Effects of surface wettability and cross section area on the evolution of an elongated bubble in a microchannel (original) (raw)
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Developments on Wetting Effects in Microfluidic Slug Flow
Chemical Engineering Communications, 2012
Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tensions forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived realistic, and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel Tjunction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed for insight 2 into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channels walls and highlight the value of numeral simulation in microfluidic design.
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International Journal of Heat and Mass Transfer, 2004
The present work investigates experimentally the bubble dynamics in two parallel trapezoidal microchannels with a hydraulic diameter of 47.7 lm for both channels. The fabrication process of the two parallel microchannels employs a silicon bulk micromachining and anodic bounding process. The results of this study demonstrate that the bubble growth and departure is generally similar to that in a single microchannel, i.e., bubbles, in general, grow linearly with time and their departure is governed by surface tension and drag due to bulk two-phase flow. For the two low mass flow rates, the growth of bubble in slug flow is also investigated. It is found that the bubble grows in the axial direction both forward and backward with its length increases exponentially due to evaporation of the thin liquid film between the bubble and heating wall. However, the coefficient of exponent is much smaller than that caused by evaporation due to the limitation effect of liquid pressure around the bubble.
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...
Chemical and Biochemical Engineering Quarterly, 2017
In this study, bubble dynamics and frictional pressure drop associated with gas liquid two-phase slug flow regime in adiabatic T-junction square microchannel has been investigated using CFD. A comprehensive study on the mechanism of bubble formation via squeezing and shearing regime is performed. The randomness and recirculation profiles observed in the squeezing regime are significantly higher as compared to the shearing regime during formation of the slug. Further, effects of increasing gas velocity on bubble length are obtained at fixed liquid velocities and simulated data displayed good agreement with available correlations in literature. The frictional pressure drop for slug flow regime from simulations are also obtained and evaluated against existing separated flow models. A regression correlation has also been developed by modifying C-parameter using separated flow model, which improves the prediction of two-phase frictional pressure drop data within slug flow region, with mean absolute error of 10 %. The influences of fluid properties such as liquid viscosity and surface tension on the two-phase frictional pressure drop are also investigated and compared with developed correlation. The higher liquid viscosity and lower surface tension value resulted in bubble formation via shearing regime.
A simple mechanism of bubble and slug formation in Taylor flow in microchannels
Chemical Engineering Research and Design, 2008
A simple mechanism is proposed to explain and predict the bubble and slug lengths in Taylor (slug) flow in microchannels. The results obtained using the proposed approach are in good agreement with a correlation based on numerical experiments , Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci, 80: 7609-7625] and available experimental data.
The pressure drop along rectangular microchannels containing bubbles
2007
This paper derives the difference in pressure between the beginning and the end of a rectangular microchannel through which a flowing liquid (water, with or without surfactant, and mixtures of water and glycerol) carries bubbles that contact all four walls of the channel. It uses an indirect method to derive the pressure in the channel. The pressure drop depends predominantly on the number of bubbles in the channel at both low and high concentrations of surfactant. At intermediate concentrations of surfactant, if the channel contains bubbles (of the same or different lengths), the total, aggregated length of the bubbles in the channel is the dominant contributor to the pressure drop. The difference between these two cases stems from increased flow of liquid through the ''gutters''-the regions of the system bounded by the curved body of the bubble and the corners of the channel-in the presence of intermediate concentrations of surfactant. This paper presents a systematic and quantitative investigation of the influence of surfactants on the flow of fluids in microchannels containing bubbles. It derives the contributions to the overall pressure drop from three regions of the channel: (i) the slugs of liquid between the bubbles (and separated from the bubbles), in which liquid flows as though no bubbles were present; (ii) the gutters along the corners of the microchannels; and (iii) the curved caps at the ends of the bubble.
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.
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.
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.
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.