Straight Microchannel Research Articles

Conjugate effect on flow boiling instability in wavy microchannel

Three-dimensional numerical study on two-phase flow boiling in rectangular microchannel with wavy vertical wall configuration is considered that can be utilized for heat dissipation in high flux electronics devices. Straight and wavy microchannels are considered for comparison and analysis. Under conjugate heat transfer situation, effect of flow boiling instability in straight and wavy microchannel is numerically studied using water as the working fluid. Behaviour of bubble growth rate and two-phase flow pattern in straight and wavy microchannel are investigated for a wide range of parameters such as waviness (γ ~ 0 - 0.267), substrate thickness to channel height ratio (δsf ~ 1 and 5), wall to fluid conductivity ratio (ksf ~ 22 - 646), and mass flux (G ~ 118 - 590 kg/m2s). It is found that smaller bubbles are formed in wavy microchannel while confined and elongated bubbles are formed in straight microchannel which often causes flow clogging and premature dryout. The work also presents a comparative analysis between single-phase and two-phase flow boiling cooling technique in straight and wavy microchannel under same geometrical parameters and applied heat flux.

Separation and Enrichment of Yeast Saccharomyces cerevisiae by Shape Using Viscoelastic Microfluidics.

Yeast Saccharomyces cerevisiae (S. Cerevisiae) is one of the most attractive microbial species used for industrial production of value-added products and is an important model organism to understand the biology of the eukaryotic cells and humans. S. Cerevisiae has different shapes, such as spherical singlets, budded doublets, and clusters, corresponding to phases of the cell cycle, genetic, and environmental factors. The ability to obtain high-purity populations of uniform-shaped S. Cerevisiae cells is of significant importance for a wide range of applications in basic biological research and industrial processes. In this work, we demonstrate shape-based separation and enrichment of S. Cerevisiae using a coflow of viscoelastic and Newtonian fluids in a straight rectangular microchannel. Due to the combined effects of lift inertial and elastic forces, this label-free and continuous separation arises from shape-dependent migration of cells from the Newtonian to the non-Newtonian viscoelastic fluid. The lateral position of S. Cerevisiae cells with varying morphologies is found to be dependent on cell major axis. We also investigate the effects of sheath and sample flow rate, poly(ethylene oxide) (PEO) concentration and channel length on the performance of the viscoelastic microfluidic device for S. Cerevisiae enrichment and separation by shape. Moreover, the separation efficiency, cell extraction yield, and cell viability after sorting operations are studied.

Study of multiphase flow inside straight and spiral microchannel and effect of two phase flow on Dean’s vortices

A combined experimental and numerical simulation study of gas–liquid two-phase flow inside straight and spiral microchannels is presented. Dynamics of Taylor bubble formation was investigated in both types of channels and it was canvassed that presence of centrifugal force in spiral channel has consequential influence on the Taylor bubble dynamics. Effect of gas–liquid flow ratio and hydrophobicity of channel wall on the Taylor bubble formation was also investigated through numerical simulations. Formation of Dean's vortices inside spiral microchannel was shown via 3D CFD based simulations. Quantification of the strength of such vortices was done by calculating vorticity (ω) which is the curl of the velocity vector field. The effect of curvature on the formation of Dean's vortices was shown using Dean Number (De) and Dean velocity (VDe). The effect of two phase flow on the vorticity was also emphasized by presenting a comparative study with single phase flow. Presence of Dean's vortices inside such spiral micro-channel can be utilized in many futuristic micro-contrivances.

The numerical study of the fluid flow along a superhydrophobic textured surface

A superhydrophobic phenomenon during fluids flow through microchannel due to the presence of the textures on the walls of the channels was considered. A numerical study of fluids flow and mixing in the microchannel at different Reynolds numbers and average roughness sizes has been carried out. The shift in the critical Reynolds number of the transition from the vortex symmetric flow regime to the vortex asymmetric (engulfment) regime towards lower values in the T-shaped microchannels with textured walls was found. The substantially unsteady structures in the straight microchannels with textured walls at Reynolds numbers of about 1000 were obtained.

Analysis of Hydraulic Mixing Efficiency in Widespread Models of Micromixers

In this paper, we present the results of a systematic numerical study of the flow and mixing modes of fluids in micromixers of various configurations, in particular, an analysis of passive micromixers, the most widely used in practice, as well as the main methods to intensify mixing. The advantages of microstructure reactors can significantly reduce reaction times and increase productivity compared to traditional bulk reactors. Four different geometries of micromixers, including the straight T-shaped microchannel, were considered. The effect of the geometrical patterns of micromixers, as well as of the Reynolds number on flow regimes and mixing efficiency were analyzed. The Reynolds number varied from 1 to 300. Unlike other studies, the efficiency of the considered mixers was for the first time compared with the cost of pressure loss during pumping. As a result, the efficiency of the most optimal micromixer in terms of hydraulic mixing and the optimal operation ranges were determined. It was shown that the maximum normalized mixing efficiency in the entire range of Re numbers was noted for mixer, in which a vortex-based intensification of mixing occurs due to the flow swirling in cylindrical chambers. This mixer allows mixing the fluids 600 times more efficiently than a straight T-mixer, while all other conditions being equal.

Numerical simulation of elasto-inertial focusing of particles in straight microchannels

Viscoelastic microfluidics has become a new trend for particle/cell manipulation in recent years. The coupling of fluid viscoelasticity and inertia has been proved to be effective for achieving particle elasto-inertial focusing at the channel centerline experimentally, which is important for downstream particle counting and detection. However, the mechanism of particle elasto-inertial focusing in viscoelastic flow is still unclear. This paper systematically explores particle elasto-inertial focusing in straight microchannels through using numerical simulation. The performance of particle focusing is studied under various control parameters. Numerical simulation is conducted under different Reynolds numbers Re, Weissenberg numbers Wi and particle diameters d to elucidate the force competition mechanisms. The results show that the increase of flow intensity and particle diameter d can speed up the focusing migration. Elasticity of the flow quantified by Wi can attenuate the particle rotation and thus weaken the lateral particle focusing towards the channel centerline. The separatrix for particle focusing towards the channel centerline or the channel walls is observed in the simulation. The results provide new insight into understanding particle elasto-inertial focusing in viscoelastic flows, and can be useful for guiding the design of viscoelastic microfluidics.

Enhancement in mixing efficiency by ridges in straight and meander microchannels

We present an analysis of the effect of incorporating ridges in straight and meander shaped pressure-driven Y-type micromixers. Triangular and rectangular ridges were incorporated on the inner walls of straight and meandered microfluidic channels, and their effect on mixing was evaluated. The incorporation of ridges greatly enhances mixing in the flow path due to strong laminar perturbations. Also, the effect of the geometry of ridges and their arrangement pattern in the microchannel was studied extensively. Incorporation of ridges in the microchannels resulted in enhanced performance of the mixer in terms of reduction in mixing time and length. Meander channels with ridges provided greater than 90 % mixing in less than 2 mm, whereas straight channels with ridges required a mixing length of 10 mm. The optimum design was fabricated using direct-laser write photolithography followed by soft lithography. The performance of the micromixer was validated using coloured solutions and biosensors. The micromixer is compact, simple in fabrication and operation, and easy-to-integrate with other microfluidic components enabling them for real-time applications. Its scope for the development of microfluidic devices where mixing is crucial has been demonstrated using glucose biosensors.

The numerical study of the flow regimes of two-phase oil and displacing agent mixtures in straight microchannels simulating a pore or crack in the rock formation

The calculated study of the flow regimes of two-phase oil and water mixture in straight microchannels simulating a pore or crack in the rock formation was carried out. It was shown as a result of the literature survey that all modern, industrially developed methods of the oil recovery from the oil reservoirs is considered as unsatisfactory in all oil-producing countries. The average value of the final oil recovery is varied from 25% to 45%, so, it is easy to count, that residual or unrecoverable oil reserves in the subsoil reach an average of 55-75% of their initial values. Also, it was found, that the use of micro and nanotechnologies in a broad sense is one of the possible and promising ways to solve these problems. A numerical method called «Volume of Fluid» (VOF) was used to simulate the two-phase flows of oil and displacing agents in the considered microchannels. It was proposed to use the dimensionless number, called Regime number, to describe the different flow regimes of oil/water mixture and transitions between these regimes. The plug flow, the slug flow, the parallel flow and the emulsion flow regimes were observed.

Influence of the Flow Rate in an Automated Microfluidic Electronic Tongue Tested for Sucralose Differentiation.

Incorporating electronic tongues into microfluidic devices brings benefits as dealing with small amounts of sample/discharge. Nonetheless, such measurements may be time-consuming in some applications once they require several operational steps. Here, we designed four collinear electrodes on a single printed circuit board, further comprised inside a straight microchannel, culminating in a robust e-tongue device for faster data acquisition. An analog multiplexing circuit automated the signal’s routing from each of the four sensing units to an impedance analyzer. Both instruments and a syringe pump are controlled by dedicated software. The automated e-tongue was tested with four Brazilian brands of liquid sucralose-based sweeteners under 20 different flow rates, aiming to systematically evaluate the influence of the flow rate in the discrimination among sweet tastes sold as the same food product. All four brands were successfully distinguished using principal component analysis of the raw data, and despite the nearly identical sucralose-based taste in all samples, all brands’ significant distinction is attributed to small differences in the ingredients and manufacturing processes to deliver the final food product. The increasing flow rate improves the analyte’s discrimination, as the silhouette coefficient reaches a plateau at ~3 mL/h. We used an equivalent circuit model to evaluate the raw data, finding a decrease in the double-layer capacitance proportional to improvements in the samples’ discrimination. In other words, the flow rate increase mitigates the formation of the double-layer, resulting in faster stabilization and better repeatability in the sensor response.

Dimensional accuracy and surface quality of micro-channels with low-frequency vibration assistance in micro-electro-discharge milling

ABSTRACT Micro-electric discharge milling (µ-ED milling) process has attracted interests of researchers in the area of fabrication of three-dimensional miniaturised features specifically in difficult-to-cut materials. However, significant electrode wear during machining degenerates the accuracy fabricated micro-features. Therefore, in this work, low-frequency workpiece vibration is introduced to assist µ-ED milling to improve the dimensional accuracy of fabricated micro-channels. Difficult-to-cut Inconel 718 superalloy was selected as a workpiece for fabricating straight micro-channels. Initially, the effect of vibration assistance on the µ-ED milling process performance was evaluated in terms of material removal rate and tool wear. Then, the effect of low-frequency workpiece vibrations on dimensional accuracy and surface quality of fabricated micro-channels was investigated using microscopic images. Overall, the low-frequency workpiece vibration assistance found to be effective in enhancing the process performance of µ-ED milling process.

Thermal analysis of high concentrator photovoltaic module using convergent-divergent microchannel heat sink design

The dense-packed high concentrator photovoltaic module (DP-HCPVM) is normally exposed to a very high heat flux concentrated from sunlight. The resultant high module operation temperature drops the overall performance and reduces the module life expectancy. Consequently, the provision of powerful cooling is compulsory for such structures. In this study, a complete three-dimensional (3D) model was developed to investigate the performance of a DP-HCPVM under the active cooling scheme to figure out its ability to attain effective heat dissipations. A new cooling convergent-divergent microchannel heat sink cases were numerically investigated and compared in terms of temperature distribution and electrical efficiency of a DP-HCPVM. The influence of heat sink design and inlet mass flowrate on the cooling performance of the heat sink device were examined. The same external dimensions are considered, and the flow was laminar with the inlet temperature of 25 °C for all designs for a reasonable comparison. The concept of variation flow direction by changing the inlet and outlet arrangement is adopted. The comparison between all studied designs was highlighted concerning surface average temperature, surface temperature uniformity, and pressure drop across the heat sink. The results indicated that the new convergent-divergent microchannel heat sink design achieved better effectiveness and heat transfer rate compared to the conventional straight microchannel heat sink design for DP-HCPVM thermal management, as it decreased the average cells temperature and temperature non-uniformity by 11.4% and 37.8%. Moreover, it was found that the electrical efficiency and Nusselt number were improved by 0.35% and 21.9% at the maximum studied inlet mass flowrate.

Co-designing electronics with microfluidics for more sustainable cooling

Thermal management is one of the main challenges for the future of electronics1-5. With the ever-increasing rate of data generation and communication, as well as the constant push to reduce the size and costs of industrial converter systems, the power density of electronics has risen6. Consequently, cooling, with its enormous energy and water consumption, has an increasingly large environmental impact7,8, and new technologies are needed to extract the heat in a more sustainable way-that is, requiring less water and energy9. Embedding liquid cooling directly inside the chip is a promising approach for more efficient thermal management5,10,11. However, even in state-of-the-art approaches, the electronics and cooling are treated separately, leaving the full energy-saving potential of embedded cooling untapped. Here we show that by co-designing microfluidics and electronics within the same semiconductor substrate we can produce a monolithically integrated manifold microchannel cooling structure with efficiency beyond what is currently available. Our results show that heat fluxes exceeding 1.7 kilowatts per square centimetre can be extracted using only 0.57watts per square centimetre of pumping power. We observed an unprecedented coefficient of performance (exceeding 10,000) for single-phase water-cooling of heat fluxes exceeding 1 kilowatt per square centimetre, corresponding to a 50-fold increase compared to straight microchannels, as well as a very high average Nusselt number of 16. The proposed cooling technology should enable further miniaturization of electronics, potentially extending Moore's law and greatly reducing the energy consumption in cooling of electronics. Furthermore, by removing the need for large external heat sinks, this approach should enable the realization of verycompact power converters integrated on a single chip.

Assessment of PMMA and polystyrene based microfluidic chips fabricated using CO2 laser machining

Laser machining could be an alternative way for the fabrication of microchannels. In this study, laser machining of polymethylmethacrylate (PMMA) and polystyrene (PS) substrates were characterized in detail. A fabrication method preventing leakage at PS microchannel inlets was developed. The effect of laser parameters (power, speed and frequency) on engraving was analyzed by scanning electron microscopy. Laser ablation mechanism of both materials was explained through thermal analysis and material properties. Defocusing the laser beam was also analyzed as an additional parameter affecting the channel profile. Two parameters affecting the resolution were analyzed which are the minimum channel size that can be achieved by the laser beam and x-y stage of the laser engraver for straight and complex microchannel geometries. The hydrophilicity of the surface before and after laser machining was tested with contact angle measurements. The capabilities/limitations of machining were revealed through some complex channel structures. Finally, a passive micromixer and a droplet generator microfluidic devices were manufactured and tested, and promising results were obtained.

Inertial migration of aerosol particles in three-dimensional microfluidic channels

In recent years, manipulation of particles by inertial microfluidics has attracted significant attention. However, most studies focused on inertial focusing of particles suspended within liquid phase, in which the ratio of the density of the particle to that of the medium is O(1). The investigation on manipulation of aerosol particles in an inertial microfluidics is very limited. In this study, we numerically investigate the aerosol particle's motion in a 3D straight microchannel with rectangular cross section by fully resolved simulation of the particle–air flow. The air flow is modeled by the Navier–Stokes equations. The particle's motions, including translation and rotation, are governed, respectively, by the Newton's second law and the Euler equations without using any approximation models for the lift and drag forces. The coupled mathematical model is numerically solved by combining immersed boundary with lattice Boltzmann method (IB-LBM). We find that the Reynolds number (Re), the particle's initial position, particle's density and diameter are the influential parameters in this process. The equilibrium positions and their stabilities of aerosols are different from those suspended in liquid.

Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics.

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC-biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC-biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC-field induced-charge electrokinetic (ICEK) phenomenon, which behaves as counter-rotating micro-vortices around the ideally polarizable surfaces of FE. Finite-element simulation is carried out by coupling the dual-frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500nm) and BSA molecules (∼10nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous-flow analyte manipulation in modern microfluidic systems.

Electromagnetohydrodynamic flows and mass transport in curved rectangular microchannels

Curved microchannels are often encountered in lab-on-chip systems because the effective axial channel lengths of such channels are often larger than those of straight microchannels for a given per unit chip length. In this paper, the effective diffusivity of a neutral solute in an oscillating electromagnetohydrodynamic (EMHD) flow through a curved rectangular microchannel is investigated theoretically. The flow is assumed as a creeping flow due to the extremely low Reynolds number in such microflow systems. Through the theoretical analysis, we find that the effective diffusivity primarily depends on five dimensionless parameters, i.e., the curvature ratio of the curved channel, the Schmidt number, the tidal displacement, the angular Reynolds number, and the dimensionless electric field strength parameter. Based on the obtained results, we can precisely control the mass transfer characteristics of the EMHD flow in a curved rectangular microchannel by appropriately altering the corresponding parameter values.

Flow boiling characteristics in different configurations of stepped microchannels

Geometrical modifications in microchannel geometry intended towards mitigation of flow instabilities generally results in increased pressure drop across the microchannel heat sink. To overcome this, stepped microchannels configurations with combination of narrow bottom (aspect ratio: AR = depth/width > 1) and wider top (AR < 1) part are investigated and their flow boiling performances have been compared with rectangular cross-section microchannels (RMC) having same hydraulic diameter (350 μm). The effect of varying wider cross-section on flow boiling instability and heat transfer performance have been explored for three configurations of stepped microchannels namely: stepped straight microchannels (SMC), stepped converging microchannels (SCMC) and stepped diverging microchannels (SDMC). These geometrical variants are fabricated on a 10 × 10 mm2 copper block having nine parallel microchannels. Flow boiling experiments have been conducted using de-ionized (DI) water as coolant for an inlet subcooling of 20 °C, coolant mass fluxes ranging from 327.6 to 777.54 kg/m2s and for wide range of heat fluxes. Combination of narrow and wider converging/diverging flow passage improves flow stability by allowing bubbles in the confined narrow bottom to grow and expand in the wider top section thus reducing their evaporative momentum. Different flow regimes govern the heat transfer and pressure drop mechanisms while the compressible volume-liquid interaction dictates flow instability. Favorable surface tension gradient in SDMC alleviates flow instability whereas reduced flow resistance at the inlet in SCMC resulted in smooth rewetting during dry out. Amplitude and fluctuation of pressure drop is found to be minimum in SCMC among all geometric configurations, while better heat transfer performance has been observed in SDMC configuration.

Three-dimensional numerical analysis of focusing and separation of diamagnetic particles in ferrofluid

Focusing and separation of particles and cells by magnetophoresis are important steps in many applications. In simple terms, the magnetophoresis can be classified into a positive one and a negative one. The most important characteristic of negative magnetophoresis is that particles and cells can be manipulated in a label-free manner. In this paper, continuous separation based on negative magnetophoresis is studied numerically using a three-dimensional model considering the interaction between particles and the ferrofluid. Firstly, the separation of two sized particles is investigated with a straight microchannel with two opposite permanent magnets for focusing particles before their separation by another bias magnet. Then the influence of size and position of permanent magnets and geometry of microchannel are investigated to achieve a better particle separation. Moreover, the effects of the concentration of the ferrofluid, the size difference of the particles, the magnet-channel distance and the flow velocity on the particle separation are analyzed.

Numerical analysis of temperature uniformity of a liquid cooling battery module composed of heat-conducting blocks with gradient contact surface angles

Battery thermal management has an important significance for electronic vehicles to maintain a suitable temperature range and reduce local temperature differences. In this study, a novel liquid cooling battery thermal management system (BTMS) that uses a straight microchannel flat tube and heat-conducting blocks with gradient contact surface angles is proposed to improve the temperature uniformity of cylindrical lithium-ion battery module. A three-dimensional transient heat transfer model is conducted to investigate the thermal performance of the proposed battery module with 48 cells arranged on both sides of a straight microchannel flat tube. The effects of contact surface angle (αi), gradient angle increment (Δα) and inlet velocity of the fluid medium on the cooling performance are discussed. The results indicated that the module with a gradient contact surface angle, which implies gradually increasing the heat transfer area, further improved the temperature uniformity of the battery module compared to that with an unchanged αi. In addition, the increase of the inlet velocity does have a positive effect on both the reduction of maximum temperatures and enhancement of uniform temperature distribution. When Δα is 15° and the inlet velocity of the fluid medium is 0.015 m/s, the temperature difference (ΔT) of the battery module can reach 2.58 °C at the end of discharge. In addition, the preheating performance of the battery module at subzero temperature is also discussed. The results show that the temperature uniformity increase with the increase of the gradient angle, thus when Δα is 15°, the temperature difference of the battery module can be controlled to 8.05 °C.

Comparative Study of the Flow Boiling Performance of the Hybrid Microchannel-Microgap Heat Sink with Conventional Straight Microchannel and Microgap Heat Sinks

In this study, the flow boiling performance of the hybrid microchannel-microgap heat sink (Hybrid 1:1), developed for improved boiling stability, is benchmarked against a conventional straight microchannel heat sink (SMC) and microgap heat sink (MG) having identical footprints of 25mm x 25mm, and microchannel and microgap dimensions of 300μm (Wmc) x 600μm (Hmc), and 25000μm (Wmg) x 600μm (Hmg) respectively. Experiments along with flow visualization are performed using deionized water as the coolant under heat fluxes and mass fluxes in the ranges of 0-128 W/cm2 and 100-399 kg/m2s respectively. SMC experiences the greatest degree of flow instabilities owing to the rapid bi-directional expansion of vapor slugs within the confined flow passages. In comparison, Hybrid 1:1 improves boiling stability particularly under low mass flux conditions on account of the expanding flow configuration that promotes forward expansion of vapor slugs. Similarly, MG also offers a comparable degree of boiling stability as Hybrid 1:1. However, SMC shows the most superior heat transfer performance at all mass fluxes followed by Hybrid 1:1 and MG. Although Hybrid 1:1 stabilizes boiling, the reduced wetted area towards the downstream compromises its heat transfer performance. Whereas the larger wetted area of SMC offsets the degrading effects of boiling instabilities. With the smallest wetted area, MG shows the most inferior heat transfer performance with severe partial dry-out conditions at high heat flux leading to early CHF, particularly under low mass flux conditions. Pressure drops of the heat sinks are in order of the extent of channel confinement, with the highest being for SMC followed by Hybrid 1:1 and MG.