反应堆核燃料组件定位格架的两相流动压降计算

燃料组件定位格架的两相流动压降计算是反应堆热工水力计算的一个重要内容.本文从两相流动的机理分析入手,根据动量平衡关系推导出了定位格架的两相流压降计算关系式.并用实验数据进行了验证,实验数据与计算公式之间的偏差不超过±30%.     

矩形通道内气水两相流摩擦阻力计算模型

常压下以空气和去离子水为工质,对横截面为1.41 mm×40 mm和3 mm×40 mm的竖直矩形通道内两相流动阻力特性进行了实验研究。利用获得的764组实验数据,对11种典型两相流摩擦阻力计算模型进行评价。结果表明：Lee-Lee模型整体预测精度最高,但在分液相雷诺数较小（Re_l<600）和较大（Re_l>8 700）区域,与实验值符合较差;在分液相紊流区(Re_l≥2 000) Chisholm B模型适用性较好,对于两实验段预测值与实验值绝对平均误差分别为6.13%和6.43%,但在分液相层流区(Re_l<2 000)其预测值与实验值偏差较大。根据压降特性提出修正两相动力黏度,并针对分液相层流区提出修正计算关系式,其预测值与实验值符合较好。     

气-液两相泡状流在摇摆状态下的摩擦压降计算

在常温下对光滑的有机玻璃管内单相水和气-液两相泡状流在两种不同摇摆角度和3种不同摇摆周期下的摩擦压降特性展开了研究.结合实验数据,提出了摇摆状态下的单相水摩阻系数的计算关系式.计算关系式不仅能够反映摩阻系数随时问变化的周期动态特征,而且计算结果与实验结果符合很好.在此基础上,运用均相流模型,对摇摆状态下两相泡状流的摩擦压降进行计算.结果发现,计算值与实验值的相对误差在±25%以内.     

棒束定位格架内单相流体三维流场研究

利用计算流体动力学(CFD)方法对带定位格架棒束内单相流体三维流场进行研究,以了解定位格架对流体流动的影响。用激光多普勒测速系统测量的实验数据对预测结果进行了校验,两者吻合较好。     

棒束定位格架两相CFD模拟方法研究

考虑气泡合并分裂,采用MUSIG模型,对3×3格架内空气-水两相分布进行计算流体力学(CFD)数值模拟研究发现,计算对入口两相分布预计不敏感,但对气泡直径大小敏感;在定位格架下游不远处,空泡份额分布由较小直径气泡起主导作用,格架下游较远处,空泡份额分布由较大直径气泡起主导作用。考虑空气-水两相流量、几何条件和压力对气泡直径的影响,本文提出针对棒束定位格架的数值模拟气泡最大直径设置关系式,并对模型选取和模拟方法给出建议。计算表明空泡份额分布曲线形状与峰值均和实验符合较好,该模拟方法能合理预测复杂通道两相数值分布。     

高压水通过加热的正方形排列棒束时两相流动特性研究

本文介绍高压高温水通过9根正方形排列的棒束及棒束定位架时的流动阻力特性实验研究.水强迫垂直向上流过棒束.分别进行了棒束不加热的单相流动实验和棒束均匀加热并出现汽液两相流动时的两相流动实验.单相及两相实验的参数范围:P=140公斤(力)/厘米~2,质量流速 ω~g=(1.5—8)×10~6公斤/米~2小时,雷诺数 Re=3×10~3—1.5×10~5,热流密度q=(0.2—1.3)×10~6千卡/米~2·小时,平均真实含汽量(?)=0.0002—0.23.在实验研究方法上,把欠热沸腾和饱和沸腾这两种工况都看成是两相流动工况,两种工况的实验数据用真实含汽量综合成统一的实验曲线和实验关系式.在测量技术方面,采用新设计的引压装置把压差信号从高压高温实验通道测孔处引入压差计.测取的信号可靠,测量误差小.实验数据全部用计算机处理.经过综合、分析、整理后,得到棒束摩阻实验曲线和实验关系式及定位架的形阻实验曲线.     

燃料组件精细化定位格架模型开发及评价

为提高燃料组件子通道内两相局部参数预测的准确性,本文基于分布式阻力方法建立精细化定位格架模型,选用合适的摩擦阻力表达式,对格架上的交混翼进行精细化建模,采用Carlucci湍流交混模型计算湍流交混速率,引入阻塞因子计算由定位格架引起的湍流交混效应,并将建立的精细化定位格架模型植入子通道分析程序（ATHAS）,对压水堆子通道和棒束实验（PSBT）基准题进行计算分析。结果表明,本文开发的精细化定位格架模型能够提高燃料组件子通道内空泡份额和温度分布的预测准确性,为棒束通道流场、焓场计算和临界热流密度（CHF）预测奠定了基础。      

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。     

竖直窄矩形通道内空气-水两相流中气弹运动速度研究

以空气和水为工质,应用高速摄像仪,对竖直窄矩形通道(3.25 mm×40 mm)内气液两相弹状流进行了可视化实验研究。气、液相表观速度分别为0.1~2.51 m/s和0.16~2.62 m/s,工作压力为常压。实验中发现窄矩形通道内弹状流与圆管中存在较大差别,气弹多发生变形,高液相流速时变形更为严重。窄边液膜含气量较高,在高液相流速时窄边液膜不下落,宽边液膜中含有由气弹头部进入和气弹尾部进入的气泡。气弹速度受气弹头部形状和宽度影响较大,受气弹长度影响较小。气弹速度可由Ishii & Jones-Zuber模型计算,但在低液相折算速度时偏差较大,其主要原因为漂移速度计算值较实验值偏小。     

矩形小通道内空气-水两相流动均相流模型研究

在机玻璃竖直矩形通道内,以空气和去离子水为工质获得实验数据。据此对竖直矩形小通道内均相流模型的适用性进行评价。结果表明,采用McAdams两相粘度时均相流模型及Chen等提出的修正均相流模型能较好用于1.41 mm间隙通道压降的预测,平均绝对误差分别为10.92%和12.20%;采用McAdams两相粘度时均相流模型对于3 mm间隙通道在两相雷诺数Re大于6000时平均绝对误差为10.04%,但气-液两相Re较低时预测偏差较大。通过实验数据分析得到了均相流模型适用于3 mm间隙通道的范围;针对两相Re较低的区域拟合得到了新的经验关系式,其预测值与实验值符合较好。     

Properties of disturbance waves in vertical annular two-phase flow

Disturbance waves play an important role in interfacial transfer of mass, momentum and energy in annular two-phase flow. In spite of their importance, majority of the experimental data available in literature on disturbance wave properties such as velocity, frequency, wavelength and amplitude are limited to near atmospheric conditions (Azzopardi, B.J., 1997. Drops in annular two-phase flow. International Journal of Multiphase Flow, 23, 1-53). In view of this, air-water annular flow experiments have been conducted at three pressure conditions (1.2, 4.0 and 5.8 bar) in a tubular test section having an inside diameter 9.4 mm. At each pressure condition liquid and gas phase flow rates are varied over a large range so that the effects of density ratio, liquid flow rate and gas flow rate on disturbance wave properties can be studied systematically. A liquid film thickness is measured by two flush mounted ring shaped conductance probes located 38.1 mm apart. Disturbance wave velocity, frequency, amplitude and wavelength are estimated from the liquid film thickness measurements by following the statistical analysis methods. Parametric trends in variations of disturbance wave properties are analyzed using the non-dimensional numbers; liquid phase Reynolds number (Re_f), gas phase Reynolds number (Re_g), Weber number (We) and Strouhal number (Sr). Finally, the existing correlations available for the prediction of disturbance wave velocity and frequency are analyzed and a new, improved correlation is proposed for the prediction of disturbance wave frequency. The new correlation satisfactorily predicted the current data and the data available in literature.     

Hydraulic Performance of Small Bending Radius Helical Coil-Pipe

Helical steam generator or helical heat exchanger is extensively used in high-temperature gas cooling reactor, fast breeder reactor, pressurized water reactor using in ship propulsion and areas of electro-power, chemical industry and petroleum industry. The purpose of this paper is to research the hydraulic performance of small bending radius helical coil-pipe used in HTR-10. Research for hydraulic performance of small bending radius helical pipe was carried out on the HTR-10 steam generator experimental facility. Based on the experimental results, it was confirmed that for helical pipe the critical Reynolds number (Re) is much greater than in a straight pipe and is a function of De. Formulas for Re of single-phase flow structure transition, friction coefficient of single-phase flow, and two-phase flow friction factor are obtained. Experience formulas of small bending radius helical pipe are recommended for design and research.     

New correlations of single-phase friction factor for turbulent pipe flow and evaluation of existing single-phase friction factor correlations

The determination of single-phase friction factor of pipe flow is essential to a variety of industrial applications, such as single-phase flow systems, two-phase flow systems and supercritical flow systems. There are a number of correlations for the single-phase friction factor. It still remains an issue to examine similarities and differences between them to avoid misusing. This paper evaluates the correlations for the single-phase friction factor against the Nikuradse equation and the Colebrook equation, respectively. These two equations are the base for the turbulent portion of the Moody diagram, and are deemed as the standard to test the explicit counterparts. The widely used correlations for smooth pipes, the Blasius correlation and the Filonenko correlation, have big errors in some Re ranges. Simpler forms of the single-phase friction factor covering large ranges are needed. For this reason, two new correlations of single-phase friction factor for turbulent flow are proposed, one for smooth pipes and the other for both smooth and rough pipes. Compared with the Nikuradse equation, the new correlation for smooth pipes has the mean absolute relative error of 0.022%, with the maximum relative error of −0.045% in the Reynolds number (Re) range from 3000 through 10⁸. It is an idea replacement of the correlations of Blasius and Filonenko. The new correlation for both smooth and rough pipes has the mean absolute relative error of 0.16% and the maximum relative error of 0.50% compared with the Colebrook equation in the range of Re = 3000-10⁸ and Rr = 0.0-0.05, which is the most simplest correlation in that error band.     

Correlations for predicting single phase and two-phase flow pressure drop in pebble bed flow channels

An experimental study was conducted on the pressure drop of the single phase and the air–water two-phase flow in the bed of rectangular cross sections densely filled with uniform spheres. Three kinds of glass spheres with different equivalent diameters (3 mm, 6 mm, and 8 mm) were used for the establishment of the test sections. The Reynolds number in the experiment ranged from a dozen to thousands for the single-phase flow and from hundreds to tens of thousands for the two-phase flow. In the present flow-regime model, the bed was subdivided into a near-wall region and a central region in order to take the wall effect into account to improve the prediction at low tube-to-particle diameter ratios. Improved correlations are obtained based on the previous study to consider the single-phase flow pressure drops for finite pebble beds with spherical particles and nonspherical particles by fitting the coefficients of that equation to both the database and the present experiment. The correlation is consistent with the observed physical behavior which explains its comparatively good agreement with the experimental data. A new empirical correlation for the prediction of two-phase flow pressure drops was proposed based on the gas phase relative permeability as a function of the gas phase saturation and the void fraction. The correlation fit well for both experimental data of spherical particles and nonspherical particles.     

Analysis of Stratified Cocurrent Flow of Gas-Liquid Mixtures in Horizontal Pipes

A very simple model, as compared with those presented thus far, is developed to describe the stratified cocurrent flow of gas-liquid mixtures in a horizontal pipe. Assuming densities of both phases are constant in location, mass and momentum equations are solved to obtain pressure gradient and volume fractions of both phases at steady state.

Several experimental data by others are found in good agreements with the calculated results by the model. When the interface is smooth, i.e. the volumetric liquid flux J_l <0.lft/s, it is a good approximation to use the Blasius equation to estimate interfacial shear stress. For 0.1?J_l ?1.0ft/s where the interface is not smooth but ripply or wavy, the Wallis correlation is found applicable. If we assume stratified flow geometry forj_l >1.0ft/s, agreements with experiment cannot be obtained, since the transition to slug or elongated bubble flow has already taken place. The transition from stratified to annular mist flow is found to occur at gas volume fraction α≈0.9 regardless of J_l .     

An improved mixing-length model for annular two-phase flow with liquid entrainment

A differential model for adiabatic, fully developed, annular two-phase flow with liquid entrainment has been developed. The model is based on a modified form of the single-phase mixing length function to account for turbulence intensity attenuation due to the presence of entrained liquid droplets in the gas core. Simultaneous predictions of pressure drop and average film thickness for given flow rates show good agreement with a wide range of experimental results for various fluids at several pressures in a range of tube diameters for both horizontal and vertical (upwards and downwards) flows.     

Measurement of Liquid Turbulent Structure in Bubbly Flow at Low Void Fraction Using Ultrasonic Doppler Method

Microscopic structure in bubbly flows has been a topic of interest in the study of fluid dynamics. In the present paper, the ultrasonic Doppler method was applied to the measurement of bubbly. The experiments were carried out for an air-water dispersed bubbly flow in a 20 mm × 100 mm vertical rectangular channel having a void fraction smaller than 3%. Two ultrasonic transducers were installed on the outer surface of the test section with a contact angle of 45^° off the vertical axis, one facing upward and the other facing downward. By applying statistical methods to the two directional velocity profiles, Reynolds stress profiles were calculated. Furthermore, to clarify the wake effect induced by the leading bubbles, the velocity profiles were divided into two types of data. The first one is for all of the liquid data and the other is the data which did not include the wake effect. For Re_m ≥1,593, it was observed that the bubbles suppressed the liquid turbulence. Furthermore, comparing with the Reynolds stress profiles in bubbly flow, it was found that Reynolds stress profiles varied with the amount of bubbles present in the flow and the effect of wake causes turbulence in the liquid.     

New Theoretical Model for Two-Phase Flow Discharged from Stratified Two-Phase Region through Small Break

A theoretical and experimental study was conducted to understand two-phase flow discharged from a stratified two-phase region through a small break. This problem is important for an analysis of a small break loss-of-coolant accident (LOCA) in a light water reactor (LWR). The present theoretical results show that a break quality is a function of h/h_b , where h is the elevation difference between a bulk water level in the upstream region and the break and b the suffix for entrainment initiation. This result is consistent with existing experimental results in literature. An air-water experiment was also conducted changing a break orientation as an experimental parameter to develop and assess the model. Comparisons between the model and the experimental results show that the present model can satisfactorily predict the flow rate and the quality at the break without using any adjusting constant when liquid entrainment occurs in a stratified two-phase region. When gas entrainment occurs, the experimental data are correlated well by using a single empirical constant.     

Frictional Pressure Drop for NaK-N2 Two-Phase Flow in Rectangular Cross Section Channel of Large Aspect Ratio

In this paper, the frictional pressure drop in an isothermal liquid metal-gas two-phase flow through a rectangular channel with large width-to-height ratio is treated semiempirically for a NaK-N₂ two-phase flow system.

The frictional pressure drop in the two-phase flow is compared with the following two reference values :

1. The frictional pressure drop in the liquid flowing alone in single phase with the same velocity as that of the liquid in the two-phase mixture.

2. The frictional pressure drop in the liquid flowing alone in single phase with the same mass flow rate as that of the liquid in the two-phase mixture.

The comparison with the former reference value is necessary for the prediction of friction loss in a liquid metal MHD generator channel whose medium would be two-phase mixture.

The semiempirical analysis was performed assuming the two-phase mixture to be a continuous medium with its properties, e.g. viscosity and density, defined by void fraction and the velocity determined by the total mass flow rate.

In the region of low slip and density ratio ρ_g/ρ_l the frictional pressure drop in the two-phase flow appeared to be smaller than that due to the liquid flowing alone with the same velocity as that of the liquid in the two-phase flow.

The experiments have been undertaken with the NaK-N₂ two-phase mixture flowing through a rectangular channel (4 × 60 mm²).

Data were taken over the following parameter range:

NaK velocity: 5～30 m/sec, Void fraction: 0～70%

Density ratio: 0.006～0.013, Quality: 0.07～1.10%.