Flow In Microchannel Research Articles

Optimised Electro-Osmotic Flow in Rectangular Microchannels with Smoothed Corners

Electro-osmotic flows are a means of circulating polar fluids through microchannels without resorting to mechanical pumping. The lack of moving parts, of noise and the ease of integration in silicon chips make them an interesting option for microchip cooling and miniaturized total analysis systems. This paper describes an optimization in terms of first- and second-law analysis of the cross-section of a microchannel subject to electro-osmotic flow. Starting from rectangular cross-sections of different aspect ratios, its corners are progressively smoothed and the resulting Poiseuille and Nusselt numbers computed. Performance evaluation criteria are then used to assess the change in, among others, heat transfer rate, temperature difference between wall and bulk fluid, and equivalent pumping power. The entropy number is also computed and the results commented. It is found that the latter criterion highlights a configuration of minimum entropy generation, whereas the trends of the heat transfer and temperature difference are opposite to that of the equivalent pumping power.

A novel microparticle size sorting technology based on sheath flow stable expansion regimes

Previously we have originally reported and experimentally explored the stable expansion phenomenon of sheath flow in straight microchannels. The stable expansion phenomenon differs from the focused stable thread and diffusion, which are expected to appear based on current understanding. It happens within a specific range of Reynolds numbers in microchannels with specific aspect ratios. However, the mechanism insight and potential applications remained poorly understood. In this study, a comprehensive numerical model, involving laminar flow, diluted species transportation, and particle tracing, was established to provide a global transparent insight. Based on the simulation results, the root cause and mechanism of the stable expansion phenomenon were revealed as a dumbbell-shape deformation of the buffer–sample–buffer interface. In addition, a microparticle size sorting application based on these findings was proposed and conducted in both simulation and experiment. The results showed that 1 and 15 μm microparticles can be totally separated with nearly 100% purity. According to the advantages and potentials of this technique, it can be promising to approach a label-free microparticle sorting with a high throughput manner.

Effects of channel wall wettability on gas–liquid dynamics mass transfer under Taylor flow in a serpentine microchannel

The wall wettability of microchannels plays an important role in the gas–liquid mass transfer dynamics under Taylor flow. In this study, we regulated the contact angle of the wall surface through surface chemical grafting polymerization under controlled experimental conditions. The dynamic changes of CO2 bubbles flowing along the microchannel were captured by a high-speed video camera mounted on a stereo microscope, whilst a unit cell model was employed to theoretically investigate the gas–liquid mass transfer dynamics. We quantitatively characterized the effects of wall wettability, specifically the contact angle, on the formation mechanism of gas bubbles and mass transfer process experimentally. The results revealed that the gas bubble velocity, the overall volumetric liquid phase mass transfer coefficients (kLa), and the specific interfacial area (a) all increased with the increase of the contact angle. Conversely, gas bubble length and leakage flow decreased. Furthermore, we proposed a new modified model to predict the gas–liquid two-phase mass transfer performance, based on van Baten's and Yao's models. Our proposed model was observed to agree reasonably well with experimental observations.

Smoothed Particle Hydrodynamics-Based Study of 3D Confined Microflows

In this study, we investigate the performance of the smoothed particle hydrodynamics (SPH) method regarding the computation of confined flows in microchannels. Modeling and numerical simulation with SPH involve the representation of flowing matter as distinct mass points, leading to particle discretization of the Navier–Stokes equations. The computational methodology exhibits similarities with other well-established particle methods, such as molecular dynamics (MD), dissipative particle dynamics (DPD), and smooth dissipative particle dynamics (SDPD). SPH has been extensively tested in the simulation of free-surface flows. However, studies on the performance of the method in internal flow computations are limited. In this work, we study flows in microchannels of variable cross-sections with a weakly compressible SPH formulation. After preliminary studies of flows in straight constant cross-section ducts, we focus on channels with sudden expansion and/or contraction. Flow models based on periodic or various inlet/outlet boundary conditions and their implementations are discussed in the context of 2D and 3D simulations. Numerical experiments are conducted to evaluate the accuracy of the method in terms of flowrate, velocity profiles, and wall shear stress. The relation between f and Re for constant cross-section channels is computed with excellent accuracy. SPH captured the flow characteristics and achieved very good accuracy. Compressibility effects due to the weakly compressible smoothed particle hydrodynamics (WCSPH) formulation are negligible for the flows considered. Several typical difficulties and pitfalls in the application of the SPH method in closed conduits are highlighted as well as some of the immediate needs for the method’s improvement.

Couple Stress Sodium Alginate-Based Casson Nanofluid Analysis through Fick’s and Fourier’s Laws with Inclined Microchannel

Casson nanofluid plays a vital role in food industries with sodium alginate nanoparticles. That is why many researchers used Casson nanofluid in their study. Due to this, the main objective of this study is to investigate the inclined microchannel flow of a Casson nanofluid based on sodium alginate (SA) under a few stresses. Because the plate at y = d is stationary and the plate at y = 0 is in motion, the fluid flows. Physically existent things utilize partial differential equations as a method of derivation. By using dimensionless variables, the underlying PDEs are dimensionless. Applying Fourier’s and Fick’s laws to the time-fractional model makes the classical model dimensionally stable by generalization. A generalized fractional model is solved using the Laplace and Fourier integral transformations. In addition, the parametric influence of other physical elements, such as the Casson parameter, coupling velocity, temperature, and stress parameters, is considered (Grashof, Schmidt, and Prandtl numbers). Concentration distributions are shown using graphs and discussed with accompanying text. We compute and describe the Sherwood number, rate of heat transfer, and skin friction. It is concluded that skin friction and Nusselt number can be enhanced by adding nanoparticle. Also, the fractional derivative makes the study more realistic by incorporating Fick’s and Fourier’s laws as compared to the classical one.

Flow in Microchannels between Sealing Surfaces of Casing Connections: LBM Simulation

Great difficulties have been encountered in the study of the seal failure of casing connection. The in situ microscopic seal surfaces under complex loads are hard to be obtained due to the engagement characteristics of thread. In this study, a method to construct the microscopic seal surfaces of a specified 7 ″ casing PTC under complex loads was proposed. Then, the lattice Boltzmann method (LBM) was applied to simulate the fluid flow inside the microchannel between the seal surfaces. The effect of pressure gradient and wettability on the flow behavior of a multicomponent fluid system at the microscopic scale was analyzed. Pressure gradient has a huge influence on the volume fraction of sealing compound and the average velocity of displacing fluid. The volume fraction of sealing compound decreases with the increase of pressure gradient. Break-through can be observed for fluid systems with relatively larger pressure gradient. For the systems with Δ p = 0.658 , Δ p = 0.439 , and Δ p = 0.219 , the break-through time steps are around 61000, 99000, and 295000, respectively. Wettability has huge impact on the distribution of sealing compound, and break-through was not observed for the fixed pressure difference Δ p = 0.110 with various wettability. It is expected to provide some new insights for understanding the seal failure of casing connections at the microscopic level.

Nanomaterials integrated with microfluidic paper-based analytical devices for enzyme-free glucose quantification

In this study, nanomaterials capable of enzyme-free glucose quantification and colorimetric readout are integrated into a microfluidic paper-based analytical devices (μPADs). Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) were utilized as a peroxidase-like nanozyme and a colorimetric probe to achieve glucose monitoring. In this developed device, glucose is oxidized by AuNPs to generate hydrogen peroxide (H2O2), which flows in the paper microchannels toward detection zones. H2O2 then etches the immobilized AgNPs to induce a color change. The intensity of color change is easily monitored using a smartphone application. Following method optimization, we obtained a linear range from 0.50 to 10.0 mmol L−1 (R2 = 0.9921) and a detection limit (LOD) of 340.0 μmol L−1. This falls in the clinically relevant range for glucose monitoring and diabetes diagnosis in humans. In addition, the total analysis time is just 20 min, which is significantly less than the same experiment performed in the solution phase. Also, our method is markedly selective; other substrates do not interfere. The recovery test in human control samples was in the range of 98.47–102.34% and the highest relative standard deviation (RSD) was 3.58%. The enzyme-free approach for glucose sensing is highly desirable for diabetes diagnosis as it replaces the more expensive enzyme with cheaper nanomaterials. Furthermore, since nanomaterials are more environmentally stable compared to enzymes, it has the potential for widespread deployment as point-of-care diagnostics (POC) in resource-limited settings.

Experimental analysis on the heat transfer performance of gas-liquid Taylor flow in a square microchannel

The flow and heat transfer behaviors of gas–liquid Taylor flow in a 1 × 1 mm square microchannel are experimentally investigated in this paper, focusing on evaluating the slug length, void fraction β, and mixing velocity on the overall performance of Taylor flow in the microchannel. T-shaped junctions of varying widths were employed to control the slug length in the Taylor flow. The experimental results indicate that reducing the junction width can heighten the bubble generation frequency and shorten the bubble/slug length. This reduction in slug length can enhance the heat transfer performance of the Taylor flow in fixed cross-section microchannels, but the degree of enhancement is significantly influenced by the void fraction. When β = 0.5, reducing the slug length can significantly improve the heat transfer coefficient while maintaining a reasonable pressure drop increment, resulting in the best overall performance. The friction pressure drop increases significantly when β = 0.67, and the level of overall performance is low. When β = 0.33, the short slugs have little effect on the performance of Taylor flow, and the change of heat transfer and the friction coefficient is only within 3%. In general, increasing the mixing velocity can improve the overall performance of the Taylor flow. However, at a relatively low void fraction, the pressure drop increase outweighs the heat transfer coefficient, leading to a decline in overall performance. Finally, a correlation is proposed to predict the Nu number under Taylor flow in the three-sided heated rectangular microchannel, with an average error of 11.77%.

Liquid–liquid two-phase flow in a wire-embedded concentric microchannel: Flow pattern and mass transfer performance

In this work, flow pattern and mass transfer of liquid–liquid two-phase flow in a wire-embedded concentric microchannel are studied using toluene–water system. Droplet flow, slug flow, oval flow and annular flow are observed in the wire-embedded concentric microchannel. The effects of embedded wires and physical properties on flow patterns are investigated. The embedded wire insert is conducive to the formation of annular flow. The flow pattern distribution regions are distinguished by the Caaq (capillary number)–Weorg (Weber number) flow pattern map. When Weorg＜0.001, slug flow is the main flow pattern, and when Weorg＞0.1, annular flow is the main flow pattern. Oval flow and droplet flow are between Weorg=0.001–0.1, and oval flow is transformed into droplet flow with the increase of Caaq. The effect of flow rate, phase ratio, initial acetic acid concentration, insert shape and flow patterns on mass transfers are studied. Mass transfer process is enhanced under annular flow conditions, the volumetric mass transfer coefficient is up to 0.36 s–1 because of the high interfacial area and interface renewal rate of annular flow.

Machine learning-based models for frictional pressure drop prediction of condensing and adiabatic flow in micro, mini and macro channels utilizing universal data

This study proposes a universal machine learning-based model to predict the adiabatic and condensing frictional pressure drop. For developing the proposed model, 11,411 data points of adiabatic and condensing flow inside micro, mini and macro channels are collected from 80 sources. The database consists of 24 working fluids, hydraulic diameters from 0.07 to 18 mm, mass velocities from 6.3 to 2000 Kg/m2s, and reduced pressures from 0.001 to 0.95. Using this database, four machine learning regression models, including “artificial neural network”, “support vector regression”, “gradient boosted regression”, and “random forest regression”, are developed and compared with each other. A wide range of dimensionless parameters as features, “two-phase friction factor” and “Chisholm parameter” are each considered separately as targets. Using search methods, the optimal values of important hyperparameters in each model are determined. The results showed that the “gradient boosted regression” model performs better than other models and predicts the frictional pressure drop with a mean absolute relative deviation of 3.24%. Examining the effectiveness of the new model showed that it predicts data with uniform accuracy over a vast range of variations of each flow parameter.

Hydrodynamics and liquid–liquid mass transfer in gas–liquid–liquid three-phase flow in a cross microchannel

Gas-liquid–liquid three-phase flow in cross microchannels has been widely used in flow chemistry and process intensification. At present, the flow and mass transfer characteristics in this type of channel are not well understood. In this work, the hydrodynamics and liquid–liquid mass transfer characteristics of gas–liquid–liquid three-phase flow were systematically investigated. For the gas–liquid–liquid hydrodynamics, the general regulation laws of bubble and droplet size were obtained by considering the synergistic effect of shearing phases, and the bubble/droplet size prediction model based on the multiphase cooperative behavior was proposed, which could be used in predicting droplet/bubble size in flow systems. For the gas–liquid–liquid mass transfer process, the experimental results showed that the mass transfer coefficient first increased and then decreased with the increase of gas flow rate when the oil phase flow rate was fixed. The semi-empirical prediction formula of the mass transfer coefficient was obtained by considering the vortex strength, the interfacial area and the effective mass transfer area at the same time, which was in good agreement with the experimental results. This work could be the comprehensive study of gas–liquid–liquid three-phase flow and liquid–liquid mass transfer under gas enhancement in a cross microchannel.

Analysis of the Current Status of Research on Fluid Flow in Microchannels

The main factors influencing the resistance to the flow of liquids in microchannels are the friction factor, the surface tension and the Poiseuille number. In this paper, the liquid itself and the surface properties inside the channel are analyzed from two perspectives. The specific influencing factors are first listed and elucidated, and then their effects are compared and corroborated by citing experimental results from other expert teams. The paper concludes with some relationships between the properties of the inner surface and the fluid resistance: the shape of the cross-sectional area of microchannels plays no important role in the fluid resistance, and the fluid is subjected to greater resistance in hydrophilic and rough inner surfaces. This can provide suggestions and guidelines for the design of microchannels.

Experimental study of dispersed flows in microchannels for 3D printing of composite materials

Experimental study of dispersed flows in microchannels for 3D printing of composite materials

Investigating simultaneous effects of temperature, surface heterogeneity and geometry on fluid mixing in electroosmotic flow considering temperature dependent properties by Nernst–Planck Poisson method

Numerical study of the effect of temperature and heat transfer as well as surface heterogeneity effects on mixing and electroosmotic flow in microchannels with convergent/divergent and straight mixing chamber is carried out. The fluid properties and surface zeta potentials are assumed to be temperature dependent. Based on the Nernst–Planck Poisson model, the governing equations of this problem include the Laplace equation, Poisson equation, Navier–Stokes and the propagation equation as well as the energy equation, respectively for the distribution of external electric field, distribution of internal electric field due to the charge of ions, fluid motion, concentration field and temperature distribution, which are solved by the finite element method. Flow rate, the mixing efficiency and a new parameter called mixing capacity, which is achieved by multiplying flow rate by mixing efficiency, are obtained for convergent/divergent and straight microchannels in a constant external electric field distributed under different wall temperatures, and performance of the geometries in flow transfer and mixing development is investigated. For electroosmotic flow, heterogeneous microchannels create vortices which improve mixing. Different arrangements of heterogeneous pieces of the surface create different patterns of vortices, each of which has different effects in the various geometries on fluid flow and mixing. Also, the dependence of the molecular diffusion on temperature causes increasing the mixing of species with the temperature increment. The results show that divergent microchannels have the maximum flow rate and the minimum mixing efficiency, but this efficiency can be increased by increasing the wall temperature. Convergent microchannels also have high mixing efficiency due to low flow rates and sufficient time for mixing even at low and near ambient temperatures, although their mixing capacity is less than the divergent microchannels.

Study on effect of gas-liquid two phase physical feature on slug flow in microchannels

At present, there are relatively few studies on the slug flow generation mode obtained by exchanging gas-liquid two-phase inlets. In this study, an experimental system combining microfluidic devices and high-speed cameras was used to study the effects of gas-liquid two-phase flow rate, liquid physical parameters, etc., On the characteristic length, generation period and other generation characteristics of slug flow, and dimensionless analysis was conducted to investigate the main factors affecting the characteristic length of gas slug. Results show that 1) when the gas flow rate affects the aeroelastic generation characteristics, the aeroelastic characteristic length increases from 443 μm when the gas flow rate increases changes to 657 μm. The generation period decreases rapidly at first and then the change amplitude slows down. The maximum value of aeroelastic generation frequency is 217 s-1; 2) when studying the effect of different liquid flow rates, increasing the liquid flow rate, the characteristic length of the gas bomb gradually decreases, and the generation period of the gas bomb gradually increases. Aeroelastic characteristic length from 770 μm changes to 378 μm. The range of aeroelastic generation cycle is 4–13.4 ms, and the maximum value of aeroelastic generation frequency is 250 s-1; 3) there is a functional relationship between the ratio of aeroelastic characteristic length to channel size L/d and dimensionless gas-liquid flow ratio Q*, Reynolds number Re, Weber number We: L/d=3.677Q*0.58/Re0.11.

Transient heat transfer and electro-osmotic flow of Carreau–Yasuda non-Newtonian fluid through a rectangular microchannel

PurposeThe purpose of this study is to investigate heat transfer and electrokinetic non-Newtonian flow in a rectangular microchannel in the developed and transient states.Design/methodology/approachThe Carreau–Yasuda model was considered to capture the non-Newtonian behavior of the fluid. The dimensionless forms of governing equations, including the continuity equation for the Carreau–Yasuda fluid, are numerically solved by considering the volumetric force term of electric current (DC).FindingsThe impact of pertinent parameters such as electrokinetic diameter (R), Brinkman number and Peclet number is examined graphically. It is observed that for increasing R, the bulk velocity decreases. The velocity of the bulk fluid reaches from the minimum to the maximum state across the microchannel over time. At the electrokinetic diameter of 400, the maximum velocity was obtained. Temperature graphs are plotted with changes in the various Brinkman number (0.1 < r < 0.7) at different times, and local Nusselt are compared against changes in the Peclet number (0.1 < ℘e < 0.5). The results of this study show that by increasing the Brinkman number from 0.25 to 0.7, the temperature along the microchannel doubles. It was observed that increasing the Peclet number from 0.3 to 0.5 leads to 200% increment of the Nusselt number along the microchannel in some areas along the microchannel. The maximum temperature occurs at Brinkman number of 0.7 and the maximum value of the local Nusselt number is related to Peclet number 0.5. Over time in the transient mode, the Nusselt number also decreases along the microchannel. By the increasing of time, the temperature increases at given value of Brinkman, which is insignificant at Brinkman number of 0.1. The simulation results have been verified by Newtonian and non-Newtonian flows with adequate accuracy.Originality/valueThis study contributes to discovering the effects of transient flow of electroosmotic flow for non-Newtonian Carreau–Yasuda fluid and transient heat transfer through rectangular microchannel. To the authors’ knowledge, the said investigation is yet not available in existing literature.

Kinetic study of uranium (VI) extraction with tributyl-phosphate in a stratified flow microchannel

Solvent extraction kinetics are of considerable importance in process and apparatus design, especially when the mass transfer is coupled with a chemical reaction. In such circumstances, the mass transfer can either be diffusion limited, reaction limited, or both in a mixed regime. Conventional extraction techniques lead to acquire an overall mass transfer coefficient for fast chemical systems. The latter brings together diffusional and reactional parts that are difficult to distinguish independently. This problem is addressed using a high velocity parallel flow microfluidic setup for the investigation of the extraction kinetics of the fast chemical system U(VI):HNO3/TBP:TPH. Experimental data were analyzed using a combination of overall mass transfer resistance calculations and advection–diffusion-reaction models. Results indicate that a chemically limited regime was obtained at a high initial concentration (119 g.l−1) of uranium (VI) due to Marangoni convections. Whereas, for low initial concentrations (10–30 g.l−1) and short residence times in the microchannel (≤13 ms), both diffusion and reaction impact the transfer rate. The obtained value of the apparent rate constant under current operating conditions ([HNO3] = 3 M, % TBP = 30 % (v/v)) was of (6.5 ± 1.7) × 10−4 m.s−1. This value is much higher than what conventional extraction techniques predicted, indicating the better mass transfer efficiency in the microfluidic setup due to the decrease of the diffusional limitation.

Study of the Hydraulic Characteristics of Slug Flow in the Microfluidic Y-shaped Junctions of Three Different Cross Angles

The slug flow in microchannels has been applied for microdroplet/bubble production in recent years. The comparison investigation of hydrodynamic properties of slug flow in three different Y-shaped microchannels was conducted by numerical methods. The numerical simulation results show that the VOF model can describe the velocity and pressure field in the Y-shaped microchannel. It was found that the pressure in the microchannel from upstream to downstream showed an overall decreasing trend, and the liquid phase pressure between the two gas slugs was less than the internal pressure of the adjacent gas slug. The midline pressure of the bubble generation moment is less than the rest of the state. Among the three Y-shaped junctions, the absolute value of the pressure difference at the moment of slug bubble formation is the smallest when the angle of the Y-junction is 90°. Compared to 120° and 90° Y-junctions, the pressure and velocity field of the 60° Y-junction is the most unstable when the slug flow generates.

Flow periodicity analysis in an additively manufactured metal microchannel heat transfer device using micro-PIV

The flow periodicity in an additively manufactured microchannel heat transfer device is analysed using the micro-particle image velocimetry technique. A metal offset strip fin (OSF) microchannel is additively manufactured using laser powder bed fusion to overcome the limitations of conventional production techniques in manufacturing small complex features. This study demonstrates the capability of measuring the velocity field in an OSF microchannel up to 1 mm deep with a resolution of 10μm and a velocity uncertainty smaller than 0.5% for laminar flows with a Reynolds number below 10. Higher measurement uncertainties are encountered in regions close to the channel walls due to particle deposition. A methodology aiming to evaluate and minimise the uncertainties is presented. The experimentally obtained velocity fields are compared to a numerical simulation of the flow in the offset strip fin microchannel. The results verify that flow periodicity can be observed and the extent of the flow development region can be studied. Nevertheless, in regions with a large velocity gradient, a considerable uncertainty on the velocity measurements exists due to manufacturing imperfections.

Transition Routes of Electrokinetic Flow in a Divergent Microchannel with Bending Walls.

Electrokinetic flow can be generated as a highly coupled phenomenon among velocity fields, electric conductivity fields, and electric fields. It can exhibit different responses to AC electric fields in different frequency regimes, according to different instability/receptivity mechanisms. In this investigation, by both flow visualization and single-point laser-induced fluorescence (LIF) method, the response of AC electrokinetic flow and the transition routes towards chaos and turbulence have been experimentally investigated. It is found, when the AC frequency ff>30 Hz, the interface responds at both the neutral frequency of the basic flow and the AC frequency. However, when ff≥30 Hz, the interface responds only at the neutral frequency of the basic flow. Both periodic doubling and subcritical bifurcations have been observed in the transition of AC electrokinetic flow. We hope the current investigation can promote our current understanding of the ultrafast transition process of electrokinetic flow from laminar state to turbulence.