场协同理论在平行通道内的数值验证

利用SIMPLE算法对两平行平板之间有凸台的湍流回流流动换热进行了数值模拟。计算采用κ-ε两方程紊流模型,对固体区域利用区域扩充法使流场在几何上规则化,对阶梯型凸台边界用壁面函数法进行了特别处理。在入口处流体Re=10000时,数值计算得出了7组不同凸台高度相对应的流场和温度场。结果表明：平行通道表面总换热量以及换热Nu随流体速度与温度梯度矢量夹角的减小而增大．验证了基于二维边界层流提出的场协同理论同样适用于复杂的湍流回流流动。     

用平行通道模型模拟异相脉动

本文主要介绍了一组沿直线排列的平行沸腾通道，通过解一维中子扩散方程来确定这个通道排列的功率分布。模型研究了在平行通道排列中导致异相脉动的条件，定性地再现了沸水堆(BWR)堆芯。在一组直线排列的通道中模拟了所有通道的情况。使用一维动力学通过联刚的中央指令处理器(CPU)来求解这些通道。根据1994年核能署对于林哈尔斯(Ringhsls)反应堆的稳定基准试验来确定通道的组别。通过在每个通道施加相同的压降为边界条件作为稳态的初始条件，对通道流量进行迭代。这个恒定的压降同时也施加在反应扰动的瞬时反馈上，这个反馈诱发了异相脉动、数值计算采用基于并行虚拟机(PVM)的并行计算方案。     

随机填充球床通道内单相流动数值模拟方法研究

利用离散单元方法(DEM)建立通道内球体随机排列几何模型,对通道内单相流动进行数值模拟并进行验证。网格划分中的曲面接触区采用搭桥法处理不仅可生成高质量网格,而且使整体网格数量也明显减少。验证结果表明:计算得到的通道内径向孔隙率分布、平均孔隙率以及流动阻力与经典关系式吻合较好;计算模型能够很好地体现出边壁效应对流动阻力的影响。     

多孔材料填充方式对通道传热效果影响的数值研究

基于结构/非结构同位网格的SIMPLE算法,编写了多孔材料内部流动和传热计算程序.程序中多孔区域动量方程采用Brinkman-Forchheimer拓展Darcy模型,能量方程采用局部热平衡模型.对恒热流边界条件下平行通道沿中心及壁面区域填充多孔材料时的流动和传热特性进行数值分析.分析结果表明,与不填充多孔材料相比,填...     

棒束定位格架空气-水两相分布数值模拟

用计算流体动力学程序CFX4．2对棒束定位格架通道内空气．水两相流体三维流动进行了数值模拟。在计算中，采用了考虑界面横向效应的两流体模型，用该模型计算模拟了通道的l/4区域和定位格架交混叶片．获得了流体区域的两相流速和相分布；并分析了定位格架交混叶片对流动和相分布的影响。结果表明．带定位格架棒束流道内空气-水两相流动数值模拟基本合理．该方法可用来初步分析复杂通道内的两相流动。     

板型元件表面结垢对堆芯最热通道流量分配影响的数值计算

运用计算流体力学程序(CFX)对板型燃料元件表面结垢对堆芯最热通道流量分配的影响进行了数值计算分析,计算结果表明．最热通道由于两边燃料板结垢,在通道间隙宽度降低10％的情况下．最热通道的流量分配系数与未结垢时相比降低18．2％。     

梯形微通道内充分发展层流摩擦阻力特性的数值研究

基于Navier-Stokes方程,利用SIMPLER算法并辅以区域扩充法,对梯形微通道内水的充分发展层流摩擦阻力特性进行数值研究。选取25个不同纵横比的梯形尺寸进行计算,并将计算的摩擦常数与H.Y.Wu和P.Cheng的实验值进行了比较。结果表明：计算值与实验值符合较好;阻力系数随纵横比（Wb/Wt）的增大而增大。同时,计算表明,Navier-Stokes方程即使在水力当量直径小到25.9μm的微通道内依然适应。     

环形通道内液态金属钠流动传热特性数值模拟

利用数值方法，使用商用软件ANSYS CFX，针对内径4~10 mm、外径7~20 mm的环形通道内不同边界条件下单相钠的流动和传热特性进行了研究，分析了流速、入口温度、内壁面热流密度和外壁面热流密度等对其流动传热的影响。结果表明，环形通道间隙对液态金属钠流动和传热影响较大，热流密度对其影响较小。将计算结果与文献中理论和实验结果进行了比较和分析，得出环形通道内液态金属钠的传热关系式。数值模拟结果与文献实验结果吻合较好，总体趋势符合良好。      

石墨慢化通道式熔盐堆的稳态热工水力计算模型

针对石墨慢化通道式熔盐堆的堆芯结构,基于COMSOL Multiphysics程序和MATLAB程序建立了堆芯稳态热工水力学计算模型。该模型对堆芯内固体区域的温度分布采用三维热传导方程进行模拟,对通道内熔盐温度采用一维单相流体模型进行计算。固体区域与熔盐通过熔盐通道壁面的对流换热边界建立热耦合。该模型基于平行通道压力损失相等的原则,分配堆芯内各熔盐通道的流量。通过对比RELAP5程序的计算结果,验证了模型对温度和流量分配计算的正确性。针对2 MWt 液态燃料熔盐堆的一种概念设计,分析了堆芯内三维温度分布和通道间流量分配。该模型可精确计算通道式熔盐堆堆芯内稳态温度分布和流量分配,对堆芯的热工水力学设计具有重要意义。     

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。     

摇摆状态下入口段和上升段对两相流动不稳定性的影响

本文对摇摆状态下并联多通道系统的两相流动不稳定性进行了初步研究。重点讨论了在摇摆状态下的并联通道入口段和上升段对管间脉动不稳定性的影响。以均匀流模型为基础,采用控制容积积分法建立了并联多通道的分析模型并建立系统方程组,同时考虑了摇摆状态的影响。用吉尔方法对系统方程组进行求解,得到了并联多通道系统在摇摆状态下的不稳定边界。发现在一定的摇摆状态下两相流动不稳定区域会同时出现在低含汽和高含汽两个区域,在高含汽率区域某些工况下会出现倍周期现象。     

轴向非均匀加热对并联通道流动不稳定性的影响

基于均相流模型建立并联通道系统的控制方程,采用交错网格技术和半隐式差分离散控制方程,并使用追赶法求解来模拟并联通道的两相流动特征。采用轴向余弦功率加热模拟轴向非均匀功率加热。运用小扰动法,获得了不同压力、入口过冷度和轴向功率加热方式下的稳定性边界（MSB）和三维不稳定性空间。对于余弦和均匀功率加热,系统稳定性均随系统压力的增大而增强。余弦功率加热在高过冷度区降低并联通道系统稳定性,而在低过冷度区增强系统稳定性。随进口阻力系数的增加,处于余弦功率加热的并联通道系统稳定性增强,MSB的拐点逐渐向高过冷度区移动。     

Laminar flow induced deflections of stacked plates

Some research and power reactors such as the Engineering Test Reactor (ETR), the Materials Test Reactor (MTR) and the Shippingport Reactor have core designs which consist of parallel, flat or curved plate fuel assemblies. The fuel is contained in the thin plates which are separated by narrow channels through which coolant flows to remove heat generated within the plates. Since the plates are flexible, the coolant flowing through the channels causes the plates to deflect. At high coolant velocities large deflections have been observed causing the plates to deform plastically leading to structural failure or plate collapse. This work examines a single plate bounded by two channels and determines the static plate deflection as a function of plate, channel and flow parameters. The deflection is due to differences in pressure and flow velocity in the channels bounding the plate and also due to different channel dimensions caused by tolerance effects. The classical thin plate equations are used with a nonlinear hydrodynamic loading function expressing the external fluid forces on the plate surfaces.     

Space-Dependent Neutron Spectrum Effects in a Fast Reactor

An analysis on the hydrodynamic instability of two-phase flow in parallel multichannels is conducted.

Occurrence of instabilities and their modes of oscillations can be evaluated by investigating into a characteristic equation, its roots and composing channel transfer functions. It is also shown that a governing matrix is reduced to a diagonal one by using its eigenvalues, the oscillation modes being divided into N (number of channels) separate fundamental modes. Characteristics of each oscillation mode are given by examining corresponding characteristic equations.

The derived equations are applied for the prediction of oscillation modes of systems composed of a few slightly different channels. The analysis successfully predicts the modes which have been experimentally observed.     

Numerical research on oscillation of two-phase flow in multichannels under rolling motion

Two-phase flow instability and dynamics of a parallel multichannels system has been theoretically studied under periodic excitation induced by rolling motion in the present research. Based on the homogeneous flow model considering the rolling motion, the parallel multichannels model and system control equations are established by using the control volume integrating method. Gear method is used to solve the system control equations. The influences of the inlet, upward sections, heating power and rolling amplitudes on the flow instability under rolling motion have been analyzed. The marginal stability boundary (MSB) under the rolling motion condition is obtained. The unstable regions occur in both low and high equilibrium quality and inlet subcooling regions. The multiplied period phenomenon occurs in the high equilibrium quality region and the chaos phenomenon appears on the right of MSB. The concept of stability space is presented.     

Thermo-hydraulic instabilities in parallel boiling channel systems: Part 1. A non-linear and a linear analytical model

Two analytical models are proposed to analyze density wave instability. One is a non-linear analytical model (PARALLEL) solved for the time elapsed and is applicable to systems with more than three channels with the same flow conditions or different flow conditions between channels. The other is a linear model (PARCOMP) solved on a complex plane and is applicable to two channel systems with or without different flow conditions. The results obtained by these models are compared with the density wave instability occurring in a twin parallel boiling channel system. The PARALLEL is applied to systems with more than three channels with the same flow conditions, and the results are compared with those in a twin channel system. Finally, the effects of the different flow conditions on the stable flow limit in a system with more than three channels are investigated analytically using PARALLEL. The approximation method by a linear model is examined and proposed to evaluate the stable flow limit in this case.     

Theoretical research on two-phase flow instability in parallel channels

Two-phase flow instability of two-channel system has been theoretically studied in the present study. Based on the homogeneous flow model, the parallel channels model and system control equations are established by using the control volume integrating method. Gear method is used to solve the system control equations. The marginal stability boundary (MSB) at different system pressure conditions is obtained. The typical MSB shape is usually a classical inclination “L” at some operation condition (i.e. the system pressure is low and the inlet resistance coefficient is small). The three-dimensional instability spaces (or instability reefs) with different inlet resistance coefficients are obtained. The three coordinates consists of phase change number (N_pch), subcooling number (N_sub) and nondimensional pressure (P⁺). The lower part of the instability space is larger than the upper one. Increasing the system pressure or inlet resistance coefficient can strengthen the system stability. However, increasing the heating power destabilize the system stability. The influence of inlet subcooling on the system stability is multi-valued.     

Linear Analysis of Thermo-hydraulic Instabilities of the Advanced Heavy Water Reactor (AHWR)

Analysis was carried out to predict the threshold of instability for Ledinegg type and density wave oscillations for the Indian Advanced Heavy Water Reactor (AHWR) which is a Natural Circulation Pressure Tube Type Boiling Water Reactor. The mathematical model considers homogeneous two-phase flow and the conservation equations are solved analytically to obtain the steady state thermo-hydraulic characteristics and flow stability map. The model was applied for the AHWR concept after it had been validated with the test data obtained from a simple forced circulation loop with small parallel boiling channels and from the High Temperature Loop (PNC). The results indicate that the proposed design configuration of the AHWR may experience both Ledinegg type (static instability) and Type-I and Type-II density wave oscillations depending on the operating condition. The effects of various geometric and operating parameters on these types of instabilities were studied. It can be seen from the results that the Ledinegg type instability is suppressed with an increase in pressure and disappears when the operating pressure is higher than 0.7MPa. But the density wave instability may occur even at 7 MPa. In addition, it is found that for parallel multiple channels operating under natural circulation condition, the stability of density wave instability is not always enhanced by increasing the throttling coefficients at the inlet of channels.