CFD analysis of microchannel emulsification: Droplet generation process and size effect of asymmetric straight flow-through microchannels

Asymmetric straight flow-through microchannel (MC) arrays are high-performance MC emulsification devices for stable mass production of uniform droplets. This paper presents computational fluid dynamics (CFD) simulation and analysis of the generation of soybean oil-in-water emulsion droplets via asymmetric straight flow-through MCs, each consisting of a microslot and a narrow MC. We also used CFD to investigate the effects of the channel size and the flow of the dispersed phase on MC emulsification using asymmetric straight flow-through MCs with a characteristic channel size of 5–400 μm. The overall shape of an oil–water interface and the time scale during droplet generation via a control asymmetric straight flow-through MC were appropriately simulated. Better insight was obtained on the flow profile of the two phases and the internal pressure balance of the dispersed phase during droplet generation. Comparison of the CFD and experiment results also provided insight into dynamic interfacial tension during droplet generation. Successful droplet generation was observed below a critical dispersed-phase velocity. In this case, the resultant droplet size was proportional to the channel size and was not sensitive to the dispersed-phase velocity applied. The maximum droplet generation rate per channel was inversely proportional to the channel size, unless the buoyancy force did not promote droplet detachment. The maximum droplet productivity per unit area of an asymmetric straight flow-through MC array was estimated to be constant, regardless of channel size.     

Computational fluid dynamics simulation of a novel bioreactor for sophorolipid production

This paper describes three-dimensional computational fluid dynamics(CFD) simulations of gas–liquid flow in a novel laboratory-scale bioreactor contained dual ventilation-pipe and double sieve-plate bioreactor(DVDSB)used for sophorolipid(SL) production. To evaluate the role of hydrodynamics in reactor design, the comparisons between conventional fed-batch fermenter and DVDSB on the hydrodynamic behavior are predicted by the CFD methods. Important hydrodynamic parameters of the gas–liquid two-phase system such as the liquid phase velocity field, turbulent kinetic energy and volume-averaged overall and time-averaged local gas holdups were simulated and analyzed in detail. The numerical results were also validated by experimental measurements of overall gas holdups. The yield of sophorolipids was significantly improved to 484 g·L~(-1)with a 320 h fermentation period in the new reactor.     

Geometry-driven coalescence of water droplets in a highly viscous fluid

This work is dedicated to numerical studies of the coalescence of water droplets moving in bitumen. The Navier–Stokes equations coupled with the volume of fluid model (VOF) using adaptive grids techniques, available in the commercial computational fluid dynamics (CFD) software Ansys Fluent 20R2, were solved numerically to investigate the behaviour of water droplets with diameters from 1 to 100 μm moving in rapidly converging and diverging microchannels of different sizes. The model has been validated against 3-D experimental data published in the literature. A good agreement has been demonstrated. The results of simulations revealed that the main parameter influencing coalescence is the bulk flow velocity in the channel. Analysis of unsteady simulations showed the existence of a critical flow velocity, above which no coalescence occurs, corresponding to capillary number Ca < 0.5 for droplets Reynolds number Re < 0.1. Besides, image processing analysis has been used for a mean droplet size estimation in different geometries. A mean size significantly increased due to the late coalescence occurring in a wider constriction.     

Evaluation of mass transport performance in heterogeneous gaseous in-plane spiral reactors with various cross-section geometries at fixed cross-section area

Due to its compactness, high heat and mass transfer rate and ease of manufacture, coiled/spiral tube has been widely used in process industries, especially as heat exchangers and chemical reactors. This study addresses the mass-transport enhancement and reaction performance in in-plane spiral reactor with various cross sections geometries, i.e. circular, half-circular, rectangular, square, trapezoidal and triangular, at fixed cross-section area at several Reynolds numbers. The mass transfer performance is compared with those of straight channel counterpart. Laminar flow of gas with catalytic reactions is investigated using a validated three-dimensional computational fluid dynamics (CFD) model. The results suggest that spiral ducts offer better reaction performance as compared to straight duct, especially at higher Reynolds number. However, it imposes higher pressure drop. Amongst various cross-section, the coil reactor with half-circular geometry yields the highest reaction performance. This study can provide insight for design guidelines of high performance coiled reactor.     

Three-dimensional computational fluid dynamics (CFD) study of the gas–particle circulation pattern within a fluidized bed granulator: By full factorial design of fluidization velocity and particle size

The fluidization velocity and mean particle size were selected to be numerically investigated pertaining to their effects on the gas–particle circulation pattern within a fluidized bed granulator by three-dimensional computational fluid dynamics (CFD) simulation applying an Eulerian–Eulerian two-fluid model. The CFD simulations were designed by full factorial design method and the developed CFD model was experimentally validated. The fluidization process was proved to reach a quasi-steady state. The gas–particle circulation pattern and particle concentration distribution were analyzed based on fluidization velocity and mean particle size. A mathematical model was developed to provide guidance on how to change fluidization level during one experiment.     

Rushton桨搅拌槽内轴弯矩的数值模拟

针对搅拌槽内流体流动、柔性结构振动和流动流体与柔性结构相互作用(流固耦合)的特征,分别采用计算流体动力学(不考虑结构振动)、计算结构动力学(不考虑流体作用)和两种计算动力学相互瞬态耦合模拟(同时考虑结构振动和流体作用)研究Rushton桨搅拌轴的弯矩幅值平均和波动特性。研究结果表明:搅拌槽内流体充当了振动的阻尼作用,抑制了搅拌桨轴侧向振动的幅度,但主体流动的低频宏观不稳定性显著地增加了搅拌桨轴旋转的不稳定性,同时搅拌桨轴侧向振动增加了搅拌桨叶片上的流体载荷不稳定性,但对不均衡性影响很小;弯矩流体成分(来源于流体压力和粘性力)与结构成分(来源于结构重力和惯性力)之间夹角是随机的,但平均夹角接近于90°;耦合模拟结果与实验数据吻合较好,且明显优于计算流体动力学和计算结构动力学分离模拟计算结果。研究结果有助于深入理解搅拌槽内流固耦合对搅拌轴弯矩的影响,对搅拌设备的机械设计具有指导意义。     

Development of a semi-analytical model to predict the pressure drop of clean pleated high-efficiency particulate air filters

The present article outlines the development of a semi-analytical model devoted to predict the pressure drop induced by clean pleated high-efficiency particulate air (HEPA) filters. Both experimental measurements and numerical simulations are used to characterize the velocity field in the pleat channel. On this basis, a semi-analytical model is derived to determine the gas flow within the pleat channel. This analytical formulation is used to predict the air pressure evolution according to filtration velocity in the pleat. This model is then validated on the basis of comparisons with measurements found in the scientific literature for different kinds of HEPA filters with different pleat geometries. This model is easy to use, fast to run compared to standard computational fluid dynamics (CFD) approaches, and is in good agreement with the experimental results.     

Studies of gas–liquid flow through reactors internals using VOF simulations

Two-phase flow through reactor internals have been experimentally and numerically studied. Experiments have been carried out with a setup running under ambient pressure for two configurations. The first configuration consists of a mixing box orifice inlet through which liquid flows as a film sheared by a gas flow. The liquid height at orifice inlet is documented over a wide range of liquid and gas flowrates. The second configuration consists of the two-phase flow through a downcomer of a distributing tray. Two and three dimensional computational fluid dynamic (CFD) simulations using the volume of fluid approach have been used to compute both flows for similar flow conditions as used in the experiments. It is shown that the agreement between experiments and calculations is very good. Based on this good agreement, it is finally discussed how CFD can be used to achieve better design rules for gas liquid reactor internals via simulations carried out for industrial process conditions.     

Numerical Simulation of Uranium Extraction from Nitric Acid Medium Using Hollow-Fiber Contactor

ABSTRACT

A two-dimensional axisymmetric computational fluid dynamics (CFD) model is presented to simulate uranium extraction from nitric acid medium using tri-n-butyl phosphate in n-dodecane in a hollow-fiber membrane contactor operated in non-dispersive solvent extraction mode. CFD model solves continuity and momentum-transport equations for the feed and shell sides and species transport equations for the feed side, shell side, as well as the membrane. Complex boundary conditions of flux continuity and concentration jump are implemented in the CFD model. The estimates of percentage of uranium extraction obtained from CFD simulations for different parametric conditions are compared with the experimental results, and a good agreement is observed. The validated CFD model is used to gain detailed insights into the hydrodynamics and mass transfer.

Abbreviations CFD: Computational fluid dynamics; NDSX: Non-dispersive solvent extraction; TBP: Tri n-butyl phosphate     

Computational fluid dynamic simulations of coal-fired utility boilers: An engineering tool

The objective of this study was to develop an engineering tool by which the combustion behavior of coals in coal-fired utility boilers can be predicted. We presented in this paper that computational fluid dynamic (CFD) codes can successfully predict performance of- and emission from- full-scale pulverized-coal utility boilers of various types, provided that the model parameters required for the simulation are properly chosen and validated. For that purpose we developed a methodology combining measurements in a 50 kW pilot-scale test facility with CFD simulations using the same CFD code configured for both test and full-scale furnaces. In this method model parameters of the coal processes are extracted and validated. This paper presents the importance of the validation of the model parameters which are used in CFD codes. Our results show very good fit of CFD simulations with various parameters measured in a test furnace and several types of utility boilers. The results of this study demonstrate the viability of the present methodology as an effective tool for optimization coal burning in full-scale utility boilers.     

CFD modeling of single‐phase flow in a packed bed with MRI validation

Computational fluid dynamics (CFD) simulations are becoming increasingly widespread with the advent of more powerful computers and more sophisticated software. The aim of these developments is to facilitate more accurate reactor design and optimization methods compared to traditional lumped‐parameter models. However, in order for CFD to be a trusted method, it must be validated using experimental data acquired at sufficiently high spatial resolution. This article validates an in‐house CFD code by comparison with flow‐field data obtained using magnetic resonance imaging (MRI) for a packed bed with a particle‐to‐column diameter ratio of 2. Flows characterized by inlet Reynolds numbers, based on particle diameter, of 27, 55, 111, and 216 are considered. The code used employs preconditioning to directly solve for pressure in low‐velocity flow regimes. Excellent agreement was found between the MRI and CFD data with relative error between the experimentally determined and numerically predicted flow‐fields being in the range of 3–9%. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

三分螺旋折流板换热器内二次流分布与强化传热关系分析

通过对倾斜角35°首尾相连的三分螺旋折流板换热器的数值模拟,展示了其壳侧通道内流体在典型切面上的流场和流线分布,以及典型切面上典型直线的流动和换热参数分布,并与性能测试结果进行了对比。结果表明,数值模拟结果与实验结果是吻合的。螺旋折流板形成的近似螺旋通道,使换热器壳侧流体受离心力和向心力共同作用形成了迪恩涡二次流,且在每个螺旋周期内都存在;二次流增强了主流区域流体与靠近壁面流体的掺混,使得壳侧典型切面上中心线和折流板外缘直线的轴向速度较大;除主流中心区域外,壳侧流体在二次流的作用下具有均匀的湍流动能;二次流所在区域内,壳侧同心柱面内典型直线上换热系数相差不大,但由于二次流能使其附近区域传热面上的流体得到不断卷吸掺混,由此强化传热。     

The Effects of Screw Geometry on the Pumping Efficiency of Micro‐Screw Pump

The design of a novel micro‐screw pump for viscous fluid is described. The device consists of a rotating screw in the centre of the channel, connected with a shaft and micro motor. The objective of this research is to investigate the effect of using various screw geometries on the pump performance. Theoretical analysis by finite volume simulations is carried out to study the influence of pitch, diameter of the screw and the thread (flight depth) to evaluate the optimal dimensions for the pump and to obtain the maximum flow rate. When the screw rotates, a net force is transferred to the fluid due to the differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace. The three‐dimensional simulations indicate a gradual increase of the average velocity with increasing the screw diameter. The maximum average velocity can be obtained when the ratio between the pitch and screw diameter (pi/d) is 0.6. Effective pumping is achieved by increasing the thread and pitch at maximum screw diameter. The numerical simulation has been validated experimentally.     

Accessing multidimensional mixing via 3D printing and showerhead micromixer design

We present a 3D metal printing showerhead mixer to blend effectively two reagent streams into a confined mixing volume. Each stream is predistributed to multiple channels to increase the contact area in the mixing zone, which enables high mixing performance with smaller pressure drop. The showerhead mixer shows excellent mixing performance owing to its ability to intersperse rapidly the two streams as characterized by the diazo coupling reactions and computational fluid dynamics (CFD) simulations. Experimental results demonstrate superior performance of the showerhead mixer compared to two common commercial micro T-mixers, especially in low Reynolds number regime. CFD results are employed to (a) help understand the mixing mechanism, (b) reproduce the experimental observations, and (c) inform the design specifications for optimal performance. Good agreement between experiments and simulations is achieved. The final design includes multiple side-fed inlets for improved mixing performance of the showerhead mixer, as suggested by the validated CFD models.     

Computational Fluid Dynamics Simulation of a Quadrupole Magnetic Sorter Flow Channel: Effect of Splitter Position on Nonspecific Crossover

In the Quadrupole Magnetic Sorter (QMS) magnetic particles enter a vertical flow annulus and are separated from non-magnetic particles by radial deflection into an outer annulus where the purified magnetic particles are collected via a flow splitter. The purity of magnetically isolated particles in QMS is affected by the migration of nonmagnetic particles across transport lamina in the annular flow channel. Computational Fluid Dynamics (CFD) simulations were used to predict the flow patterns, pressure drop and nonspecific crossover in QMS flow channel for the isolation of pancreatic islets of Langerhans. Simulation results were compared with the experimental results to validate the CFD model. Results of the simulations were used to show that one design gives up to 10% less nonspecific crossover than another and this model can be used to optimise the flow channel design to achieve maximum purity of magnetic particles.     

Hydrodynamic characteristics of valve tray: Computational fluid dynamic simulation and experimental studies

In order to better understand the hydrodynamics of valve trays, air-water operation in an industrial scale tower with 1.2 m of diameter, consisting of two 14% valve trays, was studied. Experimental results of clear liquid height, froth height, average liquid holdup, dry pressure drop, total pressure drop, weeping and entrainment were investigated, and empirical correlations were presented. Then, a three-dimensional computational fluid dynamics (CFD) simulation in an Eulerian framework for valve tray with ANSYS CFX software was done. The drag coefficient, which was used in the CFD simulations, was calculated from the data obtained in the experiments. The simulation results were found to be in good agreement with experimental data at this industrial scale. The objective of the work was to study the extent to which experimental and CFD simulations must be used together as a prediction and design tool for industrial trays.     

螺旋片导流式分离器压力简化计算

应用计算流体力学CFD(ComputationalFluidDynamics)方法,对湍流状态下不同结构参数的螺旋片导流式气液分离器螺旋结构中的流体流动场进行了数值模拟。通过分析比较螺旋个数和螺距分别对压力降的影响,从而修正达西公式,拟合出螺旋结构压力降的简化计算公式,并通过了试验验证。结果表明,用此简化计算公式计算的结果与试验数据基本一致,可以用于螺旋片导流式气液分离器螺旋结构设计计算。     

Optimal design of drainage channel geometry parameters in vane demister liquid–gas separators

Vane liquid–gas demisters are widely used as one of the most efficient separators. To achieve higher liquid disposal and to avoid flooding, vanes are enhanced with drainage channels. In this research, the effects of drainage channel geometry parameters on the droplet removal efficiency have been investigated applying CFD techniques. The observed parameters are channel angle, channel height and channel length. The gas phase flow field was determined by the Eulerian method and the droplet flow field and trajectories were computed applying the Lagrangian method. The turbulent dispersion of the droplets was modeled using the discrete random walk (DRW) approach. The CFD simulation results indicate that by applying DRW model, the droplet separation efficiency predictions for small droplets are closer to the corresponding experimental data. The CFD simulation results showed that in the vane, enhanced with drainage channels, fewer low velocity sectors were observed in the gas flow field due to more turbulence. Consequently, the droplets had a higher chance of hitting the vane walls leading to higher separation efficiency. On the other hand, the parameters affect the liquid droplet trajectory leading to the changes in separation efficiency and hydrodynamic characteristic of the vane. To attain the overall optimum geometry of the drainage channel, all three geometry parameters were simultaneously studied employing 27 CFD simulation cases. To interpolate the overall optimal geometry a surface methodology method was used to fit the achieved CFD simulation data and finally a polynomial equation was proposed.