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.     

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.     

Hydrodynamic and Hydraulic Studies on Void Fractions in Two-Phase Flow

For gas-liquid two-phase flow, two kinds of fundamental equations have been proposed by the methods of hydrodynamics and hydraulics. The hydrodynamic equation, to avoid analytical difficulties, does not deal with two-phase flow itself, but with a hypothetical liquid flow which has actual physical quantities in the region where the liquid phase really exists and has the imaginary physical quantities of liquid in the region where the gas exists, for each instant, and with a hypothetical gas flow similarly. The hydraulic equation differs from the usual form in the treatment of friction term or interaction, and has better physical insight. These equations are analyzed mathematically and some relationship between physical quantities centering void fraction are discussed. Hydrodynamic equation shows the void fraction distribution to have relation to pressure loss and velocity distributions, and, for the case of laminar two-phase flow with axial symmetry, explicit expression is presented. By hydraulic equation, the relation between void fraction and other variables are discussed. For the upward two-phase flow with constant flow area, gas-liquid interaction force is discussed theoretically and experimentally.     

矩形通道中两相流变密度模型空泡份额计算

基于Bankoff的圆管内和无限长平板间两相流变密度模型空泡份额计算式的推导，结合流体在管道中的流场分布特征，建立了矩形通道中两相流流场分布规律方程，导出了变密度模型在矩形通道中空泡份额的计算式，并对3种通道计算的结果进行了对比分析。计算结果与原有Bankoff模型符合得很好。     

Void Fraction and Pressure Drop in Liquid Metal Two-Phase Flow

This paper describes a new correlation for predicting a two-phase frictional pressure drop multiplier, and discusses the pressure level effects and the mass velocity effects. This correlation predicts satisfactorily the frictional pressure drop not only for liquid metals but also for ordinary fluid two-phase flow in a wide range of flow variables.

The authors' void fraction correlation previously proposed is also compared with published data of void fraction for liquid metal two-phase flow, and is found to represent well the mass velocity effects. Wettability and magnetohydrodynamic effects are discussed briefly in relation to the hydrodynamic characteristics of liquid metal two-phase flow.     

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.     

受热管内空泡率空间分布与流型的关系

介绍了运用RBI双探头光学探针,在加热上升管内对蒸汽-水两相流空泡率径向分布测量的情况,同时对不同实验工况下两相流流型进行了识别。根据实验结果,总结出加热上升管内空泡率径向分布与流型的关系。     

周期性力场作用下窄通道内两相流压降特性

通过实验研究了摇摆造成的周期性附加惯性力作用下矩形窄通道内空气 水两相流压降特性。按分液相雷诺数将流动分为层流区（Re_f <800）、过渡区（800≤Re_f≤1 400）及湍流区（Re_f >1 400）3个区域,并对各区域内附加压降、重位压降和摩擦压降平均值及瞬态值进行了比较。结果表明,附加惯性力对窄通道内两相流整数倍周期内平均摩擦阻力无明显影响。周期性附加惯性力作用下(摇摆周期16 s,摇摆振幅30°),层流区及过渡区气相表观速度、液相表观速度、质量含气率及摩擦压降随时间周期性波动,波动周期等于摇摆运动周期;瞬时摩擦压降相对于其平均值的波动幅值随气液两相流速的增加而减小。湍流区两相流动参数周期性波动不明显。     

基于漂移流模型的矩形通道内泡状流-弹状流转变准则

以空气和水为工质,对竖直向上矩形通道(40 mm×1.41 mm,40 mm×10 mm)两相流流型特性进行了可视化研究。气液相表观速度分别为0.01~0.59 m/s和0.02~3.72 m/s。基于3个经典的泡状流向弹状流转变准则,考虑矩形通道的尺寸效应,导出了泡状流向弹状流转变时的临界空泡份额为0.23。以窄边宽度2.5 mm为界,将矩形通道分为小通道和常规通道两类,对泡状流向弹状流转变准则进行修正,修正准则能很好地预测实验值。为进一步验证修正准则的准确性和适用性,将修正准则与Mishima、Wilmarth和Sadatomi等的实验数据进行了对比,结果显示修正准则同样具有较好的预测效果。     

Vold Fraction Correlation for Boiling and Non-Boiling Vertical Two-Phase Flows in Tube

Abstract

Applicability of previously proposed Vold fraction correlation was studied on the boiling steam-water flows in tube. The correlation was found to be applicable to the data of Sekoguchi (d=13.55 mm) and Bartolomei (d=25mm), i.e. the value of parameter K in the correlation equals to unity. The average value of K for pipes of smaller diameter (d= 6.1, 7.7 mm) was 0.57 for both adiabatic and diabatic steam-water flows. These values of k are not dependent on flow regime, heat flux and superficial water velocity within the investigated ranges. A criterion was established with a dimensionless group Eoλ which determines the K-value as: K=1.0 for Eoλ≧2×10⁶ and K=0.57 for Eoλ<2×10⁶.

A comparison between the predicted values by this method and experimental values at pressures from atmospheric to 80 kg/cm² abs., heat fluxes 0–1.5×10⁶ kcal/m²-hr, pipe diameters 5–76.2mm and gas-water volumetric flow ratios 0.06–10⁴, showed that the present correlation is adequate within ±15% of deviation.     

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.     

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 .     

Flow Behavior and Phase Distribution in Two-Phase Flow around Inverted U-Bend

Experimental results are presented on the flow behavior, phase distribution, average void fraction and slip ratio in air-water two-phase mixture flowing through an inverted U-bend. The curved test section of transparent acrylic resin tubing with 24 mm I. D. was used, comprising a riser, an inverted U-bend with radius of 96 mm, and a downcomer. From the visual observation, a flow map was derived, which indicates the relation between the flow patterns and velocities. Distributions of local void fraction along a diameter lying in the central plane of the bend and over the whole cross section of bend tube were measured, and those distribution mechanisms were explained by the effect of centrifugal and gravitational forces. The average void fraction around the bend was obtained by numerical integration of the measured local void fraction, and the slip ratio was calculated using the average void fraction.     

内螺纹管中汽-液两相流体摩擦压降特性研究

在压力为9～22MPa,质量流速为600～1200kg/(m2·s),含汽率为0～1的工况范围内,对Φ38.1mm×7.5mm的6头内螺纹管中汽-液两相流体的摩擦压降特性进行了试验研究。试验段采用水平绝热布置。试验结果表明:压力对两相流摩擦压降的影响很大,随压力增加,两相流摩擦压降倍率减小,在临界压力附近,两相流摩擦压降倍率趋近于1;随含汽率增加,两相流摩擦压降倍率先增加,然后有减小的趋势;随质量流速增加,两相流摩擦压降倍率减小。用于计算单相水摩擦压降系数以及用于计算汽-液两相流体摩擦压降的试验关联式被提供。     

窄矩形通道内两相流动压降特性研究

以空气和水为实验工质,分别在40mm×1.6mm和40mm×3mm的矩形通道中对竖直向上气-液两相流动阻力特性进行了实验研究。该研究还对比了现有的两相流动阻力计算关系式,结果表明,对于窄缝为1.6mm的通道,传统的阻力计算关系式均不适用;而窄缝为3mm的通道,除Friedel模型和Tran模型外,其余模型与实验值符合较好。为此结合实验数据,以分液相雷诺数为依据将流动分为层流区、过渡区和湍流区3个区域,分别对Chisholm关系式进行修正,结果表明：C为当量直径的线性函数,当量直径越大,C越小。修正关系式与实验数据的误差较小,能很好地预测本次实验结果。     

Measurement of void fraction in bubbly-slug flow with a constant electric current method

In several void fraction measurement methods, a constant electric current method which is one of conductance methods is focused in the present study. By using this method, void fraction can be measured with higher temporal resolution. However, it has been mainly applied to annular flow in previous studies. In the present study, Maxwell's estimation, Bruggemann's estimation, low void fraction approximation and new estimations which consider the bubble shape are applied in order to measure more accurately void fraction of dispersed bubbly flow and slug flow. To understand the effect of bubble shapes and flow patterns, void fraction was measured by the constant electric current method for a rising single spherical bubble and a rising single slug bubble without a forced convection. In addition, void fraction was also measured in bubbly flow and bubbly-slug flow with a forced convection. Then, effects of flow patterns on the proposed estimations of void fraction and the accuracy of their estimations were discussed with the measurement results. From the result, the new estimations which consider a bubble shape are more accurate than the previous estimation in a slug bubble and bubbly-slug flow.     

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.     

Void fraction distributions of inert gas jets across a single cylinder with non-wetting surface in liquid sodium

Little work on the void fraction behaviors along structural materials with poor-wettability for liquid metals has been performed. In the present study, void fraction behaviors around a single cylinder with non-wetting surface condition were quantitatively discussed by using a gas jet–cylinder system where the impinging jet flow, the boundary layer flow, the separation flow, and the wake flow appear. One cylinder with a non-wetting surface and two cylinders with a wetting surface were used to vary the wettability for liquid sodium, and void fraction distributions were measured around the cylinders. In the case of wetting condition, void fraction distributions around the cylinder decrease clearly in the backward region of the cylinder, and liquid-rich region is formed due to bubble separation from the cylinder surface. On the other hand, under non-wetting condition, because of two-phase flow without bubble separation on the cylinder surface, void fraction distributions show almost steady values around the cylinder compared to those with wetting surface. The void behaviors on a non-wetting surface were also confirmed by a visualization experiment conducted in water. The observed differences can be basically attributed to the work of adhesion required for liquid–solid interfacial separation.