三角形排列的紧密栅元棒束内流动行为的数值模拟

对三角形排列紧密栅元通道内的空气湍流流动进行了数值研究,系统考察了涡粘性和雷诺应力两类湍流模型模拟紧密栅元通道内流动特征的适用性.结果表明:SSG雷诺应力模型对流动有较好的模拟,这说明湍流各项异性的模拟在紧密栅元中十分重要;不同雷诺数和几何结构下的模拟显示,二次流的大小和雷诺数的相关性不大.但随着棒间距和棒径比(P/D)的增大,二次流减小.     

摇摆条件下典型通道间湍流的流动传热特性

利用Fluent软件分析了摇摆条件对典型四棒束间的湍流流体流动和传热特性的影响机理。摇摆运动会对棒束间流体的流动传热特性产生一定影响,但不会对绝热通道与加热通道内流体流动相似性产生影响。而当摇摆幅度较大时,径向附加力会使通道横截面上的参数分布发生显著的变化,进而影响流体的流动与传热特性。在摇摆条件下,随着P/D（棒间距/棒直径）的逐渐减小,尤其是小于1.1时,典型棒束间流体的流动传热特性发生明显变化。     

液态铅铋合金在绕丝燃料棒组件子通道间湍流交混数值模拟

液态铅铋合金（LBE）是第四代液态金属核反应堆候选冷却剂,由于LBE热物性具有一定的特殊性,亟待对LBE在燃料组件子通道中的流动与传热过程开展研究。本文对LBE在带绕丝燃料棒组件中湍流流动进行数值模拟与分析,将燃料棒壁面温度的数值模拟结果与响应的实验数据相比较,2者具有较高的吻合度,说明数学模型及数值结果具有较高的可靠性与准确性;使用湍流交混系数β表征LBE在不同子通道间、不同燃料棒间隙宽度与燃料棒直径比（S/D）结构下的湍流交混情况,结果表明,不同子通道间β波动程度具有差异性,β的大小与S/D呈负相关。基于不同S/D与雷诺数的计算结果,拟合出不同子通道间β关联式,为绕丝燃料棒三角形排列方式的燃料组件子通道分析程序开发提供交混模型。      

三角形子通道超临界水热工水力特性数值分析

目前国际上对超临界水冷堆进行了大量的研究,但对其堆芯内超临界流体流动传热的认识还十分有限.本文采用CFX对超临界水冷堆典型三角形子通道内的流动传热特征进行了CFD研究,对比分析了包壳壁面等热流密度和燃料芯块等体积热流密度两种情况.计算结果表明,不锈钢包壳层的周向导热显著强化了燃料棒圆周上温度分布和传热系数的均匀性,但对二次流和湍流脉动的影响不大.间隙区的湍流脉动主要受几何参数P/D的影响,当P/D<1.3时,湍流交混系数在0.02～0.025之间,当P/D>1.3时,湍流交混系数较小,在温度拟临界点附近区域,存在交混系数的突变.     

微小通道内流动及气溶胶沉积特性实验研究

事故后气溶胶通过安全壳微小通道穿透会发生滞留现象，在不同载气流动状态下，主导沉积机制不同，会影响气溶胶的穿透率，因此需要研究微小通道内气溶胶的载气流动特性及气溶胶沉积特性。本文建立了微小通道气溶胶穿透与滞留实验装置，开展了雷诺数为40～3 600的载气流动与气溶胶沉积实验研究。在雷诺数约为700时，微小通道内会发生层流转捩。层流流动下，在大突缩比的矩形微通道中，随着雷诺数的增大，入口碰撞效应减弱，穿透率增大；小突缩比薄壁毛细管微通道入口效应弱，随着雷诺数的增大，气溶胶几乎完全穿透。在雷诺数为800～3 600的湍流流动下，气溶胶沉积主要处于涡流扩散-碰撞区及惯性主导区，受到湍流涡的影响气溶胶穿透率随雷诺数的增大而减小。     

棒束内湍流流动特性的PIV与CFD研究

在压水堆燃料组件的定位格架下游,局部扰动沿流动方向逐渐衰减,流场最终趋于稳定。光滑棒束区冷却剂的湍流流动和交混特性是影响反应堆经济性和安全性的重要因素,有必要进行深入研究。本文采用粒子图像测速（PIV）与数值模拟(CFD)相结合的方法,对3×3小规模棒束内水的流动特性进行研究,得到了一阶平均流速以及二阶湍流统计信息。结果表明,中心子通道的速度明显高于棒间隙区,但轴向均方根速度呈现出相反的变化趋势。在相邻子通道横向速度梯度的作用下,棒束内出现了大尺度的流量脉动现象,且脉动波长随雷诺数的增加而增大。此外,实验得到的湍流交混系数较压水堆采用的Castellana公式预测值偏高10%左右,这一偏差随雷诺数的增加有减小的趋势。     

不同棒束结构稠密栅元通道内的湍流CFD研究

采用URANS（UnsteadyReynoldsAveragedNavierStokes）方法对不同棒束结构稠密栅元通道（P/D=1.001～1.2）内的湍流流动进行CFD模拟。研究分析了不同Re（Re=5000~215000）的湍流流动的主流速度、壁面剪应力、湍动能等参数。研究表明：在较稠密的棒束（P/D<1.1）通道内,P/D的变化对子通道内主流速度和剪应力分布均有较大影响。本文的模拟结果也验证了在达到临界P/D前（即使δ/D<0.011）,交混因子Y和δ/D成反比关系。对于固定的棒束结构（P/D=1.062）,当Re达到一定值（Re=9600）时,子通道内主流速度和剪应力分布对Re的变化不敏感。     

三角形棒束内铅铋湍流普朗特数模型研究

为准确预测低普朗特数流体在燃料组件棒束子通道内的传热特性,需选取合适的湍流普朗特数模型。针对5种不同的湍流普朗特数模型,基于三角形棒束换热关联式,研究采用剪切应力传输（Shear Stress Transfer,SST）k-ω湍流模型,分析不同的棒束子通道结构,并与液态铅铋实验验证的换热关联式计算结果进行对比,分析不同棒径与节径比条件下各种湍流普朗特数模型的适用性。分析研究结果表明,整体湍流普朗特数模型不仅与雷诺数Re、贝克莱数Pe有关,还与节径比P/D有关;在节径比1.3～1.7范围内Kays学者提出的局部湍流普朗特数模型模拟结果与Mikityuk关系式计算值较为吻合;各种湍流普朗特数模型均有最佳的节径比适用范围。因此,相关模型能够用于不同节径比条件下三角形棒束子通道内铅铋传热特性的预测。     

紧密栅棒束子通道间周期性大尺度涡的CFD模拟

为准确评估紧密栅棒束子通道间的搅混现象,采用开源计算流体力学（CFD）软件OpenFOAM 2.0并基于k-ω的显式几何雷诺应力湍流模型对两种子通道内的周期性大尺度涡结构进行模拟,研究了紧密栅子通道间周期性大尺度涡波长、峰值频率等参数的变化规律。结果表明,周期性涡结构存在一个很强的峰值频率,其平均最大频率随雷诺数（Re）呈线性增加,但其平均波长（λ）不随Re变化,只与子通道的结构参数有关;周期性涡结构导致两个子通道间存在很强的周期性的流动震荡,是紧密栅子通道湍流搅浑得到强化的主要原因。      

燃料棒束对称近壁子通道内湍流结构的模拟实验研究

描述了棒束子通道内流速分布,壁面剪应力分布和湍流雷诺应力张量分布的实验研究。由四根棒组成的棒束平行对称地布置在一个矩形流道内。试验棒的中心距与棒直径之比为:P/D=1.148,而壁距与棒直径之比分别为W_1/D=1.045和W_2/D=1.074。两种不同几何条件下,实验中雷诺数分别为6.11×10~4和7.07×10~4。实验结果表明,棒束子通道内的湍流结构与圆管内的湍流结构有很大差别。特别是在棒和通道壁之间的窄缝区存在着相当强的轴向和周向湍流强度,因而那里也有相当强的湍流动能,这显然是由于通过棒-壁窄缝处强烈的湍流脉动流所造成的。和过去进行的非对称布置的子通道实验(子通道具有相同几何参数P/D及W/D,但与相邻子通道几何非对称地布置于同一矩形通道内)相比,发现对称子通道情况下子通道之间通过棒-棒窄缝处的湍流动量迁移则很小,可以忽略不计。壁面剪应力分布的实验值和用VELASCO程序计算结果相对比,发现两者之间有明显的差异,尤其是在棒-壁窄缝区,差异更大。建议有必要发展比现有程序更为完善的分析计算程序,以便提高对棒束子通道湍流流动的计算精度。     

Characteristics of Counter current Gas-Liquid Two-Phase Flow in Vertical Tubes

Investigations into the flow pattern and the void fraction for countercurrent air-water flow in vertical tubes of diameter D = 40 and 80 mm were reported. The flow maps were presented and showed slug flow regime occupied larger portion on them. The void fraction was measured by the quick-closing valve technique, in bubbly and slug flow regime.

The void fraction data available in the literatures as well as present work for counter- current flow in vertical tubes were correlated in terms of dimensionless groups. The experimental results of the present work were also compared with the drift flux model.     

Vertically Downward Two-Phase Flow, (II)

Following Part (I) of the present paper, which presented the experimental results obtained on the void distribution and average void fraction shown by nearly fully-developed, vertically downward two-phase flow of air-water mixture, this Part (?) covers the flow regime transition criteria among the three basic flow regimes : bubbly, slug and annular flows. The annular flow further was divided into two subregions of falling film flow and annular drop flow. The general situation of the transition criteria is as follows : (1) bubbly-to-slug flow transition occurs when the local void fraction in the central region of the tube is 0.3; (2) slug-to-annular drop flow transition criterion is given as a case which equations giving average void fraction for the slug flow and the annular flow are simultaneously satisfied; (3) slug-to-falling film flow transition occurs when the pressure difference between the crest of large wave and the bottom overcomes the surface tension; (4) the occurrence of liquid droplets from wave crests gives the transition criterion between the falling film flow and the annular drop flow.

These criteria were correlated to predict each flow regime boundary respectively considering flow mechanisms or from experimental results. The correlations obtained were compared with published flow regime maps for atmospheric air-water flow and showed satisfactory agreement.     

Combinated Flow Pattern Map for Cocurrent and Countercurrent Air-Water Flows in Vertical Tube

Flow patterns for cocurrent and countercurrent air-water flows in vertical tubes (40 and 80mm I.D.) at volumetric flux densities of air and water in the ranges ?115–158 and ?100–102 cm/s were observed. A flow pattern map presenting the entire data of the observed flow patterns, i.e. bubbly, slug and annular flow for each mode of flow operation (upflow, countercurrent flow and downflow) were presented on the j_l vs. j_g plane. The flow pattern maps showed significant difference of flow pattern transition boundaries with upflow, countercurrent flow and downflow. Flow pattern transition curves were smoothly continuous with the change of the direction of water flow, on the other hand the change of flow direction of air showed complicated effect on flow pattern transition near zero j_g . Comparison of the present flow pattern data with the reported general flow pattern maps for upflow showed that the correlation of Taitel et al. for bubble-slug flow transition is applicable to both cocurrent and countercurrent air-water flows.     

摇摆状态下气液两相流流型转变的实验研究

通过可视化观察和数码照片对摇摆状态下光滑有机玻璃管内气液两相流流型进行分类和定义,并分析了不同管径、摇摆角度以及摇摆周期对流型之间转变的影响.结果表明:在液相折算流速一样的情况下,管径增加、摇摆周期缩短或摇摆角度减小会使得环状流形成需要更高的气体折算流速;弹状流向搅混流转变所需气相流量则随着管径的减小、摇摆周期的增加或摇摆角度的减小而增加.而在气相折算流速一样的条件下,管径增加、摇摆周期缩短或摇摆角度增大会使泡状流产生需要更高的液相流量.     

Study on intermittent flow behavior in a vertical channel under low-pressure condition

The intermittent flow behavior in a vertical annulus under a low-pressure condition was experimentally studied using a scaling experiment facility. The temperature and pressure variations in the channel had been obtained under the heat load ranging from 0 to 2.0 kW, initial subcooled water temperature ranging from 50 to 90 °C and length–diameter ratio ranging from 1.6 to 50. The effects of the heat load and length–diameter ratio of channel on the flow characteristics were investigated in detail. The experimental results showed that the steam bubbles erupted more frequently and regularly at a high heat load. The intermittent flow period decreased with increase of the heat load and aspect ratio. Based on the mechanism analysis, an empirical model considering the steam oscillation and the vapor–liquid interface rupture based on the experimental data was proposed. It was found that the accumulated steam basically increased linearly. The oscillation of the pressure and velocity decreased gradually with continuous steam accumulation. The Reynolds number of the liquid within the rising section was very small at the stagnation state since there was no forced circulation flow. Finally, a blockage was engendered in the pipeline with the steam accumulated.     

Mechanistic Model of Slugging Onset in Horizontal Circular Tubes

A new model of the transition mechanism from stratified gas-liquid two-phase flow into a slug pattern in horizontal circular tubes was developed. This model incorporated the contribution of liquid kinematic energy to wave growth on the interphase surface. The transition limit was numerically determined so as to allow a wave crest to reach the top wall of the horizontal tubes.

Air-water experiments were performed to obtain data for comparison with model predictions. Three test sections were used. They had different inner diameters and the largest test section had a rod bundle inside it. Predictions of liquid flow rate at the flow pattern transition boundary, with given gas flow rates, were within data scatter of the experiments. This suggested that the present model satisfactorily described effects of tube size and internal structure on slugging onset in horizontal circular tubes.     

Numerical study of the MHD flow characteristics in a three-surface-multi-layered channel with different inlet conditions

A 3D MHD flow simulation was conducted to clarify the effects of the inlet flow conditions on the results of the validation experiment carried out previously and on the design window of the first wall using a three-surface-multi-layered channel. MHD pressure drop was largely influenced by the inlet condition. The numerical model with turbulent velocity profile showed qualitatively good agreement with the experimental result. The first wall temperature and pressure distributions obtained by the 3D simulation corresponded well to those obtained by the 2D simulation assuming fully developed flow. This suggested that complicated three-dimensional inlet flow condition generated in the L-shape elbow would not affects the existing design window.     

Vertically Downward Two-Phase Flow, (I)

Fully developed vertically downward two-phase flow of air-water mixture was investigated on void distribution and average void fraction among the three basic flow regimes; bubbly, slug and annular flows. The annular flow further was divided into two regimes of falling film flow and annular drop flow. Test channel is in form of inverted U-tube and tests were carried out at 100 tube diameters downstream from the curved part. Distributions of local void fraction were measured by means of a conductance needle probe method and the average void fraction was obtained from numerical integration of the measured local void fraction According to the results, profiles of local void fraction in bubby and slug flows showed characteristic natures with a peak in the middle region between the center and the wall of tube The average void fraction in downward flow depended greatly on the flow regimes. Accordingly correlation for each flow regime was developed to predict the average void fraction, based on flow mechanisms and experiments. The correlations were compared with experimental results for atmospheric air-water flow and showed satisfactory agreement.     

低压自然循环系统流动闪蒸过程流型研究

本文采用高速摄影和网格电导传感器对低压自然循环系统垂直上升段内闪蒸诱发的两相流流型演变开展研究。针对不同的流动状态,分别给出了稳定和不稳定两相流动条件下上升段内的流型种类。基于上升段内流体温度沿轴向的变化规律,确定流体温度沿轴向位置的转折点为闪蒸发生的起始位置。采用无量纲过冷数和闪蒸数,对低压自然循环系统的流动状态进行了划分;在入口过冷数小于12、闪蒸数介于4～5之间时,系统处于稳定的两相自然循环流动状态。     

Effects of Wall Roughness and Entry Length on Void Profile in Vertical Bubbly Flow

An experimental study of upward air-water bubbly two-phase flow in an entry region was performed with various rough wall test tubes. The objective of the work is to clarify the effects of wall roughness and entry length on void profile. The fluid flows in the vertical circular test tube of 25 mm I.D. under nearly atmospheric pressure, at room temperature. The void profile changes from a pattern similar in appearance to the saddle shape which has local void peaks near the wall, into the power law shape whose curve is approximated by a power law formula, with increasing wall roughness and/or entry length. That is, wall roughness and entry length have a similar effect upon void profile. There are two patterns in the power law shape, a pattern with sharp center peak and a pattern with obtuse center peak. As wall roughness and/or entry length increase, the void profile changes from the former pattern to the latter pattern. At enough long entry length (L/D ? 150), every void profile has almost the same power law shape independent of wall roughness. Some void profiles are asymmetric to the axis.