Slug flow in microchannels: Numerical simulation and applications (original) (raw)

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.

Thermal Performance Analysis of Slug Flow in Square Microchannels

Thermal Performance Analysis of Slug Flow in Square Microchannels, 2020

Thermal performance enhancement of microchannel heat sinks for electronics cooling is becoming more and more necessary. To this end, microchannels with noncircular cross-sections conveying immiscible droplets have been employed in this study and geometric manipulations are applied to enhance fluid mixing and consequently achieve a higher rate of heat removal. Three-dimensional numerical simulations are performed using volume of fluid method for channels of 100 mm hydraulic diameter. Constant wall temperature is chosen as the boundary condition for heating section of the channels. Effects of parameters such as adding curvature to the side walls and varying the entering velocity of the base liquid on heat transfer rate are studied. A performance coefficient is used to evaluate the relative impact of increase in both the Nusselt number and pressure drop as a result of adding curvature to the channel walls. Results of the study showed that slug flow in curved channels is capable of improving the thermal performance in comparison with single liquid flow in straight channels and in best case, can improve the performance up to 50%.

An Analytical-Numerical Model for Two-Phase Slug Flow through a Sudden Area Change in Microchannels

Journal of Applied Fluid Mechanics

In this paper, two new analytical models have been developed to calculate two-phase slug flow pressure drop in microchannels through a sudden contraction. Even though many studies have been reported on two-phase flow in microchannels, considerable discrepancies still exist, mainly due to the difficulties in experimental setup and measurements. Numerical simulations were performed to support the new analytical models and to explore in more detail the physics of the flow in microchannels with a sudden contraction. Both analytical and numerical results were compared to the available experimental data and other empirical correlations. Results show that models, which were developed based on the slug and semi-slug assumptions, agree well with experiments in microchannels. Moreover, in contrast to the previous empirical correlations which were tuned for a specific geometry, the new analytical models are capable of taking geometrical parameters as well as flow conditions into account.

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.

Slug Flow-Heat Transfer in Parallel Plate Microchannel Including Slip Effects and Axial Conduction

Energy Procedia, 2013

In this paper the forced convection with slug flow model in parallel plate microchannel is studied. In the slug flow model, it is assumed that the velocity of the fluid is uniform across any cross-section perpendicular to the axis of the microchannel. The microscale analysis is taking into account by including the jump temperature at the surface. The axial conduction within the fluid is included but the viscous dissipation is neglected. The energy equation is solved by finite integral transform technique. The isoflux and isothermal boundary conditions were applied. The local and fully developed Nusselt numbers have been obtained in terms of the Knudsen number and Peclet number.

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...

Flow and heat transfer in slug flow in microchannels: Effect of bubble volume

International Journal of Heat and Mass Transfer, 2019

Flow and heat transfer in gas-liquid slug flow in small diameter channels have been studied extensively in the last few decades because of its unique ability to segment the flow and enhance heat transfer by the internal recirculation in the liquid phase. The segmentation of the continuous liquid phase is achieved by the gas bubble of the size of the channel. While the hydrodynamics and heat transfer for long Taylor bubbles having volume more than that of a sphere that can fit in the channel has been studied extensively, very little attention has been paid to the bubbles having smaller volume but almost spanning the channel. The bubble volume can be represented by a non-dimensional equivalent sphere radius, ratio of the radius of a sphere having same volume as that of the bubble and channel radius. In this work, we study the hydrodynamics of the slug flow for a range of bubble volumes keeping all other parameters constant for non-dimensional equivalent sphere radius close to 1, between 0.72 and 1.55. The bubble shape, pressure distribution, bubble velocity and flow field in the liquid slug has been investigated. The effect of Reynolds number on the bubble shape for short as well as Taylor bubbles has also been studied. Heat transfer without phase change for constant heat flux boundary condition at the wall has been investigated and the Nusselt number is found to be highest for the non-dimensional equivalent sphere radius close to one. The heat transfer results have also been compared with a simple phenomenological model available in literature for heat transfer in slug flow.

Liquid-liquid Slug Flow in a Microchannel Reactor and its Mass Transfer Properties -A Review

Mass transfer is a basic phenomenon behind many processes like reaction, absorption, extraction etc. Mass transfer plays a significant role in microfluidic systems where the chemical / biological process systems are shrinkened down to a micro scale. Micro reactor system, with its high compatibility and performance, gains a wide interest among the researchers in the recent years. Micro structured reactors holds advantages over the conventional types in chemical processes. The significance of micro reactor not limited to its scalability but to energy efficiency, on-site / on-demand production, reliability, safety, highly controlled outputs, etc. Liquid-liquid two phase reaction in a microreactor system is highly demandable when both reactants are liquids or when air medium cannot be suitable. This article overviews various liquid-liquid flow regimes in a microchannel. Discussions on the hydrodynamics of flow in micro scale are made. Considering the importance of mass transfer in liquid-liquid systems and the advantage of slug regime over other regimes, the article focuses especially on the mass transfer between two liquid phases in slug flow and the details of experimental studies carried out in this area. The advantages of slug flow over other flow regimes in micro structured reactor applications are showcased.