Aspect Ratio Microchannels Research Articles

Performance analysis of a low-pressure cryogenic vaporizer for liquified hydrogen supply system

In this paper, we theoretically investigated the thermal performance of a low-pressure cryogenic vaporizer for liquified hydrogen supply system. A performance analysis model was developed by the modified effectiveness-NTU method considering the axial conduction and phase change of liquified hydrogen. In the analysis model, a plate-fin heat exchanger (PFHE) was chosen as the type of a cryogenic vaporizer. The inlet temperature and pressure conditions of liquified hydrogen were 20 K and 10 bar, respectively and the outlet temperature condition of hydrogen was a room temperature. Based on the theoretical results, we presented effects of geometric parameters on the performance of the vaporizer under the fixed pumping power condition. Especially, it is shown that the effectiveness of the hot fluid and cold fluid exhibit opposite trends depending on the axial conduction. In addition, we presented the effect of the aspect ratio of microchannels on the effectiveness considering both cases where the microchannel heights of the EGW mixture and hydrogen were either the same or different. Also, we showed the effect of the vaporizer length on the performance of the vaporizer. Finally, geometric parameters that can satisfy the two target performances are suggested.

Constructal design of a hybrid heat sink with rectangular microchannel and porous fin in a 3D electronic device with artificial neural-network, NSGA-II and different decision-making methods

Constructal theory is used to build a hybrid heat-sink model with rectangular microchannel and porous fin in a 3D electronic device. Firstly, constructal design is conducted with objective of minimizing linear weighted sum of maximum temperature-difference and pump power consumption with fixed total volumes of 3D electronic device, porous fin and microchannel. The optimal microchannel aspect ratio and elemental number are gained. Secondly, multi-objective-optimization is conducted based on NSGA-II, which uses the “gamultiobj” function of Matlab's artificial neural-network toolbox. The findings indicate that when microchannel aspect ratio and elemental number are 0.29 and 38, respectively, complex function achieves its double minimum of 0.810, which is decreased by 19.0% compared to its original value. Therefore, optimizing microchannel aspect ratio and elemental number simultaneously contributes to improving the comprehensive performance of 3D electronic device. Based on LINMAP, TOPSIS and Shannon Entropy decision-making strategies, the multi-objective solution set is compared. The third decision-making method has the smallest deviation index. The corresponding optimal microchannel aspect ratio and elemental number are 0.35 and 32, respectively, which can be used as the optional structure design scheme. The research findings can serve as a new theoretical reference for designing hybrid cooling system in a 3D electronic device.

Flow boiling characteristics of HFE-7000 in high aspect ratio microchannels with the effect of flow orientation

The aim of the present experimental study is to investigate the flow boiling characteristics in a high aspect ratio microchannel for two different channel orientations. Hydrofluoroether 7000 (HFE-7000) flowing within a rectangular glass microchannel (with a hydraulic diameter of 909 μm and a width-to-height aspect ratio of 10) coated with a tantalum layer to provide uniform heating along the channel was the configuration investigated. The data were recorded for mass flux of 14 − 42 kg m−2 s−1, while the heat flux was varied between 3.5 and 24.3kWm−2 under horizontal and vertical upward flow. Analysis of the experimental data reveals that thermal performance is influenced by both mass and heat fluxes, as well as flow orientation, and these are presented and discussed. The wall temperature mapping and heat transfer coefficient analysis showed that for low mass flow cases, the dry out occurs at a lower heat flux and for either flow orientation. At medium and higher heat fluxes, though, it is found that a horizontal flow produces dry out events more easily compared to vertical upward flow. The observed phenomenon during flow boiling also shows the importance of nucleate boiling and thin film evaporation on the heat transfer performance. In addition, pressure fluctuations during flow boiling were also recorded. A Fourier transform analysis of pressure fluctuations data showed that vertical upward flow produces a higher dominant frequency than horizontal flow. Finally, the analysis of thermal characteristics and pressure fluctuations can provide further insights into the design of devices where flow orientation plays a major role.

Elasto-inertial focusing and particle migration in high aspect ratio microchannels for high-throughput separation

The combination of flow elasticity and inertia has emerged as a viable tool for focusing and manipulating particles using microfluidics. Although there is considerable interest in the field of elasto-inertial microfluidics owing to its potential applications, research on particle focusing has been mostly limited to low Reynolds numbers (Re<1), and particle migration toward equilibrium positions has not been extensively examined. In this work, we thoroughly studied particle focusing on the dynamic range of flow rates and particle migration using straight microchannels with a single inlet high aspect ratio. We initially explored several parameters that had an impact on particle focusing, such as the particle size, channel dimensions, concentration of viscoelastic fluid, and flow rate. Our experimental work covered a wide range of dimensionless numbers (0.05 < Reynolds number < 85, 1.5 < Weissenberg number < 3800, 5 < Elasticity number < 470) using 3, 5, 7, and 10 µm particles. Our results showed that the particle size played a dominant role, and by tuning the parameters, particle focusing could be achieved at Reynolds numbers ranging from 0.2 (1 µL/min) to 85 (250 µL/min). Furthermore, we numerically and experimentally studied particle migration and reported differential particle migration for high-resolution separations of 5 µm, 7 µm and 10 µm particles in a sheathless flow at a throughput of 150 µL/min. Our work elucidates the complex particle transport in elasto-inertial flows and has great potential for the development of high-throughput and high-resolution particle separation for biomedical and environmental applications.

Microdroplet formation of water and alumina nanofluid in a T-junction microchannel

A scarcity of studies about nanofluids’ utilization in droplet formation inside microdevices currently hovers in the literature although potential applications of nanoparticles in a microfluidic environment are foreseen. For this purpose, experimentally assessing both nanofluid and microdroplet characteristics is fundamental. This work reports a series of experimental tests on the microdroplet formation of distilled water (DIW) and DIW-based aluminum oxide (Al2O3) nanofluid in a microfluidic T-junction. While water and nanofluid are used as the dispersed phase, mineral oil is used as the continuous phase. Microdroplet formation in the squeezing, transitional, and dripping regimes is characterized and scaling laws for the non-dimensional droplet volumes are presented. The effects of flow rate, capillary number, microchannel aspect ratio, and nanoparticle concentration are investigated. The addition of Al2O3 nanoparticles to the water is observed to have a major impact in the transitional regime (up to 40% increase), whereas in the dripping regime its influence is lower, with less than 10% difference. This was attributed to the nanofluid's enhanced interfacial tension and viscosity compared to the DIW, as well as possible adsorption at the surface.

Optimization of Manifold Microchannel Heat Sink Based on Design of Experiment Method

Abstract The manifold microchannel heat sink (MMCHS) shows high potential to cope with the rapidly increasing power density of electronic devices. It is of significance to search for an optimization method to comprehensively consider multiparameter effects on the performance of microchannel. Here, we have studied the temperature and flow characteristics of the MMCHS via experimental method. Then, the effect of geometric parameters including microchannel aspect ratio (α), hydraulic diameter (Dh), and manifold block numbers (Nm) on the performance of a MMCHS were studied by numerical method. Subsequently, the design of experiment (DOE) method was proposed to optimize the geometric parameters of the MMCHS. The relation of friction coefficient and heat transfer coefficient with respect to the three geometric parameters was gained by the mathematical regression model. The statistically optimized manifold microchannel achieved a 10.0% reduction in temperature rise, 59.9% enhanced in the heat transfer coefficient, and the figure of merit (FOM) of 1.6. Our work shows a novel method to consider the effect of structure parameters on the performance of the manifold microchannel and provides a novel optimization design method for the manifold microchannel.

On conjugate heat transfer in microchannel heat sinks

This work aims to study the anisotropic thermal impacts of employing liquid metal fluids including Na and NaK as coolants in two metallic microchannel heat sinks. The two studied microchannel heat sinks have aspect ratios of 1 and 2 while their hydraulic diameters are 267 µm and 400 μm. In the first stage, the accuracy and reliability of the developed numerical model are verified against benchmark problems and experimental data. In the second stage, the utilized verified and validated numerical model is extended to forced convection of Na and NaK in metallic microchannel heat sinks. The investigated metallic microchannel heat sinks are assumed to be out of stainless steel and Nickel with thermal conductivities of 17 W/mK and 90 W/mK, respectively. Obtained results revealed that NaK provides higher Nusselt numbers than Na regardless of the microchannel aspect ratio and hydraulic diameter. The effect of the microchannel heat sink solid substrate thermal conductivity on the temperature and heat flux distributions over the microchannel walls are also investigated comprehensively. For the investigated steady state Na and NaK flow within metallic microchannel heat sinks, the thermal conductivity of the heat sink solid substrate is observed to have a significant effect on the local temperature and heat flux distributions over microchannel walls.

Enhancing structural replication of microfluidic chips: Parameter optimization and mold insert modification

AbstractThe demolding process in micro‐injection molding constitutes a critical phase, exerting a decisive influence on the quality of polymer microstructures. In this study, the demolding forces of PMMA microfluidic chips were measured under different demolding temperatures, packing pressures, and demolding speeds by using pure nickel (Ni) mold insert. The dimensional deviations of the microchannels with different aspect ratios were analyzed. In addition, the effect of mold insert modification on the demolding force was further analyzed. The results showed that the effect of demolding temperature on the demolding force was the most significant, and the peak demolding force decreased with the decrease in temperature. The width of the microchannels increased and the depth decreased after demolding, while the opposite results were observed for the micro‐mixing structures. However, the dimensional deviation of the micro‐mixing structures was larger than that of the microchannels. Using the optimal molding parameters, the peak demolding force could be reduced from 114.0 N at 110°C to 47.0 N, a decrease of 58.8%. It is noteworthy that, when the microchannel was narrower than 100 μm, achieving precise replication of microchannel dimensions became progressively more challenging as the microchannel width decreased. Using the Ni‐WS2 mold insert with low surface energy and low friction coefficient, the demolding force could be further reduced by 38.3%. The effect of the molding quality of microfluidic chip microstructures on the mixing performance of heavy metals was demonstrated by micro‐mixing experiments.Highlights Demolding temperature crucially affects microstructure dimensions. Optimal parameters and modified mold insert reduce demolding force by 38.3%. Dimensional deviation increases with the increase of microchannel aspect ratio. Optimized microfluidic chip enhances Co2+ chemiluminescence by 6.42%.

Electrohydrodynamic (In)Stability of Microfluidic Channel Flows: Analytical Expressions in the Limit of Small Reynolds Number

We study electrohydrodynamic (EHD) linear (in)stability of microfluidic channel flows, i.e., the stability of interface between two shearing viscous (perfect) dielectrics exposed to an electric field in large aspect ratio microchannels. We then apply our results to particular microfluidic systems known as two-liquid electroosmotic (EO) pumps. Our novel results are detailed analytical expressions for the growth rate of two-dimensional EHD modes in Couette–Poiseuille flows in the limit of small Reynolds number (R); the expansions to both zeroth and first order in R are considered. The growth rates are complicated functions of viscosity-, height-, density-, and dielectric-constant ratio, as well as of wavenumbers and voltages. To make the results useful to experimentalists, e.g., for voltage-control EO pump operations, we also derive equations for the impending voltages of the neutral stability curves that divide stable from unstable regions in voltage–wavenumber stability diagrams. The voltage equations and the stability diagrams are given for all wavenumbers. We finally outline the flow regimes in which our first-order-R voltage corrections could potentially be experimentally measured. Our work gives insight into the coupling mechanism between electric field and shear flow in parallel-planes channel flows, correcting an analogous EHD expansion to small R from the literature. We also revisit the case of pure shear instability, when the first-order-R voltage correction equals zero, and replace the renowned instability mechanism due to viscosity stratification at small R with the mechanism due to discontinuity in the slope of the unperturbed velocity profile.

Dean vortex-enhanced blood plasma separation in self-driven spiral microchannel flow with cross-flow microfilters

Point-of-care (POC) diagnostic devices have been developing rapidly in recent years, but they are mainly using saliva instead of blood as a test sample. A highly efficient self-separation during the self-driven flow without power systems is desired for expanding the point-of-care diagnostic devices. Microfiltration stands out as a promising technique for blood plasma separation but faces limitations due to blood cell clogging, resulting in reduced separation speed and efficiency. These limitations are mainly caused by the high viscosity and hematocrit in the blood flow. A small increment in the hematocrit of the blood significantly increases the pressure needed for the blood plasma separation in the micro-filters and decreases the separation speed and efficiency. Addressing this challenge, this study explores the feasibility of diluting whole blood within a microfluidic device without external power systems. This study implemented a spiral microchannel utilizing the inertial focusing and Dean vortex effects to focus the red blood cells and extract the blood with lower hematocrit. The inertial migration of the particles during the capillary flow was first investigated experimentally; a maximum of 88% of the particles migrated to the bottom and top equilibrium positions in the optimized 350 × 60 μm (cross-sectional area, 5.8 aspect ratio) microchannel. With the optimized dimension of the microchannel, the whole blood samples within the physiological hematocrit range were tested in the experiments, and more than 10% of the hematocrit reduction was compared between the outer branch outlet and inner branch outlet in the 350 × 60 μm microchannel.

Oscillating high aspect ratio micro-channels can effectively atomize liquids into uniform aerosol droplets and dial their size on-demand.

Ultrasonic atomization of liquids into micrometer-diameter droplets is crucial across multiple fields, ranging from drug delivery, to spectrometry and printing. Controlling the size and uniformity of the generated droplets on-demand is crucial in all these applications. However, existing systems lack the required precision to tune the droplet properties, and the underlying droplet formation mechanism under high-frequency ultrasonic actuation remains poorly understood due to experimental constraints. Here, we present an atomization platform, which overcomes these current limitations. Our device utilizes oscillating high aspect ratio micro-channels to extract liquids from various inlets (ranging from sessile droplets to large beakers), bound them in a precisely defined narrow region, and, controllably atomize them on-demand. The droplet size can be precisely dialled from 3.6 μm to 6.8 μm by simply tuning the actuation parameters. Since the approach does not need nozzles, meshes or impacting jets, stresses exerted on the liquid samples are reduced, hence it is gentler on delicate samples. The precision offered by the technique allows us for the first time to experimentally visualise the oscillating fluid interface at the onset of atomization at MHz frequencies, and demonstrate its applications for targeted respiratory drug delivery.

High-quality fabrication of diamond straight microchannels heat sink with large aspect ratio microchannels using UV nanosecond laser based on multi-feed method

The high heat flux density of modern chips presents a major challenge to their development. Single-phase or two-phase flow heat dissipation techniques using high thermal conductivity materials are seen as a potential solution. However, creating microfluidic channels from ultra-high thermal conductivity diamond materials is a significant challenge. In this paper, we present an experimental study on the fabrication of diamond heat sinks with high aspect ratio microchannels using a UV-nanosecond laser processing system, along with two multi-feed processing techniques for the collecting grooves and microchannels. We used a 1 μm spacing grid path combined with a polished multi-feed process to fabricate a 1600 μm deep collecting groove with a smooth bottom morphology. We also proposed a multi-area multi-feed fabrication technique for microchannels with an aspect ratio of 8:1 and a depth of 1600 μm. Finally, we have successfully fabricated a diamond straight microchannels heat sink, featuring a high aspect ratio and low phase change damage.

Mixing intensification in an acoustofluidic micromixer aided with micro-pillars

In this work, we present an acoustofluidic micromixer that employs the combination of acoustic waves and micro-pillars to achieve enhanced mixing performance. We have shown that micro-pillars can generate micro-vortices, which lead to intense stretching and folding of the fluid trapped inside the micromixer and subsequently result in the formation of lamellar structures consisting of layers of the stretched fluids having steep concentration gradients between them. High concentration gradients coupled with the increased interfacial area result in higher rates of molecular diffusion. A numerical study is conducted, testing the performance of micro-pillars of three different designs in comparison to a simple rectangular acoustofluidic micromixer without any micro-pillars. The performance of the device in terms of improving the reaction kinetics of a chemical process has also been analyzed. For wide aspect ratio microchannels, our acoustofluidic micromixer helps in achieving rapid and homogeneous mixing while maintaining a high throughput.

Flow Fluctuation during Flow Boiling of Binary Mixtures in High Aspect Ratio Microchannel

Flow boiling performance is affected by several factors, such as channel characteristics and working fluid types. It is found that there is still limited study that discusses the use of binary mixtures combined with high aspect ratio microchannels. The aim of this study is to investigate the flow fluctuation during flow boiling of binary mixtures in rectangular microchannels. Here, a 6 mm width and 0.3 mm depth rectangular channel was utilised, and it represents a hydraulic diameter of 571 μm and an aspect ratio of 20. In the present works, a mass flux of 10 kg m-2 s-1 was used, and the heat flux ranged from 15.2 and 21.0 kW m-2. The image processing technique was applied to track the bubble tail movement. In addition, the thermal camera was utilised to gather the wall temperature distribution of the channel. The preliminary results show that the use of binary mixtures influences the vapour fraction in the channel and the flow fluctuation characteristics. Some differences are observed in terms of wall temperature characteristics. However, the rapid increase of wall temperature is found in the outlet region for high flux cases under all liquid types which suggests the dominance of dry out event.

Flow study of Dean’s instability in high aspect ratio microchannels

Dean’s flow and Dean’s instability have always been important concepts in the inertial microfluidics. Curved channels are widely used for applications like mixing and sorting but are limited to Dean’s flow only. This work first reports the Dean’s instability flow in high aspect ratio channels on the deka-microns level for De>162. A new channel geometry (the tortuous channel), which creates a rolled-up velocity profile, is presented and studied experimentally and numerically along with other three typical channel geometries at Dean’s flow condition and Dean’s instability condition. The tortuous channel generates a higher De environment at the same Re compared to the other channels and allows easier Dean’s instability creation. We further demonstrate the use of multiple vortexes for mixing. The mixing efficiency is considered among different channel patterns and the tortuous channel outperforms the others. This work offers more understanding of the creation of Dean’s instability at high aspect ratio channels and the effect of channel geometry on it. Ultimately, it demonstrates the potential for applications like mixing and cell sorting.

Numerical study of bacteria removal from microalgae solution using an asymmetric contraction-expansion microfluidic device: A parametric analysis approach

Microalgae have been remarkably taken into account due to their wide applications in the biopharmaceutical, nutraceutical and bio-energy fields. However, contamination of microalgae with bacteria still appears to be a concern, adversely impacting products’ quality and process efficiency. Microalgae decontamination with conventional techniques is usually expensive and time-consuming. Moreover, damage to microalgae cells is highly possible. Asymmetric contraction-expansion microchannels (Asym-CEMCs) are promising passive microfluidic devices that can overcome conventional techniques' drawbacks with their standing-out features. However, the flexibility of Asym-CEMCs performance arising from their various tunable geometrical parameters results in the fact that their performance for separating a target particle cannot be predicted without an investigation. In this work, for the first time, Asym-CEMCs were numerically studied for the removal of a very conventional bacteria, B. subtilis (1 μm), from one of the most popular microalgae, C. vulgaris (5.7 μm). The influences of the microchannel aspect ratio, length and width ratios of the expansion-to-contraction zones, and the total flow rate on the separation resolution and focusing width of the particles were investigated by a 3D numerical model. The aspect ratio had the strongest influence on the Asym-CEMC performance, however, the length ratio had no considerable effect on the results. A decrease in the aspect ratio augmented the shear-induced lift force and Dean drag force, leading to a significant separation resolution improvement. Microalgae decontamination was also enhanced by an increase in the total flow rate and expansion-to-contraction width ratio. Finally, a locally optimized Asym-CEMC with an aspect ratio of one and expansion-to-contraction width and length ratios of 4.7 and 2.07, respectively, was proposed, leading to complete microalgae decontamination with a high normalized separation resolution of 0.6. In a word, Asym-CEMCs with tailored dimensions are promising for successfully decontaminating microalgae from bacteria.

Numerical investigation of heat transfer and pressure drop characteristics of flow boiling in manifold microchannels with a simple multiphase model

The Manifold Microchannel (MMC) heat sink is an emerging micro-scale electronic cooling technology with high heat dissipation potential and application prospects. This paper numerically studied the subcooled flow boiling in MMC heat sinks. The feasibility of using the Mixture multiphase flow model to simulate the subcooled flow boiling in the MMC heat sink is validated. As the vapor-liquid interfaces are not reconstructed in this model, less computational resources are needed compared with other multiphase models. The results show that at constant channel width, the maximum and average temperatures of the MMC decrease with the increase of the microchannel aspect ratio. For cases with an inlet velocity of 0.35 m/s, the average wall temperature decreases from 388.53 K to 374.26 K when the aspect ratio increases from 6.67 to 16.67. The difference between the maximum and the minimum temperature on the heated surface increases with the increase of channel aspect ratio, resulting in an uneven temperature distribution. The thickness of temperature boundary layer near the divider is thicker than that near the heated base at low aspect ratio. Low pressure drops are obtained for cases with high aspect ratios due to the increase in cross-sectional area. The pressure drop of MMC with aspect ratio of 6.67 is nearly twice of the case with aspect ratio of 16.67. Although the Mixture model could not characterize the bubble shapes, the numerical results are similar to those obtained by the VOF model and the experimental visual results. A complex case of subcooled flow boiling in Z-type MMC with a hotspot heat flux of 800 W/cm2 is also conducted using the whole computational domain. Mal-distributions of mass flux, wall temperature and vapor void fraction are observed for this complicated problem. The difference between the maximum and the minimum temperatures is about 12 K. The Mixture model is more suitable to obtain reasonable numerical results of flow boiling in complex MMC with much less computational resource.

Experimental investigation of bubble dynamics and flow patterns during flow boiling in high aspect ratio microchannels with the effect of flow orientation

Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFE−7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg m−2 s−1 and 42 kg m−2 s−1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW m−2 to 18.7 kW m−2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film.

Basic Guide to Multilayer Microfluidic Fabrication with Polyimide Tape and Diode Laser.

For normal operations, microfluidic devices typically require an external source of pressure to deliver fluid flow through the microchannel. This requirement limits their use for benchtop research activities in a controlled static environment. To exploit the full potential of the miniaturization and portability of microfluidic platforms, passively driven capillary microfluidic devices have been developed to completely remove the need for an external pressure source. Capillary microfluidics can be designed to perform complex tasks by designing individual components of the device. These components, such as the stop valve and trigger valve, operate through changes in microchannel dimensions and aspect ratios. A direct, maskless fabrication protocol that allows the precise fabrication of microchannels and other microfluidic components is introduced here. A diode laser and polyimide tape on a PMMA substrate are the only components needed to start fabrication. By varying the laser power used and the number of laser repetitions, various depths and widths of the microchannel can be quickly created to meet specific needs. As an example of a functional unit, a trigger valve was fabricated and tested, as proof of the validity of the fabrication protocol.

Two-phase flow patterns investigation in large aspect ratio microchannel with T-mixer

Two-phase flow patterns investigation in large aspect ratio microchannel with T-mixer