深宽比对微通道内气液二相流空隙率的影响

微通道内气液二相流的空隙率对管路内热质传递具有重要影响。利用高速摄像仪对4种不同深宽比的矩形截面微通道内空气-水二相流的流型进行实时观测和记录,对二相流型图进行了分析和计算,得到了不同流型时的空隙率。4种微通道尺寸(深度×宽度)分别为100μm×200μm,100μm×400μm,100μm×800μm,100μm×2 000μm。结果表明:对于弹状流,空隙率随着微通道截面深宽比的减小而降低,当流型为液环流时,空隙率随着微通道截面深宽比的减小而增加。在实验基础上,将截面深宽比作为参数提出了一个新的空隙率预测模型,计算结果显示:模型能对矩形截面微通道内气液二相流动空隙率进行很好的预测。     

Investigating the effect of pressure on a vertical two-phase upward flow with a high viscosity liquid

This article presents void fraction and pressure gradient data for sulfur hexafluoride (SF₆) with gas densities of 28 and 45 kg/m³ and oil (with viscosity 35 times that for water) in a 127 mm diameter pipe. The superficial velocities of gas ranged from 0.1 to 3 m/s and those for liquid from 0.1 to 1 m/s, respectively. Measurements of void fraction data were recorded using a capacitance wire mesh sensor (WMS) system, which permits the 3D visualization of the flow patterns. All the data were obtained with a data acquisition frequency of 1,000 Hz. A differential pressure transducer was used to measure the pressure drops along the length of the pipe. The WMS provide time and cross-sectionally resolved data on void fraction and from an analysis of its output, flow patterns were identified using the characteristic signatures of probability density function (PDF) plot of time series of void fraction. The PDF plots showed the single peak shapes associated with bubbly and churn flows but not the twin-peaked shape usually seen in slug flows. This confirms previous work in larger diameter pipes but with less viscous liquids. For the bubble and churn flows investigated, the pressure gradient was observed to decrease with an increase in gas superficial velocity. Nevertheless, there was an insignificant observed effect of pressure on void fraction below certain transitional flow rates, the effect however became significant beyond these values. In the present work, wisps appear to be smaller, which might be due to the different fluid properties of the working fluids employed. In addition, wisps are easily revealed as long as there is a transition between churn and annular flows regardless of the pressure. Experimental data on void fraction and pressure gradient are compared against existing data. Reasonably good agreements were observed from the results of the comparison.     

Characterization of slurry flow regime transitions by ERT

Slurry flows in a pipe have been visualized using electrical resistance tomography (ERT). Complementary to image reconstruction technique, a strategy based on the direct interpretation of the raw ERT measurements has been developed to analyze slurry flow regimes in a horizontal pipe. Experiments have been conducted using a lab-scale pipe loop equipped with a commercial ERT device. The images of slurry concentration have been reconstructed using an iterative Landweber method and compared to a direct linear back-projection (LBP) method available in the ERT software. The slurry concentration has been estimated and the homogeneity of the concentration distribution has been assessed directly from resistance measurements and a simple resistance model. It is shown that this procedure, combining the image reconstruction and the direct interpretation of ERT measurements, allows analyzing the slurry flow regimes and transitions for slurry concentrations up to 20% v/v and velocities up to 2.2 m/s.     

水平管道气液两相流中空隙率对动态压降信号的影响

引　言在化工、石油及动力等工程中 ,气液两相流动现象极为普遍 ,其研究得到了人们的广泛重视 .气液两相流动中的管道平均压降与其他流动条件及参数之间的关系已有较多的研究 ,得到了大量数学模型和经验公式可资工程设计应用[1,2 ],但对管道压降动态特性的研究还不多见 .而对于两相流系统安全性能的设计和运行状态监控等方面 ,管道压降的动态特性无疑是极为重要的参数 .另一方面 ,气液两相流动体系中的空隙率是表示气相浓度 (含气率 )的常用指标之一 ,它对确定气液两相流系统的流型、气液分相流量以及管道中的摩擦压降、重力压降和惯性压降…     

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h⁻¹ (0–6.10⁻³ m s⁻¹). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.     

微通道内伴有化学吸收气液两相流的空隙率与压力降

微通道内气液两相流空隙率与压力降对微反应器的热质传递性能有显著影响,是微反应器的重要设计参数。采用高速摄像仪和压力测量系统分别对矩形微通道内单乙醇胺水溶液化学吸收CO₂过程的空隙率和压力降进行了研究,考察了弹状流下气液两相流量与化学反应速率对空隙率及压力降的影响。结果表明:当液相流量一定时,微通道内空隙率和压力降均随着气相流量的增大而增大,空隙率随化学反应速率的增大而减小,压力降随化学反应速率的增大而增大;当气相流量一定时,随着液相流量和化学反应速率的上升,微通道内空隙率下降,而压力降上升。提出了微通道内伴有化学吸收的空隙率和压力降的半理论预测模型,模型平均误差分别为15.79%和11.12%,显示了良好的预测性能。     

Analysis of gas holdup in bubble columns with non-Newtonian fluid using electrical resistance tomography and dynamic gas disengagement technique

This paper presents an experimental analysis of the influence of the liquid rheology on the gas flow pattern in a bubble column reactor. Aqueous solutions of xanthan are selected as an example of non-Newtonian shear thinning fluid. Averaged gas holdup is determined by two experimental techniques: parietal pressure probes and electrical resistance tomography (ERT). ERT is also used to provide 2D images of the gas phase distribution in a column cross-section. Bubble size distributions are evaluated by a gas disengagement technique using the parietal pressure probes. All these techniques clearly show the gas flow pattern is different in Newtonian and non-Newtonian fluids. Gas holdup values decrease when increasing the liquid viscosity and reach a minimum or a plateau. Homogeneous flow regime, observed in water at low gas velocities, tends to disappear when viscosity increases. This evolution is visualized by a much less isotropic distribution of the gas phase within cross-section of the column and by the appearance of much larger bubbles due to an increased coalescence phenomenon.     

Gas holdup in pulp fibre suspensions: Gas voidage profiles in a batch-operated sparged tower

Gas holdup in a semi-batch operated slurry (pulp fibre suspension) bubble column was investigated for two pulp types (softwood and hardwood kraft pulps) over a range of suspension mass concentrations (C_m=0–9% by mass) and superficial gas velocities (U_g=0.0027–0.027 m/s). Three techniques were used: height difference between gassed and ungassed operation; pressure difference as a function column height; and electrical resistance tomography (ERT). Depending on the technique used the average, axial and radial holdup profiles could be determined. In the pulp suspensions, the ERT determined gas holdups correlated well with those determined using the differential height method. In water, the ERT determined gas holdups were significantly lower, but the agreement was significantly improved by increasing the background conductivity by adding 1 g/L salt to the water. This, however, reduced the overall gas holdup due to the effect of the electrolyte on bubble coalescence. Other differences between the three measurement techniques were attributed to limitations in the detection methods and the averaging procedures used to compare results.The presence of pulp fibres reduced gas-holdup at all gas flow rates and suspension concentrations studied and is attributed to increased bubble coalescence which increases bubble size and consequently bubble rise velocity through the suspension. Gas holdup (as determined by ERT) increased with column height. The radial gas profiles were non-uniform and more peaked than the corresponding water profiles. At low suspension concentrations this was attributed to asymmetric suspension recirculation within the column. As suspension concentration increased, channels formed in the suspension with the average void fraction leveling off to a plateau.     

Destabilisation of homogeneous bubbly flow in an annular gap bubble column

Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102 m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014–0.200 m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift‐flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity.     

An experimental study of gas void fraction in dilute alcohol solutions in annular gap bubble columns using a four-point conductivity probe

The influence of alcohol concentration on the gas void fraction in open tube and annular gap bubble columns has been investigated using a vertical column with an internal diameter of 0.102 m, containing a range of concentric inner tubes, which formed an annular gap; the inner tubes had diameter ratios from 0.25 to 0.69. Gas (air) superficial velocities in the range 0.014–0.200 m/s were investigated. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8–300 ppm by mass, were used as the working liquids. Radial profiles of the local void fraction were obtained using a four-point conductivity probe and were cross-sectionally averaged to give mean values that were within 12% of the volume-averaged gas void fractions obtained from changes in the aerated level. The presence of alcohol inhibited the coalescence between the bubbles and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition.     

加压气液鼓泡塔的CFD数值模拟与ERT实验验证

在内径0.3 m,高6.6 m的加压鼓泡塔内,采用计算流体力学(CFD)数值模拟与气泡群平衡模型(PBM)耦合法进行塔内流体力学模拟,并将数值模拟结果与基于电阻层析成像技术(ERT)的实验结果对比分析,将通过ERT实时采集的横截面气含率分布和时间序列图与模拟结果进行比较。结果表明:ERT技术测量结果与CFD计算结果吻合良好,能很好地表示鼓泡塔内气液流动状态,进一步表明ERT技术对加压鼓泡塔内气液两相流进行可视化与实时测量是可行的。     

Void characteristics in turbulent fluidized beds

Void properties (size, rising velocity) in the turbulent flow regime have been determined in a 0.1 m-ID X 3.0 m high Plexiglas column of glass beads (d_p = 0.362, mm) by using an optical fiber probe system. The bubble size increases with an increase in gas velocity in the slugging flow regime but it sharply decreases in the turbulent flow regime. The mean amplitude of pressure fluctuations is linearly related to the bubble or void size in the bed. The void rising velocity is almost constant in the turbulent flow regime. Uniform condition of the bed structure in the turbulent flow regime can be determined from the void distribution coefficient in the bed. In addition, the bed condition in the turbulent How regime has been evaluated from the variations of the void velocity coefficient and the propulsive power of a rising void with gas velocity.     

无泄漏的管壳式换热器壳侧局部含气率的研究

在常温常压下 ,应用电导探针对无泄漏的TEMA -E型管壳式换热器壳侧气液两相流的局部含气率进行了测量 ,研究发现在壳侧错流区折流板的迎流面和背流面附近 ,气液分布有着明显的差异 ,而且两者均不等同于错流区中心区域的气液分布。通过对局部含气率的加权体积积分得到了体积平均含气率 ,并采用马蒂内利参数及全液相佛鲁德数进行关联 ,得到的关联式与实验数据较吻合     

Adiabatic two-phase flow in narrow channels between two flat plates

Adiabatic cocurrent flow of air and water through a narrow passage between two flat plates 240 mm long and 99mm wide with gap-widths of 0.778 mm and 1.465 mm was investigated for six different orientations: Vertically upward and downward, 45^° inclined upward and downward, and horizontal flows between horizontal plates and between vertical plates. Except for horizontal flow between vertical plates, the effects of gap width and flow channel orientation on flow pattern, void fraction and friction pressure drop were found to be small in narrow channels. The void fraction and two-phase friction multiplier data could also be reasonably correlated in terms of the Martinelli parameter. For horizontal flow between vertical plates, both the void fraction and friction multiplier data showed strong mass velocity effects. Several friction pressure drop correlations were tested for applicability to the narrow channels including a separated flow model proposed in this work.     

垂直同心环形管内上升气液两相环状流含气率与压降预测

对垂直同心环形管内上升气液环状流的截面含气率及压降预测进行了研究.根据Kelessidis有关团状流向环状流转换的思路以及Wallis有关圆管内环状流积分分析的方法,考虑环形管的结构特征及环状流的流型特征,通过两相动力学理论建立了新的预测截面含气率及总压降的模型.将新模型及现有模型与实验数据的分析比较表明新模型对于总压降的预测效果较好.     

基于空隙率的油气两相流流量测量的研究

利用文丘里管进行油气两相流流量测量的研究,在考虑了气液滑移比对流量测量的影响,对均相流流量测量模型进行了修正,得到了空隙率和文丘里管两端差压与两相流总质量流量及液相质量流量的关系式。实验结果表明利用这两个关系式,在干度小于2％,空隙率在15%～83％范围内,总质量流量测量和液相质量流量测量的均方根误差均小于5％。针对不同的流型,对关系式中的系数作了修正计算,进一步降低了测量误差。     

CFD modeling and validation for two-phase medium viscosity oil-air flow in horizontal pipes

This study presents the results of a Computational Fluid Dynamics (CFD) simulation of two-phase medium viscosity oil-air flow in a 50.8?mm internal diameter horizontal pipe. Void fraction and pressure gradient predictions were validated using experimental data for four different oil viscosities (0.039, 0.06, 0.108 and 0.166?Pa s) and different flow rates varying from 0.1 to 2.9?m/s for the gas phase and from 0.01 to 2.95?m/s for the liquid phase, where four flow patterns were predicted (stratified, dispersed bubble, bubble elongated and slug flow). The obtained results of void fraction and pressure gradient presented a mean relative error of 30.04 and 21.38%, respectively. Furthermore, the CFD results were compared against 66 empirical correlations and predictions from OLGA. It was found that between the three studied methods (CFD, OLGA and empirical correlations) the CFD model outperformed the other two methods regarding the predicted flow patterns, pressure gradients and void fractions on most cases.     

Void Fraction Measurement for Two‐Phase Flow Using Electrical Resistance Tomography

Measurement of void fraction of two‐phase flows remains a challenging area. In this paper the application of an electrical resistance tomography (ERT) system for this purpose has been studied. A new approach through the direct use of the voltage data measured by the ERT system is presented. The measured voltage data are first compressed through a feature extraction, and a polynomial regression procedure is followed to obtain the relationship between the void fraction and the feature extracted. Both simulation and experiment are carried out to verify the approach. The methodology of the new approach, simulation and experimental results are presented in the paper.     

Gas-solid trickle flow hydrodynamics in a packed column

The pressure gradient and the static and the dynamic hold-up have been measured for a system consisting of a Fluid Cracking Catalyst (FCC) of 30–150 × 10⁻⁶ m diameter, trickling over a packed bed and with a gas streaming in countercurrent flow. The experiments were carried out at ambient conditions using a glass column of 25 × 10⁻³ m diameter. The packing material consisted of 8 × 8 × 3 mm ceramic Raschig rings, a mixture of 7 × 7 × 1 mm glass Raschig rings and 5 × 5 mm catalyst pellets and of stacked Kerapak ceramic mixing units of Sulzer, each unit 50 mm long and 25 mm in diameter. Four different gases have been tested. A correlation for the pressure gradient in the preloading region is derived based on the Ergun equation and taking into account the internal gas recirculation due to the solids trickles. The void fraction of the trickles is found to be independent of the physical properties of the gas phase. The behaviour of the GSTF-system in the preloading regime and the phenomena of loading and flooding are discussed. A correlation is given which relates the boundary between preloading and loading with the particle and gas properties and the solids flow rate.