Time-dependent flow structures and Lagrangian mixing in Rushton-impeller baffled-tank reactor

The object of this work is to investigate the role of large-scale convective structures in promoting mixing in a stirred tank. We focus on a standard geometry (flat bottom, four-baffle reactor stirred by a six-blade Rusthon impeller) and we use an Eulerian-Lagrangian approach to investigate numerically the dispersion of fluid particles. The three-dimensional, time-dependent, fully developed flow field is calculated with a computationally efficient procedure using a RANS solver with k-ε turbulence modeling and the flow field is assessed precisely against experimental data. Then, fluid parcels are tracked in the calculated flow field. Analyzing the trajectory of fluid parcels, the segregated regions within the flow are identified and mixing indicators are calculated (mixing time, circulation length and sojour time distribution). A physical explanation is thus proposed to establish a link between large-scale mixing and complex fluid dynamics generated by the interactions of radial-discharge jet, ring vortices, and upper counter rotating vortex.     

Dynamics of gas–liquid flow in a rectangular bubble column: experiments and single/multi-group CFD simulations

Several flow processes influence overall dynamics of gas–liquid flow and hence mixing and transport processes in bubble columns. In the present work, we have experimentally as well as computationally studied the effect of gas velocity, sparger design and coalescence suppressing additives on dynamics of gas–liquid flow in a rectangular bubble column. Wall pressure fluctuations were measured to characterize the low frequency oscillations of the meandering bubble plume. Bubble size distribution measurements were carried out using high-speed digital camera. Dispersed gas–liquid flow in bubble column was modelled using Eulerian–Eulerian approach. Bubble population was represented in the model with a single group or multiple groups. Bubble coalescence and break-up processes were included in the multi-group simulations via a suitable population balance framework. Effect of superficial gas velocity and sparger configurations was studied using single-group simulations. Model predictions were verified by comparison with the experimental data. Role of bubble size in determining plume oscillation period was studied. Multi-group simulations were carried out to examine evolution of bubble size distribution. An attempt is made to understand the relationship between local and global (over all the dispersion volume) bubble size distribution. The models and results reported here would be useful to develop and to extend the applications of multi-group CFD models.     

布置成涡结构微混合器内的流动与混合特性

基于成涡结构强化混合原理设计了一种改进型的平面被动式微混合器。采用商业软件CFD-ACE+对该结构微混合器的混合特性进行了三维数值模拟和结构优化,进一步揭示该微混合器结构对通道内流体流动与混合特性的影响。结合混合通道内流体的浓度和流型分布的数值和实验结果可知,该新型微混合器在布置成涡结构的弯曲通道内形成了扩展涡、分离涡和Dean涡,实现了涡系的叠加和强化,加大了流体间的扰动,从而增加了流体的接触面积强化混合;在综合考虑流体混合强度和压降分布等因素下,成涡结构正向布置且缝宽比W_d/W=1/4,厚宽比B/W=3/10,布置角度θ_a=120°的该微混合器在较广Re范围内的混合效果显著。     

An experimental and CFD study of liquid jet injection into a partially baffled mixing vessel: A contribution to process safety by improving the quenching of runaway reactions

Thermal runaway remains a problem in the process industries with poor or inadequate mixing contributing significantly to these incidents. An efficient way to quench such an uncontrolled chemical reaction is via the injection of a liquid jet containing a small quantity of a very active inhibiting agent (often called a stopper) that must be mixed into the bulk of the fluid to quench the reaction. The hazards associated with such runaway events mean that a validated computational fluid dynamics (CFD) model would be an extremely useful tool. In this paper, the injection of a jet at the flat free surface of a partially baffled agitated vessel has been studied both experimentally and numerically. The dependence of the jet trajectory on the injection parameters has been simulated using a single-phase flow CFD model together with Lagrangian particle tracking. The comparison of the numerical predictions with experimental data for the jet trajectories shows very good agreement. The analysis of the transport of a passive scalar carried by the fluid jet and thus into the bulk, together with the use of a new global mixing criterion adapted for safety issues, revealed the optimum injection conditions to maximise the mixing benefits of the bulk flow pattern.     

Modelling of Self-Induced Oscillations in the Mixing Head of a RIM Machine

In this paper, the self-induced oscillations in the mixing head of a RIM machine were modelled. An analytical and simple correlation was derived between the Strouhal number and the averaged pressure drop along the amplitude of the oscillation in the mixing chamber. This correlation theoretically identified that the frequency of the oscillations could be well correlated by the jet Reynolds number, Re_d, and the dimensionless distance between the two jets as proposed by Denshchikov et al., Fluid Dyn. 3 , 460-462 (1983). The flow field dynamics in the mixing head was simulated successfully by Fluent and the computed pressure fluctuations were used to calculate the frequencies of the oscillations in the mixing head. The calculated Strouhal numbers are in good agreement with the dominant frequency from the power spectra of the measured velocity component u_x (Santos, 2003). Finally, the effect of Re_d and Froude number, Fr, on the Strouhal number was investigated in the impingement region. The average Strouhal number showed a decrease with the Reynolds numbers, due to the increasing randomness of flow field in the impingement region. It was also found that the operations at lower values of Fr presented an increasing stability up to the point where the system is unable to present dynamic evolution. The model in this paper provides a theoretical starting point towards understanding of the quantities of the oscillatory flow in the mixing head, as well as a numerical approach to evaluate the dominant frequency in the mixing chamber.     

正反馈式流体振荡器内部流动特性的数值模拟

为探索正反馈式流体振荡器内部流动特性,建立了正反馈式流体振荡器的三维数值模型。采用数值模拟的方法对流体振荡器的内部流动特性进行了研究,通过监测特征点的压力变化和特征面的质量流率变化,对振荡器的内部流动过程进行量化并分析其内部的流动过程和原理。结果表明,在柯恩达效应和涡流的共同作用下,该正反馈式流体振荡器内部流场呈周期性振荡,振荡腔和反馈通道中的流体流动过程具有良好的周期性和稳定性,且频率与振荡器振荡频率一致。     

The NETmix reactor: Pressure drop measurements and 3D CFD modeling

NETmix is a new static mixing technology based on a network of mixing chambers interconnected by channels. Three NETmix reactors with different geometries were used to obtain experimental data for pressure drop and a generalized model for pressure drop in NETmix reactors has been developed. This model features a single adjustable parameter and it is only dependent on the geometric configuration of the NETmix design. The Z factor and the power number were also determined to compare the performance of different NETmix configurations with other existing mixers. The dynamic measurement of pressure drop was used to evaluate the mixing dynamics in the NETmix chambers and, above the critical Reynolds number, the natural oscillation frequency was quantified. Furthermore, a three-dimensional computational fluid dynamic transport model was also developed and validated. The energy performance of the three NETmix prototypes was quantified and shown to be very competitive with the compared existing static mixers. The developed 3D CFD transport model, validated by the reported experimental data, enables the computation of transport properties for any geometrical design and fluid properties, and avoids the need for experimental data each time a new NETmix configuration is designed.     

Analysis of dominant flow structures and their flow dynamics in chemical process equipment using snapshot proper orthogonal decomposition technique

Snapshot proper orthogonal decomposition (POD) technique has been applied to reveal the dominant flow structures, their dynamics and length scales in six widely used industrial equipments (stirred tank, bubble column, Taylor-Couette flow (annual contactor), ultrasonic reactor, jet reactor, and channel flow). The variation in length scale of structures within an equipment, with change in its operating conditions (Reynolds number and power input) or change in its geometric configuration (sparger and impeller designs), has been brought out in this work. The planar data set for POD analysis was obtained from particle image velocimetry (PIV) and large eddy simulation (LES) studies. The dominant spatial topology was analyzed by using the velocity and vorticity POD modes. The modes have revealed the following flow structures: the ascending streaks and bursts in channel flow, the vortex tube and leading edge vortices in jets, the irregular small chaotic vortices in Taylor-Couette flow, the variation in plume oscillation and flow structures in the vortical region of bubble column resulting from changes in sparger design, the high intensity vortices near the source of ultrasound in the ultrasonic reactor and the effect of impeller designs on dominant flow structures and near blade vortices in the stirred tank. The length scales of structures are obtained by applying image processing on the spatial modes. The dynamics of these flow structures in each of the items of equipment is captured by reconstructing the flow field using appropriate spatial and temporal modes that contribute to these structures. Further, a unique attempt has been made to correlate the length scale distribution with the mixing time.     

撞击流反应器内涡特性研究进展

撞击流技术具有良好的混合效果,广泛应用于能源、环保、化工等工程领域。由于撞击流反应器流场内存在大量无序的湍流涡结构,使其具有良好的混合效果。本文基于撞击流的混合原理,详细叙述了撞击流反应器内不同混合尺度下的混合过程以及涡的演变对混合的影响。结合实验和数值模拟等研究结果,阐述了不同类型撞击流反应器和撞击流反应器多相流场涡特性,归纳了撞击流反应器流场涡的特点。论述了撞击流反应器涡的产生和脱落机理。着重对圆柱射流、平板射流和撞击流流场内涡特性的本征正交分解（POD）分析进行总结,利用流场能量的角度揭示涡演化和消散规律。最后,对开发新型撞击流反应器、优化分析方法等研究前景进行展望。     

氯化法钛白氧化反应器气体错流混合

氧化反应器是氯化法制取钛白粉的关键设备，混合区的流场设计直接影响反应产品TiO₂颗粒的质量（如粒径和粒径分布）.利用无干扰流场测试技术——粒子图像测速仪（PIV）研究了不同气体错流混合区的结构设计对混合区域流场的影响，考察了气流动量比、射流狭缝宽度以及射流角度对混合区流场的影响.结果表明，较大的气流动量比、窄射流狭缝宽度和接近90°的射流能获得更好的混合区速度分布.进一步利用计算流体力学（CFD）手段对相应的流场进行了模拟，与PIV实验结果吻合很好，验证了模型对冷态流动的预测能力.     

射流强化螺旋通道内流体流动与换热的数值模拟

采用CFD软件模拟了射流作用下圆形截面螺旋通道内流体的流动及强化传热特性,模拟结果与实验结果吻合较好。研究了无量纲曲率?=0.061、无量纲螺距?=0.121的螺旋通道内复合涡旋结构及其演变过程,考察了射流入射角度?=π/6~π/3、射流速比?j=3~6时射流对螺旋通道换热的强化效果。结果表明,射流的初始阶段,射流的冲击作用抑制了单一螺旋通道内的离心二次涡旋,生成一对与其旋转方向相反的射流诱导涡旋,随流动发展,射流诱导涡旋先由两涡演变为单涡结构而后逐渐耗散消失。射流作用显著强化了螺旋通道内侧壁面附近流体的换热,随着?减小或?j增大,强化传热效果增强。?j≥4时,不考虑射流流量增加时综合强化传热因子JF1=1.26~1.67,考虑射流流量增加时JF2=1.008~1.19。     

Interactions of phase equilibria,jet fluid dynamics and mass transfer during supercritical antisolvent micronization

Supercritical antisolvent (SAS) precipitation has been successfully used in the micronization of several compounds. Nevertheless, the role of high-pressure vapor–liquid equilibria, jet fluid dynamics and mass transfer in determining particle size and morphology is still debated. In this work, CO₂ has been adopted as supercritical antisolvent and elastic light has been used to acquire information on jet fluid dynamics using thin wall injectors for the investigation of the liquid solvents acetone and DMSO at operating conditions of 40 °C in the pressure range between 6 and 16 MPa. The results show that two-phase mixing after jet break-up is the phenomenon that characterizes the jet fluid dynamics at subcritical conditions. When SAS is performed at supercritical conditions a transition between multi-phase and single-phase mixing is observed by increasing the operating pressure. Single-phase mixing is due to the very fast disappearance of the interfacial tension between the liquid solvent and the fluid phase in the precipitator. The transition between these two phenomena depends on the operating pressure, but also on the viscosity and the surface tension of the solvent. Indeed, single-phase mixing has been observed for acetone very near the mixture critical point, whereas DMSO showed a progressive transition for pressures of about 12 MPa.In the second part of the work, a solute was added to DMSO to study the morphology of the microparticles formed during SAS precipitation at the different process conditions, to find a correlation between particle morphology and the observed jet. Expanded microparticles were obtained working at subcritical conditions; whereas spherical microparticles were obtained operating at supercritical conditions up to the pressure where the transition between multi- and single-phase mixing was observed. Nanoparticles were obtained operating far above the mixture critical pressure. The observed particle morphologies have been explained considering the interplay among high-pressure phase equilibria, fluid dynamics and mass transfer during the precipitation process.     

Experimental Study of the Mixing Time in a Jet‐Mixed Gas‐Liquid System

In the present work, the impeller in the conventional gas‐liquid mixed vessels was replaced by a fluid jet as the mixer. Using an experimental setup, the effect of several parameters on the mixing time as a measure of the liquid‐phase mixing intensity, which is one of the required transport characteristics for designing gas‐liquid mixed systems, was studied. The results show that gas injection decreases the mixing time in comparison with the ungassed condition, but the mixing time is not necessarily decreased by increasing the gassing rate. On the basis of the amount of the jet Reynolds number and gassing rate, and thus the created circulation pattern, the mixing time may be decreased or increased. Also, the location of the probe for cases in which there are more dead zones in the vessel have a considerable effect on the measured mixing time. With increasing uniformity of the velocity domain, the influence of the probe location was reduced. Also, by increasing the jet flow rate and decreasing the nozzle diameter, the length of the jet, the amount of entrained bulk fluid, and the intensity of recirculation flow increased, and thus the mixing time decreased.     

Liquid mixing intensification by adding swirling flow in the transverse jet mixer

In this study, effect of swirling addition on the liquid mixing behavior of multiorifice-impinging transverse jet mixer has been investigated by planar laser-induced fluorescence as well as large eddy simulation (LES). In the case of swirling addition into the jet flow, there exists an optimized swirling jet angle or optimized jet-to-cross velocity ratio for the fixed mixer configuration. A larger swirling jet angle will make the flow dominated by the swirling, resulting in a slower mixing process. Interaction of swirling crossflow with no-swirling-injected streams, or with swirling-injected streams in the opposite direction is beneficial for the mixing. LES predictions show that many small vortices are produced homogenously due to intensified impingement in the case of opposite swirling directions, leading to a relative fast mixing process in few milliseconds. Whereas the mixing is restrained when the swirling directions of two flows are the same.     

Computational evaluation of effective transport properties of differential microcellular structures

This study presents a combined implementation of three-dimensional (3D) advanced imaging and computational fluid dynamics (CFD) modeling and simulation techniques to interpret the effective transport properties of single and stacked samples of differential microcellular structures. 3D morphological analysis software (ScanIP) was used to create representative elemental volumes via high-resolution tomography data for samples of tetrakaidekahedron-shaped Inconel and bottleneck-type aluminum foams. Pore-structure-related information for single and stacked differential samples were obtained with the aid of image analysis software, while their effective transport properties were attained by computationally resolving the pressure drop developed across these materials for superficial fluid velocities in the range from 0 to 6 m s⁻¹. Model validation was demonstrated by tolerable agreement between resulting CFD predicted results and experimentally measured values of flow properties. With these techniques, contributory effects were identified for pore-structure-related properties, pore density, and flow entrance on the flow dynamics of microcellular structures. This approach could prove useful in the design of highly efficient porous metallic components for applications specific to fluid transport.     

偏心射流-刚柔组合桨搅拌器内混沌混合行为研究

搅拌反应器内普遍存在混合隔离区,是实现高效混合的一大障碍。流场耦合诱发流体的混沌现象,可减少混合隔离区,提高流体混合效率。结合Matlab软件,探究偏心空气射流-单层刚柔组合桨体系的混合行为演变规律,对比分析了不同偏心率下桨叶类型、桨叶离底高度、空气射流量以及转速对流体混沌混合的影响。结果表明,刚柔组合桨通过其自身刚-柔-流的多体运动与偏心空气射流的流场耦合,破坏了流体混合过程中出现的对称性流场,使更多的流体进入混沌状态。刚-柔组合桨（RF-RDT、RF-IRDT）比刚性桨（RDT、IRDT）的LLE值大,其中RF-RDT相比于其他3种类型的搅拌桨（IRDT、RDT、RF-IRDT）,其LLE值分别提高了约42.8%,27.0%、6.9%;空气射流的偏心率等于0.6时,其最大LLE值相比于其他偏心率（0.8、0.4、0.2、0）,依次提高了6.5%、2.4%、17.6%、25.1%。该研究结果可为刚柔组合桨的优化设计提供理论依据。     

ANALYSIS OF AXIAL MIXING IN A GAS-LIQUID RECIPROCATING PLATE COLUMN

Axial mixing in the continuous phase in a Landau reciprocating-plate column (LRPC) has been investigated for both single-phase and two-phase gas-liquid flow conditions. A hydrodynamic model is proposed in which axial mixing is described as a process consisting of a backflow through the plate plus longitudinal mixing within the stage. The region in the proximity of the plates is almost perfectly mixed, beyond which there is a low-intensity mixing zone that varies in height and degree of mixing depending on phase velocities as well as the plates design and oscillation velocity. The presence of the dispersed phase affects axial mixing in both the well- and poorly mixed regions of each stage in two opposite ways: it decreases the backflow between the stages due to the hindrance effect caused by the presence of gas bubbles, and it increases the axial dispersion coefficient in the second stage by increasing the turbulence and phase entrainment caused by circulation and bubbles rising. The model adjustable parameters were determined from an experimentally measured dispersion coefficient over a wide range of operating conditions using the transient tracer injection method. The predictions of the model compare favorably with experimental data and can be applied for describing axial mixing in the continuous phase in an LRPC with±14% accuracy.     

Incompressible Smoothed Particle Hydrodynamics Modeling and Investigation of Fluid Mixing in a Rectangular Stirred Tank with Free Surface

In the present study, a robust modified Incompressible Smoothed Particle Hydrodynamics (ISPH) method is developed and applied to model a fluid mixer with a rotating stirrer and a tank body with free surface and horizontal oscillations. The mixer consists of a rectangular tank in which a stirrer rotates to mix the fluid. For the fluid in the tank, the free surface condition is considered. According to the Reynolds number, it is assumed that the mixing process is turbulent, and so a turbulent viscosity is defined. Although the smoothed particle hydrodynamics (SPH) method can result in some complexities, it is generally an easy and appropriate method for modeling mixing flow, free surface flow, and moving body problems, which will simultaneously be applied in the present study. The method is improved with kernel gradient corrective tensor, shifting particle algorithm, and turbulent viscosity, and it is validated against other well-known test cases and problems and is applied to model the mixing phenomenon. This study aims to increase the mixing rate and decrease the mixing time of the described mixer. To do so, the effect of the rotation of the stirrer on the mixing process is first investigated, after which the linear oscillation of the body is added and then the optimal shape of the tank is examined. The results show that the stirrer’s rotating speed, the linear oscillation of the tank body, and streamlining all have major effects on the mixing rate.