Low concentration sand transport in multiphase viscous horizontal pipes: An experimental study and modeling guideline

Low concentration particle transport in multiphase horizontal pipes in the presence of a viscous liquid is experimentally investigated. The experiments were conducted for a wide range of liquid and gas flow rates in both intermittent and stratified flows. Critical flow rates (velocity) is defined as the minimum required liquid and gas flow rates (velocities) to keep particles constantly moving in the pipe. The effects of physical parameters such as sand concentration, sand size, pipe size, and liquid viscosity are also experimentally investigated. It is observed that that critical velocity is a function of sand concentration and sand size and increases by increasing either within the ranges of particle concentrations and sizes examined. Regarding the effect of carrier liquid viscosity, the experimental data reveal that by increasing viscosity the minimum required flow rates to constantly move sand along the pipe increases within the range examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1821–1833, 2016     

CFD–DEM modeling of gas fluidization of fine ellipsoidal particles

Particle characteristics are important factors affecting gas fluidization. In this work, the effects of both particle size and shape on fluidization in different flow regimes are studied using the combined computational fluid dynamic–discrete element method approach. The results are first analyzed in terms of flow patterns and fluidization parameters such as pressure drop, minimum fluidization, and bubbling velocities. The results show that with particle size decreasing, agglomerates can be formed for fine ellipsoidal particles. In particular, “chain phenomenon,” a special agglomerate phenomenon exists in expanded and fluidized beds for fine prolate particles, which is caused by the van der Waals force. The minimum fluidization velocity increases exponentially with the increase of particle size, and for a given size, it shows a “W” shape with aspect ratio. A correlation is established to describe the dependence of minimum fluidization velocity on particle size and shape. Ellipsoids have much higher minimum bubbling velocities and fluidization index than spheres. © 2015 American Institute of Chemical Engineers AIChE J, 62: 62–77, 2016     

利用一维波和段塞稳定性模型预测分层流向段塞流的转变

利用气液二相流一维波模型和段塞稳定性模型,对直径2.54 cm水平管内空气-水二相流出现段塞流时的各相临界表观速度和临界液层高度进行了理论预测。计算中发现,2种模型分别适用于不同的流速区域,在较低的气相流速下,一维波模型的预测结果比较理想,但是在较高的气速条件下不太适合,而利用段塞稳定性模型可以较好地获得高流速下分层流向段塞流的流型转变条件。因此,结合这2种模型对发生流型转变时的临界参数作了分析,并且应用于40 mm和50 mm水平管道的油气二相流实验。将理论计算的结果和实验测得的流型数据进行了对比,并且对影响流型的管径、流速等因素作了分析,结果表明计算得到的特征参数和实验数据比较吻合。     

Flow regimes in a gas–liquid–solid three-phase moving bed

The gas–liquid–solid three-phase moving beds could supply a potential solution for multiphase reactions with catalyst easily deactivated, and the flow regimes in it were studied by optical method and pressure drop measurement. Results showed that taking the trickle flow as the initial flow regime, the flow channels were more obvious as the particle velocity increased. When the initial flow regimes were pulse flow and bubble flow respectively, the pulse-to-trickle and bubble-to-pulse flow transitions mainly occurred at moderate-to-high particle velocities (0.01–0.04 m s⁻¹ under conditions used in this work). Moreover, the flow regime map in the three-phase moving bed was constructed and shown that the region of trickle flow increased and the region of bubble flow decreased. Finally, the application of three-phase moving beds was discussed, and it could be suitable for those reactions, which had to operate in the pulse flow, bubble flow, and transition zone.     

湿气管道积液临界气速预测的新模型

气田开发经常采用湿气集输方案。针对湿气输送管道出现的积液问题,基于分层流最小界面剪切应力准则,利用气液平界面分层流液膜区的速度分布规律,建立了求解积液临界气速的新机理模型。由分层流液膜区的流场描述和气相动量方程得到气液界面剪切应力的表达式;利用界面剪切应力函数曲线特性,可以通过界面剪切应力关于持液率求导获得临界气速。以不同文献中收集的临界气速实验数据,对新模型和其他具有代表性的湿气管道积液模型进行验证对比,表明新模型的预测精度要优于其他模型。     

A prediction model for the critical gas velocity and pressure gradient of continuous liquid-carrying in inclined pipes

A generic model for the prediction of critical gas velocity and pressure gradient in slightly inclined pipes (β ≤ 6°) is presented in this article. The gas–liquid configuration was determined based on the minimum energy principle and consideration of wettability and surface tension. A visualization experiment was conducted to obtain the critical gas velocity and the critical pressure gradient of a gas–liquid flow through the 40 and 60-mm pipe diameter. The theoretical study shows that the configuration is close to a convex interface shape at the critical conditions, which is in accord with the experimental phenomenon. Experimental study shows interfacial waves are the main cause of increased interfacial friction factor and a linear functional relationship between the inclination angle and the flow correction factor f(β). The results demonstrate that the new model is capable of providing satisfactory prediction results for the critical gas velocity, pressure drop, and liquid holdup.     

Velocity of isolated particles along a pipe in stratified gas–liquid flow

The average velocity of isolated grains of sand was experimentally measured in smooth stratified flow in slightly declined pipes. Isolated particles in smooth stratified flow behave similarly to isolated particles propelled by both hydraulic conveying and intermittent gas/liquid flow. In all three cases, particle velocity is linear with respect to the average liquid velocity of the flow (or the average fluid velocity in the slug body for intermittent flow) and has a gradient of approximately one. The data in stratified flow are successfully correlated dimensionlessly (Eq. 7). The correlation is extrapolated to zero particle velocity to estimate the conditions required to ensure sand transport in a flowline in smooth stratified flow. The experimental results suggest that particle velocity is strongly governed by the size of a particle relative to the depth of the viscous sublayer at the pipe wall. If a particle is larger than the viscous sublayer, it is exposed to more coherent turbulent structures and therefore experiences a greater drag.     

Micromechanic modeling and analysis of the flow regimes in horizontal pneumatic conveying

Pneumatic conveying is an important technology for industries to transport bulk materials from one location to another. Different flow regimes have been observed in such transportation processes, but the underlying fundamentals are not clear. This article presents a three‐dimensional (3‐D) numerical study of horizontal pneumatic conveying by a combined approach of discrete element model for particles and computational fluid dynamics for gas. This particle scale, micromechanic approach is verified by comparing the calculated and measured results in terms of particle flow pattern and gas pressure drop. It is shown that flow regimes usually encountered in horizontal pneumatic conveying, including slug flow, stratified flow, dispersed flow and transition flow between slug flow and stratified flow, and the corresponding phase diagram can be reproduced. The forces governing the behavior of particles, such as the particle–particle, particle‐fluid and particle‐wall forces, are then analyzed in detail. It is shown that the roles of these forces vary with flow regimes. A general phase diagram in terms of these forces is proposed to describe the flow regimes in horizontal pneumatic conveying. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

水平管内气液段塞流液塞长度发展规律的实验和模拟研究

In petroleum industry, the slug flow is a fre-quently encountered flow regime in multiphase flowpipeline. For pipeline designers, the liquid slug lengthdistribution is important for the proper design ofdownstream facilities, such as slug catcher and sepa-ration system. However, for its transient and unsteadynature, it is a great challenge for engineers to correctlypredict the flow parameters of slug flow, especiallythe maximum liquid slug length. The unit cell model for slug flow in horizontal…     

基于颗粒流运动论的喷动流化床气固流动行为三维模拟

1 INTRODUCTION Spout-fluid beds have been of increasing interest in the petrochemical, chemical and metallurgic indus-tries since spout-fluid beds can reduce some of the limitations of both spouting and fluidization by su-perimposing the two type of systems[1―4]. In recent years, spout-fluid beds have become an alternative for gas/solid contactors in coal gasification. Spout-fluid bed coal gasifiers have been adopted for APFBC-CC (advanced pressurized fluidized bed combus-tion-combined…     

基于CCDM的流态化制冰的多相流动与传热特性

流态化制冰是一种新型的直接接触式动态制冰方式,通过建立联合连续和离散模型的CCDM(combined continuum and discrete model)全耦合模型,采用一种非结构网格颗粒搜索法,计算了流态化制冰过程所涉及的颗粒-流体多相系统的流动和传热特性。结果表明:基于CCDM模型计算出的结果与实验数据有较好的一致性,模型的准确度高;冻结程度关键值的提出,简化了颗粒间碰撞模型的判断和颗粒相表面传热量的等效计算;颗粒间的碰撞、聚并对颗粒粒径的作用,会进一步影响颗粒的运动速度、停留时间以及轴向浓度分布的变化,难以忽略其对于流态化制冰过程的多相流动与换热特性的影响。     

A new mechanistic model to predict gas–liquid interface shape of gas–liquid flow through pipes with low liquid loading

Devising a new mechanistic method to predict gas–liquid interface shape in horizontal pipes is concerned in this article. An experiment was conducted to find the pressure gradients of air–water flow through a 1‐in. pipe diameter. Comparing results of model with some experimental data available in the literature demonstrates that the model provides quite better predictions than existed models do. This model also predicts flow regime transition from stratified to annular flow better than Apparent Rough Surface and Modified Apparent Rough Surface models for both 1‐ and 2‐in. pipe diameters. The model also leads to reliable predictions of wetted wall fraction experimental data. Although one parameter of new model was evaluated based on air–water flow pressure loss experimental data for 1 in. pipe, it was considerably successful to predict pressure drop, liquid holdup, stratified‐annular transition and wetted wall fraction for other gas–liquid systems and pipe diameters. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1043–1053, 2015     

Hydrodynamics of a gas-driven gas-liquid-solid spouted bed with a draft tube

A cylindrical gas-liquid-solid spouted bed, driven exclusively by gas flow, has been developed with a high potential for use in biochemical processes, such as a biological wastewater treatment. A plexiglass column with a 152 mm inner diameter was used in combination with a 53 mm inner diameter plexiglass draft tube. Three particle types were studied with densities ranging from 1044 kg/m³-1485 kg/m³ and average particle sizes ranging from 0.7-2.5 mm. Four flow regimes were observed when increasing the gas velocity, including fixed bed, semispouted bed, full spouted bed, and internal circulating fluidized bed. The transition gas velocities between those regimes were experimentally measured and termed as minimum spouting velocity, full spouting velocity, and minimum circulating velocity, respectively. A measurement of the downward particle flux in the annulus was used to identify the minimum spouting velocity, while the particle velocity and dense phase retraction in the annulus were monitored for the full spouting and minimum circulating velocities. All regime transition velocities increased with more dense particles and longer draft tubes. The minimum spouting velocity and full spouting velocity were not affected when varying the nozzle-tube gap, while the minimum circulating velocity increased with longer nozzle-tube gaps. Experiments without a draft tube were carried, though the spouting stability was significantly reduced without the draft tube.     

A SIMPLE MODEL FOR PREDICTING THE VOID FRACTION OF GAS/NON-NEWTONIAN FLUID INTERMITTENT FLOWS IN UPWARD INCLINED PIPES

In this work, a simple correlation, which incorporates the mixture velocity, drift velocity, and the correction factor of Farooqi and Richardson, was proposed to predict the void fraction of gas/non-Newtonian intermittent flow in upward inclined pipes. The correlation was based on 352 data points covering a wide range of flow rates for different CMC solutions at diverse angles. A good agreement was obtained between the predicted and experimental results. These results substantiated the general validity of the model presented for gas/non-Newtonian two-phase intermittent flows.     

Mixing characteristics of wet granular matter in a bladed mixer

We performed numerical simulations of dry and wet granular flow inside a four-bladed mixer using the discrete element method (DEM). A capillary force model was incorporated to mimic the complex effects of pendular liquid bridges on particle flow. The simulations are able to capture the main features of granular flow, which is substantiated by the comparison of our results with experimental data.It was found that mean and fluctuating velocity fields for wet and dry particles differ significantly from each other. Our results indicate a strong increase in heap formation for wet particles and hence velocity fluctuations in the vertical direction become more pronounced. We observe that mixing in bladed mixers is strongly heterogeneous for wet granular matter due to the formation of different flow regimes within the mixer. The analysis of mixing quality shows that the spatial distribution of mixing intensity is influenced by the moisture content. This can lead to locally and even globally higher mixing rates for wet particles compared to dry granular matter.     

Trajectory modeling of gas–liquid flow in microchannels with stochastic differential equation and optical measurement

The numbering‐up of microchannel reactors definitely faces great challenge in uniformly distributing fluid flow in every channel, especially for multiphase systems. A model of stochastic differential equations (SDEs) is proposed based on the experimental data recorded by a long‐term optical measurement to well quantify the stochastic trajectories of gas bubbles and liquid slugs in parallel microchannels interconnected with two dichotomic distributors. The expectation and variance of each subflow rate are derived explicitly from the SDEs associated with the Fokker–Planck equation and solved numerically. A bifurcation in the trajectory is found using the original model, then a modification on interactions of feedback and crosstalk is introduced, the evolutions of subflow rates calculated by the modified model match well with experimental results. The established methodology is helpful for characterizing the flow uniformity and numbering‐up the microchannel reactors of multiphase system. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4028–4034, 2015     

水力旋流器内非牛顿流体多相流场的数值模拟

利用一种非牛顿流体黏度修正模型描述水力旋流器内高浓度矿浆的非牛顿流动特性,并结合雷诺应力模型(RSM)、混合多相流模型(Mixture)以及拉格朗日颗粒追踪模型(LPT)建立了一种适用于模拟水力旋流器内非牛顿流体多相流场的数学模型。模拟结果与报道的实验值的相对误差均在10%以内,表明了该模型的可靠性。结果表明,非牛顿流体黏度的空间分布与矿浆密度的空间分布类似。沿零轴速包络面(LZVV)的轮廓存在一个高密度环,其原因为某粒径范围内的颗粒受到的径向合力为零,颗粒群沿LZVV做高速旋转运动。分散相的空间分布取决于不同粒径的颗粒受力。对于不同粒径的单位质量颗粒,向外离心力的数值大约为向内压力梯度力的两倍左右,使得大颗粒进入下行流并在底流口收集。随着颗粒粒径的减小,总体向内且具有波动性的流体曳力呈指数增长。向内的流体曳力将部分颗粒推向轴心,经上行流逃逸,同时也增强了颗粒运动的随机性。当颗粒粒径小于一定值后,流体曳力远远大于离心力和压力梯度力,颗粒运动的随机性非常强,宏观表现为均匀分布。     

大颗粒三相环隙气升式环流反应器流体力学行为

研究了大颗粒体系气升式环流反应器的流体力学行为,考察了表观气速和颗粒质量分数对床层膨胀高度、循环液速和固含率分布的影响。实验结果表明,按颗粒的运动状态不同可以将反应器内的流动分为3个区域,即固定床区域、膨胀床区域和循环床区域,各流动区域内的流动行为存在显著差异。随着颗粒质量浓度的增大,起始流化气速和最小循环气速均显著增大。基于三相流化床的流化模型和环流反应器的特点建立了相应的数学模型,对大颗粒三相气升式环流反应器的起始流化气速和最小循环气速进行了预测,模型预测值与实验测量值吻合良好。     

Computational study of particle in-flight behavior in the HVOF thermal spray process

A computational framework is developed for the multiphase flow in a high velocity oxygen-fuel (HVOF) thermal spray coating process with steel powders as the feedstock. The numerical model includes continuum-type differential equations that describe the evolution of gas dynamics and multi-dimensional tracking of particle trajectories and temperature histories in the turbulent reacting flow field. The numerical study shows that the particle temperature is strongly affected by the injection position while the particle velocity is less dependent on this parameter. Moreover, both particle velocity and temperature at impact are strongly dependent on particle size, although the spatial variation of these two variables on the substrate is minimal. It is also found that not all the particles are deposited on the substrate perpendicularly (even if the spray angle is 90°), due to substantial radial fluid velocities near the stagnation point. A statistical distribution of particle velocity, temperature, impinging angle and position on the substrate as well as particle residence time is obtained in this work through independent random tracking of numerous particles by accounting for the distributed nature of particle size in the feedstock and injection position as well as the fluctuations in the turbulent gas flow.