Mass Transfer In Microchannels Research Articles

Thin-film evaporation in microchannels: A combined analytical and computational fluid dynamics approach to assessing meniscus curvature impact

Computational fluid dynamics (CFD) is employed to investigate the effects of variation in bulk meniscus curvature on evaporative heat and mass transfer in microchannels. Evaporation in microchannels presents a classic multiscale problem, encompassing both microscopic and macroscopic length scales in the thin-film and bulk meniscus regions, respectively. Therefore, the liquid-vapor interface or meniscus shape is first determined through thin-film evaporation analysis using numerical approaches found in the literature. However, a new approach for establishing boundary conditions is also proposed, allowing for the representation of varying curvatures within the bulk meniscus region. Then, the obtained meniscus thickness profiles are exported to a two-dimensional CFD framework. The primary advantage of this developed CFD framework lies in its capability to simultaneously analyze evaporative heat and mass transfer from both microscopic and macroscopic regions, a feature that enhances research endeavors concerning micro and miniature heat pipes. The developed model is applied to various rectangular microchannel sizes, ranging from 10 to 100 μm in width, with wall superheats varying from 0.1 to 3.0 K. The aim is to quantify the effects of variations in bulk meniscus curvature on the evaporative characteristics of the extended meniscus (comprising both thin-film and bulk meniscus regions), using the concept of thermal resistance. Consequently, based on the CFD simulation results, multiple regression analysis is employed to formulate the total thermal resistance in terms of independent parameters, namely the microchannel width, wall superheat, and the radius of curvature (Rc). It is observed that the radius of curvature has a marginal impact on the total thermal resistance, allowing its influence to be expressed by a power-law function of (Rc/Rm)0.102, where Rm represents the minimum radius of curvature.

Numerical investigation on Taylor bubble mass transfer in microchannel based on CO2 gas with the consideration of gas compressibility

This paper focused on the mass transfer of Taylor bubbles composed of pure carbon dioxide in organic solvent in microchannels. Combined with the Henry’s law, the carbon dioxide concentration at the two-phase interface was dynamically updated by using a compressible fluid model and considering the variation of gas density in the bubble. The critical bubble shape factor could help to judge the dominant mechanism of bubble deformation. The entire mass transfer process was divided into three stages: the rapid dissolution stage, the two-end dissolution stage, and the diffusion-like dissolution stage. The gas–liquid mass transfer capacity and influencing factors of each stage were analyzed, and the dominant mass transfer mechanism of each stage was summarized. It was found that the non-uniform degree of gas density in the bubble was related to the bubble Reynolds number and bubble volume by analyzing the distribution variation of solute density.

Mass Transfer Characteristics of CO2 and Blended Aqueous Solutions of [C2OHmim][Lys]/MDEA in a Microchannel

In this paper, the two-phase flow and mass transfer process of CO2 and [C2OHmim][Lys]/MDEA mixed aqueous solution in a T-shaped microchannel were investigated systematically. By using a high-speed camera connected to a computer, the two-phase flow patterns were recorded online, including the Taylor flow and a small amount of Taylor-bubbly flow. And the effects of ionic liquid concentration and two-phase flow ratio (QG/QL) on bubble length, specific surface area, gas holdup, and mass transfer coefficient were studied experimentally. The results indicate that the blended solution of [C2OHmim][Lys]/MDEA could significantly enhance two-phase mass transfer compared to single N-methyldiethanolamine aqueous solution. The reaction mechanism between CO2 and ionic liquid/amine mixed aqueous solution was also analyzed. On the basis of experimental data, some new correlations for predicting the bubble initial length and the volumetric mass transfer coefficient were proposed, which matched well with the experimental data. This work is helpful to provide new ideas for the design of CO2 absorbents and further improve the data of gas–liquid mass transfer in microchannels.

Quantification of interfacial mass transfer during droplet formation in a microfluidic T-junction using VOF

Taylor flow in microchannels is characterized by interfacial mass transfer rates which are two to three orders of magnitude larger than in conventional reactors. During the droplet formation stage vortices are observed which further contribute to interfacial mass transfer. In this work, we quantify the contribution of the droplet formation stage to the overall mass transfer by integrating a compressive continuum species transfer model into a volume-of-fluid (VOF) solver. It is observed that the interfacial mass transfer depends solely on the vortex intensity. Moreover, altering the flow rate ratio and viscosity significantly affects the interfacial mass transfer performance. Overall, the interfacial mass transfer during the droplet formation stage contributes up to ∼70% of the overall mass transfer at large relative velocities between the dispersed and continuous phases. The present findings will guide applications involving interfacial mass transfer in microchannels.

Transforming Navier‐stokes equation to a fourth order nonlinear differential equation for the study of electrolyte flow in a circular microchannel

AbstractIn this paper, analytical solutions are derived for the study of steady‐state electrolyte flow in a circular microchannel under the simultaneous effects of an external electric field and uniform wall injection. The Navier‐Stokes equations containing an axial linear electric body force are transformed to a fourth‐order nonlinear differential equation and solved analytically using a series solution method. The most important results are presented and discussed in terms of radial Reynolds number and a fundamental non‐dimensional electric field parameter. The Electrical Double Layer theory is employed to model the effects of external electric field. Analytical relations are given for velocity profiles, exit flow rate, and total pressure drop in the microchannel. The results show that a high‐intensity electric field can completely reverse the velocity profile or make it more flat in the presence of wall injection. This electric effect can be used in practical applications to control the shear stress, heat, or mass transfer in microchannels or microelectromechanical systems. In addition, various graphs are presented showing simultaneous effects of electric field and wall injection of the same fluid on the flow physics.

Numerical Investigation on the Heat and Mass Transfer in Microchannel with Discrete Heat Sources Considering the Soret and Dufour Effects.

Heat-transfer enhancement in microchannel heat sinks (MCHS) has been a hot topic in the last decade. However, most published works did not focus on the heat sources that are discrete, as in most microelectronic devices, and the enhancement of heat and mass transfer (HMT) due to the Soret and Dufour effects being ignored. Based on a heterogeneous two-phase model that takes into consideration the Soret and Dufour effects, numerical simulations have been performed for various geometries and heat sources. The numerical results demonstrate that the vortices induced by a heat source(s) can enhance the heat transfer efficiency up to 2665 W/m2·K from 2618 W/m2·K for a discrete heat source with a heat flux q = 106 W/m2. The Soret effect can affect the heat transfer much more than the Duffour effect. The integrated results for heat transfer due to the Soret and Dufour effects are not sampled superpositions. Discrete heat sources (DHS) arranged in microchannels can enhance heat transfer, especially when the inlet velocity of the forced flow is less than 0.01 m/s. This can provide a beneficial reference for the design of MCHS with DHS.

Biochemical Reaction Acceleration by Electrokinetic Mixing in a Microfluidic Chip.

In the past two decades, a large number of micromixers have been studied and reported to solve the low mass transfer in microchannels. However, there is still a lack of research on how much biochemical reaction efficiency or detection sensitivity can be improved by microfluidic rapid mixing. In this study, using our previously developed ultrafast microelectrokinetic turbulent (μEKT) mixing method, taking glucose oxidation reaction as the research object, we investigated the effect of increasing microfluidic mass transfer efficiency on the efficiency and sensitivity of biochemical reactions. The results showed that fast mixing could improve the enzymatic reaction efficiency by improving the mass transfer efficiency. Further detection of glucose in glucose solutions and blood samples showed that the fast mixing could significantly improve the sensitivity of detection. These results indicate that mixing enhancement can be a significant step of microfluidics-based applications such as POCT, liquid biopsy, food safety, and so on.

Hydrodynamics of Liquid-Liquid Flows in Micro Channels and Its Influence on Transport Properties: A Review

Hydrodynamics plays a major role in transport of heat and mass transfer in microchannels. This includes flow patterns and flow regimes in which the micro-channels are operated. The flow patterns have a major impact the transport properties. Another important aspect is the pressure drop in micro-channels. In the present review, the experimental and Computational Fluid Dynamics (CFD) studies covering all the above aspects have been covered. The effect of geometrical parameters like shape of channel, channel size, material of construction of channels; operating parameters like flow velocity, flow ratio and fluid properties have been presented and analyzed. Experimental and analytical work of different pressure drop models has also been presented. All the literature related to influence of flow patterns on transport properties like volumetric mass transfer coefficients (VMTC) and heat transfer coefficients (HTC) have been presented and analyzed. It is found that most works in Liquid-Liquid Extraction (LLE) systems have been carried out in slug flow and T-junctions. Models for coupled systems of flow and mass transfer have been presented and works carried out for different coupled systems have been listed. CFD simulations match experimental results within 20% deviations in quantitative and qualitative predictions of flow phenomena for most research articles referred in this review. There is a disparity in prediction of a generalized regime map and a generalized regime map for prediction of flow patterns for various systems would need the help of Artificial Intelligence.

Numerical study on gas–liquid two-phase flow and mass transfer in a microchannel

Abstract A numerical study of the gas–liquid two-phase flow and mass transfer in a square microchannel with a T-junction is carried out in this work. Through numerical simulation methods, the flow patterns of bubble flow, slug flow and annular flow are determined. By proposing a new flow pattern conversion relationship with different media and different speeds, 100 sets of CO2-water flow patterns and 100 sets of CO2-ethanol flow patterns are obtained. The effects of surface tension on flow pattern, bubble length and liquid plug length are studied. The pressure distribution and pressure drop are analyzed, and mass transfer is obtained through slug flow simulation, and the influencing factors of gas–liquid mass transfer are studied. The results show that the effect of surface tension on the length of the bubble and the length of the liquid plug is completely opposite, the pressure distribution is stepped, and the pressure drop increases with the increase of the gas–liquid velocity. In addition, it was found that the volumetric mass transfer coefficients of the bubble cap and the liquid film gradually decreased with time, and eventually stabilized. The increase in bubble velocity accelerates the mass transfer rate, while the increase in unit cell length slows the mass transfer rate. However, the influence of film thickness and liquid film length on mass transfer varies with time.

Intensification of gas-liquid two-phase flow and mass transfer in microchannels by sudden expansions

The intensification of mass transfer of CO2 absorption into [Bmim][BF4] aqueous solution by continuous sudden expansion units was investigated experimentally in microchannels. The influences of two-phase flow rates and the number of sudden expansion units on pressure drop and volumetric mass transfer coefficient were studied systematically. The results indicate that the sudden expansion structure could remarkably enhance gas-liquid mass transfer. The enhancement factor of mass transfer increases with increasing the number of sudden expansion units and gas-liquid flow rate ratio, while the pressure drop has only slight elevation. The effective mass transfer enhancement efficiency was assessed according to the energy consumption and enhancement factor, which could achieve from 1.2 to 2.4, showing excellent mass transfer intensification effect of the sudden expansion structure. Moreover, a new correlation was proposed for predicting liquid side volumetric mass transfer coefficient.

Two-phase flow and mass transfer in microchannels: A review from local mechanism to global models

Microreaction technology is preferential in process intensification and chemical synthesis, especially in multiphase applications. A thorough understanding of the hydrodynamics and mass transfer is the pre-requisite for implementing such applications. This review discusses the recent progress on the flow and mass transfer of two-phase systems in microchannels, in a multi-scale view from local mechanism to global behavior. The flow patterns, the formation of bubbles/droplets, and the manipulation of bubbles/droplets are presented and discussed in detail. The mass transfer aspects include the velocity profile and mixing inside droplets/slugs, as well as the interfacial mass transfer mechanism and simulative models. The aim of the review is to show directly the physical ingredients which determine the transport phenomena, help explain the observed behavior, and guide reactor design. To simplify the physical ingredients, the attention is focused on straight channels while phenomena in other channels (e.g., meandering channel) are neglected.

Enhancement of gas-liquid mass transfer in microchannels by rectangular baffles

The configuration of microchannel with staggered rectangular baffles was constructed to improve the gas-liquid mass transfer efficiency. The influences of the baffle blockage length, baffle interval, channel width and the gas and liquid superficial velocities on the volumetric mass transfer coefficient, pressure drop and efficient energy efficiency for CO2/water system were investigated experimentally. The enhancement mechanism of mass transfer was analyzed according to the gas-liquid two-phase flow pattern in baffled microchannels. The comparison of the volumetric mass transfer coefficient between the microchannels with and without baffles was performed to present the intensification of baffled structure to mass transfer. The increases of baffle blockage length and channel width, and the reduction of baffle interval are conducive to enhancing gas-liquid mass transfer, with the maximum enhancement factor up to 2.8. Although the baffled structure could lead to an increase in pressure drop within 0.5 kPa in absolute value or 23% in relative value, which is an acceptable rise for practical application. Moreover, effective energy efficiency could reach to 2.25 in the case of the maximum enhancement of mass transfer. This study could provide theoretical guidance for the development and optimization of rectangular baffled microdevices for process intensification.

A comprehensive review on liquid–liquid two-phase flow in microchannel: flow pattern and mass transfer

Liquid–liquid two-phase flow in microchannel is very common in micro-chemical and micro-biological system, etc. Deep understanding of the liquid–liquid two-phase flow mechanisms and mass transfer in microchannel can promote industrial applications significantly. To summarize the recent research progress on the liquid–liquid two-phase flow in microchannel, this paper collects research work about this topic, especially focusing on flow pattern and mass transfer. To begin with, flow patterns observed in various conditions are identified and factors which influence the flow patterns are analyzed. Then, mass transfer in liquid–liquid two-phase flow is discussed, especially the mass transfer during droplet flow, with both experiments and simulations. Furthermore, energy dissipation involved in liquid–liquid two-phase flow in microchannel is also briefly discussed. Finally, future needs are proposed for extending the researches on liquid–liquid two-phase flow and enlarging its application fields.

Controlled retention of droplets and the enhancement of mass transfer in microchannel with multi-groove structure

A simple and novel droplet-based microfluidic device with multi-groove structure has been developed to achieve stay-collision-transfer process of droplets. After summarizing the scaling law of droplet diameter and formation frequency with two phase flow rates, we got the optimum operating range of continuous phase flow. During the retention of droplet in groove, the existence of internal vortex was observed, which lead to the homogeneity of internal concentration field. For the process of solvent-microextraction with high phase ratio, due to multi-groove structure, the actual volume phase ratio in microchannel can be much smaller than injection phase ratio to improve extraction efficiency. And two-phase acid-base reaction and fluorescence extraction experiments further validate the enhancement of mass transfer process by this kind of microchannel.

Experimental study of the contribution of liquid film for liquid-liquid Taylor flow mass transfer in a microchannel

Liquid-liquid two-phase Taylor flow mass transfer in microchannels, which has many advantages for heat and mass transfer operations, should be studied to develop practical applications in various chemical processes. To understand the effects of various parameters on mass transfer and develop an estimation method of mass transfer performance, mass transfer in microchannels was examined by an evaluation of phenol extraction from dodecane to water. Under stable Taylor flow conditions in a 1mm inner diameter glass channel, the organic phase (dodecane) formed liquid droplets and the aqueous phase (water) formed liquid slugs with a liquid film enclosing the organic phase droplets. The effect of the droplet length on mass transfer was investigated with a microchannel device that consists of a circular channel with a 1mm inner diameter and union tees with inner diameters of 2.0mm and 2.4mm. Although the droplets lengths were varied in the range of 7.4mm to 30.5mm, approximately the same volumetric mass transfer coefficients (Ka) were obtained. This result suggested that the side interface of the droplets that contact the thin liquid film becomes a mass transfer interface as well as an end interface. This occurs because the side interface area increases with increasing droplet length and droplet volume while the specific interfacial area remains approximately constant. The volumetric mass transfer coefficients were obtained for various cross-sectional shapes and hydraulic equivalent channel diameters at various flow velocities. The mass transfer coefficients were derived by dividing the volumetric mass transfer coefficients by the specific interfacial area. It was found that the Sherwood number (Sh), which is a dimensionless expression of the mass transfer coefficient, has excellent correlation with the Reynolds number (Re); thus, the estimation of the mass transfer performance for a given channel and given operational conditions can be determined using the correlation between Sh and Re.

Towards a new membrane micro reactor system for direct synthesis of hydrogen peroxide

The approach of producing hydrogen peroxide (H2O2) by direct catalyzed reaction of H2 and O2 has been extensively studied over the past decades. The process offers the potential to establish a “green” alternative to the conventional large-scale anthraquinone process. However, the implementation of this reaction still faces major hurdles including safety, selectivity and productivity of the reaction. In this work, we report results on the development of a novel intensified suspension-flow membrane micro reactor system for direct synthesis of hydrogen peroxide as well as on a related flexible micro reactor system for transfer hydrogenation which served as a starting point. In the new reactor system for H2O2 direct synthesis safety issues are greatly reduced by the introduction of a membrane to separate the gaseous reactants. Furthermore productivity is maximized by increased mass transfer in micro channels and constant reactant feeding over the length of the reactor. Finally, the selectivity can be optimized by controlling the reactant distribution in the liquid phase. A rigorous two-dimensional single-phase model based on ANSYS Fluent was adopted to study the influence of the design on the reaction. This model had already been validated in a previous study. By expanding the model to describe the multiphase phenomena we were able to get further insight on the behavior of the overall system.

A Lagrangian approach for modelling electro-kinetic mass transfer in microchannels

The computational modelling of multiphysics mass transfer in a microchannel is a challenging endeavour. This article has proposed and validated a Lagrangian methodology for modelling coupled physics using near optimal computational efforts. This new development has been verified and compared with a reference Eulerian finite difference model. A heuristic theory for computational mass transfer phenomena has been studied. At high Peclet numbers when the reference Eulerian model fails, the Lagrangian model resolves electro-osmotic mass transfer, showing a good quantitative agreement with theoretical analysis. The Lagrangian model also helps to estimate necessary parameters so that an optimal electro-osmotic pumping can be designed in a microchannel. The computational efficiency of the Lagrangian model has been examined, showing that an increase of the Peclet number by a factor of 32 increases the global computational complexity by about a factor of 104 if the reference Eulerian model were used. This verifies the optimal performance of the Lagrangian model.

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Numerical study of the coupling between reaction and mass transfer for liquid-liquid slug flow in square microchannels

While the benefits of miniaturization on processes have been widely demonstrated, its impact on microfluidics and local mechanisms such as mass transfer is still little understood. The coupling between reaction and mass transfer in microchannels is simulated for liquid-liquid slug flow. First, the extrapolation to confined flow of the classical model used to calculate interfacial mass fluxes in reactive infinite media was studied. This model consists in estimating transferred fluxes between two phases as a function of the enhancement factor E. Its expression depends on the model used to represent interfacial mass transfer. In infinite media, Lewis and Whitman's stagnant film theory is generally preferred for its simplicity and its reliability. In the case of confined slug flow, the limitation of such a model to predict interfacial fluxes is highlighted. Second, the case of liquid-liquid competitive consecutive reactions in microchannels is considered. The unfavorable impact of the length between droplets on selectivity is emphasized. This is a direct consequence of mass-transport mechanisms in microchannels. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Enhanced liquid–solid mass transfer in microchannels by a layer of carbon nanofibers

AbstractThis paper demonstrates that the observed rate of reaction of the liquid-phase selective hydrogenation of an alkyne is higher for an open and rough carbon nanofiber (CNF) layer positioned on a microchannel wall than for an unsupported flat plate catalyst or dense and smooth CNF layers. This enhancement can be contributed to two phenomena: (1) the kinetic reaction rate increases with a factor of 1.4, due to an increase of the open and freely accessible surface area of the CNF layer available for catalyst deposition; (2) the liquid–solid mass transfer increases with a factor of 2.8, due to an increase in the surface roughness of the CNF layer and the formation of large spherical islands of clustered CNFs. A thin layer of CNFs on a microchannel wall can therefore successfully be used to increase the overall rate of reaction per reactor volume compared to an unsupported flat plate catalyst.