of pile sodium boiling experiment run at the Nuclear Research Center of Karlsruhe, Germany.     

高温热管内钠蒸发冷凝特性分子动力学研究

高温钠热管是热管堆中进行非能动热量传输的核心部件。为深入理解热管内工质钠的蒸发机理及气液交界的传热传质特性,用分子动力学软件LAMMPS模拟了600 K下钠的蒸发,统计了质量调节系数,定为0388 7。随后变更壁温,打破体系内热质输运平衡,进行非平衡态模拟,观察液膜变化,求解气液交界处的净蒸发通量和换热系数。结果表明,9～10 ns后,底部的液膜厚度、气液交界处的净蒸发通量及换热系数分别在01～052 nm、003～007 kg/(m2·s)、22～39 kW/(m2·K)范围波动,此时上部液膜厚度在6 nm左右,其气液交界的净蒸发通量在10-4量级,换热系数为0028 kW/(m2·K),至末期降为0003 5 kW/(m2·K)。本文为钠热管启动阶段的数值模拟提供了参考。     

Mechanistic modeling for ring-type boiling water reactor fuel spacer design (3) run-off effect and model formulation

The fuel spacer is one of the components of a fuel rod bundle and its role is to maintain an appropriate rod-to-rod clearance. The fuel spacer influences the liquid film flow distribution in the fuel rod bundle, so that the spacer geometry has a strong effect on thermal hydraulic characteristics of BWR such as critical power and pressure drop in the fuel bundle. In this paper, liquid film flow characteristics were experimentally investigated in a circular channel with ring-type spacers using air and water as test fluids in order to be compared with the analytical results introducing the mechanistic spacer model. The spacer model was composed of three effects; the drift flow effect, the narrow channel effect and the run-off effect. The drift flow effect and the narrow channel effect were discussed in the previous works and the run-off effect is done in this paper. This paper shows the formulation around the spacer with use of the three effects. The proposed model explains well the experimental results of liquid film flow rates and thickness carried out in reference to the spacer thickness, the gap between the channel wall and the spacer.     

丝网芯内钠薄液膜蒸发与毛细特性研究

高温碱金属丝网芯热管主要应用于核能、航空航天等领域。由于碱金属工质与常规低温工质在热物性上存在显著差异,且丝网芯热管内部运行机理复杂,碱金属工质在丝网芯表面的相变行为亟待研究。本文建立了以前驱膜理论和蒸发弯月面液膜理论为基础的丝网芯毛细蒸发薄液膜模型,考虑分离压力的电子分量与钠液膜热传导效应,对液态钠工质在丝网芯表面的毛细现象进行了分析,进而探究了不同运行参数对钠液膜传热传质与毛细特性的影响。研究表明,液态钠工质主要在本征弯月面内发生相变,运行温度通过改变工质热物性影响液膜的传热传质能力。丝网芯的动态调整能力与丝径和孔径等几何参数有关,通过改变液膜接触线位置,进而改变蒸发面积和三相接触线长度,从而影响丝网芯受力情况和传热传质能力,改变丝网芯表面液膜提供的毛细力。     

Investigation on heavy liquid metal cooling of ADS fuel pin assemblies

In the framework of accelerator driven sub-critical reactor systems heavy liquid metals are considered as coolant for the reactor core and the spallation target. In particular lead or lead bismuth eutectic (LBE) exhibit efficient heat removal properties and high production rate of neutrons.However, the excellent heat conductivity of LBE-flows expressed by a low molecular Prandtl number of the order 10⁻² requires improved modeling of the turbulent heat transfer. Although various models for thermal hydraulics of LBE flows are existing, validated heat transfer correlations for ADS-relevant conditions are still missing. In order to validate the sub-channel codes and computational fluid dynamics codes used to design fuel assemblies, the comparison with experimental data is inevitable.Therefore, an experimental program composed of three major experiments, a single electrically heated rod, a 19-pin hexagonal water rod bundle and a LBE rod bundle, has been initiated at the Karlsruhe Liquid metal Laboratory (KALLA) of the Karlsruhe Institute of Technology, in order to quantify and separate the individual phenomena occurring in the momentum and energy transfer of a fuel assembly.     

Two Phase Pressure Drop Multipliers and ICE Technique Applied in a Multibubble Slug Ejection Model

Two-phase flow of coolant is described in subassemblies of liquid metal fast breeder reactors by a one-dimensional two-phase multibubble slug ejection model. Vapor flow of sodium between the slugs is modeled as annular flow with a moving liquid film of variable thickness wetting structural surfaces. The two-phase pressure drop multiplier concept introduced by Lockhart-Martinelli is used in the frame of the slug ejection model with an algorithm that simultaneously computes interfacial friction coefficient and liquid film axial velocity distribution. The implicit continuous Eulerian (ICE) technique developed by Harlow and Amsden for computing pressure distribution in a continuum medium is applied in heterogeneous two-phase flow to the governing equations for the vapor phase. This solution method is more stable than the numerical solution by finite differences of the vapor momentum equation.     

钠热管中气液交界质量调节系数的分子动力学研究

钠热管是热管堆内的关键热量传输部件,深入理解内部工质的蒸发与冷凝换热机理对于钠热管的设计优化具有重要意义。本研究用分子动力学软件LAMMPS,结合钠热管的启动和运行工况,模拟了600、700、800、900 K下液态钠薄膜的平衡态蒸发,求解了质量调节系数（MAC）。并在模拟中引入了氩气作为非凝结性气体,研究了非凝结性气体对MAC的影响。研究结果表明,4组工况下的MAC分别为0.388 6、0.211 9、0.261 5、0.241 6;非凝结性气体存在时,MAC分别为0.282 9、0.254 3、0.129 5、0.107 2。本研究为钠热管的数值模拟提供了参考借鉴和理论支撑。     

Matrix-assisted pulsed laser evaporation of polymeric materials: a molecular dynamics study

Matrix-assisted pulsed laser evaporation (MAPLE) has been recently developed to deposit high-quality thin films for a wide range of polymeric materials. To analyze the evaporation of polymer molecules in MAPLE, we present a molecular dynamics (MD) simulation of laser ablation where the target material is modeled as a solution of polymer molecules in a molecular matrix. The breathing sphere model is used for MD simulations of laser ablation of the molecular matrix. Polymer molecules are described using a bead-spring model, where each bead represents one or several polymer groups. The initial stage of polymer ejection is investigated at different laser fluences and pulse durations. The influence of polymer molecules on the stability of clusters formed in the plume and the processes that can lead to polymer decomposition are discussed.     

Method of Critical Power Prediction Based on Film Flow Model Coupled with Subchannel Analysis

A new method was developed to predict critical powers for a wide variety of BWR fuel bundle designs. This method couples subchannel analysis with a liquid film flow model, instead of taking the conventional way which couples subchannel analysis with critical heat flux correlations. Flow and quality distributions in a bundle are estimated by the subchannel analysis. Using these distributions, film flow rates along fuel rods are then calculated with the film flow model. Dryout is assumed to occur where one of the film flows disappears. This method is expected to give much better adaptability to variations in geometry, heat flux, flow rate and quality distributions than the conventional methods.

In order to verify the method, critical power data under BWR conditions were analyzed. Measured and calculated critical powers agreed to within ±7%. Furthermore critical power data for a tight-latticed bundle obtained by LeTourneau et al. were compared with critical powers calculated by the present method and two conventional methods, CISE correlation and subchannel analysis coupled with the CISE correlation. It was confirmed that the present method can predict critical powers more accurately than the conventional methods.     

Dynamics of liquid sodium pool spreading under sodium fire conditions

The spreading of burning liquid sodium has been investigated using a depth-averaged shallow water equation for isothermal and non-isothermal (burning) conditions. In the latter case, the spreading is one-way coupled with the flame through a separate energy equation for the pool, with appropriate source terms for radiative and conductive heat transfer from the flame, and a sink term (for the continuity equation) to account for loss due to burning. Pool fires on soil and concrete surface have been considered with appropriate friction and heat transfer terms in the momentum and energy equations, respectively. Using this model, numerical simulations have been carried out for a wide range of leak rates, and for a range of burning rates of liquid sodium. Results obtained from the non-isothermal model show that the non-isothermal effects of liquid sodium spreading can safely be neglected for the case or spreading of burning liquid sodium on a typical ground surface such as concrete or soil. Based on these conclusions, dimensionless correlations are proposed for the prediction of spreading parameters such as, equilibrium pool radius, pool formation time, and for mass inventory under pool fire conditions for liquid sodium. These parameters which are obtained from the spreading code can be specified, as input parameters for the existing sodium fire safety codes.