Converging-diverging Microchannel Research Articles

Enhancement of concentrator photovoltaic system through convergent-divergent microchannels

Concentrated photovoltaic technology is one of the evolving technologies that converts solar to electrical energy with a high efficiency percentage. Increasing concentrated photovoltaic cell’s temperature significantly reduces the performance of the system. Thus, in order to properly functioning of system, thermal management will be a rudimentary issue. This article focuses on the cooling performance of high-concentration photovoltaic system using microchannels attached to the bottom plate of solar cell. To this end, the effect of convergent-divergent microchannels at three amplitudes and three wave lengths at different Reynolds numbers is investigated, and results are compared with simple microchannels. The output results are presented via temperature, pressure, velocity contours as well as Nusselt number, friction factor, and effectiveness ratio. Compared to the simple microchannels, convergent-divergent microchannel lowers the wall temperature about 0.4–2 Kelvin by making velocity gradients along the channel. This leads to boosting the heat transfer capability up to 3 times in the best case and up to 1.5 times in the worst case. Increasing the wave length enhances the performance of system, while the opposite trend has been seen for greater wave amplitudes. For case A with lowest wave length, Nu increases 1.92 and 2.4 times at lowest and highest Re numbers while friction factor increases due to the higher contact surface area along the fluid stream. Moreover, increasing the wave amplitude leads to higher Nu number, higher friction factor and consequently lower effectiveness ratio. Finally, effectiveness ratio as a criterion to choose best case has shown that a higher wave length alongside with lower wave amplitude offers better heat transfer enhancement and lower pressure drop compared to simple channel. Case C-1 as a best case offers 0.8K lower wall temperature, 1.6 times better Nu number with only 1.3-time higher friction factor.

Hotspot thermal management of silicon-based high power hetero-integration

Extremely uneven thermal distribution due to the hotspots generating at the transistor channel regions on GaN power chip is one of the major obstacles to high power hetero-integration on silicon. In this work, a 12 mm × 18 mm × 0.7 mm embedded converging-diverging microchannel (CDMC) cooler silicon interposer (SI) is put forward to address the hotspot cooling. A 5.3 mm × 3.5 mm × 0.08 mm GaN monolithic microwave integrated circuit (MMIC) 3-stage power amplifier (PA) chip with DC heating power of ∼ 101 W was integrated on the SI to evaluate the on-chip hotspot cooling performance. The average hotspot heat flux was up to 306.8 ∼ 307.1 W/mm2, and the average chip heat flux achieved 545.4 ∼ 546.3 W/cm2. The maximum junction temperature of the GaN chip on CDMC SI was 137.2 ∼ 147.4 °C with the pressure drop of 122.8 ∼ 394.3 kPa and pumping power of 0.41 ∼ 3.29 W at 70 °C ambient temperature. The thermal and hydraulic characteristics of the CDMC SI are analyzed based on field synergy principle and thermodynamic properties using finite element method (FEM). The heat transfer coefficient (HTC) of the CDMC SI has 22.4 ∼ 35.4% increase compared with that of a typical microjet array (MJA) SI. The cooling performance and efficiency of the CDMC SI have been validated theoretically and practically to be prior to those of the MJA SI, and it is a promising solution for high power hetero-integration on silicon with brilliant hotspot thermal management capability.

Enhanced solvent extraction in a serial converging-diverging microchannel at high injection ratio

A microchannel with serial converging–diverging geometries, inspired by porous structures of rock, was designed to enhance the cross-interfacial mass transfer of two-phase flow at a high phase injection ratio up to 200:1. It was experimentally demonstrated that the design enhanced mass transfer coefficient by approximately four times compared to classic microchannel design. The superior performance of the microchannel was attributed to the retention of dispersed fluid before the converging geometry due to the capillary pressure barrier that resulted in velocity contrast between the two fluids and reduced the effective mass-transfer distance in continuous phase. Additionally, the deformation and dispersal of droplets at the diverging geometry further increased the interfacial area. The flow patterns in this geometry were experimentally identified. The serial converging–diverging design can be used for high phase ratio extraction applications.

Investigation of hydrodynamic cavitation induced reactive oxygen species production in microchannels via chemiluminescent luminol oxidation reactions

Hydrodynamic cavitation was evaluated for its reactive oxygen species production in several convergent-divergent microchannel at the transition from micro to milli scale. Channel widths and heights were systematically varied to study the influence of geometrical parameters at the transitory scale. A photomultiplier tube was used for time-resolved photon detection and monitoring of the chemiluminescent luminol oxidation reactions, allowing for a contactless and in situ quantization of reactive oxygen species production in the channels. The radical production rates at various flow parameters were evaluated, showing an optimal yield per flow rate exists in the observed geometrical range. While cavitation cloud shedding was the prevailing regime in this type of channels, the photon arrival time analysis allowed for an investigation of the cavitation structure dynamics and their contribution to the chemical yield, revealing that radical production is not linked to the synchronous cavitation cloud collapse events. Instead, individual bubble collapses occurring throughout the cloud formation were recognized to be the source of the reactive oxygen species.

Hydrothermal and Second Law Analyses of Fluid Flow in Converging-Diverging (Hourglass) Microchannel

A three-dimensional numerical investigation is carried out to understand the mechanism of hydrothermal performance and second law analysis for a flow through converging-diverging (hourglass) microchannel. The fluid flow and convective heat transfer characteristics are investigated as a function of convergence-divergence angle, width ratio (ratio of bigger width to smaller width), and Reynolds number. The present study uses various parameters such as performance evaluation index, entropy generation, Bejan number, and augmented entropy generation number to examine the hydrothermal and thermodynamic performance characteristics of an hourglass microchannel. The parametric analysis reveals that the Nusselt number increases with an increase in convergence-divergence angle; however, it decreases with an increase in width ratio. Increasing the convergence-divergence angle effectively reduces the overall temperature gradient, whereas the width ratio has an opposite effect. The results also indicate that the hourglass microchannel shows better hydrothermal and thermodynamic performance than the uniform microchannel. For a given range of parameters, the maximum value of the performance evaluation index is 1.34, and the maximum reduction in entropy generation is 28% relative to a uniform microchannel. The findings obtained in this study may serve as a valuable roadmap for designing an hourglass microchannel in diverse heat transfer applications.

Oscillative Trapping of a Droplet in a Converging Channel Induced by Elastic Instability.

Permanent trapping of an oscillating, nonwetting droplet is observed in a converging-diverging microchannel when aqueous, viscoelastic fluids are injected. Classical theories based on the balance between capillary and viscous forces suggest that the droplet size should decrease with increasing flow rates of a displacing Newtonian fluid, and the droplet should be completely displaced at high injection rates. However, droplets in viscoelastic fluids cannot be removed by increasing flow rates due to the oscillation. The oscillation amplitude linearly increases with the Deborah number (De), which further inhibits the droplet's passing through the constriction, "permanently." Our microfluidic experiments show that the onset of oscillation is determined by a critical De, which is near 1. We derive a linear relationship for the trapped droplet length with Ec^{1/3}, where Ec is the elastocapillary number, by introducing the elastic force into the force balance in addition to the capillary and viscous forces.

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.

Experimental Analysis of Wall Flow Pattern and Fluid Shear Stress Effects on Creeping Flow Field in Convergent-Divergent Microchannels

The flow of fluids including micron particles in micro channels and manufacturing microfluidics tools have been one of the important topics in last decades and are utilized in different industries. The research procedure is experimental in this work. The experimental tests are carried out on the flow in a convergent-divergent microchannel having 200 $mu$m height. The utilized experimental method in this research is particle tracking based on the xerography technique, observation, record and data processing. The results showed that the velocity of the particles in Y-direction has a great effect on their motion in the convergent-divergent channel. Study of the particles' sedimentation in different Reynolds showed that it can be eliminated the destructive effect of channel obstruction using convergent-divergent channel which is originated from increasing sedimentation in low Reynolds. The shear stress and velocity gradient are two main factors of the particles' velocity in the convergent-divergent channel. The results showed that the shear stress has an excessive impact on the particles' velocity than the velocity gradient. Moreover, besides the upper and bottom walls, side walls affect the particles' velocity. The flow of fluids including micron particles in micro channels and manufacturing microfluidics tools have been one of the important topics in last decades and are utilized in different industries. The research procedure is experimental in this work. The experimental tests are carried out on the flow in a convergent-divergent microchannel having 200 $mu$m height. The utilized experimental method in this research is particle tracking based on the xerography technique, observation, record and data processing. The results showed that the velocity of the particles in Y-direction has a great effect on their motion in the convergent-divergent channel. Study of the particles' sedimentation in different Reynolds showed that it can be eliminated the destructive effect of channel obstruction using convergent-divergent channel which is originated from increasing sedimentation in low Reynolds. The shear stress and velocity gradient are two main factors of the particles' velocity in the convergent-divergent channel. The results showed that the shear stress has an excessive impact on the particles' velocity than the velocity gradient. Moreover, besides the upper and bottom walls, side walls affect the particles' velocity.

Numerical investigation and simultaneous optimization of geometry and zeta-potential in electroosmotic mixing flows

Effects of zeta potential distribution and geometrical specifications on flow rate and mixing performance of electroosmotic flows in straight, converging, diverging, and converging-diverging microchannels are investigated by lattice Boltzmann method. Modified Navier-Stokes equations along with the Nernst–Planck equation for ionic distributions are solved and validated against the available electroosmotic flow solutions. The geometrical effects on flow patterns and mixing efficiencies in the presence of patched zeta potential distributions are examined in search for the simultaneously enhanced mixing performance and mass flow rate. Numerical results indicate that converging channel leads to a sizable increase in mixing efficiencies, while at the same time decreases the flow rate. In contrast, diverging channels increase the flow rate, where the mixing performance deteriorates. Thus, it is expected to achieve a balance between the mixing efficiency and mass flow rate using converging-diverging geometries. In this regard, the response surface methodology (RSM) is employed to maximize the mixing efficiency and flow rate of the converging-diverging microchannel. According to numerical results, the optimum parameters predicted by RSM are in excellent agreement with full numerical simulations.

AC Electric Field-Induced Trapping of Microparticles in Pinched Microconfinements.

The trapping of charged microparticles under confinement in a converging-diverging microchannel, under a symmetric AC field of tunable frequency, is studied. We show that at low frequencies, the trapping characteristics stem from the competing effects of positive dielectrophoresis and the linear electrokinetic phenomena of electroosmosis and electrophoresis. It is found, somewhat unexpectedly, that electroosmosis and electrophoresis significantly affect the concentration profile of the trapped analyte, even for a symmetric AC field. However, at intermediate frequencies, the microparticle trapping mechanism is predominantly a consequence of positive dielectrophoresis. We substantiate our experimental results for the microparticle concentration distribution, along the converging-diverging microchannel, with a detailed theoretical analysis that takes into account all of the relevant frequency-dependent electrokinetic phenomena. This study should be useful in understanding the response of biological components such as cells to applied AC fields. Moreover, it will have potential applications in the design of efficient point-of-care diagnostic devices for detecting biomarkers and also possibly in some recent strategies in cancer therapy using AC fields.

Convergent–divergent micromixer coupled with pulsatile flow

An efficient mixing system for a microfluidic platform that uses periodic sinusoidal characteristics in space and time is proposed in this work. A convergent–divergent channel with sinusoidal walls (space-periodic) represented the most effective coupling with pulsatile (time-periodic) flow among the five tested geometries. The effects of sinusoidal pulsing on mixing performance in the convergent–divergent microchannel were examined using unsteady Navier–Stokes equations. The mixing performance was analyzed using the Reynolds and Strouhal numbers, and the ratio of pulsing amplitude to steady flow velocity. The micromixer shows interesting mixing behaviors with these parameters in terms of instantaneous mixing patterns as well as overall mixing performance. The micromixer with pulsing inputs achieved mixing index, 0.92 within two periods of the sinusoidal walls.

Numerical Simulation of Heat Transfer Enhancement in Periodic Converging-diverging Microchannel

Heat transfer performance and fluid flow in three-dimensional converging–diverging microchannel of circular cross-section are studied numerically. Three different types of microchannel, diameter 100μm and length 6.125mm each are used for simulation for the Reynolds number range of 50 to 1000. The effect of Reynolds Number, converging/diverging angle and converging-diverging cross-section length on pressure drop and heat transfer in proposed microchannels are investigated and discussed. The result indicated that the flow in the converging-diverging cross-section induces stronger recirculation and flow separation, which decreases with decreasing the converging and diverging angle and increases with increasing aspect ratio. However, the local and global heat transfer performances in the channels are improved by the converging-diverging cross-section at the expense of higher pressure drop compared to the straight channel with the same cross-section. Conversely, special attentions are given to analyze the thermal performance of microchannels through variation of thermal resistance with pumping power.

Controlled splitting and focusing of a stream of nanoparticles in a converging-diverging microchannel.

We demonstrate the potential of a converging-diverging microchannel to split a stream of nanoparticles towards the interfacial region of the dispersed and the carrier phases, introduced through the middle inlet and through the remaining two inlets respectively, while maintaining a low Reynolds number limit (<10) for the flow of both phases. In addition to the splitting of passive tracer particles, such as polystyrene beads as used herein, the present setup has the potential to be utilized for a controlled reaction and thereby the separation of products towards an intended location, as observed from the experimentation with silver-nanoparticles and hydrogen-peroxide solution. Moreover, the microscale dimension of the channel allows controlled deposition of the reaction product over the bottom surface of the channel, allowing the possibility of bottom-up fabrication of microscale features. We unveil the underlying hydrodynamics that lead to such behaviours through numerical simulations, which are consistent with the experimental observations. The phenomenological features are found to be guided by the splitting of the intrinsic streamlines conforming to the flow geometry under consideration.

DNA molecule stretching through thermo-electrophoresis and thermal convection in a heated converging-diverging microchannel

A novel DNA molecule stretching technique is developed and tested herein. Through a heated converging-diverging microchannel, thermal convection and thermophoresis induced by regional heating are shown to significantly elongate single DNA molecules; they are visualized via a confocal laser scanning microscopy. In addition, electrophoretic stretching is also implemented to examine the hybrid effect on the conformation and dynamics of single DNA molecules. The physical properties of the DNA molecules are secured via experimental measurements.

Effects of microchannel geometry on preconcentration intensity in microfluidic chips with straight or convergent–divergent microchannels

Preconcentration microfluidic devices are fabricated incorporating straight or convergent-divergent microchannels and hydrogel or Nafion membranes. Sample preconcentration is achieved utilizing concentration-polarization effects. The effects of the microchannel geometry on the preconcentration intensity are systematically examined. It is shown that for the preconcentrator with the straight microchannel, the time required to achieve a satisfactory preconcentration intensity increases with an increasing channel depth. For the convergent-divergent microchannel, the preconcentration intensity increases with a reducing convergent channel width. Comparing the preconcentration performance of the two different microchannel configurations, it is found that for an equivalent width of the main microchannel, the concentration effect in the convergent-divergent microchannel is faster than that in the straight microchannel.

Dielectrophoretic choking phenomenon in a converging-diverging microchannel.

Experiments show that particles smaller than the throat size of converging-diverging microchannels can sometimes be trapped near the throat. This critical phenomenon is associated with the negative dc dielectrophoresis arising from nonuniform electric fields in the microchannels. A finite-element model, accounting for the particle-fluid-electric field interactions, is employed to investigate the conditions for this dielectrophoretic (DEP) choking in a converging-diverging microchannel for the first time. It is shown quantitatively that the DEP choking occurs for high nonuniformity of electric fields, high ratio of particle size to throat size, and high ratio of particle's zeta potential to that of microchannel.

Transient electrophoretic motion of a charged particle through a converging–diverging microchannel: Effect of direct current‐dielectrophoretic force

Transient electrophoretic motion of a charged particle through a converging-diverging microchannel is studied by solving the coupled system of the Navier-Stokes equations for fluid flow and the Laplace equation for electrical field with an arbitrary Lagrangian-Eulerian finite-element method. A spatially non-uniform electric field is induced in the converging-diverging section, which gives rise to a direct current dielectrophoretic (DEP) force in addition to the electrostatic force acting on the charged particle. As a sequence, the symmetry of the particle velocity and trajectory with respect to the throat is broken. We demonstrate that the predicted particle trajectory shifts due to DEP show quantitative agreements with the existing experimental data. Although converging-diverging microchannels can be used for super fast electrophoresis due to the enhancement of the local electric field, it is shown that large particles may be blocked due to the induced DEP force, which thus must be taken into account in the study of electrophoresis in microfluidic devices where non-uniform electric fields are present.

Pressure-driven transport of particles through a converging-diverging microchannel

Pressure-driven transport of particles through a symmetric converging-diverging microchannel is studied by solving a coupled nonlinear system, which is composed of the Navier-Stokes and continuity equations using the arbitrary Lagrangian-Eulerian finite-element technique. The predicted particle translation is in good agreement with existing experimental observations. The effects of pressure gradient, particle size, channel geometry, and a particle's initial location on the particle transport are investigated. The pressure gradient has no effect on the ratio of the translational velocity of particles through a converging-diverging channel to that in the upstream straight channel. Particles are generally accelerated in the converging region and then decelerated in the diverging region, with the maximum translational velocity at the throat. For particles with diameters close to the width of the channel throat, the usual acceleration process is divided into three stages: Acceleration, deceleration, and reacceleration instead of a monotonic acceleration. Moreover, the maximum translational velocity occurs at the end of the first acceleration stage rather than at the throat. Along the centerline of the microchannel, particles do not rotate, and the closer a particle is located near the channel wall, the higher is its rotational velocity. Analysis of the transport of two particles demonstrates the feasibility of using a converging-diverging microchannel for passive (biological and synthetic) particle separation and ordering.