带热套管的T型接管内流动换热的数值模拟和实验研究

为了分析核反应堆冷却剂系统中带热套管T型接管内由于注入非等横向射流导致的构件热冲击状况，本文应用计算流体力学商用软件FLUENT5．3进行了紊流流动换热的数值模拟，分析了主管及接管与热套间环腔内的流动换热特性，针对套管上开有通流小孔，并采用凸台支撑的热套管结构形式，模拟了射流与主流流速比为0．05及0．5两种典型工程，传热实验，研究了主管及接管内壁近壁区域的传热特性，并讨论了热套管尺寸变化对接管热冲击的影响，结果表明，数值模拟与实验数据吻合良好，热套管对构件的热保护程度与热套管结构形式及流速比密切相关，适当减小流速比有利于改善构件热应力状况。     

压水堆主系统T型三通管传热实验研究

对射流垂直向下入射至主流中，射流管与主管直径比为0．16时，在宽广的射流与主流流速比范围内（R＝0．01－1．5），测量了3个不同区域内加热装套管前后三通管传热情况，通过实验得到了不同流速比条件下不同区域的近壁流体温度及换热系数，比较了热套管对于三通管传热的影响，通过实验发现，热套管的主要影响区域是射流与主管连接区及射流管内，对于射流管下游区域的影响很小，在流速比较高时，热套管的影响作用更明显一些。     

斜向三通管内射流混合特性的三维数值模拟

针对反应堆安全注射过程中的热冲击问题,应用基于有限差分的有限容积法和k-ε紊流模型,分别对支管倾角为30°60°、90°、120°和150°的倾斜三通管内的流动和换热进行三维数值模拟研究,得到了三通管内的流动结构、压力分布与温度分布.分析了三通管几何结构、流动条件和流体温度变化对三通管内的流动结构、温度分布及热应力的影响.结果表明,主流与支管射流的温度差、支管倾角对三通管内的热应力的影响较大,在三通管的迎流侧热应力表现更为明显.     

反应堆冷却剂系统主管道安注斜接管等温横向射流流动特性的研究

针对核反应堆冷却剂系统中的主管道安注斜接管等温横向射流问题,应用计算流体力学商用软件CFX进行等温横向射流流动的数值模拟,得出了典型运行工况下的三维流场分布.深入研究了射流与主流在不同流速比情况下等温横向射流的流动特性、影响区域及主要影响因素,在所研究的参数范围内,得出了流速比是决定等温横向射流流动特性的最重要因素,同时将数值模拟结果与流动可视化试验结果进行了比较,二者吻合良好.     

直接安注反应堆压力容器下降环腔内射流传热试验研究

失水事故工况 (LOCA)下反应堆下降环腔内的流动和传热研究 ,对反应堆压力容器 (RPV)的安全具有重要的意义。通过对一种直接安注的反应堆压力容器内流动和传热的研究 ,将流动分为横穿射流和冲击射流 ,比较了在两种射流下下降环腔内流动和传热的特点 ,分析了流速比和对流换热系数及温度的关系 ,当流速比在 1～ 1 0时 ,流动属于横穿射流 ,对流换热主要由环腔流速决定 ;流速比大于 1 0后 ,属于冲击射流 ,环腔内对流换热主要决定于安注流速 ,此时局部对流换热能力随安注流速的增加而增加     

不同倾角射流时近壁区流动与传热特性研究

针对反应堆冷却剂系统中不同倾角三通构件射流时近壁区流动与传热特性,在试验研究的基础上,比较分析了流速比在0～2范围内,射流以45°、90°倾角入射主流时三通构件不同区域的流动特性、近壁区混合函数及无量纲换热系数。结果表明,不同入射角对混合函数及无量纲换热系数的变化趋势影响不大,控制变化趋势的关键参数依然是流速比,入射角仅对混合函数及无量纲换热系数的幅值大小有一定影响。     

压水堆受热冲击三通构件中流速比的选取

从传热学的角度分析了压水堆中的三通构件所受到的热冲击作用，探讨了流速比对于构件所受热冲击的影响，实际运行中，为降低构件受到的热冲击，最佳流速比范围应在0．04－0．1之间。     

斜接管嘴非等温横向射流时近壁流体温度的数值模拟

针对反应堆主管道45°安注斜接管嘴,基于1:9的比例模型,采用计算流体力学程序(CFX)软件,进行了常压条件下安注斜接管嘴主、射流温差分别为30℃和70℃、射流和主流的流速比为0.87～40、构件不同区域内非等温横向射流时的壁温变化及其分布特征的数值模拟研究,得出了构件内主接管相交区、射流下游区、主管侧面区及主管底部各区域测点近壁流体的混合函数。通过对射流与主管流体两种温差下各区域测点混合函数的比较发现,在本次研究中,30℃、70℃两种温差下各点混合函数基本保持不变,各点混合函数的大小与温差关系不大。将本文混合函数的计算结果与同期试验结果进行了比较,二者符合良好。     

斜接管射流流动特性数值模拟

采用数值模拟方法对受限斜射流的流动特性、射流发展影响区域、射流发展关键因素及射流涡特性进行研究.研究表明:受限斜射流存在附壁斜射流、离升斜射流和冲击斜射流3种基本流型.流速比(V<,R>)是斜接管射流流动特性的关键特征参数;射流影响区域随V<,R>的增大而越大;在高V<,R>下,射流强烈冲击主管底面,并在上游形成明显回...     

基于热-流-固耦合的燃料元件性能分析方法研究

中空棱柱形燃料元件形式和运行工况特殊,没有现成的燃料性能分析软件能够满足计算要求,需要建立新的分析方法。本研究基于COMSOL软件二次开发,采用颗粒增强复合材料的等效物性模型和共轭传热技术实现中空六棱柱形燃料的三维热-流-固耦合计算,通过与美国通用电气公司数据的对比证明了该分析方法的有效性。采用该方法计算了多种燃料元件尺寸和不同轴向功率分布下的热应力和温度,结果表明侧棱处温度最高而内壁面壁厚最薄处热应力最大,壁厚越薄、长度越长,燃料元件的最大热应力和温度越小,展平入口段的轴向功率分布也能够略微降低最大热应力和温度。以上分析方法可以用于新型中空棱柱形燃料元件的优化设计。      

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.     

Developed single phase turbulent flow through a square-pitch rod cluster

An experimental study of developed single phase turbulent flow through a square pitched array of rod bundles is described. Measurements were made at two spacings (p/d = 1.194, 1.107) of the mean velocity distribution and wall shear stress variation, together with the six terms of the symmetrical Reynolds stress tensor. The departure of the turbulent flow structure from axisymmetric pipe flow, particularly in the rod gap region, was found to depend strongly on the (p/d) ratio.     

管型对波动管热分层现象的影响分析

为分析评价压水堆核电厂稳压器波动管管型对热分层现象的影响,提出采用螺纹管来减弱热分层的措施。利用计算流体力学（CFD）分析方法,对升温、升压阶段波动管原型和改进模型的热分层现象进行数值模拟,得到两种模型不同波动流速下沿波动管轴线方向的截面最大温差分布以及流场分布。对比分析结果表明：波动管结构由光管改为螺纹管后流场紊动加强并出现涡流,冷热流体间的混合增强,与原型相比可使波动管的截面温差减小约1/3,从而有效地减弱热分层的影响。     

Single Scattering Calculations of Sky-Shine with Cylindrical Container Penetrations

Loss by deposition of aerosol particles in an air sampling pipe causes error in the estimation of aerosol concentration in the atmosphere.

For a horizontal pipe, the deposition fraction for laminar flow can be estimated by equations of deposition governed by gravity settling and diffusion. For turbulent flow, there are two methods available—one using the equation by Yoshioka et al, to express deposition velocity, the other being the “extrapolation method” proposed by the present authors.

The present paper examines the validity of the two methods, with particular reference to the contribution of gravity settling to the deposition, and the effect of roughness of the pipe wall on the deposition from turbulent flow.

The deposition fraction in a horizontal straight metal pipe can be estimated with deviation from experimental values not exceeding a factor of 2, throughout the whole region covered by the study, extending over both laminar and turbulent ranges. Use of a suitable friction factor to account for the roughness of the pipe wall gives a reasonable value of deposition fraction in the turbulent region. The deposition from turbulent flow is mainly governed by gravity settling when the Reynolds number is not very large (Re?10⁴).     

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.     

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.     

New explicit correlations for turbulent flow friction factor

Two new correlations of single-phase friction factor for turbulent pipe flow are shown in this paper. These two formulas are actually explicit approximations of iterative Colebrook's relation for calculation of flow friction factor. Calculated friction factors are valid for whole turbulent flow including hydraulically smooth and rough pipes with special attention on transient zone of turbulence between them. Hydraulically smooth regime of turbulence does not occur only in total absence of roughness of inner pipe surface, but also, four new relations for this theoretical regime are presented. Some recent formulas for turbulent flow friction calculation are also commented.     

Experimental investigation on heat transfer characteristics in a safety injection nozzle component in PWR

Thermal fatigue is a potentially significant degradation mechanism in Nuclear Power Plants (NPP). For the fatigue analysis, the thermal load information about components must be determined firstly. In this paper, an experimental study was carried out to obtain local fluid temperatures and local heat transfer coefficients for the safety injection nozzle component in reactor coolant system (RCS). In this mixing tee component a hot jet issues into a cold cross-flow stream from an oblique pipe and the turbulent mixing of two fluids induces local cycling stresses on the adjacent piping wall. Experiments were performed using a special-made heat fluxmeter, which can measure the mixed fluid temperature close to the wall and the heat transfer coefficient between the fluid and the wall. Plexiglass and metallic 1/9-scale mockups were manufactured for flow visualization and heat transfer tests, respectively. All tests were conducted at range of 0–40 for the jet-to-cross-flow velocity ratio. The flow visualization test has obtained general pattern of the flow and identified sensitive zones in the component where the jet and cross-flow interact intensively to cause thermal fatigue more possibly. In the heat transfer test, heat fluxmeters were positioned in the wall at these sensitive zones. The measurement results of temperatures and heat transfer coefficients have been discussed in detail in the paper. These experimental results allow us improving the state of knowledge of the thermal load to be used in the industrial mixing tees in operating for long lifetime assessment and for the design in the basic Nuclear Power Plants.     

Laser doppler measurements of flow in a rod bundle

A two component laser doppler velocimeter with polarized beams and frequency shift was used to measure the turbulent flow field for axial flow between the rods of a nine rod, square pitch rod bundle. Parameters measured include mean axial and lateral velocities, turbulence intensities and the friction factor. The axial velocities for 10000 to 40000 Reynolds number are slightly higher than those reported by Rowe. The maximum lateral velocities measured are about 1% of the bulk velocity; somewhat larger than suggested by earlier authors. Axial and lateral turbulence intensities are larger than those in pipe flows.