In this paper, we have introduced a negative Dielectrophoresis based microfluidic system using a novel arrangement of microelectrodes to perform switching of micro objects. Both the experimental and numerical results are presented. Two sets of interdigitated electrodes, extending slightly into the microchannel from each sidewall, are embedded on the bottom of the microchannel. A finite element model in COMSOL Multiphysics 5.2a was developed to demonstrate switching of Red Blood Cells in the microchannel followed by multiple parametric studies to study the effect of several parameters on cell trajectories and optimize the design parameters. To verify numerical results, a PDMS-based microfluidic device on glass wafer was fabricated. The switching of Red Blood Cells in the microfluidic device with a single inlet and three outlets was also demonstrated.

In this paper, a two-dimensional Dissipative Particle Dynamics (DPD) technique to simulate the poiseuille flow in a microchannel is developed using an in-house code. The calculated Reynolds number is reduced via adjusting the DPD parameters. The obtained velocity profile is compared with the analytical results and a good agreement is found. The drag force and the drag coefficient on a stationary cylinder exerted by the fluid particles are obtained using the developed DPD code. The calculated drag coefficient exhibits a close match with already published data in the literature.

Experiment and simulation studies were performed to investigate the flow characteristics of pressure-driven Nitrogen flow in the micro-channels of Printed Circuit Heat Exchanger (PCHE). The core of the PCHE was made using diffusion bonding method with ten stainless steel 316L plates, where hemispherical 1 mm in diameter channels were chemically etched. On one of the plates, four-teen more channels, 1 mm in diameter, were milled to make pressure taps to measure local pressure drops. Then, one inlet tube and four-teen tubes for pressure tabs were attached by welding on the top and sides of PCHE core, respectively. The PCHE was connected to Nitrogen tank with pressure regulator, Coriolis flowmeter and differential pressure gauge, and data was acquired with DAQ system. By varying the velocity of Nitrogen gas from 1 to 35 m/s, differential pressure drops were measured between two different locations. The local pressure drops were analyzed theoretically and pressure loss coefficients could be calculated in straight and serpentine channels, respectively. In addition, simulation work using ANSYS Fluent was also performed to understand flow characteristics especially at corners of channels.

The increase in the power of chips and microchips has resulted in the generation of heat fluxes to be dissipated of the order of 100 W/cm 2 in very small areas [1], therefore, dissipating this heat has become a priority for the proper functioning of these dispositives. Thus, the proposition of new geometries and dissipation methods has become an area of great interest in scientific research. This research presents the analysis of a new geometry for a heat sink capable of dissipating very high energy flows by means of liquid cooling. The analyzes are based on previous analyzes of serpentine type geometries, where the heat flux to dissipate was 10 W/cm 2 [2]. The operating conditions, such as velocity and pressure drop, as well as heat transfer are analyzed. Water is used as the dissipation fluid at an inlet velocity of 0.1 m/s. The geometry to be analyzed, called “Aztec Geometry,” is a radial type geometry that was originally designed for fuel cells, and has small fins arranged in three radial stripes. The results indicate that the pressure drop is on the order of 85 Pa, which is less than the pressure drop when radial coil microchannels are used (between 100 and 870 Pa). The dissipated heat is larger than the heat dissipated in radial coil microchannels (205 W versus 145 and 194 W), proving that the proposed radial geometry has a greater dissipation capacity at a lower cost than previously proposed geometries in the market.

The goal of this research is to present an analytical model of nanostructures and study the effects of their geometry on the performance of micro channels. The pressure drop experienced by micro channels is of interest as it presents a limit on forced convection heat transfer. This work will demonstrate how the presence of nanostructures alleviates pressure drop and results in enhanced cooling capabilities. Multiple transient analyses were performed in ANSYS FLUENT to ascertain performance characteristics of microchannels without the presence of hydrophobic nanostructures. The results were compared to the analytical model developed in this study.

To minimize the computational and optimization time, a numerical simulation of 3D microchannel heat sink was performed using surrogate model to achieve the optimum shape. Latin hypercube sampling method was used to explore the design space and to construct the model. The accuracy of the model was evaluated using statistical methods like coefficient of multiple determinations and root mean square error. Thermal resistance and pressure drop being conflicting objective functions were selected to optimize the geometric parameters of the microchannel. Multi objective shape optimization of design was conducted using genetic algorithm and the optimum design solutions are presented in the Pareto front. The application of the surrogate methods has predicted the performance of the heat sink with the sufficient accuracy employing significantly lower computational resources.

In this study, three-dimensional numerical simulations are performed to investigate heat transfer enhancement in multi-harmonic micro-scale wavy channels. The focus is on the influence of channel surface-topography, modeled as multi-harmonic sinusoidal waves of square cross-sectional area, on the enhancing mechanisms. A single-wave device of 0.5 mm × 0.5 mm × 20 mm length, is used as baseline, and new designs are built with harmonic-type surfaces. The channel is enclosed by a solid block, with the bottom surface within the sinusoidal region being exposed to a 47 W/cm 2 heat flux. The numerical solutions of the governing equations for an incompressible laminar flow and conjugate heat transfer are obtained via finite elements. By using the ratio of the Nusselt number for wavy to straight channels, a parametric analysis — for a set of cold-water flowrates (Re = 50, 100, and 150) — shows that the addition of harmonic surfaces enhances the transfer of energy and that such ratio achieves the highest value with wave harmonic numbers of n = ±2. Use of a performance factor (PF), defined as the ratio of the Nusselt number to the pressure drop, shows that, surprisingly, the proposed wavy multi-harmonic channels are not as efficient as the single-wave geometries. This outcome is thought to be, primarily, due to the uncertainty associated with the definition of the Nusselt number used in this study, and establishes a direction to investigate the development of a more accurate definition.

Microfluidic applications may involve the control of flows through a series of junctions. For example, flow branching is typically used to deliver specific quantities of fluid to various locations and converging flows are associated with mixing processes. The shape of the fluid interface of the converging flows at and downstream of the flow junction is the focus of this study. In particular, we investigate the transition from planar stratified flow to annular flow. We report results for microchannel configurations which have a main channel and an intersecting (at 90 or 45 degrees) daughter channel. Two 90 degree channels are investigated, one with cross sectional dimensions 129 × 100 um and the other 200 × 100 um. The 45 degree channel dimensions are 161 × 100 um. We compare flows of water over various Reynolds numbers (based on total flow) of 5–400. Flow visualization is achieved using confocal fluorescence microscopy. Flow modules are fabricated using soft lithography techniques and are enclosed by bonding plasma cleaned glass slides to the PDMS module. At lower Reynolds numbers, e.g., < 20, stratified flow is achieved with some curvature at the interface. As the Reynolds number increases, the flow transitions to have large degrees of curvature and eventually into annular flow. This transition is influenced by Reynolds number, flow ratio (daughter/total flow), channel geometry (cross section) and configuration (45 vs. 90 degrees). Flow regime plots are produced for all microchannel configurations. This study should provide insight into intersecting microfluidic flows when the Reynolds number is high enough to produce a non-planar fluid interface at the junction.

With the advance of microfluidic platforms and due to the need to solve different implications that still exist on the transport of electrically conducting fluids, the analysis on strategies in micropumps that involve a simplicity in its structure, absence of mechanical moving parts, flow reversibility and low power requirement is current. Therefore, the present investigation contributes with the analysis of the combined magnetohydrodynamic/pressure driven flow of multilayer immiscible fluids in a microchannel formed by two parallel flat plates. The mathematical model is based in a steady fully developed flow and the pumped fluids follow the power law model to describe the pseudoplastic fluids rheology, while magnetic effects on the flow are given from the Lorentz forces. The velocity profiles and flow rate are obtained in the limit of small Hartmann numbers by solving analytically a closed system of ordinary differential equations, together to the corresponding boundary conditions at the solid-liquid interfaces in the channel walls and at the liquid-liquid interfaces between the fluid layers. The results show that the flow field is controlled by the dimensionless parameters that arise from the mathematical modeling being a parameter that indicates the competition between pressure to the magnetic forces, magnetic parameters related to Hartmann numbers, viscosities ratios between the fluids, flow behavior indexes and the dimensionless position of the liquid-liquid interfaces.

Phase separation has been proven to be beneficial to air-cooled parallel flow microchannel condensers for air conditioning systems. The inlet to the condenser with phase separation is located at the middle of the condenser height. After the first pass, in the vertical second header of the condenser, vapor phase separates from liquid phase mainly due to gravitational effects. In ideal case vapor should go to the top exit and liquid to the bottom exit, resulting in increased heat transfer. Due to interaction between vapor and liquid, separation is not perfect, expressed through the separation efficiency. This paper presents a parametric study of phase separation efficiency in the intermediate headers, with the target to improve separation efficiency. Header prototypes which have two exits are made with transparent PVC to simulate the real header and provide visual access. Using R-134a as a baseline, the measurement of separation efficiency and its general trend will be shown first. The results are compared to those of a mechanistic model based on flow regime and force balance analysis. Inlet mass flux in simulation is controlled at 87 kg·m −2 ·s −1 – 311 kg·m −2 ·s −1 and inlet quality at 0.05–0.25. The observed flow patterns in header are compared with the modeling results as well. Then, the header diameter is increased, which effectively improves the separation efficiency due to reduction of vapor velocity in header. Finally, R245fa and R32 are modeled in comparison with R-134a to discuss the effect of fluid properties on separation efficiency.

The recirculating wake behind the obstacle at moderate Reynolds numbers was devoid of particles, this was discovered by Haddadi et al. (J. Fluid Mech., 2014). However, only one obstacle and narrow Reynolds numbers were considered in their work. In this work, we constructed more confined environment, where the suspensions of solid fraction 0.25% with different particle diameters of 1, 5 and 10 μm flow past the critical confined low-aspect-ratio cylinder arrays ( H/D = 0.3 ) with different arrangements were experimentally carried out. Reynolds numbers performed in the experiments ranged from 14∼550, and different flow patterns were observed. It was found that particles could flow into the region behind cylinder at low flow rate. Then, particle-depleted wake zone was formed behind the cylinder when increasing Re , which is similar with reported literature. It was interesting to find that when increasing Re further, the particles could flow into the recirculating wake zone behind cylinder. We generalized the particle behavior behind cylinder as from “entry” to “particle-free” and to “re-entry”. Additionally, the influence of different layout modes with inline and staggered cylinder arrays were also investigated. We found the particle-depleted wake zones behind the in-line cylinder array were larger than the one of the staggered cylinder array as the velocity were the same at “particle-free” stage. The in-line cylinder array possessed higher ability of allowing 1 μm particles to fill with the recirculating wake, whereas there were always existing particle-free zones in the core of recirculating wake of staggered cylinder array at “re-entry” stage. In order to understand particle-free mechanism better, microparticle image velocimetry (μPIV) technique was utilized to quantitatively measure the flow fields of in-line and staggered arranged cylinders. The obtained fluorescence pictures demonstrated that few particles flow into the zone behind cylinder at the “particle-free” stage, and fluorescence particles can flow into the wake at “re-entry” stage. That is, fluorescence particles also experienced the stages from “entry” to “particle-free” and to “re-entry”.

Previous research has proved Double-layer Microchannel Heat Sinks (MHSs) to be efficient ways to improve the cooling performance of electronic devices. However, the cooling potential of the upper working liquid cannot be fully utilized to cool down the substrate with the heated elements. In this sense, a concept of staggered double-layer MHS is proposed and designed. The parallel and counter flow directions are considered to investigate the flow arrangement effect. The Reynolds number effect, Nusselt number and pressure drop are analyzed in detail and compared with those of a parallel straight double-layer MHS. It is found that the staggered double-layer MHSs exhibit much better heat transfer enhancement and overall thermal performance compared with the parallel straight double-layer MHS. For the staggered double-layer MHSs, the counter flow case is superior to the parallel flow case. This research provides a new structure design to enhance the heat transfer in microchannel heat sinks and broad application prospects for heat sinks in the thermal management of high power density electronic devices.

An experimental investigation was conducted on a single phase microchannel heat sink, in which the bottom surface of the microchannel was modified with hybrid micro-sandblasting of elliptical patterns (HSEP) and fully sandblasting (FS) to passively enhance the microchannel heat transfer performance. The dimension of the microchannel is measured as 26 mm (L) × 5mm (W) × 0.35 mm (H), which results in a hydraulic diameter of 654 μm. Deionized water was used as the coolant, and the Reynolds number range between 85 to 650 was tested. The experimental results show that fully sandblasting (FS) bottom surface of the microchannel only slightly improved the heat transfer performance. However, the modified surface with HSEP enhanced the heat transfer performance substantially, compared to the benchmark results obtained with the bare surface (BS) microchannel. The pressure drops of the HSEP increased slightly compared to the BS and FS, due to the flow resistance of the microstructures. The proposed surface for enhancement of heat transfer will be useful in many high heat flux engineering applications. In the future, this study will be further extended to two-phase microchannel heat transfer.

Combustion characteristics at small scales have been studied continuously due to the potential applications in portable power devices. It is known that heat release impacts at small scales result in different flame behavior as compared to conventional scales. The impacts of geometry, stoichiometry, flow rates, wall temperatures, etc. are widely studied in literature. However, dilution impacts still need to be further studied due to its important role on controlling the flame behavior and subsequent pollutants emissions at these scales. In this work, premixed hydrogen/air combustion is simulated at an axis-symmetric micro channel (with diameter D = 0.8 mm and length L = 10 mm ), where detailed chemical kinetics are implemented in simulations (32 species and 173 reactions). The heat transfer on the wall is considered by imposing a hyperbolic temperature profile on the wall, where the wall temperature increases from 300 K at the inlet to 1300 K at the outlet. With this setup, a range of equivalence ratios including a typical fuel-lean regime (ϕ = 0.7), stoichiometric regime (ϕ = 1.0) and and two cases at an ultra-rich regime (ϕ = 2.0 and ϕ = 3.0) are investigated. For each equivalence ratio, excess dilution (using N2) is introduced to the mixture and its impact is compared with other cases. With that, the impacts of dilution variations on the combustion characteristics of premixed hydrogen/air are investigated for different equivalence ratios. More specifically, several incidents such as flame dynamics, flame stabilization, extinctions and NOx emissions are studied for the aforementioned operating conditions.

Cavitation process in microchannels has been widely studied from variety of perspectives. In diesel engines, smaller droplets provide a larger surface area and this leads to more efficient combustion. It has been observed that the atomization process is influenced by turbulence and cavitation in the nozzle. These factors therefore contribute to the size of the droplets, the spray angle, penetration of the fuel and other characteristics. In the present study, a numerical simulation of cavitation process in microchannels similar to nozzles is conducted using ANSYS Fluent. Cavitation phenomenon has been analyzed numerically for a variety of downstream pressures and L/D ratios. The effect of the geometry of the nozzle (length to diameter ratio) and downstream flow pressure conditions on the cavitation are also studied. The results can be used for optimal design of diesel injectors.

In recent years, particle hydrodynamic focusing and ordering in a confined microchannel has been developed as a promising technique in lab-on-a-chip systems such as microparticle/cell separation, on-chip flow cytometry and detection. During the focusing, the uniformly distributed finite-sized particles from the channel inlet migrate across streamlines to several equilibrium positions according to the balance between series of hydrodynamic lift forces. While most studies in literature focus on single particle’s motion in the hydrodynamic focusing process by considering particle-liquid interactions, very few of them investigate particle-particle interactions, which is important in particle ordering and manipulation at high throughput. In this study, we use an immersed boundary (IB) - lattice Boltzmann (LB) coupled model to investigate the dynamic behaviors of a particle pair traveling through a square microchannel. The utilized numerical model retains the advantages of both LBM and IBM, which can accurately model the momentum exchange between liquid and particles, and conveniently treat complex geometry and movement of liquid-particle surfaces. By conducting numerical simulations, the time-dependent dynamic behaviors of particle pairs, including trajectories and interactions from initial rest condition to final quasi-equilibrium condition are obtained. Influences of important factors, such as Reynolds number and particle sizes on particles’ motions are discussed and the underlying physical mechanisms of particle-particle interactions are revealed in-depth.

The investigation of the dynamics of a droplet traversing through constricted channels is an interesting field of research. In the present work, the motion and deformation of a neutrally buoyant droplet in planar constricted micro channels containing another immiscible fluid is studied computationally using an open-source finite-volume fluid flow solver, Gerris. The important non-dimensional parameters pertaining to such type of flow are — the Capillary number, which gives the relative importance of the viscous force over the surface tension force, the viscosity ratio between the dispersed and suspending medium and the ratio of the drop diameter to the channel height. Both symmetrical and asymmetrical constrictions are considered and results obtained are compared with a straight channel without any constriction. The parametric studies are conducted to study the effect of droplet size, viscosity ratio, Capillary number and presence of constriction in the channel. Depending on the parameters chosen, the drop extends to a maximum length as its front passes through the constriction and the deformation increases with the increase in number of constrictions. In the case of large drops, it is observed that the droplet disintegration rate increases in the presence of constrictions.

Passive micromixers have application in the biosciences area. In particular, passive micromixers that may be used as part of point-of-care (POC) diagnostic testing devices are becoming common-place and have application in developed, developing, and relatively undeveloped locales. Characterizing and improving mixing efficiency in these devices is an ongoing research effort. Different channel geometries and flow obstacles lead to varying degrees of mixing effectiveness and serve to increase chaotic advective mixing in contrast to the molecular diffusive mixing that occurs even in the absence of these obstacles. Entropy is generated due to these, and other, irreversible processes. Efficient micromixer design is of interest to biomedical and mechanical engineers working in the biosciences area. The entropy generation rate, we contend, can provide an aid in determining how thoroughly mixed fluids in the channel have become, as well as provide insight into improving channel design to maximize desired outcomes, such as mixing, and minimizing losses due to heat transfer and power consumption. In this paper, we focus our analysis on numerical simulations conducted using computational fluid dynamics (CFD) on a supercomputer-cluster to do simulations with extended mesh refinement and very small residuals. This enabled us to test a wide range of flows with varying Reynolds numbers. The configuration of flow and species parameters within the simulations were compared to experimental results to confirm their validity. We show that varying the geometry of the channel can lead to a measurable increase in entropy generation via the Second Law of Thermodynamics. Further, we show that this increase in entropy is linked to mixing from obstacle-induced chaotic advection and diffusion. We provide evidence of a positive correlation between the efficiency of the mixing process and entropy generation. These findings will aid in the design of more efficient portable health care-related devices, particular in remote or underdeveloped regions where power utilization is a critical concern.

This paper presents the analytical solution of a combined electroosmotic/pressure driven flow of three viscoelastic immiscible fluids in a parallel flat plate microchannel. The mathematical model is based in the Poisson-Boltzmann equation and Cauchy momentum conservation equation. In the steady state analysis, we consider that the three fluids are electric conductors and obey to the simplified Phan-Thien-Tanner rheological model; therefore, different conditions at the interface between the fluids as electric slip, surface charge density and electro-viscous stresses balance are discussed in detail. Results show the transport phenomena coupled in the description of the velocity profiles, by the analyzing of the dimensionless parameters obtained, such as: the electric slips, the electric permittivities ratios, the surface charge densities, the zeta potentials at the walls, the interfaces positions, the viscosity ratios, the viscoelastic and electrokinetic parameters, and the term involving the external pressure gradient. Here, the presence of a net electric charges balance at the interface, breaks the continuity of shear viscous stresses, modifying the flow field; hence, for the established electric conditions at the interface through the values of the electric slips and the surface charge densities, play a role like a switch on the flow behavior. This investigation extends the knowledge about the techniques on the control of immiscible non-Newtonian fluids in microescale.

The work presented here is motivated by the recent growing interest in using additive manufacturing to fabricate micro-channels networks. Distorted shapes and rough geometries influence hydrodynamic characteristics of micro-channels by increasing their flow resistance and pressure drop or altering wall shear stresses inside them. Since geometric conformity and shape fidelity of micro-channels networks are greatly influenced by manufacturing process, this work is focused on dimensional characterization of micro-channels fabricated using additive manufacturing. In this work, circular and rectangular cross-section micro-channels are 3D printed. Shapes and dimensions of 3D printed micro-channels are examined using Scanning Electron Microscope (SEM) imaging. In this work, 500 μm diameter and 200 μm square transparent PolyLactic Acid (PLA) micro-channels are 3D printed with average errors 0.25% and 1.65%, respectively. SEM images confirmed geometric conformity and shape fidelity of the 3D printed circular and rectangular cross-section micro-channels. Statistical analysis is performed on multiple prints to verify reproducibility and shape conformity. Results show that factors such as printing direction play essential role in the shape conformity and geometric fidelity of the micro-channels. Although 3D printing is a promising route for attaining micro-channels there are still significant improvements that can be made to the precision of the printer in the XY plane for printing small geometric figures. This improvement will likely come as the printing technology and software both improve to allow the operator more control over the outcome of the print. Additionally, new 3D printing materials may open the gate for new applications in different fields such as thermal management and microfluidics.