Developments on Wetting Effects in Microfluidic Slug Flow (original) (raw)
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Slug flow in microchannels: Numerical simulation and applications
Journal of Industrial and Engineering Chemistry, 2018
This paper reviews the state-of-the-art numerical techniques employed in the literature for modelling slug flow in microchannels. The proposed solutions in literature for overcoming some of the drawbacks of the numerical methods are presented. Additionally, literature covering specific applications such as enhancement of heat transfer and mixing is reviewed to provide further insight into the transport mechanisms and their applications. Digital microfluidics, as a means of slug manipulation and control, is introduced in the following section of the paper. The application of thermocapillary, magnetic, electric, optical and acoustic forces is elaborated in particular.
Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels
The hydrodynamics of liquid slugs in gas-liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L s /w < 1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U TP . The product of the liquid slug residence time and the recirculation rate is independent of U TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.
Gas-liquid slug formation at a rectangular microchannel T-junction: A CFD benchmark case
Central European Journal of Engineering, 2011
Computational fluid dynamics (CFD) is an important tool for development of microfluidic systems based on gasliquid two-phase flow. The formation of Taylor slugs at microchannel T-junctions has been studied both experimentally and numerically, however discrepancies still exist because of difficulties in correctly representing experimental conditions and uncertainties in the physics controlling slug flow, such as contact line and velocity slip. In this paper detailed methods and results are described for the study of Santos and Kawaji [1] on the comparison of experimental results and numerical modeling. The system studied consisted of a rectangular microchannel Tjunction nominally 100 µm in hydraulic diameter, used to generate Taylor slugs from air-water perpendicular flow. The effect of flow rates on parameters such as slug length, velocity slip, void fraction and two-phase frictional pressure drop were studied. Numerical simulation was performed using FLUENT volume-of-fluid (VOF) model. It is proposed in this paper that this microfluidic problem be taken up by researchers in the field as a benchmark case to test other numeric codes in comparison to FLUENT on the prediction of micro-scale multiphase flow, and also to model in more detail the experimental system described to obtain greater accuracy in prediction of microfluidic slug formation.
Experimental study of slug flow for condensation in a single square microchannel
Experimental Thermal and Fluid Science, 2012
Local condensation heat transfer for slug flow in a single silicon square microchannel is investigated. Chromel–alumel microthermocouples are located in the rectangular microgrooves formed in the silicon wafer and covered with Pyrex glass for measuring the surface temperature. Various condensation flow patterns are identified in the microchannel: mist flow, churn flow, annular flow, slug flow, liquid ring flow, and annular/bubbly flow. Our attention is focused on the analysis of local heat transfer, and hydrodynamic characteristics of slug flow because it is one of the basis two-phase flow pattern in condensation in the microchannel. Experimental results obtained from images processing show that bubbles velocity is significantly influenced by the departure of each new bubble followed with the new liquid slug from the microchannel entrance. The coalescence phenomena between the neighboring bubbles contribute to increase the bubbles velocity. The experimental data are compared with co...
Journal of Micromechanics and Microengineering, 2003
Experimental observations of liquid microchannel flows are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and turbulence effects on the friction factor are discussed. The experimental findings are classified into three groups. Group I emphasizes 'flow instabilities' and group II points out 'viscosity changes' as the causes of deviations from the conventional flow theory for macrochannels. Group III caters to studies that did not detect any measurable differences between micro- and macroscale fluid flow behaviors. Based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and particle suspension flows. The wall slip effect is negligible for liquid flows in microconduits. Significant surface roughness effects are a function of the Darcy number, the Reynolds number and cross-sectional configurations. For relatively low Reynolds numbers, Re < 2000, onset to turbulence has to be considered important because of possible geometric non-uniformities, e.g., a contraction and/or bend at the inlet to the microchannel. Channel-size effect on viscous dissipation turns out to be important for conduits with Dh < 100 µm.
Numerical Investigations of Two-phase Flows through Enhanced Microchannels
Chemical and Biochemical Engineering Quarterly, 2016
Microfluidic devices are quite important for process industries, as these devices can intensify heat and mass transfer in two-phase reaction systems. Two-phase reaction systems, such as gas-liquid and liquid-liquid reactions with certain limitations have already been carried out in microfluidic systems by a few authors. However, these concepts are still under development and a detailed understanding of the hydrodynamics involve is required. Hydrodynamics studies are inherently crucial to provide precise reaction conditions and identify asymptotic performance limits. In the present work, Computational Fluid Dynamics (CFD) simulation was carried out to investigate the hydrodynamics involved in the T-junction enhanced microchannel. The slug formation, slug size, slug shape, and pressure drop in the enhanced microchannel were predicted using the volume of fluid (VOF) for water-cyclohexane system. The effects of obstruction spacing on pressure drop, slug lengths, and mixing within the slug were also examined. This study revealed that mixing enhances tremendously within the slug and at the interface in the enhanced microchannel, but with slightly greater pressure drop. However, an increase in obstruction spacing affects the slug formation, unit slug length, and pressure drop.
Fluid mechanics of flow through rectangular hydrophobic microchannels.
In this study, the effect of two important parameters have been evaluated for pressure driven liquid flows in microchannel in laminar regime by analytical modeling, followed by experimental measurement. These parameters are wettability conditions of microchannel surfaces and aspect ratio of rectangular microchannels. For small values of aspect ratio, the channel was considered to a have rectangular cross-section, instead of being two parallel plates. Novel expressions for these kinds of channels were derived using Eigen function expansion method. The obtained two-dimensional solutions based on dual finite series were then extended to the case of a constant slip velocity at the bottom wall. In addition, for large values of aspect ratio, a general equation was obtained which is capable of accounting for different values of slip lengths for both upper and lower channel walls. Firstly, it was found that for low aspect ratio microchannels, the results obtained by analytical rectangular 2-D model agree well with the experimental measurements as compared to one dimensional solution. For high aspect ratio microchannels, both models predict the same trend. This finding indicates that using the conventional 1-D solution may not be accurate for the channels where the width is of the same order as the height. Secondly, experimental results showed that up to 2.5% and 16% drag reduction can be achieved for 1000 and 250 micron channel height, respectively. It can be concluded that increasing the surface wettability can reduce the pressure drop in laminar regime and the effect is more pronounced by decreasing the channel height.
EFFECT OF SURFACE WETTABILITY AND GAS/LIQUID VELOCITY RATIO ON MICROSCALE TWO-PHASE FLOW PATTERNS
Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.