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.     

水平管内油气水三相流动摩擦阻力压降特性研究

对水平管内油气水三相流动的摩擦压降特性进行系统的理论和实验研究 ,建立了泡状流和环状流摩擦压力降的理论模型 ,揭示了摩擦压力降随折算气速、折算液速、油水混合物含水率及管径的变化规律。实验结果与理论计算结果相吻合。     

矩形截面螺旋通道内气液两相流局部含气率分布实验研究

应用电导探针测量技术,对矩形截面螺旋通道内气液两相流局部含气率进行实验研究。在不同的气相折算速度下,应用电导探针测量了弹状流弹单元的长度,并与可视化方法进行对比,验证了电导探针的可靠性,并为信号处理选择合适的阈值。分别在泡状流、弹状流及环状流三种流型的条件下,分析了气相与液相折算速度对局部含气率分布的影响。实验结果发现,螺旋通道气液两相局部含气率呈非对称的抛物线形分布,这种非对称性受流型和液相折算速度的影响。     

扰流型喷枪顶吹管内特殊两相流流型实验

主要对自主设计的渐缩管、渐扩渐缩管、螺旋管和四孔管进行水-空气两相流混合顶吹实验,并得出了扰流型喷管的管内流型变化规律。实验通过对可视化特殊喷管内的气液两相进行高速拍摄,并调节水与空气两相各自的体积流量,获取不同喷管中出现的特殊流型照片及视频。实验结果表明：在表观气速和表观液速变化时,除渐缩管外,其他特殊喷管的流型转变均有一定规律性;渐扩渐缩管内截面半径变化较大,易产生环状-搅拌流,并有典型泡状流出现,螺旋管由于轴向环流速度的影响,会产生大密度泡状流并逐渐过渡到有旋流趋势的环状流型,特殊结构的四孔管中流型较稳定,短暂出现泡状流、弹状流后形成稳定环状流。四孔管的设计最利于冶金熔炉中柴油-氮气混合两相流喷吹,形成的气泡群中单个气泡直径较小,柴油被充分细化打散,渣层中的还原反应更充分,能有效提高柴油对渣层中磁性铁的还原率。     

Viscosity and rheological behavior of microbubbles in capillary tubes

The viscosity of microbubbles has been measured in capillary tubes. Experiments were conducted in tubes of different diameters and lengths, with a constant microbubble concentration. The effects of bubble void fraction and size distribution on the viscosity of microbubbles were also investigated. Microbubbles demonstrate shear‐thinning non‐Newtonian behavior. The viscosity of microbubbles decreases with a decrease in tube diameter and bubble void fraction, and with an increase in tube length. Although viscosity changes with tube dimensions, the flow index (n^′) is only influenced by the microbubble void fraction. It is also found that bubble size distribution in the range (1–12 µm) used in this study does not affect the viscosity of microbubbles. The data were then used to develop a correlation to predict viscosity of microbubbles, which represents the experimental viscosity data with an absolute average relative deviation less than 1.3%. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2660–2669, 2014     

Hydrodynamic behavior of single particles in a draft-tube bubble column

Velocities of single particles (magnetic tracer particles) were measured in the draft tube region and annular region of a draft-tube bubble columns by use of the magnet-detector coil technique. The relative particle to liquid velocity was also obtained. The observed distribution of particle velocity was broader than that of liquid velocity. The relative particle to liquid velocity depended significantly on superficial gas velocity. Those for the draft tube region had minimums with increasing gas velocity, while those for the annular region decreased monotonously. The effective drag coefficients for the particles were correlated with particle Reynolds number and gas holdup.     

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.     

Void fraction measurement and calculation model of vertical upward co-current air–water slug flow

The focus of this paper is on the measurement and calculation model of void fraction for the vertical upward co-current air–water slug flow in a circular tube of 15 mm inner diameter. High-speed photography and optical probes were utilized, with water superficial velocity ranging from 0.089 to 0.65 m·s^-1 and gas superficial velocity ranging from 0.049 to 0.65 m·s^-1. A new void fraction model based on the local parameters was proposed, disposing the slug flow as a combination of Taylor bubbles and liquid slugs. In the Taylor bubble region, correction factors of liquid film thickness C_δ and nose shape C_Z^* were proposed to calculate α_TB. In the liquid slug region, the radial void fraction distribution profiles were obtained to calculate α_LS, by employing the image processing technique based on supervised machine learning. Results showed that the void fraction proportion in Taylor bubbles occupied crucial contribution to the overall void fraction. Multiple types of void fraction predictive correlations were assessed using the present data. The performance of the Schmidt model was optimal, while some models for slug flow performed not outstanding. Additionally, a predictive correlation was correlated between the central local void fraction and the cross-sectional averaged void fraction, as a straightforward form of the void fraction calculation model. The predictive correlation showed a good agreement with the present experimental data, as well as the data of Olerni et al., indicating that the new model was effective and applicable under the slug flow conditions.     

内置水平管式鼓泡流化床动力学的数值模拟

基于欧拉双流体模型,在水平管表面采用渐进式网格,对内置水平管式鼓泡流化床的动力学特性进行数值模拟,研究表观气速对时均空隙度与膨胀率的影响。模拟结果显示:气泡在水平管排区域存在合并、破裂等运动行为;在管排区域随着表观气速的增加,时均空隙度增大,并且相比于无管式流化床沿径向方向更加均匀;内置水平管式流化床的时均膨胀率高于无沉浸管式流化床的时均膨胀率,且随表观气速的增加有逐渐接近的趋势。     

流体物性对壳侧不互溶双组分两相流动特性的影响

选用空气-水和空气-柴油2种不互溶双组分两相混合物物系,实验研究了液相物性对管壳式换热器壳侧两相流流型及其转变和两相流动特性的影响.研究表明,液相物性对间歇流向环状流的转变有显著影响,对于向泡状流转变的影响则不如在管内流动明显.研究还表明,液相物性特别是粘度和表面张力对截面含气率和两相摩擦压降有显著影响.     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

Effect of tube diameter on two‐phase flow patterns in mini tubes

The effect of tube diameter on two‐phase flow patterns was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6, and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01–50 and 0.01–3 m/s, respectively. The gas and liquid flow rates were measured and the two‐phase flow pattern images were recorded using high‐speed CMOS camera. The flow patterns observed were dispersed bubbly, bubbly, slug, slug‐annular, wavy‐annular, stratified, and annular flows. These flow patterns were not observed in all the test diameters, but were found to be unique to particular tube diameters, confirming the effect of tube diameter on the flow pattern. The data obtained were compared to existing experimental data and flow regime transition maps which show generally reasonable overall agreement at the larger diameters, but significant differences were observed with the smaller diameter tubes.     

Experimental determination of the drag coefficient in a swarm of bubbles

Simultaneous measurements of liquid velocity by laser Doppler velocimetry and bubble velocity, diameter, and void fraction by a double optical probe are performed in a bubble column to study the influence of the void fraction on the relative velocity of a swarm of gas bubbles. Bubble diameters d_b vary from 2 to 10 mm and local void fractions α_loc can reach 35%. It is found that, for α_loc<15%, the relative bubble velocity is determined by the hindrance effect and consequently decreases with the void fraction. Beyond this critical value, the aspiration of bubbles in the wake of the leading ones dominates the hindrance effect and the relative velocity thus increases suddenly. The contribution of the bubble diameters to this evolution is also determined. Finally, a drag correlation, valid for the whole range of void fraction and for pure water-air systems, is proposed.     

Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank☆

Vertical distributions of local void fraction and bubble size in alr–water dispersion system were measured with a dual conductivity probe in a fully baffled dished base stirred vessel with the diameter T of 0.48 m, holding 0.134 m3 liquid. The impel er combination with a six parabolic blade disk turbine below two down-pumping hy-drofoil propel ers, identified as PDT+2CBY, was used in this study. The effects of the impel er diameter D, rang-ing from 0.30T to 0.40T (corresponding to D/T from 0.30 to 0.40), on the local void fraction and bubble size were investigated by both experimental and CFD simulation methods. At low superficial gas velocity VS of 0.0077 m·s?1, there is no obvious difference in the local void fraction distribution for al systems with different D/T. However, at high superficial gas velocity, the system with a D/T of 0.30 leads to higher local void fraction than systems with other D/T. There is no significant variation in the axial distribution of the Sauter mean bubble size for al the systems with different D/T at the same gas superficial velocity. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of the impel er diameter on the gas–liquid flows. The local void fraction predicted by the numerical simulation approach was in reasonable agreement with the experimental data.     

Tomographic imaging of counter-current bubbly flow by wire mesh tomography

The objective of this work is to investigate characteristics of counter-current bubbly flow in a circular pipe with an inner diameter of 50 mm by using wire mesh tomography (WMT). The accuracy of WMT on void fraction measurement is also clarified by comparing the result with a non-intrusive optical method. The accuracy is within ±10%. Local void fractions of many flow conditions are reported. Local void fraction profile affected by superficial liquid velocity and bubble size is shown and discussed. Furthermore, intrusive effects, including bubble break-up and bubble deceleration, are also investigated. Bubbles passing a transparent wire mesh sensor (WMS) are investigated by the optical method. It is shown that bubbles are broken and decelerated by wires of the sensor. It can be concluded that the bubble break-up rate increase with increasing of bubble velocity. However, the bubble deceleration is not depending on the bubble velocity.     

Void fraction in vertical gas-liquid slug flow: Influence of liquid slug content

In this study we develop a model for computing the mean void fraction and the liquid slug void fraction in vertical upward gas-liquid intermittent flow. A new model for the rate of gas entrained from the Taylor bubble to the liquid slug is formulated. It uses the work done by the pressure force at the rear of the Taylor bubble. Then an iterative approach is employed for equating the gas entrainment flux and the gas flux obtained via conservation equations. Model predictions are compared with experimental data. The developed iterative method is found to provide reasonable quantitative predictions of the entrainment flux and of the void fraction at low and moderate liquid slug void fraction conditions. However, with an increased liquid slug void fraction experimental data indicate that the flow in the liquid slug transits to churn-heterogeneous bubbly flow thus gas entrainment flux tends to zero. Considering this effect in the iterative model significantly improved the predictions for large liquid slug void fraction conditions.     

Enhancement in gas holdup in bubble columns through use of vibrating internals

Including internals in bubble columns is known to enhance the gas holdup. In this paper, a method to achieve this objective substantially has been proposed via the use of vibrating helical spring internals. Experimental observations on effect of vibrating internals such as vibrating helical springs on gas holdup in bubble columns are presented. Effects of superficial gas velocity, H/D ratio (height of the static liquid to column diameter ratio), volume fraction of helical springs, and thickness of the helical spring wires on hydrodynamics parameters are studied. Increase in gas holdup up to 135% is observed by using vibrating helical spring internals in bubble columns compared to bubble columns without internals. This method offers a simple, cost‐effective, and easy way to enhance gas holdup even at high gas fluxes. It has been reported that this enhancement stems from the fact that the vibrating springs breakup the gas into fine bubbles, which effectively reduces their rise velocity and enhances their average residence time in the liquid column.     

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.     

DENSE PHASE CIRCULATING FLUIDISED BEDS

A central draught tube in a Ruidised bed can be used to promote solids circulation and prevent the formation of stagnant regions. The dominant driving force for circulation is the difference in voidage between the draught tube and annulus, caused by supplying a larger superficial gas velocity to the base of the draught tube than to the base of the annulus. A theoretical model has been developed, taking into account the effects of shear stress, bubble growth in the draught tube, and the flow of solids from the annular downcomer to the central draught tube. Comparison with experimental work, the results of earlier workers, and unpublished work by British Gas, shows satisfactory agreement over a wide range of bed size with a variety of particles.     

基于截面气含率的文丘里湿气压降模型

运用两相流理论对湿气中的气相与液相流动进行分析,在分层流与环雾状流的条件下推导了两相流通过水平标准文丘里流量计的理论模型.通过分别考虑截面气含率、相间摩擦以及液滴夹带等因素,在文丘里轴向对湿气流动中的气相动量方程进行求解.通过对水平直管中的截面气含率公式进行修正,建立了适用于收缩管道的截面气含率模型,并在此基础上模拟了湿气流经标准文丘里时其两个取压孔之间的轴向静压分布.实验证实,使用修正后的截面气含率公式将使模型对文丘里压降的预测准确度明显提升,其相对误差在15%以内.该模型以湿气两相流在水平文丘里中的流动形态为依据,具有较充分的物理背景,而且在推导过程中较少依赖特定实验装置与数据,为建立具有一定普适性的文丘里湿气计量模型奠定了基础.