核电厂管线中的热分层现象

由于阀门渗漏使核电厂安注系统冷水注入到充满热水的连接安注系统与主管道的支管中，而发生的热分层和温度振荡现象的研究对于确保核电厂的安全和可靠运行具有重要意义。运用计算流体力学软件CFX，采用k－ε湍流模型，以研究某核电厂安注系统支管中热分层现象的实验为对象，模拟了阀门渗漏冷水进入含有高温水的支管以后所发生的热分层现象，数值模拟的结果与实验测量结果吻合。在此基础上，通过改变阀门渗漏冷水的流量、支管的结构等参数，进一步研究支管中热分层现象与这些参数的内在关系，从而得出了影响热分层现象的主要原因及热分层现象发生的一些规律。     

稳压器波动管考虑热分层影响的疲劳分析

在核电厂中,稳压器波动管及波动管热段三通是保证核电厂反应堆冷却剂压力边界完整性的重要设备.其属于核安全1级设备,承受内压、自重、热胀、地震及各种正常加异常工况下的温度和压力瞬态,特别对于压水堆核电厂的波动管,还会承受热分层导致的总体和局部载荷.热分层现象的反复出现增加了管道及接管嘴处出现疲劳失效(贯穿管壁裂纹)的可能性.本文阐述了对波动管热分层实施温度测量的方案,及对测量结果的分析处理;建立分析热分层整体应力和局部应力,以及波动管疲劳分析的计算模型;确立合理且切实可行的波动管疲劳分析所需的分析瞬态.上述方法已在"300 MWe PWR NPP稳压器波动管热分层"课题研究得到鉴定,并在实际的寿命管理等工程项目中发挥了重要作用.     

基于应变与加速度响应的核电厂小支管振动评价

核电厂存在大量的仪表管、取样管、排气管等小支管,其机械振动疲劳断裂是商业运行核电厂经常出现的问题。本文以阳江核电站二号机组GRE023管线的实测振动应力对其振动疲劳寿命进行了评价,采用有限元方法以加速度时程二次积分获得的位移时程为输入对其振动疲劳寿命进行了分析,获得了小支管振动疲劳瞬态动力分析关键参数设置方法,同时也为类似结构的减振改造设计积累了经验,为核电厂小支管振动治理提供了参考。     

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。     

基于温度场分析的安注管线止逆阀内漏监测方法研究

核管道安注管线止逆阀内漏会造成相关管段的热分层与冷热交替,并引发热疲劳开裂。而止逆阀内泄漏流量的监测对相关管段的热疲劳在线监测以及泄漏的及时发现与处理至关重要。本文提出了一种基于止逆阀前管外壁温度测量的安注管线止逆阀内漏流量监测方法,并以热安注管线为例进行了分析讨论。首先通过流固耦合计算获取了已知主管道流体温度与流量、泄漏流体温度与流量的温度场,定义了止逆阀阀前监测截面热分层特征温度参数,接着通过多变量回归计算,获取了热分层特征温度参数与主管道流体温度与流量、泄漏流体温度与流量的关系式。在实际使用时,只要根据监测位置测量的管外壁温度计算得到热分层特征温度参数,即可利用该关系式,根据电厂现有工艺参数(主管道流体温度与流量、泄漏流体温度)得到泄漏流量。分析表明,热分层特征温度参数与主管道流体温度与流量、泄漏流体温度与流量具有良好的关联性,拟合公式与模拟计算最大误差小于10%,可满足核管道安注管线止逆阀内泄漏流量监测要求。     

疲劳监测系统在核电厂老化管理及延寿中的应用研究

核级管道热疲劳失效是核电厂金属部件失效的主要形式之一,也是核电厂老化管理及延寿中的关注重点之一。本文采用核电厂疲劳监测系统测得管道的温度数据,掌握其真实的温度影响过程,对监测区域管道的疲劳状态进行评估,实现管道热疲劳状态寿命管理,最终可以为核电厂延寿工作提供数据支持。     

两环路核电厂试验支管振动疲劳研究

针对国内二代改进型的两环路核电厂试验用支管中存在的振动疲劳问题,文章提出了一种确定疲劳振动的测量和计算分析方法,并运用该方法对国内某两环路核电厂小支管进行了工况分析、振动测量和最大有效振动速度计算,同时采用结构力学有限元程序ANSYS软件对小支管的振动疲劳应力进行了分析。结果表明,该方法能够很好地诊断出小支管中存在的第一类敏感管和第二类敏感管,从而为判断在两环路核电厂试验中支管是否为敏感管提供了理论依据。     

稳压器波动管内热纹振荡现象数值模拟研究

对波动管内热纹振荡现象展开研究,获得波动管内热纹振荡现象的产生原因以及启堆升功率工况下热纹振荡现象造成波动管内壁面温度振荡的幅值和频率。采用计算流体力学(CFD)方法中的动态Smagorinsky涡粘模型对波动管内热纹振荡现象进行了数值模拟。结果表明:波动管内产生热振荡现象的原因是冷热流体相对流动所产生的涡状流使得冷热交界面产生上下振荡;波动管内的热纹振荡现象使得波动管内壁面温度的振荡幅值在4~20℃范围内,频率低于5 Hz。     

快堆中心测量柱高周热疲劳评定方法

流体温度振荡是快堆中的常见现象。快堆中心测量柱下封头处于流体温度振荡严重的位置,流体温度振荡会降低反应堆结构材料的寿命,威胁反应堆的安全。为了研究流体温度振荡对中心测量柱下封头寿命的影响,将中心测量柱下封头简化为平板模型,提出了一套热疲劳评定方法。利用Miner法则对结构材料的疲劳损伤因子进行计算,并针对中心测量柱下封头进行分析。本文计算结果符合Miner法则的要求,可为工程应用中的材料选取和厚度选择提供参考。     

核电厂管道热疲劳机理与防治

管道热疲劳是管道受交变热应力长期影响而产生管道裂纹或破裂的现象,虽然热疲劳原因引起的管道破裂事件在核电厂发生的概率很小,但管道破裂有可能引起一回路破口等事故,因此需要引起重视.本文对管道热疲劳产生的机理进行分类并进行分析,根据管道热疲劳产生机理的特征,提出核电厂设计、在役运行阶段应采取管道热疲劳预防与检测的措施.     

Experimental study on fluid mixing phenomena in T-pipe junction with upstream elbow

Temperature fluctuation in fluid causes high cycle thermal fatigue in structure materials according to temperature distributions and time variations. A mixing tee is one of typical geometries where temperature fluctuation occurs. In the nuclear reactors and general plants, an elbow is often used near the mixing tee and it brings biased velocity distribution and also the secondary flow. In this study, influences of upstream elbow in the main pipe were studied in a water experiment of mixing tee with the elbow. Temperature distribution in the mixing tee was measured by a movable thermocouple tree and velocity field was measured by a high speed PIV. The temperature fluctuation at the mixing tee with the upstream elbow had the large component at low frequency in comparison with the straight case. The effect of the upstream elbow is significant to evaluate the high cycle thermal fatigue in the mixing tee because of larger importance of low frequency fluctuation on the point of fluid temperature-stress conversion.     

Structural response function approach for evaluation of thermal striping phenomena

A rational analysis method for thermal stress induced by fluid temperature fluctuation is developed, by utilizing frequency response characteristics of structures. High frequency components of temperature fluctuation are attenuated during a transfer process from fluid to structures. Low frequency components hardly induce thermal stress, since temperature homogenization in structures. Based on investigations of the frequency response mechanism, a frequency response function of structures was derived, which can predict stress amplitudes on structural surfaces from fluid temperature amplitudes and frequencies. It is formulated by multiplication of the effective heat transfer and the effective thermal stress functions. The frequency response function was applied to fatigue analysis of nuclear components, and clarified relation of fatigue damage to thermal hydraulic and structural design parameters.     

Thermal stress analysis for fatigue damage evaluation at a mixing tee

Fatigue cracks have been found at mixing tees where fluids of different temperature flow in. In this study, the thermal stress at a mixing tee was calculated by the finite element method using temperature transients obtained by a fluid dynamics simulation. The simulation target was an experiment for a mixing tee, in which cold water flowed into the main pipe from a branch pipe. The cold water flowed along the main pipe wall and caused a cold spot, at which the membrane stress was relatively large. Based on the evaluated thermal stress, the magnitude of the fatigue damage was assessed according to the linear damage accumulation rule and the rain-flow procedure. Precise distributions of the thermal stress and fatigue damage could be identified. Relatively large axial stress occurred downstream from the branch pipe due to the cold spot. The variation ranges of thermal stress and fatigue damage became large near the position 20° from the symmetry line in the circumferential direction. The position of the cold spot changed slowly in the circumferential direction, and this was the main cause of the fatigue damage. The fatigue damage was investigated for various differences in the temperature between the main and branch pipes. Since the magnitude of accumulated damage increased abruptly when the temperature difference exceeded the value corresponding to the fatigue limit, it was suggested that the stress amplitude should be suppressed less than the fatigue limit. In the thermal stress analysis for fatigue damage assessment, it was found that the detailed three-dimensional structural analysis was not required. Namely, for the current case, a one-dimensional simplified analysis could be used for evaluating the fatigue damage without adopting the stress enhancement factor K_t quoted in the JSME guideline. The results also suggested that, for a precise assessment of the fatigue damage at a mixing tee, the effect of multi-axial stress on the fatigue life together with the mean stress effect should be taken into account.     

Numerical analysis of thermal striping induced high cycle thermal fatigue in a mixing tee

Thermal striping, characterized by turbulent mixing of two flow streams of different temperatures that result in temperature fluctuations of coolant near the pipe wall, is one of the main causes of thermal fatigue failure. Coolant temperature oscillations due to thermal striping are on the order of several Hz. Thermal striping high-cycle thermal fatigue that occurs at tee junctions is one of the topics that should be addressed for the life management of light water reactor (LWR) piping systems. This study focuses on numerical analyses of the temperature fluctuations and structural response of coolant piping at a mixing tee. The coolant temperature fluctuations are obtained from Large Eddy Simulations that are validated by experimental data. For the thermal stress fatigue analysis, a model is developed to identify the relative importance of various parameters affecting fatigue-cracking failure. This study shows that the temperature difference between the hot and cold fluids of a tee junction and the enhanced heat transfer coefficient due to turbulent mixing are the dominant factors of thermal fatigue failure of a tee junction.     

Study on mixing behavior in a tee piping and numerical analyses for evaluation of thermal striping

Water experiments were carried out for thermal hydraulic aspects of thermal striping in a mixing tee, which has main to branch diameter ratio of 3. Detailed temperature and velocity fields were measured by a movable thermocouple tree and particle image velocimetry. Flow patterns in the tee were classified into three groups; wall jet, deflecting jet, and impinging jet, which had their own temperature fluctuation profiles, depending on a momentum ratio between the main and branch pipes. Non-dimensional power spectrum density (PSD) of temperature fluctuation showed a unique profile, when the momentum ratio was identical. Numerical simulation based on finite difference method showed alternative vortex development, like Karman vortex series, behind the jet from the branch pipe in the wall jet case. The prominent frequency of the temperature fluctuation in the calculation was 0.2 of St number based on the branch pipe diameter and in good agreement with the experimental results. Mixing behavior in the tee was characterized by the relatively large vortex structures defined by the diameters and the velocities in the pipes.     

采用格林函数求解二维管内壁温度导热反问题

核电厂某些管道系统不允许通过开孔安装温度传感器来测量管道内壁温度,需要通过间接无损的方法来获得管道内壁的温度波动。基于格林函数法进行导热反问题分析,利用二维管外壁温度反向推导内壁温度。通过算例验证,并与共轭梯度法进行对比。结果表明,采用格林函数法能够准确地获得管道内壁温度波动;对于采用共轭梯度法难以收敛的厚壁管导热反问题也同样适用,并且由于无需迭代,因此计算效率高很多,更适用于核电厂疲劳监测计算。      

On the relevance of low side flows for thermal loads in T-junctions

The mixing of coolant streams of different temperatures in pipe junctions leads to temperature fluctuations that may cause thermal fatigue in the pipe wall. Numerous T-junction experiments are known from literature, which were performed to study the nature of thermal loads in the pipe walls occurring during the mixing of hot and cold liquid. It is common to all known experiments that the experimental boundary conditions are set to reflect cases, in which the flow velocities in both main and side branches of the T-junctions are of the same order of magnitude. In the present experiments, carried out using wire-mesh sensors, it was observed that very low flow velocities in the side branch compared to the main pipe may lead to conditions potentially severe for thermal fatigue due to the low frequency of the temperature fluctuations occurring. The T-junction presented here consists of a perpendicular connection of two pipes of 50 mm inner diameter. The straight and the side branches are supplied with water of different electrical conductivities, to enable performing generic, isothermal tests on turbulent mixing with the idea to model the temperature fluctuations in thermal mixing processes. A pair of wire-mesh sensors, each with a grid of 16 × 16 measuring points, are used to record conductivity distributions in the downstream of the T-junction as well as directly at the junction in both branches. At very low flow rates in the side branch, a characteristic entrainment of liquid from the main branch into the side branch was found. Typically the entrainment flow in the side branch results in relatively high fluctuations at the low-frequency range. While the sensor in the main flow shows fluctuations with a power spectrum similar in character to mixing experiments with comparable flow velocities in both branches of the T-junction. The phenomenon of entrainment of water from the main branch into the side branch against the main flow direction vanishes at a certain critical velocity in the side branch.     

Thermal fatigue damage evaluation of a PWR NPP steam generator injection nozzle model subjected to thermal stratification phenomenon

Thermal stratification phenomenon with the same thermodynamic steam generator (SG) injection nozzle parameters was simulated. After 41 experiments, the experimental section was dismantled; cut and specimens were made of its material. Other specimens were made of the preserved pipe material. By comparing their fatigue tests results, the pipe material damage was evaluated. The water temperature layers and also the outside pipe wall temperatures were measured at the same level. Strains outside the pipe in 7 positions were measured. The experimental section develops thermal stratified flows, stresses and strains caused enlargement of material grain size and reduction in fatigue life.