Visualization and measurement of two-phase flow by using neutron radiography

To apply neutron radiography (NR) technique to fluid research, high frame-rate NR with a steady thermal neutron beam has been developed in the present research program by assembling up-to-date technologies for neutron source, scintillator, high-speed video and image intensifier. This imaging system has many advantages such as a long recording time (up to 21 min), high-frame-rate (up to 1000 frames s⁻¹) imaging and no need for triggering signal. Visualization of air-water two-phase flow in a metallic duct was performed at the recording speeds of 250, 500 and 1000 frames s⁻¹. The qualities of those consecutive images were good enough to observe and measure the flow structure and the characteristics. It was demonstrated also that some characteristics of two-phase flow could be measured by using the present imaging system. Image processing technique enabled measurements of various flow characteristics in two ways. By utilizing geometrical information extracted from NR images, data on flow regime, rising velocity of bubbles, and wave height and interfacial area in annular flow were obtained. By utilizing attenuation characteristics of neutrons in materials, measurements of void profile and average void fraction were performed. It was confirmed that this new technique may have significant advantages both in visualizing and measuring high-speed fluid phenomena when the other methods such as an optical method and X-ray radiography cannot be applicable.     

Three-dimensional visualization of void fraction distribution in steady two-phase flow by thermal neutron radiography

Three-dimensional void fraction distributions of a steady air–water two-phase flow in a 4×4 rod-bundle with circular ferrule type spacers were measured by neutron radiography using a CT method. The high flux thermal neutron radiography system at JRR-3M in JAERI was used. Two-phase flow was visualized with a SIT tube camera and time-averaged one-dimensional cross sectional averaged void fraction distributions were calculated. Visualization with high spatial resolution up to 0.18 mm was carried out by using a cooled CCD camera. Projections in 250 directions were obtained and were reconstructed by a filtered back projection method after using some image processing techniques. Animations were made to show the three-dimensional distributions. One-dimensional and three-dimensional void fraction distributions of the steady state two-phase flow in the rod bundle near the spacer were clearly visualized.     

Development of high-frame-rate neutron radiography and quantitative measurement method for multiphase flow research

In relation to the establishment of thermal neutron radiography as a measurement method with high accuracy and reliability, this paper reviewed the present status on the development of high-frame-rate neutron radiography with a steady thermal neutron beam and its application to multiphase flow researches. This review included also the present progress on the quantification of neutron radiographic image at Kyoto University, i.e. (1) quantitative method to measure void fraction of two-phase flow with thermal neutron radiography (Σ-scaling method), (2) influence of scattered neutrons on void fraction measured by neutron radiography, (3) measurement error of neutrons in a low neutron flux field, (4) error in void fraction measurement due to low gray level, and (5) measurement error due to low imaging speed Moreover, a new experimental approach on a total macroscopic cross section for thermal neutrons measurement by neutron radiography was presented. This paper revealed neutron radiography to be a promising visualization and measurement method in thermal hydraulic research.     

Study on flow characteristics in gas-molten metal mixture pool

As part of basic research on the flow characteristics of a two-phase mixture pool under severe accident of fast breeder reactor (FBR), visualization and measurement of nitrogen gas-molten lead/bismuth two-phase flow in a rectangular pool were performed by using the neutron radiography technique. Measurements of drag coefficient of a single bubble and bubble shape regime showed that the relationship between the shape, size and the rising velocity of a single isolated nitrogen bubble in the molten lead/bismuth was not much different from that for an ordinary one. Appropriate correlation for drift velocity and drag coefficient between phases were recommended based on the drift flux correlation of measured pool void fraction. One- and two-dimensional analyses were performed by using a next generation computational code for safety analysis of severe accident of FBRs, SIMMER-III with various drag coefficient models. It was revealed that Kataoka–Ishii’s equation was suitable basically for estimation of drift velocity, namely, drag force between phases.     

环形流道两相流特性实验研究

在日本原子力研究所的一个两以试验台架上，进行了环形流道空气－水丙相流窟泡份额和逆向流试验，观察了其流型。通过试验和分析，提出了漂移流模型中分布系数Co和气相漂移速度Vgi的经验关系式；得到了逆向流Wallis表达式的拟合参数。对试验结果的分析表明，当流道间隙很小（Bond数小于1）时，流型和两相流动与间隙较大时有所不同。     

Availability of MCNP & MATLAB for reconstructing the water-vapor two-phase flow pattern in neutron radiography

The China Advanced Research Reactor (CARR) is scheduled to be operated in the autumn of 2008. In this paper, we report preparations for installing the neutron radiography instrument (NRI) and for utilizing it efficiently. The 2-D relative neutron intensity profiles for the water-vapor two-phase flow inside the tube were obtained using the MCNP code without influence of γ-ray and electronic-noise. The MCNP simulation of the 2-D neutron intensity profile for the water-vapor two-phase flow was demonstrated. The simulated 2-D neutron intensity profiles could be used as the benchmark data base by calibrating part of the data measured by the CARR-NRI. The 3-D objective images allow us to understand the flow pattern more clearly and it is reconstructed using the MATLAB through the threshold transformation techniques. And thus it is concluded that the MCNP code and the MATLAB are very useful for constructing the benchmark data base for the investigation of the water-vapor two-phase flow using the CARR-NRI.     

热中子照相的MCNP数值模拟初步研究

中子照相的数值模拟在中子照相技术研究中有着重要的作用。介绍了MCNP程序在数值模拟中子照相中的应用,给出了样品的模拟图像结果,并利用MCNP模拟分析了散射中子对中子照相成像结果的影响,对以后的中子照相实验具有指导意义。     

Void fraction estimation within rod bundles based on three-fluid model and comparison with x-ray CT void data

An interfacial shear stress equation in the dispersed-annular two-phase flow regime has been developed, which is based on a three-fluid model consisting of a liquid film on a rod, vapor and entrained liquid associated with a vapor flow. It is an extension of J.G.M. Andersen's procedure that provides a two-fluid interfacial shear stress equation using the drift flux parameters C₀ and V_gj. This interfacial shear stress equation can take into account a phase and velocity distribution through an equivalence between the drift flux parameters and the interfacial shear stress.

Using the three-fluid subchannel analysis code TEMPO with the three-fluid interfacial shear stress model, the capability of a three-fluid calculation using the drift flux parameters C₀ and V_gj that reproduce a measured void fraction is demonstrated. A comparison was made with advanced X-ray computed tomography (CT) void fraction data within a 4×4 rod bundle in diabatic 1 MPa pressure conditions. The three-fluid velocity field was estimated to be in good agreement with the experimental result of a void fraction.     

形态滤波在快中子图像降噪处理中的应用

在基于CCD耦合闪烁光纤阵列的快中子照相系统中,快中子图像受噪声污染较为严重.针对快中子图像中的椒盐噪声和泊松噪声,研究了形态滤波技术在降噪方面的应用.结果表明在对快中子图像处理过程中,基于二维多方向结构元素的形态滤波在滤除噪声和保持图像细节等方面效果较佳.     

Study on the stability behaviour of two-phase natural circulation systems using a four-equation drift flux model

Theoretical investigations were carried out to study the influence of two-phase flow parameters such as friction factor multiplier, drift velocity and void distribution parameter on the stability of boiling two-phase natural circulation systems. The theoretical model considers a four-equation drift flux model which solves the linearised conservation equations of mass, momentum and energy applicable to boiling two-phase natural circulation systems. The model was applied to three boiling natural circulation loops wherein Type I and Type II instabilities were observed over a wide range of operating pressures. The two-phase friction loss was predicted using different friction factor multiplier models available in literature. It was found that these models influence the steady state and threshold powers for stability, especially the Type II instabilities in natural circulation significantly. Since the void fraction depends on the drift velocity and the void distribution parameter in two-phase flow, these parameters were varied and their effects on the natural circulation flow stability were investigated. It was found that an increase in either the drift velocity or the void distribution parameter reduces the unstable regions observed in the Type I or Type II flow instabilities in two-phase natural circulation systems.Further, investigations were carried out to study the effect of loop diameter on the Type I and Type II instabilities in natural circulation. This study is important to reveal the capability of the reduced diameter scaled facilities of the prototype systems to simulate natural circulation instabilities. The results indicate that with increase in the loop diameter, the threshold power of the Type I instability and the Type II instability increases. Moreover, the stability of natural circulation greatly enhances with increase in the diameter of the loop.     

SPRR-300反应堆中子照相装置准直系统的研制

准直器是中子照相装置的关键组件之一．利用石墨为慢化剂．铋为γ过滤器。采用光阑直径、准直器长度、石墨塞长度可调以实现准直比和镉比的大幅度调节．钢结构水箱快门完成了SPRR—300准直系统设计。实验结果显示设计是合理的．成像质量达ASTM-86达二级以上．能满足不同成像技术和不同样品的中子照相需求。     

Distribution parameter and drift velocity for two-phase flow in a large diameter pipe

In view of practical importance of the drift flux model for two-phase flow analysis in general, and in the analysis of nuclear reactor transients and accidents in particular, the distribution parameter, and the drift velocity have been studied for two-phase flow in a vertical large diameter pipe. In this, study, local measurements were performed on flow parameters, such as void fraction, gas velocity and, liquid velocity in a vertical upward air–water two-phase flow in a pipe of 200 mm inner diameter and, 25 m in height by using the local sensor techniques such as hot-film probes, optical multi-sensor, probes and differential pressure gauges. Two-phase flow regimes in a vertical large diameter pipe, were classified into bubbly, churn and slug flows according to the visual observation. The values of the, distribution parameter and the mean drift velocity were determined directly by their definition using experiment data of the local flow parameters in a two-phase flow in a large diameter pipe. Various existing drift flux correlations were compared with the present experimental results and experimental data obtained by other researchers. A detailed discussion on the problems of these correlations was presented in this paper.     

垂直上升两相流漂移流模型研究

漂移流模型在两相流空泡率计算中处于非常重要的地位,长期以来,研究者对模型中分布参数及漂移速度的确定持不同的观点.本文采用理论分析与实验研究相结合的方法,总结了分布参数和漂移速度的一般规律,提出了垂直上升两相流漂移流模型分布参数和漂移速度必须满足的限制条件,根据理论分析以及实验数据的验证,得出了漂移流模型推荐关系式.     

Measurement System of Bubbly Flow Using Ultrasonic Velocity Profile Monitor and Video Data Processing Unit, (II)

The authors have developed a measurement system which is composed of an ultrasonic velocity profile monitor and a video data processing unit in order to clarify its multi-dimensional flow characteristics in bubbly flows and to offer a data base to validate numerical codes for multi-dimensional two-phase flow. In this paper, the measurement system was applied for bubbly countercurrent flows in a vertical rectangular channel. At first, both bubble and water velocity profiles and void fraction profiles in the channel were investigated statistically. Next, turbulence intensity in a continuous liquid phase was defined as a standard deviation of velocity fluctuation, and the two-phase multiplier profile of turbulence intensity in the channel was clarified as a ratio of the standard deviation of flow fluctuation in a bubbly countercurrent flow to that in a water single phase flow. Finally, the distribution parameter and drift velocity used in the drift flux model for bubbly countercurrent flows were calculated from the obtained velocity profiles of both phases and void fraction profile, and were compared with the correlation proposed for bubbly countercurrent flows.     

A void fraction correlation for generalized applications

A void fraction correlation has been developed to cover the full range of pressures, flows, void fractions, and fluid types (steamwater, air-water, hydrocarbons, and oxygen). The correlation, referred to here as the Chexal-Lellouche correlation, has been qualified against several sets of steady-state two-phase/two-component flow test data that cover a wide range of thermodynamic conditions and geometries typical of PWR and BWR fuel assemblies as well as for pipes up to 450 mm in diameter. The correlation is based on a drift flux model and determines the drift flux parameters, C_o and V_gj both cocurrent and countercurrent two-phase flows for the full range of pressures, flows and void fractions. The correlation is available as a source code module for inclusion into any thermal-hydraulic computer program, and as an interactive personal computer program. The correlation is continuous and does not depend on flow regime maps or spline fitting.     

Simultaneous measurement of void fraction and fundamental bubble parameters in subcooled flow boiling

Visualization was performed for the vapor bubbles in water subcooled flow boiling in a vertical heated tube to measure simultaneously the void fraction and the four fundamental bubble parameters: nucleation site density, bubble release frequency, bubble lifetime and bubble size. Using the mass flowrate and liquid subcooling as the experimental parameters, the changes of void fraction and bubble parameters with the wall heat flux were measured. The results of image analysis showed that the vapor void fraction could be approximated by the function of nucleation site density and bubble lift-off diameter; the bubble lift-off diameter was more influential than the nucleation site density. It was hence concluded that the bubble lift-off diameter could be regarded as the key parameter to determine the vapor void fraction under the present experimental conditions. The strong relation of bubble lift-off diameter to superheated liquid layer thickness was indicated for the future model development studies of bubble lift-off diameter.     

中国先进研究堆间接中子照相方法的初步研究

中国先进研究堆（CARR）的建成为我国间接中子照相技术的发展及应用提供了重要的实验平台。本文基于CARR开展了间接中子照相方法的初步研究。首先利用蒙特卡罗方法计算分析了基于CARR开展间接中子照相测量的条件;然后根据这些条件,通过解析计算确定了间接成像两次曝光过程的实验参数;最后利用CARR的热中子孔道,以核燃料元件模拟件为样品进行了间接中子照相实验,并研究了间接中子照相成像数据的处理及分析方法。     

Evaluation of Delayed Neutron Fraction β for Thermal Fission of U-235 Based on Integral Experiments Using SHE

Countercurrent gas-liquid flow is theoretically and experimentally evaluated for a boiling system simulating a BWR core. In a single channel, flow patterns are determined from the mass balance equations and pressure drop under steady state conditions is calculated for each flow pattern using a drift flux model, where the distribution parameter and drift velocity are correlated as functions of void fraction and hydraulic diameter from void fraction data. The calculated pressure drop shows a similar trend to that of the data for the effects of bypass leak flow rate and heater power. Countercurrent behavior in three boiling channels under slow transient conditions is also predicted from the single channel characteristics and close agreement is obtained between the predicted and experimental results. The results show that steam up-flow or cocurrent up-flow easily occurs in a channel with low pressure drop, namely, with a large entry orifice, high power or low bypass leak flow rate.     

基于可移动紧凑加速器D-T中子源的热中子层析实验研究

中子照相是十分重要的无损检测方法之一,尤其是针对含氢材料、同位素等的无损检测,中子照相技术具有其他射线成像不可比拟的优势。中国工程物理研究院核物理与化学研究所基于紧凑型D-T中子源,研发了可移动中子成像检测仪,成功实现了热中子照相和快中子照相实验检测。为确定基于该装置开展热中子层析检测的可行性,本文进行了数值模拟计算,利用该仪器开展了针对轻重材料模拟件的热中子层析成像实验,利用采集的181幅投影图像,在图像信噪较低和采集幅数较少条件下,成功重建了铝和聚乙烯材料包裹下的0.2 mm直径的钆丝。     

Television Imaging System for Fast Neutron Radiography Using Baby Cyclotron

Abstract

A television imaging system for fast neutron radiography (FNR-TV) developed using the fast neutron source reactor YAYOI was applied to the baby-cyclotron based fast neutron source to get images of thick objects quickly. In the system the same technique as a current television imaging system of thermal neutron radiography was applied, while the luminescent converter was used to detect fast neutrons.

Using the CR39 track etch method it took about 7 h to get an image, while the FNR-TV only 20 s enough for taking the same object. However the FNR-TV imaging result of the simulation model of a large explosive device for the space launch vehicle of H-2 type was not so good as the image taken with the CR39 track etch method. The reason was that the luminescence intensity of the FNR-TV converter was a quarter of that in the YAYOI.