Circular Cross-section Microchannel Research Articles

Heat transfer and pressure drop analysis of a microchannel heat sink using nanofluids for energy applications

Abstract The present research aims to enhance heat transfer in straight and wavy profile heat sinks using the same length and hydraulic diameter with different microchannel geometries (triangular, rectangular, trapezoidal, semi-circular, and circular) for uses in electronics, inkjet printing, high heat flux cooling of lasers, and other domains. The nanofluid employed is water/aluminum oxide (water/Al2O3), and the flow regime is laminar. The range of Reynolds number (Re) in this study was 220 ≤ Re ≤ 550, and the concentrations of nanoparticle Al2O3 with Heavy Water (2H2O) were 1.2 % volume. This investigation uses 3-dimensional Computational Fluid Dynamics (CFD) simulation software to investigate the heat transfer characteristics of several cross-sectioned microchannels. The numerical investigation utilizes the finite volume approach, and the CFD analysis is validated with accessible literature with different wavy profiles. According to the CFD simulation results, the microchannel with a circular cross-section has the highest heat transfer performance (up to 18 %) among the other cross-sections. The circular cross-section microchannel seemed to have the most significant increase in coolant temperature (by 9–22 %). The analysis outcomes prove that the microchannel with a circular cross-section has the highest performance for heat transfer; the triangular channel has the lowest performance under the same geometric parameters and boundary conditions. So, it is suggested that a circular microchannel can be used for a heat-carrying capacity of 150 W/cm2, a hydraulic diameter of 500 µm, and a Reynolds number equal to 500.

Fabrication of Circular Cross-Section Microchannels with 3-D Lattice Arrangement and Their Use as On-Off Valves

In this paper, circular cross-section microchannels with 3-D lattice arrangements are designed and fabricated using the Mosquito method in order to construct on-off valves. The 3-D microchannels with on-off valves consist of two types of lines: the flow lines for chemical liquid flow and the control lines to activate the valves. We confirmed that both a circular cross-section and a PDMS with low elastic modulus used as the microchannel material contribute to a valve that can be closed with a lower pressure. Then, we demonstrated liquid flow to evaluate the functionality of the valve. Fluorescein solution was flown into a flow line. We found that the fluorescence intensity decreases at the intersection between the flow and control lines when the flow line is closed by the inflation of the control line, experimentally confirming the functionality of the valve microchannels fabricated via the Mosquito method.

Effect of liquid cooling on PCR performance with the parametric study of cross-section shapes of microchannels

In recent years, PCR-based methods as a rapid and high accurate technique in the industry and medical fields have been expanded rapidly. Where we are faced with the COVID-19 pandemic, the necessity of a rapid diagnosis has felt more than ever. In the current interdisciplinary study, we have proposed, developed, and characterized a state-of-the-art liquid cooling design to accelerate the PCR procedure. A numerical simulation approach is utilized to evaluate 15 different cross-sections of the microchannel heat sink and select the best shape to achieve this goal. Also, crucial heat sink parameters are characterized, e.g., heat transfer coefficient, pressure drop, performance evaluation criteria, and fluid flow. The achieved result showed that the circular cross-section is the most efficient shape for the microchannel heat sink, which has a maximum heat transfer enhancement of 25% compared to the square shape at the Reynolds number of 1150. In the next phase of the study, the circular cross-section microchannel is located below the PCR device to evaluate the cooling rate of the PCR. Also, the results demonstrate that it takes 16.5 s to cool saliva samples in the PCR well, which saves up to 157.5 s for the whole amplification procedure compared to the conventional air fans. Another advantage of using the microchannel heat sink is that it takes up a little space compared to other common cooling methods.

Analogue tuning of particle focusing in elasto-inertial flow

We report a unique tuneable analogue trend in particle focusing in the laminar and weak viscoelastic regime of elasto-inertial flows. We observe experimentally that particles in circular cross-section microchannels can be tuned to any focusing bandwidths that lie between the “Segre-Silberberg annulus” and the centre of a circular microcapillary. We use direct numerical simulations to investigate this phenomenon and to understand how minute amounts of elasticity affect the focussing of particles at increasing flow rates. An Immersed Boundary Method is used to account for the presence of the particles and a FENE-P model is used to simulate the presence of polymers in a Non-Newtonian fluid. The numerical simulations study the dynamics and stability of finite size particles and are further used to analyse the particle behaviour at Reynolds numbers higher than what is allowed by the experimental setup. In particular, we are able to report the entire migration trajectories of the particles as they reach their final focussing positions and extend our predictions to other geometries such as the square cross section. We believe complex effects originate due to a combination of inertia and elasticity in the weakly viscoelastic regime, where neither inertia nor elasticity are able to mask each other’s effect completely, leading to a number of intermediate focusing positions. The present study provides a fundamental new understanding of particle focusing in weakly elastic and strongly inertial flows, whose findings can be exploited for potentially multiple microfluidics-based biological sorting applications.

Thin liquid films formation and evaporation mechanisms around elongated bubbles in rectangular cross-section microchannels

Thin film evaporation is the dominant mechanism of heat transfer in the microchannel flow boiling process. Hence, its understanding is paramount to development of predictive models and more efficient microchannel heat exchangers. Although microchannels mostly have a rectangular cross-section, the existing models are developed for circular cross-section microchannels. The non-uniformity of the liquid film thickness in microchannels with a rectangular cross-section has made prediction of the onset of dryout and heat transfer coefficient greatly challenging. Here, a test device capable of measuring the wall heat flux is utilized to fully characterize the thin film formation and evaporation process. The effects of flow capillary number (Ca) and channel aspect ratio on the liquid film thickness are determined in microchannels with 300 μm width and 300, 150, and 75 μm heights, representing aspect ratios of 1, 2, and 4, respectively. It is shown that the only existing mechanistic model for the elongated bubble regime in microchannels fails to predict the experimental heat transfer coefficient (HTC) both quantitively, by a 50–120% margin, and qualitatively, due to a fundamental difference in evaporation and dryout of a variable versus a uniform film assumed in the model. The results presented here pave the way for development of a more accurate mechanistic model for the thin film evaporation heat transfer mechanism in microchannels with a rectangular cross-section.

Numerical simulation and experiment of droplet formation in circular cross-section micro-channels

In recent decades, microfluidics in biological applications have experienced significant growth due to their advantages of small volume, low cost, short reaction time and high throughput. Almost all cross-section shapes of micro-channels in microfluidic chips are rectangular or triangular by the existing chip fabricating technologies, including hot embossing, lithography, etching and injection molding, etc. However, compared with the above micro-channel shapes, the circular one has the advantages in aspects of fluid flow, droplet generating, heat transfer and its replication for blood vessels. This paper presents a T-junction droplet microfluidic chip with circular cross-section micro-channels. The effect of micro-channel wettability, interfacial tension, velocity and flow rate of continuous phase on droplet size are simulated and mechanism of droplets generating process is explored. Comparing with continuous phase viscosity and interfacial tension, flow rate plays a decisive role in determining the droplet size which is in the range of 100–350 [Formula: see text]m according to the simulation result. The Capillary number is affected by the above three parameters and an estimating numerical method for generated droplet size was proposed according to the above simulation results and calculated by Capillary number. The droplets, the sizes of which were in the range of 20–400 [Formula: see text]m, were produced by varying the parameters of water and oil flow rates in the designed T-junction droplet microfluidic chip with circular cross-section micro-channels.

Fabrication of sandwich-like microfluidic chip with circular cross-section micro-channels

In microfluidic chips, most micro-channel cross-section shapes are rectangular or triangular in existing chip fabricating technologies, including hot embossing, lithography, etching, injection molding, etc. However, compared with the above micro-channel shapes, a circular shape has advantages in aspects of fluid flow, droplet generation, heat transfer and blood vessel replication. This paper presents a sandwich-like microfluidic chip with circular cross-section micro-channels. The sandwich-like structure includes three layers. The top and bottom layers are PDMS material while the middle layer is composed of micron glass capillaries (used as micro-channels) with circular cross-sections. The glass capillaries are made of borosilicate tube by a glass heating process. Sphere shaped paraffin wax is used as a sacrificial material to form micro-channel junctions. To test the functions of the fabricated micro sandwich-like microfluidic chip, a droplet generating experiment was conducted in the T-junction chip. The droplet size can be controlled in the range of 20–400 [Formula: see text]m by varying the water and oil flow rates. This proposed microfluidic chip structure has the advantages of short processing cycle, low cost and small flow resistance.

Microfab in a Microwave Oven: Simultaneous Patterning and Bonding of Glass Microfluidic Devices

We report a new method for fabrication of research-grade glass microfluidic devices using a household microwave oven. The transformative step in this technique is the formation of the microchannel and fusion bonding of the glass layers in a single step. This is demonstrated here using a non-photolithographic technique for both high aspect ratio rectangular and circular cross-section microchannels. In this paper, the microfluidic devices are fabricated utilizing a thin paper cutout as the sacrificial channel layer, glass blanks, and a microwave kiln sold commercially for arts and crafts. Glass microfluidic devices are fabricated in a microwave oven in as short as 2 min.

New portable microchannel molding system based on micro-wire molding, droplet formation studies in circular cross-section microchannel

This work describes a new appliance for design and fabrication of various 3D microchannels by a quick process known as micro-wire molding. The method provides several possibilities including simultaneous curing and molding without any displacement or a curing oven. Both metallic and Nylon micro-wires can be used in this method. The method is designed so that the molding container needs minimal polydimethylsiloxane providing possibility to produce channels with different cross-sections, shapes and geometries. Inside a circular cross-section channel by 800 μm diameter, a T-junction is used to generate micro-droplets of water in oil with two different examined liquids as the continuous phases. The effect of relative flow rate and viscosity of the phases have been studied and demonstrated that these parameters affect the shape, size, velocity and separation of the droplets inside the running phase within the channel.

マイクロチャネル内でのゲルビーズを利用した線虫の高精度回転マニピュレーション

We presented rotational manipulation of Caenorhabditis elegans (C. elegans) for higher precision by a circular cross section microchannel. The C. elegans was trapped within a biocompatible alginate gel bead to rotate by a rotational motor. Without immobilization treated by a sodium azide, the C. elegans move during solidification of alginate. Then, a cavity was happened in an alginate gel bead. On the other hand, the C. elegans was trapped without any cavity after immobilization treatment. The nematode’s immobilization and recovering from immobilization were evaluated quantitatively by considering worm bending frequency. Finally, the eccentricity of rotational manipulation was evaluated with/without centering by a microchannel. The eccentricity was 35% higher with the microchannel and high precision rotation was achieved at 0.03 degrees/step of actuator.

Modelling the solidification of a power-law fluid flowing through a narrow pipe

We develop a mathematical model to simulate the solidification process of a non-Newtonian power-law fluid flowing through a circular cross-section microchannel. The initial system consists of three partial differential equations, describing the fluid flow and temperature in the liquid and solid, which are solved over a domain specified by the Stefan condition. This is reduced to solving a partially coupled system consisting of a single partial differential equation and the Stefan condition. Results show qualitative differences, depending on the power law index and imposed flow conditions, between Newtonian and non-Newtonian solidification. The model behaviour is illustrated using power law models for blood and polyethylene oxide.

Mass transfer behavior of liquid–liquid slug flow in circular cross-section microchannel

An alkaline hydrolysis reaction was used as the model reaction to investigate the performance of liquid–liquid slug flow microchannel. The specific interfacial area was determined through the photographic snapshot method physically by means of measuring the lengths of relevant slugs. The overall volumetric mass transfer coefficients were calculated through the Danckwerts’ model chemically. The influences of various operating conditions on the slug length, the overall volumetric extraction rate and the mass transfer coefficient were investigated quantitatively. A decreasing trend of volumetric mass transfer coefficients along the channel length was found. The linear dependence of the volumetric extraction rate on the volumetric mass transfer coefficient indicates that the overall rate of the process is determined by the mass transfer process. In addition, the volumetric mass transfer coefficients were correlated for different channel lengths.

Numerical study of axial heat conduction effects on the local Nusselt number at the entrance and ending regions of a circular microchannel

The effects of duct wall thermal conductivity and thickness on the local Nusselt number at the entrance and ending regions of a circular cross-section microchannel in a conjugate heat transfer problem are numerically investigated. The simultaneously developing laminar flow with a constant heat flow rate per unit length on the outside surface of the duct wall is considered. The results show that the axial heat conduction in the duct wall decreases the local Nusselt number at the entrance and ending regions of the microchannel with respect to their values for microchannels without conjugate effects. The effects of various parameters on the local Nusselt number at the ending region of the microchannel are analyzed and three generalized correlations are proposed for predicting the ending length and corresponding local and average Nusselt numbers. Comparisons with other numerical results show suitable accuracy of the correlations.

Heat transfer characteristics of flow boiling in a single horizontal microchannel

Heat transfer characteristics of subcooled flow boiling of FC-72 in a single horizontal circular cross-section microchannel (480 μm i.d., 800 μm o.d., 102 mm long) are presented. Different flow patterns, both in the stable and unstable flow boiling regimes, have been captured using high speed video camera. Data in small, medium, high and very high heat flux cases under small, medium and high mass flux has been presented. Convective heat transfer coefficients in each flow boiling situation have been calculated and presented. Stable flow boiling with alternating bubbly/slug flow, slug/annular flow and annular/mist flow have been observed for heat flux of 150 kW/m 2 or higher and mass flux of 1500 kg/m 2 s or higher. Back and forth oscillations with flow instabilities have been observed in cases of lower heat and mass fluxes. However, no complete reverse flow in upstream direction has been observed.