Slug Flow In Microchannels Research Articles

A peculiar segmented flow microfluidics for isoquercitrin biosynthesis based on coupling of reaction and separation

A segmented flow containing a buffer-ionic liquid/solvent in a micro-channel reactor was applied to synthesize isoquercitrin by the hesperidinase-catalyzed selective hydrolysis of rutin, based on a novel system of reaction coupling with separation. Within the developed microchannel reactor with one T-shaped inlet and outlet, the maximum isoquercitrin yield (101.7±2.6%) was achieved in 20min at 30°C and 4μL/min. Compared with a continuous-flow reactor, reaction rate was increased 4-fold due to a glycine–sodium hydroxide:[Bmim][BF4]/glycerol triacetate (1:1, v/v) system that formed a slug flow in microchannel and significantly increased mass transfer rates. The mass transfer coefficient significantly increased and exhibited a linear relationship with the flow rate. Hesperidinase could be efficiently reused at least 5 times, without losing any activity. The bonding mechanism and secondary structure of hesperidinase indicated that hesperidinase had a greater affinity to rutin at a production rate of 4μL/min in this segmented flow microreactor.

An optical approach for quantitative characterization of slug bubble interface profiles in a two-phase microchannel flow

A new measurement technique is developed for quantitatively mapping the liquid–gas interface profiles of air bubbles in an adiabatic microchannel slug flow environment. Water seeded with 0.5μm-diameter fluorescent polystyrene particles is pumped through a single acrylic microchannel of 500μm×500μm square cross section. A periodic slug flow is achieved by the controlled injection of air into the channel. Particles are constrained to the liquid phase, and their distribution in the flow is visualized through an optical microscope in an epifluorescent configuration with pulsed laser illumination to resolve the instantaneous liquid–gas interface profile to within ±2.8μm in the focal plane.This approach is able to identify the interface profile within individual focal planes at various depths within the channel, unlike conventional backlit optical profile detection approaches that can only resolve the interface at the midplane. A similar particle-tracking technique was previously demonstrated for interface reconstruction in annular flows; however, the additional noise within images due to the reflection and refraction of background light at the compound-curvature interfaces characteristic of slug bubbles requires texture-based image analysis to obtain interface profiles. The varying interface profile of the slug bubbles in the streamwise direction also greatly complicates the tracking procedures for achieving a three-dimensional reconstruction of slug bubbles based on the measured two-dimensional interface profiles, which requires spatial alignment of the streamwise position of liquid–vapor interfaces realized at varying depths within the channel. This is addressed during reconstruction by using the measured mid-plane slug bubble cap curvature to assign the relative streamwise positions of interface profiles obtained at other measured depths. The characterization of two different selected bubble morphologies presented here demonstrates a critical improvement in metrological capability which can provide greater insight into microchannel flow phenomena in the slug-flow regime.

SLUG FLOW HEAT TRANSFER IN MICROCHANNELS: A NUMERICAL STUDY

Two-phase microchannel slug flow has attracted significant interest among researchers due to its wide range of applications. This paper will present some as yet overlooked features of slug flow heat transfer in microchannels, using numerical simulation. Numerical simulations are carried out using ANSYS FLUENT with the volume of fluid (VOF) method with a 2D axisymmetric geometry for a 100 µm inner diameter microchannel with constant wall temperature boundary conditions. Effects of slug flow parameters on heat transfer are discussed showing the heat transfer rate follows a similar trend to existing numerical results. Our results show a significant increase in Nusselt number with liquid−liquid two-phase flow compared to single-phase flow, by up to 200%. The study revealed that capillary number, size of the liquid slugs, as well as the liquid film thickness have an important impact on heat transfer.

CFD Simulation on Hydrodynamics of Liquid-Liquid Slug Flow in Microchannel

Hydrodynamics in a liquid-liquid slug flow in a T-junction microchannel of 600μm diameter, operated under a squeezing regime, was simulated with the computational fluid dynamics method. The slug flow generation simulated shows very good agreement with experimental snapshots where the clear slug formation takes place in the vicinity of the T-junction. The internal circulation within each slug was also obtained, which could make us better understand the hydrodynamics of liquid-liquid slug flow in microchannel.

Stability Analysis of Reactive Multiphase Slug Flows in Microchannels

Conducting multiphase reactions in micro-reactors is a promising strategy for intensifying chemical and biochemical processes. A major unresolved challenge is to exploit the considerable benefits offered by micro-scale operation for industrial scale throughputs by numbering-up whilst retaining the underlying advantageous flow characteristics of the single channel system in multiple parallel channels. Fabrication and installation tolerances in the individual micro-channels result in different pressure losses and, thus, a fluid maldistribution. In this work, an additional source of maldistribution, namely the flow multiplicities, which can arise in a multiphase reactive or extractive flow in otherwise identical micro-channels, was investigated. A detailed experimental and theoretical analysis of the flow stability with and without reaction for both gas-liquid and liquid-liquid slug flow has been developed. The model has been validated using the extraction of acetic acid from n-heptane with the ionic liquid 1-Ethyl-3-methylimidazolium ethyl sulfate. The results clearly demonstrate that the coupling between flow structure, the extent of reaction/extraction and pressure drop can result in multiple operating states, thus, necessitating an active measurement and control concept to ensure uniform behavior and optimal performance.

Drag reduction of slug flows in microchannels by modifying the size of T-junctions

Pressure drop measurements of air–water slug flows in three circular microchannels (d=0.486mm) with various T-junctions (0.136, 0.194 and 0.252mm) were performed. The measured pressure drops were compared with common predictive models, and it was found that there were deviations between the models and measurements. The tendencies of the deviation between the models and measurements were different for different channels, implying the existence of a T-junction size effect. For confirmation of this effect, a scaling analysis based on the Laplace pressure and Bretherton law was performed. The analysis results showed that the inner pressure of a bubble increased when the T-junction size became small, resulting in drag reduction of the slug flow in the microchannel. Consequently, a modified predictive model for the pressure drop of slug flows was proposed. However, the drag reduction effect decreased when the T-junction was smaller than d/2 because the bubbles cannot maintain their shape without the depressurization due to the geometrical restriction.

Hydrodynamics and mass exchange in gas-liquid slug flow in microchannels

The present state of hydrodynamics and mass transfer studies in segmented gas-liquid flow in microchannels has been analyzed. It has been shown that such parameters as gas bubble velocity, gas hold-up, relative gas bubble length, pressure drop, mass transfer coefficients from gas bubbles to liquid slugs and to liquid film, as well as mass transfer coefficient from liquid to channel wall can be satisfactorily predicted. Nevertheless, some correlations were obtained under definite conditions and should be summarized. The purpose of further research is to develop reliable methods for calculation of mass transfer coefficients as functions of channel geometry, phase properties, and phase velocities in mini- and microchannels.

Direct carbonylation of nitrobenzene to phenylisocyanate using gas–liquid slug flow in microchannel

A microreaction system was developed for non-phosgene direct isocyanate synthesis from nitrobenzene and CO gas at pressure (9.5 bar) much lower than those in conventional ones (>100 bar). A gas–liquid slug flow of the reactant mixture was formed in the microchannel for efficient mass transfer across the gas–liquid interfaces. The isocyanate yield of the microflow reaction was shown to be three to six times higher than that of the batch reaction, depending on the inner diameter (i.d.) of the microtube. Higher isocyanate yield was obtained in a narrow-bore tube (0.5 mm i.d.) than in a wide-bore tube (1.0 mm i.d.). The results were interpreted in terms of the length of the liquid slug monitored through transparent PFA tubes. The liquid slug length in the narrow-bore tube (0.6 ± 0.2 mm) was found to be shorter than that in the wide-bore tube (1.1 ± 0.1 mm). This is consistent with the higher isocyanate yield obtained in the narrow-bore tube, because the shorter liquid slug has the larger gas–liquid interfacial area per unit volume.

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number ( Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.

Pressure drop of slug flow in microchannels with increasing void fraction: experiment and modeling

Pressure drop in a gas-liquid slug flow through a long microchannel of rectangular cross-section was investigated. Pressure measurements in a lengthy (∼0.8 m) microchannel determined the pressure gradient to be constant in a flow where gas bubbles progressively expanded and the flow velocity increased due to a significant pressure drop. Most of the earlier studies of slug flow in microchannels considered systems where the expansion of the gas bubbles was negligible in the channel. In contrast, we investigated systems where the volume of the gas phase increased significantly due to a large pressure drop (up to 1811 kPa) along the channel. This expansion of the gas phase led to a significant increase in the void fraction, causing considerable flow acceleration. The pressure drop in the microchannel was studied for three gas-liquid systems; water-nitrogen, dodecane-nitrogen, and pentadecane-nitrogen. Inside the microchannel, local pressure was measured using a series of embedded membranes acting as pressure sensors. Our investigation of the pressure drop showed a linear trend over a wide range of void fractions and flow conditions in the two-phase flow. The lengths and the velocities of the liquid slugs and the gas bubbles were also studied along the microchannel by employing a video imaging technique. Furthermore, a model describing the gas-liquid slug flow in a long microchannel was developed to calculate the pressure drop under conditions similar to the experiments. An excellent agreement between the developed model and the experimental data was obtained.

Enhanced production of ethyl pyruvate using gas–liquid slug flow in microchannel

The use of a gas–liquid slug flow in a microreactor has several advantages such as a large surface area for contacting gas and liquid phases and circulation flow inside slugs. These advantages accelerate the mass transfer between the two phases, resulting in a high concentration of dissolved gas. The enhanced mass transfer is useful in carrying out liquid phase oxidation that uses oxygen gas as the oxidizing agent. The gas–liquid slug flow in a microreactor system is applied to the oxidation of ethyl lactate using an oxy-vanadium species for producing ethyl pyruvate. The reactor system includes two T-shaped micromixers: one for mixing the substrate with a catalyst solution and the other for generating slug flow by the addition of oxygen. The oxidation reaction proceeds right after the slug flow is generated in the second mixer, because the substrate and the catalyst mix rapidly in the first micromixer. Moreover, a high concentration of dissolved oxygen due to improved mass transfer in slug flow increases the oxidation reaction rate. Therefore, compared to a batch reaction, the synthesis using slug flow provides high yields of ethyl pyruvate per unit time and achieves satisfactory productivity at the reaction temperature of 323K, which is lower than that employed in conventional syntheses. The proposed system enables the production of ethyl pyruvate using a simple reactor setup with reduced energy consumption.

Method for calculating the void fraction and relative length of bubbles under slug flow conditions in capillaries

A method for calculating the void fraction and relative size of bubbles at the known flow rates of phases is constructed using the mathematical model of a gas—liquid slug flow in capillaries that was developed earlier. The results of the calculations are in good agreement with the experimental data of other authors. The boundedness of linear approximations such as the Armand formula by the small values of the capillary numbers is revealed. It is shown that the void fraction depends not only on the dynamic gas holdup, but also on the capillary number and the Weber number, as well as on the direction of the flow. It is found that the ratio of the dynamic gas holdup to the void fraction varies from 1 to 2.5 as the capillary number increases. A tenfold error in the experimental determination of the length of liquid slugs by a simplified procedure is revealed. A simple calculation relationship that relates the dynamic gas holdup to the void fraction is derived from the Liu—Vandu—Krishna approximation. The theoretical explanation of the causes of the abnormal dependence of the void fraction on the dynamic gas holdup in microchannels with sizes of less than 100 fum is given. The specific features of a slug flow in microchannels that are caused by the disintegration of a film into drops are explained. The developed calculation method can also be applied to liquid—liquid systems.

21009 T型微細混合部内における気液二相スラグ流の形成過程(OS10 マイクロ・ナノ熱流体工学(2),オーガナイズドセッション)

Recently, two-phase slug flow in microchannel is investigated for utilization as a microbubble generator or microreactor and so on. In previous studies slug flow formation process is investigated in high wettability channels. However PDMS channels which are popular material for microchannel device has low wettability surface to water. Therefore purpose of this study is to investigate the relation of slug length to wettability. In the present experiments N_2 gas is used as gas phase and pure water as liquid phasse. Slug flow and measured its length by high speed video camera in time series. Forming process of slug flow is observed by high speed video camera. The slug length is measured from observed images. In this study, three kind of formation process of gas slug flow are observed in rectangular microchannel. Formation processes of gas slug and liquid slug flow are compared.

3-D numerical simulation of contact angle hysteresis for microscale two phase flow

Understanding the physics of microscale two-phase flow is important for a broad variety of engineering applications including compact PEM fuel cells and heat exchangers. The low Bond number and confined geometry make it critical to consider both the surface tension at the liquid–gas interfaces and the surface forces acting at the channel boundaries. Within the framework of a numerical volume of fluid (VOF) approach, the present work proposes a model to account for surface adhesion forces by considering the effects of contact angle hysteresis. A transient model is developed by correcting boundary force balances through specification of the local contact angle and instantaneously updating the local angle values based on the variation of the volume fraction from previous time steps. The model compares very well with new data provided here for droplets on a rotating disk and liquid slug flow in microchannel. The simulation reveals that the contact angle distribution along the slug profile in the microchannel flow can be approximated using a piecewise linear function. This study indicates that the asymmetric distribution of the contact angle might be responsible for several phenomena observed in the microchannel experiments, including slug instability.

Visualization and Tracking of Spontaneous Liquid-Liquid Slug Flow in Microchannels

Visualization and Tracking of Spontaneous Liquid-Liquid Slug Flow in Microchannels

Gas Absorption of Slug Flow in Microchannel (1st Report, Experimental Study with C02/Water System)

Gas absorption and pressure drop characteristics of a slug flow in microchannel were experimentally studied with CO2 and water system in expectation of high absorption performance by large specific interface area, toroidal flow in slug and high heat transfer capability of microchannel. Absorption rate was measured by detecting volume change of gas bubble along the channel with photo-interrupters, and then absorption time constant was formulated as a function of inner diameter, bubble velocity, and liquid slug length. Pressure drop was also formulated by fitting experimental data to an equation modeled as a sum of Poiseuille flow part and toroidal flow part. Based on the formulas, high mass transfer performance and compactness of the proposed system was demonstrated by comparison with typical gas absorption systems such as falling liquid film and dispersion bubbles. The microchannel gas absorber, of which channel length was set to keep outlet liquid concentration a specified value, was estimated to be more compact by reducing a channel diameter with a slight increase of pressure drop.

00/03189 Gas industry development in Egypt

This study uses high-speed imaging to characterize microchannel slug flow boiling using a novel experimental test facility that generates an archetypal flow regime suitable for high-fidelity characterization of key hydrodynamic and heat transfer parameters. Vapor and liquid phases of the fluorinated dielectric fluid HFE-7100 are independently injected into a T-junction to create a saturated two-phase slug flow, thereby eliminating the flow instabilities and flow-regime transitions that would otherwise result from stochastic generation of vapor bubbles by nucleation from a superheated channel wall. Slug flow boiling is characterized in a heated, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a uniform heat flux via Joule heating. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles under heat fluxes ranging from 30 W/m2 to 5160 W/m2. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics. The experimental approach demonstrated in this study provides a unique platform for the investigation of microchannel slug flow boiling transport under controlled, stable conditions suitable for model validation.