Liquid-liquid Two-phase Flow Research Articles

CFD simulation of liquid-liquid extraction mechanism and enhancement schemes in microchannels based on three mass transfer models

Microreactors are highly efficient devices with a large specific surface area to improve the mass transfer efficiency. In this paper, a comprehensive three-dimensional CFD model was constructed based on three mass transfer theories to simulate the liquid-liquid two-phase flow and mass transfer process in a microchannel reactor, and two enhancement schemes were proposed. The multiphase flow and mass transfer characteristics were investigated by visualization and extraction experiments. The results indicated that the extraction efficiencies (E) and overall mass transfer coefficients (KLa) calculated by the penetration model and surface renewal model were within ±15 % errors compared to the experimental values. Moreover, E primarily increases with the increase of fluid residence time, while KLa increases with increasing flow rate. As the flow rate increases from 0.3 ml/min to 1.5 ml/min, E decreases by 15 %, and KLa rises from 0.78 s⁻¹ to 2.65 s⁻¹. Whereas the channel width decreases from 0.8 mm to 0.3 mm, E decreases by 3 %, and KLa rises from 0.44 s⁻¹ to 2.77 s⁻¹. Finally, microchannel with necked structure and baffles in mixing zone both improve the mass transfer efficient to some extent.

Experimental study of liquid–liquid dispersion patterns in T-inlet microchannels with different junction angles

Liquid-liquid two-phase flow in T-inlet microchannels with different junction angles (θ = 30°, 60°, 90°, 120° and 150°) was investigated experimentally. Four flow regimes of the dispersed phase were identified, i.e., parallel flow, jetting, dripping and squeezing, and the distribution of flow regimes for the dispersed phase corresponding to variations in the junction angle was plotted. The consequences of varying junction angle and flow conditions in the squeezing regime on the generated droplet size were analyzed. The results indicate that low capillary number and large flow rate ratio are conducive to the formation of large-size droplets. For constant flow conditions, junction angle θ = 90° is detrimental to the formation of squeezing microdroplets. The increase in microchannel junction angle causes the droplet size to decrease until θ > 90°, where the droplet size increases with the junction angle. On the basis of experimental results, the scaling law correlation equations containing the junction angle for predicting the droplet length and droplet volume are proposed, respectively. The predicted values match well with the experimental data. The results of this work contribute to the enhancement of the monodispersity of microdroplets and the precise control over a wide range of the generated droplet size by adjusting the junction angle.

Characterization of liquid-liquid two-phase flow patterns and mass transfer coefficients in Human-shaped microchannels

Based on the principle of splitting and recombining, this study designed and fabricated millimeter-scale Human-shaped microchannels. The research focused on investigating the flow patterns and mass transfer characteristics of liquid-liquid two-phase flow within the Human-shaped microchannels. The experiment utilized a water and n-butanol system to investigate the flow behavior of liquid-liquid two-phase in the channel with a flow rate ratio of 1. When the total volumetric flow rate increases from 120 µl/min to 2400 µl/min, the flow pattern experiences slug flow, slug-droplet flow, parallel flow and non-steady flow patterns respectively. The study investigated the impact of factors including total flow rate, feed method, angle, curvature, channel width, phase ration extraction efficiency and mass transfer coefficients. The Human-shaped channel exhibited superior mass transfer performance compared to conventional microchannels due to its advantageous configuration. The higher the included angle is, the stabler the flow pattern within the channel, resulting in a better mass transfer performance. Furthermore, the absence of curvature is more favorable for achieving higher mass transfer efficiency. In the experiments, the overall volume mass transfer coefficient ranged from 0.007 to 0.1742 s−1. This study can provide insights for the development and structural optimization of microchannels based on the split-recombine principle.

Mass transfer phenomena of partially miscible liquids under liquid-liquid slug flow in a circular microchannel

Microreactors have been demonstrated to become an effective tool for increasing mass and heat transfer in heterogeneous chemical processes that are unachievable in batch or continuous stirred tank reactors. The key to superior performance is the availability of a large specific surface area (A/V) in the microsystem for mass and heat transfer between phases. Among the flow patterns generated in the two-phase liquid-liquid flow, slug is an ideal flow with a high stability characteristic, regular velocity, and uniform dimension throughout a microchannel system. The flow behavior gives slugs great potential to increase mass and heat transfer between phases, so optimizing the utilization of microreactors in many chemical process applications needs an in-depth understanding of the liquid-liquid hydrodynamic. Therefore, the current work aims to determine the mass transfer coefficient under liquid-liquid slug flow and the influence of slug dimensions, channel material, volumetric flow rate, and the flow rate ratio of the organic-aqueous phase on the mass transfer coefficient. The experiments were performed in 1 mm (ID) circular PTFE and silicone tubes, with a 30–40 ml/hour flow rate, and sodium hydroxide concentrations of 0.1, 0.13, and 0.15 M were used. The results exhibit that the most prominent overall mass transfer coefficient was 0.121/s, attained at 0.15 M sodium hydroxide concentration, with 40 ml/hour total discharge within the silicone channel. Microchannels generated smaller slug sizes at a higher aqueous phase flow rate than the organic phase. The smaller size provided a large specific surface area (A/V) for enhancing the mass transfer coefficient.

A method of predicting the two-phase liquid-liquid flow resistance in a horizontal pipe with a layer of metal foam

The study reported here is experimental. Its aim was to determine the method applicable for prediction of pressure losses of an unstable two-phase liquid-liquid system flow through a horizontal pipe with a short section of the pipe containing a metal foam packing. However, the flow disturbance resulting from the presence of foam does not lead to a change in the type of the mixture (as there is no phase inversion) nor a permanent emulsification of the flow system. The calculation procedure developed in the study takes into account both the peculiarities that accompany the flow of a two-phase liquid-liquid system in an empty pipe (i.e. type of continuous phase, phase slip, hold-up), as well as geometric parameters of the foam skeleton (i.e. cell and pore dimensions, surface roughness of fibers). The results of experimental research were applied for the purposes of determining the specific form of the equations, consistent with the mathematical model adopted and described in detail in this paper. They were conducted for two foams: aluminum and nickel, which were characterized by the same degree of pore packing (PPI20). The mean dimensions of the elements forming metal foams skeletons were determined by computed tomography. The horizontal flow tube had a diameter of 10 mm and the foam packing occupied it only over a a section with a certain length. In the liquid-liquid system pumped to the pipe, the hydrophilic phase was tap water, while the hydrophobic phase was typical machine oil. The adopted range of flux changes of both liquids corresponded to the conditions at which unstable two-phase systems of the W/O, O/W and W+O type were formed in the empty pipe. The pressure loss of the flowing substance was measured in the foam layer. The values of constants and exponents of the adopted model equation were determined separately for single-phase flow and two-phase systems of various types. The results of the statistical evaluation of the performance of the developed dependencies justify their application in the design procedures of selected heat or mass transfer devices. The equations are dimensionless, and therefore, under the conditions of validity, they should also be effective for other liquid-liquid systems, other foams and flow in a pipe with a different diameter. The developed method is based on a new approach to the description of the two-phase flow resistance through the porous material filling the pipe. In addition, it concerns the flow of a liquid-liquid system, which still forms the area that has been tackled in few publications.

Controlled synthesis of ZIFs by microemulsion method at high-throughput in a chaotic microreactor

The average size and particle size distribution (PSD) of zeolitic imidazolate frameworks (ZIFs) (such as ZIF-8, ZIF-67) are closely related to their physical and chemical properties. However, it is difficult to prepare small ZIFs with narrow PSD in traditional batch reactors owing to poor mixing efficiency. Here, a four-stage oscillating feedback microreactor (FOFM) was developed to prepare ZIFs by microemulsion method. CFD simulations and visualization experiments were conducted to investigate liquid-liquid two-phase flow patterns involved, demonstrating that chaotic advection can be effectively produced, thus dispersed phase is broken up into small droplets and homogeneous microemulsions are obtained. The proposed microemulsion method based on FOFM enables preparation of ZIF-8 with small average size (38.2 nm) and narrow PSD (σg = 1.24) without surfactant at a flow rate of 90 mL/min.

Study on hydrodynamic diffusion law of the swelling particle slurry in karst pipeline

The swelling particle grouting material has demonstrated remarkable plugging effectiveness in high-pressure and large-flow burst water within karst pipelines. Currently, current research on the rheological model, flow computation theory, and plugging mechanism of this material is lacking. The conventional grouting slurry diffusion process, using the liquid-liquid two-phase flow method, fails to accurately simulate high solubility slurry and particle swelling. To address these limitations, this study established a precise constitutive model to describe the swelling particle slurry diffusion process in dynamic water. Additionally, a coupling calculation method was proposed to analyze the spatiotemporal heterogeneity of viscosity during slurry diffusion by considering the migration of slurry and the changes in viscosity. To investigate the interaction between particle swelling and flow field changes, a Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) coupling model was developed for the diffusion of swelling particle slurry. It is demonstrated that slurry viscosity increases exponentially within the diffusion front as the particle swelling rate rises, and the drag force exhibits an intriguing behavior of initially increasing and then decreasing as the slurry flows through the pipeline. Furthermore, the CFD-DEM coupling model proved to be more accurate in describing viscosity distribution and diffusion distance compared to the finite element solution. The primary objective of this paper is to reveal the plugging mechanism and provide theoretical support for the engineering application of the swelling particle grouting material.

Mass Transfer and Droplet Behaviors in Liquid-Liquid Extraction Process Based on Multi-Scale Perspective: A Review

Liquid-liquid extraction is an important separation technology in the chemical industry, and its separation efficiency depends on thermodynamics (two-phase equilibrium), hydrodynamics (two-phase mixing and contact), and mass transfer (molecular diffusion). For hydrodynamics, the dispersion size of droplets reflects the mixing of two phases and determines the mass transfer contact area of the two phases. Therefore, a deep understanding of the droplet dispersion mechanism can help guide process intensification. The mass transfer and droplet behaviors in the liquid-liquid extraction process are reviewed based on three scales: equipment, droplets, and the interface between two liquids. Studies on the interaction between mass transfer and other performance parameters in extraction equipment as well as liquid-liquid two-phase flow models are reviewed at the equipment scale. The behaviors of droplet breakage and coalescence and the kernel function of the population balance equation are reviewed at the droplet scale. Studies on dynamic interfacial tension and interaction between interfaces are reviewed at the interface scale. Finally, the connection among each scale is summarized, the existing problems are analyzed, and some future research directions are proposed in the last section.

Hydrodynamics of liquid-liquid two-phase flow accounting for the coupling of dean vortices and Poiseuille flow

Hydrodynamics of liquid–liquid (L-L) two-phase flow in a confined helical microchannel are investigated via a VOF-CSF model, and further the influences of continuous phase flowrate, interfacial tension, viscosity ratio, helical screw pitch, helical radius as well as the diameter ratio of droplet to channel were systematically ascertained. To achieve the precise control over the interface deformation in a confined helical microchannel, the correlation equations with several key dimensionless numbers are established to predict the droplet aspect ratio, eccentricity and the minimum liquid film thickness. Especially, the minimum film thickness can be used to distinguish the flow field structures, and when the dimensionless outer minimum liquid film thickness (δ*out) is greater than 0.16, there is only one complete vortex system structure inside the droplet. Otherwise, two vortices are observed. Besides, the Q-criterion is used to quantitatively confirm the disappearance of vortices inside the droplet. Finally, the force field characteristics are also discussed to reveal the corresponding droplet interface deformation mechanisms, and it is found that, the droplet is driven forward by the differential pressure force, and deformed by the shear force along its flow direction. Both of the Dean force and Coriolis force usually control the interface deformation of droplet in the radial direction. The results in our study offer valuable guidance for designing the L-L two-phase flow microsystems to realize the goal of process intensification.

Measurement and Prediction of Droplet Entrainment in Inclined Liquid–Liquid Flow by PLIF Method

Inclined liquid-liquid two-phase flows widely exist in petroleum, chemical, and other important industrial processes. The study of droplet entrainment in inclined flow is of great significance for investigating the heat/mass transfer between phases and optimizing the industrial production processes. In this study, a planar laser-induced fluorescence (PLIF) system is designed to visualize the inclined liquid-liquid flows. The silicone oil (organic phase) and water/glycerol mixture (aqueous phase) are used as the experimental media. The length and amplitude of the interfacial waves are derived from the flow visualizations. The relationship between the critical wavelengths and Froude number is explored to indicate the transition from stratified flow (ST) to stratified flow with mixing at the interface (ST&MI). The wave aspect ratio is derived to characterize the instability and breakage of the interfacial wave. Meanwhile, the entrainment ratios of dispersed droplets in the continuous phases are detected, and the effects of the flow rate and the interfacial shear on the droplet entrainment are studied. Finally, a physical model is developed to predict the droplet entrainment ratios based on the force balance analysis of the interfacial wave. In general, the developed model is effective in predicting the droplet entrainment ratio with mean absolute errors of 0.015 and 0.023 for the organic and aqueous droplets, respectively.

Numerical study of liquid-liquid two-phase flow through coiled flow inverters: Effect of volume fraction, dean number and orientation

The present study deals with the numerical investigation of void fraction and flow patterns for liquid–liquid (oil–water) two-phase flow through a coiled flow inverter (CFI) with different flow conditions and geometrical orientations (horizontal and vertical). A CFI comprising 90° bends between two consecutive helical coils, and works on complete flow inversion due to directional change in centrifugal force. The velocity and phase distribution profiles for liquid–liquid two-phase flow in CFI are analysed by varying Reynolds and Dean numbers from 637−5033 and 202–1592, respectively. The orientation significantly influences overall friction factor based on mixture at low inlet velocities, while at higher inlet velocity it is nearly similar despite of CFI orientation. The inherent mechanism of vorticity distribution, to predict fluid rotational motion, is also reported for each phase in CFI. The change in CFI orientation resulted into an increase in the surface area(interface)-to-volume ratio of oil by ≈32%.

Hydrodynamics and mass transfer of liquid-liquid two-phase flow in circular milli-channels: Sizing-up effect

Background: Microfluidic is one of the most promising technologies for safety production in chemical industry while scale-up is a major challenge for its industrialization. Methods: In this work, hydrodynamics and mass transfer of liquid-liquid two-phase flow in different circular milli-channels were experimentally and numerically studied to investigate the sizing-up effect. Significant findings: Slug flow was observed in toluene-water system and the droplet length was measured. The results showed shorter droplets tended to present under higher flow rates, inlet mode one and smaller T-mixers. The buoyancy exerted on the toluene phase accelerated droplet detachment. For n-butanol-water system, a repetition of ‘parallel flow - annular flow’ was observed due to the prominent gravitational effect. The flow was asymmetric and the phenomenon was especially notable in the serpentine channel with a larger diameter. The detailed velocity field was demonstrated by numerical simulation. Mass transfer enhancement was observed in smaller channels and serpentine channels, resulting from shorter diffusion distance and secondary flow, respectively.

Hydrodynamics and mass transfer studies of liquid-liquid two-phase flow in parallel microchannels

Parallel or multiple microchannels (PMC) are being tried for high throughput operations. Extremely fewer reports on liquid/liquid two-phase flow in PMC exists in the open literature compared to the gas/liquid flow especially utilizing the new designs. In this work, a bifurcation-based PMC was employed to study hydrodynamics and the mass transfer aspect of solvent extraction of propionic acid. Slug formation, slug volume, specific surface area, extraction efficiency and mass transfer coefficients were evaluated. PMC exhibited better results than the single microchannels (SMC), making them superior in performance than the SMC. The standard deviation for uniform flow distribution is within 2%. A maximum specific surface area was obtained in PMC than the single channels, which is 10513 m2/m3. Similarly, the maximum extraction efficiency of 98% is achieved in PMC at flow rates 16 times higher than the SMCs. The overall pressure drop in PMC is significantly less than in the SMCs. Thus PMC is a suitable choice for process intensification in process industries.

Localization of jet electrochemical additive manufacturing with a liquid confinement technique

Electrochemical additive manufacturing (ECAM) is a flexible additive manufacturing method to deposit metallic/metallic-alloys microstructures without any thermally induced residual stresses. The conventional ECAM process suffers from stray deposition leading to reduced dimensional accuracy. Therefore, this paper proposes a novel liquid confinement technique to improve the deposition localization in Jet-ECAM process. Dichloromethane was employed as a second liquid phase to confine and control the flow field distribution of electrolyte as well as localize electric field on the substrate. Thus, the stray deposition induced by spreading electrolyte and hence spreading electric field could be avoided, and the deposition localization was improved. Liquid-liquid two-phase flow model was developed to analyze the flow field characteristic of two-phase flow formed by electrolyte and dichloromethane. Both simulation and observation results indicated that the flow region of the electrolyte on the substrate was well confined by the dichloromethane. The experimental results showed that the deposition localization was improved about 43.84 % compared to that in traditional Jet-ECAM. The inter-electrode gap (IEG) had a significant effect on both electrolyte flow region and current density distribution on the substrate, which affected deposition quality. Additionally, high current density leads to coarsened grains and reduces the deposition quality. Moreover, increasing the reciprocating speed of nozzle could refine the grains of microstructure, but the deposition efficiency was reduced. By employing process parameters of IEG of 100 μm, electric current of 0.0015 A, and the reciprocating speed of 300 μm/s, micro-patterns including both lines and curves were well deposited with a width of 500 μm, height of 60 μm, and a radius of 2 mm (letters G and U) and 2.5 mm (letter D).

Experimental and numerical studies of separation intensification in segmented flow microreactors

Process intensification studies have already been well-established in continuous operations, particularly achieved by using microreactors. Separation intensification benefits from the application of process intensification and shows great potential, yet it is still not sufficiently studied. In this work, liquid-liquid two-phase segmented flow capillary microreactors and simulation models based on finite element method were used to perform experimental and numerical studies on the extractive separation intensification of the Zn-Cd-Mn system, respectively. The influence of hydrodynamics including metal ion concentration, plug velocity, and initial pH of the aqueous phase on the separation factor was explored. By regulating the hydrodynamic conditions, higher separation factors are obtained in shorter times to achieve separation intensification. The Damkhler number (Da) was introduced to clarify the rate-determining step and elaborate the separation mechanism. During the simulations, the results obtained from the model based on the finite element method agree well with the experimental ones. Taking advantage of the strong complementarity between numerical and experimental studies, separation intensification can be more deeply understood.

Experimental and numerical studies of liquid-liquid two-phase flows in microchannel with sudden expansion/contraction cavities

The addition of micro cavities is a promising passive method to enhance mass transfer performance in microchannels. Experimental and numerical methods are adopted to investigate the flow characteristics of velocity field, droplet shape and vorticity for the liquid–liquid two-phase flows in microchannels with micro cavities. The results indicate that the droplet expands in cavities for slug and transitional flow patterns with droplet length longer than the channel width, while the expansion is not obvious for droplet flow patterns. Dimensionless expansion width increases with decreasing Capillary number and with increases in droplet length. For long droplet with length more than 2.5 times of the channel width, the expansion width is nearly the same with increasing droplet length. Due to the sudden decrease and increase of cross-section areas in cavities, the droplet moving speed fluctuates in the cavities, leading to the pulsating flow in cavity-shaped microchannels. When the droplet passes through the cavity structure, the continuous phase near the wall enters the cavity and induces small vortexes in the cavities. The intensity of vortex is more pronounced with longer droplet due to the blocking effect of droplet expansion in cavities. The vorticity magnitude in cavities is always higher than the value in straight microchannel, indicating that micro cavities can enhance the fluid mixing in the microchannel.

Distribution of liquid-liquid two-phase flow and droplet dynamics in asymmetric parallel microchannels

The numbering-up of microchannels in parallel is usually engineered to achieve high-throughput production of droplets, and the flow distribution of fluids and uniformity of droplets in parallel microchannels are core issues for droplet formation. In this work, the distribution of liquid–liquid two-phase flow and droplet dynamics in asymmetric parallel microchannels are investigated by using a high speed camera, and the factors affecting the stability and uniformity of droplets are explored. The dynamic behavior of droplet swarm in the rear cavity and its feedback effect on droplet generation at T-junctions are revealed. The microfluidic devices with various configurations are designed to investigate its effects on flow distribution. In addition, based on the principle of hydrodynamic resistance for the liquid-liquid two-phase flow in microchannels, the prediction for the distribution of liquid-liquid two-phase flow under slug flow is proposed.

水―パーフルオロカーボン系の水平管内二相流の層状流形成

水―パーフルオロカーボン系の水平管内二相流の層状流形成

Experimental and numerical studies of liquid-liquid slug flows in micro channels with Y-junction inlets

The immiscible liquid-liquid two-phase flows in microchannels with 4 different inlet angles (30°, 60°, 90°, 120°) were analyzed using numerical and experimental methods. Silicone oil was used as the discrete phase, and distilled water containing 0.5 wt% SDS was used as the continuous phase. The influences of two-phase flow rates and inlet angles on droplet lengths were analyzed in detail. The two-phase pressure and the velocity vectors of the continuous phase vary periodically during the formation of droplets. Droplet length increases with increasing volume flow rate of the dispersed phase and the ratio of the volume flow rate of the dispersed phase to the continuous phase, and decreases with increasing volume flow rate and capillary number of the continuous phase. The smallest droplets are generated with 60° inlet angle at the same flow rate. Based on the multiple regression analysis, the correlation equation for the prediction of dimensionless droplet length was proposed using the ratio of dispersed phase to continuous phase volume flow rate, the capillary number of continuous phase and the inlet angle as parameters.

An experimental study of interfacial dynamics during stratified oil-water draining from storage tank

Oil-water stratification is an inevitable phenomenon in the petroleum industry. The present study describes stratified oil-water draining from a square tank through a 9 mm diameter drain pipe. The purpose of this study is to understand the flow physics of oil as well as air entrainment in drain pipe during drainage of water. In order to observe the influence of the viscosity of oil on the entrainment and flow structure, two different oil-water stratifications are selected. In these two cases, the viscosity ratios of oil to water are 1.64 and 55, respectively. While draining, the flow of single-phase liquid in the drainpipe is transformed to two-phase liquid-liquid flow and then three-phase flow. A pointed tip of oil is formed as the oil-water interface approaches the drainpipe inlet. Different flow patterns, namely bubbly flow, slug flow, churn flow, elongated slug flow, jet flow, and annular flow, are observed as air and oil enter the drain pipe. Reverse flow is observed in the case of a higher viscosity ratio. An empirical correlation based on non-dimensional numbers has been proposed for describing the condition of no air entrainment in the drain pipe. Optical probes are used successfully to detect the presence of several regimes of air-oil-water and oil-water in the drainpipe.