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。     

气泡在液态铅铋合金内上升行为的数值模拟

为利用气泡提升泵的概念设计来增加铅铋合金非等温回路的自然循环能力,需对充入回路的气泡大小进行选择。气泡的初始直径能显著影响气泡提升泵提升自然循环的能力。利用VOF模型,对不同初始直径的氦气泡在液态铅铋合金中的提升作用进行了数值模拟研究。结果表明,对于单个气泡,随着初始直径增大,提升能力呈先增加后趋于不变的趋势;初始直径较大的气泡易分裂生成较小的子气泡附着在管壁上,有影响传热效率的潜在可能。研究了在静止液态铅铋合金中,不同初始直径的氦气泡上升速度和上升高度与时间的关系及气泡分裂的原因。     

溶液堆内气-液两相流流动及换热特性数值研究

在溶液堆台架模型数值模拟研究的基础上,对实际堆结构的堆芯内气-液两相流流动及冷却盘管与堆内溶液间的换热特性进行了数值模拟研究.采用欧拉两相流模型描述堆芯内气-液两相流流动,MUSIG(MUltiple-SIze-Group)模型描述堆芯内气泡尺度分布和相互作用,流-固耦合模型描述溶液与盘管间换热.数值计算得到了堆芯内的温度、速度、气泡组分等分布及冷却盘管的换热效率.数值计算结果表明:在有气泡扰动时堆内温度分布比没有气泡时均匀,冷却盘管可将堆内产生的85%热量带出,与试验测量结果一致.额定功率时,不同气体产生量对于冷却盘管换热影响的研究表明,随着堆内气泡产生量的增加,溶液与冷却盘管之间的换热得到强化.     

气液两相流工况下核主泵的正转全特性研究

在冷却液流失事故(LOCA)事故发生期间,核主泵将处于两相混合运行状态。主要对核主泵的正转全工况不同含气率冷却介质的泵水力性能、流道内部气体体积分布情况及流体流态进行研究,并采用计算流体力学(CFD)模拟计算与气液两相流试验进行验证。研究发现:在正转逆流制动工况,核主泵的扬程曲线随含气率增加整体向下偏移,但其变化规律基本相同。在正转水泵工况和正转正流制动工况,随着流量增加,含气率对核主泵扬程特性的影响逐渐减小,且同流量下核主泵的扭矩和冷却剂介质密度成较为明显的正比关系。     

振动工况下环管内气液两相流参数分布实验研究

为模拟地震对反应堆堆芯内两相流动的影响,将振动产生装置与实验段结合,并利用电导探针技术实验研究了振动工况下环管内气液两相流局部参数的分布特性。振动装置所采用的偏心轮的偏心距为15.875mm,并可通过调节电机转速获得不同的振动周期。环管实验段内径和外径分别为19.1mm和38.1mm,总长度为2.32m。实验以空气和水为工质,流动工况覆盖了泡状流、弹状流及搅混流等流型。实验结果表明,振动对管内气液两相流局部参数如含气率和界面浓度等分布具有重要影响。随振动周期的减小,管内两相流参数分布发生明显变化。另外振动对低含气率的泡状流影响较大,随含气率的增加,流型逐渐向搅混流过度,振动对两相流参数的影响逐渐减小。     

水平管泡状流液相速度分布与相分布

用热膜探针测量了内径为 3 5mm的水平管泡状流中的液流速度 ,同时用双头电导探针测量了有关的相界面参数 ,结果表明 ,局部液流平均速度在靠管道下部的分布与单相液流的分布类似 ,在靠管道上部突然减小 ;局部液流平均速度在靠管道下部随气流折算速度的增加而增加 ,在靠管道上部则随气流折算速度的增加而减小 ;局部含气率和气泡频率随气流折算速度的增加而增大 ,在靠近管道上壁处有一峰值 ,其分布规律随液流速度的增加而趋向均匀     

针对REPEC加热实验的RELAP5程序模拟与分析

为研究压力容器外部流道的冷却能力及流动传热过程,在反应堆压力容器外部冷却(REPEC, Reactor Pressure vessel External Cooling)实验台架前期加热实验的基础上,采用RELAP5程序对实验工况进行模拟和对比。模拟结果与实验数据一致性较好。随加热热流、进出口面积的增加,系统内自然循环流量也增加;入口欠热度对自然循环流量的影响不是很明显;近饱和沸腾条件下,系统出现明显的两相不稳定流动。     

中国先进研究堆堆芯流量分配计算

针对中国先进研究堆(CARR)正常运行强迫循环工况和自然循环工况下堆芯内冷却剂流动方向相反的特点,开发了堆芯流量分配计算程序。程序针对这两种运行工况进行了全堆芯的数值模拟,得出堆芯流量分配计算结果和非对称冷却条件下板状燃料元件的温度场。计算发现两种工况下堆芯内各通道的流量份额变化不大,表明流量分配主要取决于通道几何形状和尺寸,基本可以忽略功率分布不均的影响。     

垂直上升管内泡状流压力波传播

研究了垂直上升管内气液两相泡状流压力波的传播速度和衰减规律,为了提高压力波测量精度．实验中设计了不影响两相流动结构的调频式压力扰动装置．实验结果表明,随着含气率的增加,泡状流中压力波波速开始陡降,当含气率大于0．05以后波速缓慢下降;衰减系数随含气率的增加连续增加：工质的流速对压力波的传播没有影响;压力波的传播速度及其衰减与扰动频率有关．随着扰动频率的增加,波速及其衰减都增加本文实验验证了泡状流压力波色散特性的临界频率现象．即高于临界频率．压力波的色散特性消失．在本试验条件和参数范围内．临界扰动角频率为300Hz．     

开式自然循环系统出口排热管内流型演化研究

开式自然循环系统作为新型非能动余排系统最终热阱排放回路，其安全稳定运行对于事故工况下堆芯余热安全导出至关重要，本研究通过可视化实验方法观察了开式自然循环系统出口排热管内流型演化特性，发现随着加热功率增加，开式自然循环逐步建立，该系统出口排热管内依次出现单相流、间歇性汽泡流、弥散泡状流、弹状流和间歇喷射流5种典型流型，分析了出口排热管内流型与系统稳定运行之间的关系，发现了开式自然循环系统剧烈振荡的根源，为提高开式自然循环系统流动稳定性提供了参考。      

Two-dimensional numerical simulation of single bubble rising behavior in liquid metal using moving particle semi-implicit method

Gas-lift pump in liquid metal cooling fast reactor (LMFR) is an innovative conceptual design to enhance the natural circulation ability of reactor core. The two phase flow characteristics of gas–liquid metal make significant improvement of the natural circulation capacity and reactor safety. It is important to study bubble flow in liquid metal. In present study, the rising behaviors of a single nitrogen bubble in 5 kinds of common stagnant liquid metals (lead bismuth alloy (LBE), liquid kalium (K), sodium (Na), potassium sodium alloy (Na–K) and lithium lead alloy (Li–Pb)) and in flowing lead bismuth alloy have been numerically simulated using two-dimensional moving particle semi-implicit (MPS) method. The whole bubble rising process in liquid was captured. The bubble shape, rising velocity and aspect ratio during rising process of single nitrogen bubble were studied. The computational results show that, in the stagnant liquid metals, the bubble rising shape can be described by the Grace's diagram, the terminal velocity is not beyond 0.3 m/s, the terminal aspect ratio is between 0.5 and 0.6. In the flowing lead bismuth alloy, as the liquid velocity increases, both the bubble aspect ratio and terminal velocity increase as well. This work is the fundamental research of two phase flow and will be important to the study of the natural circulation capability of Accelerator Driven System (ADS) by using gas-lift pump.     

Flow characteristics of natural circulation in a lead-bismuth eutectic loop

Lead and lead-alloys are proposed in future advanced nuclear system as coolant and spallation target.To test the natural circulation and gas-lift and obtain thermal-hydraulics data for computational fluid dynamics (CFD) and system code validation,a lead-bismuth eutectic rectangular loop,the KYLIN-Ⅱ Thermal Hydraulic natural circulation test loop,has been designed and constructed by the FDS team.In this paper,theoretical analysis on natural circulation thermal-hydraulic performance is described and the steady-state natural circulation experiment is performed.The results indicated that the natural circulation capability depends on the loop resistance and the temperature and center height differences between the hot and cold legs.The theoretical analysis results agree well with,while the CFD deviate from,the experimental results.     

Flow characteristics of natural circulation in a lead–bismuth eutectic loop

Lead and lead-alloys are proposed in future advanced nuclear system as coolant and spallation target.To test the natural circulation and gas-lift and obtain thermal-hydraulics data for computational fluid dynamics(CFD) and system code validation, a lead–bismuth eutectic rectangular loop, the KYLIN-Ⅱ Thermal Hydraulic natural circulation test loop, has been designed and constructed by the FDS team. In this paper, theoretical analysis on natural circulation thermal-hydraulic performance is described and the steady-state natural circulation experiment is performed. The results indicated that the natural circulation capability depends on the loop resistance and the temperature and center height differences between the hot and cold legs. The theoretical analysis results agree well with,while the CFD deviate from, the experimental results.     

Measurement and modeling of average void fraction, bubble size and interfacial area

The local void fraction, bubble size and interfacial area concentration for co-current air-water bubbly flow through a horizontal pipe of 50.3 mm internal diameter were investigated experimentally using the double-sensor resistivity probe method. The local and area-averaged void fractions and interfacial area concentrations were analyzed as a function of liquid and gas flow rates. These parameters were found to increase systematically with decreasing liquid flow and increasing gas flow. However, variations with the liquid flow were not as significant as with the gas flow. A consistent variation of the gas phase drift velocity and distribution parameter with the liquid flow rate was observed. It was demonstrated that presentation of the average void fraction in terms of flowing volumetric concentration was more appropriate for horizontal bubbly flow. Several bubble break-up mechanisms were discussed. It was concluded that average pressure fluctuations generated by the turbulent liquid fluctuations acting across a bubble diameter are the only mechanism which causes distortion of a bubble. Based on this force and the competing surface tension force, a theoretical model was developed for mean bubble size and interfacial area concentration. The theoretically predicted mean bubble size and interfacial area concentration were found to agree reasonably well with those measured by the double-sensor resistivity method.     

环形通道内液态铅铋合金流动换热特性实验研究

通过对环形通道内液态铅铋合金的流动换热特性进行实验研究,得到了气泡泵注气对液态金属流动的影响,并拟合出环形通道内液态铅铋合金的摩擦系数关系式和换热特性关系式。结果表明：采用气泡泵注气能有效提升铅铋合金的质量流速;相同Reynolds数下环形通道内液态铅铋合金的摩擦系数大于由布拉休斯公式计算得到的摩擦系数;液态铅铋合金对流换热过程中,导热项占主导地位,并且Nusselt数随Peclet数的增大而增大。     

基于汽泡壅塞流模型的竖直圆管临界热流密度理论研究

为研究单管壅塞流的临界热流密度（CHF）现象,建立了基于近壁处汽泡壅塞机理的CHF计算模型。模型通过求解相应的质量、动量和能量方程,再结合汽泡直径脱离模型、壁面临界空泡份额等模型,从而计算得到CHF。将模型计算结果同实验值比较,吻合良好,验证了模型的正确性。在此基础上,以建立的CHF模型为基础,研究了进口焓差、质量流速、管径和加热长度对CHF的影响,为预测壅塞流CHF提供依据。      

基于扩散界面法的液态铅铋合金中气泡上升行为模拟

基于扩散界面法,对单个氮气气泡在液态铅铋合金内从静止到充分发展整个过程中的动力学行为进行数值模拟,得到气泡形变特性和气泡上升速度随时间的变化关系,将模拟结果与Grace经验关系图对比,发现模拟得到的气泡形变结果在Grace经验关系图中均可找到且很好地吻合,从而验证了扩散界面法在模拟液态铅铋合金中气泡上升行为的可行性和准确性。同时基于界面扩散法的模拟,对比了5种不同初始直径的氮气泡在液态铅铋合金中的上升行为,发现初始直径较小的气泡在上升过程中扰动会更剧烈,初始直径较大的气泡在上升过程中易发生分裂现象。     

运动条件下铅铋反应堆热工水力特性研究

为研究运动条件下铅铋反应堆热工水力特性,开发了运动条件铅铋反应堆瞬态分析系统程序,并完成了对设计的5 MW自然循环小型模块化铅铋反应堆的建模,分析了运动条件对反应堆自然循环热工水力特性的影响。计算结果表明,倾斜条件下,堆芯流量减小,堆芯出口温度升高,在计算最大倾斜角度下,流量减小20%,冷却剂堆芯出口温度升高20 ℃。起伏条件下,起伏幅度和起伏周期越大,对反应堆影响越大,由于系统阻力影响,流量变化较起伏加速度有小于1 s的延时。摇摆条件下,摇摆角度越大和摇摆周期越小,对反应堆影响越大,燃料包壳峰值温度较稳态值高20 ℃以内,对反应堆正常运行时安全性影响较小。     

ADS铅铋自然循环热分层现象研究

为保障加速器驱动的次临界系统（ADS）的安全,采用计算流体力学分析方法,对ADS铅铋自然循环热分层现象进行数值模拟。研究结果表明：铅铋自然循环中,热分层最严重的区域存在于变温段,且在回路中热分层状态不同。回路温差较大时,流速提高,热分层现象较明显。回路管径较大时,流速降低,热分层现象不明显。流速较低时,局部区域热分层现象趋于消失;流速较大时,最大温差截面温差加大。