Configurations Of Microchannels Research Articles

Effect of variation in heights of cylindrical pinfins on the thermo-hydraulic performance of an open-microchannel heat sink

This article investigates the impact of different heights of cylindrical micropillars on the thermo-fluid properties of an open microchannel heat sink. The dimensions of the microchannel considered in this study are 0.1 mm × 0.2 mm × 10 mm. Computational fluid dynamics has been employed to simulate seven distinct microchannel layouts. These configurations are Case 1 (Closedmicrochannel with full height pinfins), Case 2 (Openmicrochannel with reduced height pinfins), Case 3 (Openmicrochannel with half height pinfins), Case 4 (Open microchannel with reduced height pinfins at upstream and half height pinfins at downstream), Case 5 (Open microchannel with reduced height pinfins at upstream and half height pinfins at downstream). Case 6 (Open microchannel with two half-height pinfins at upstream and downstream regions, respectively). Case 7 (Open microchannel with two reduced height pinfins at upstream and downstream regions, respectively). Nusselt number, pressure drop, and overall thermal performance of all the considered open configurations of microchannels are evaluated and compared with the plain and closed microchannel for the range of Reynolds number between 150 and 350 by considering water as a working fluid. The results depict that the maximum augmentation in the Nusselt number is about 16.8% achieved by the open microchannel heatsink (Case 2) corresponding to the closed microchannel (Case 1) with a reduction in pressure drop of about 7.32%. To evaluate the mutual effect of heat transfer and pressure drop, overall thermal performance is evaluated. At a lower Reynolds number, the highest performance is associated with Case 2 but at the elevated range of Reynolds number, the maximum performance is acquired by Case 4.

Hybrid acoustic materials through assembly of tubes and microchannels: design and experimental investigation

PurposeThe purpose of this paper is the design and experimental investigation of compact hybrid sound-absorbing materials presenting low-frequency and broadband sound absorption.Design/methodology/approachThe hybrid materials combine microchannels and helical tubes. Microchannels provide broadband sound absorption in the middle frequency range. Helical tubes provide low-frequency absorption. Optimal configurations of microchannels are used and analytical equations are developed to guide the design of the helical tubes. Nine hybrid materials with 30 mm thickness are produced via additive manufacturing. They are combinations of one-, two- and four-layer microchannels and helical tubes with 110, 151 and 250 mm length. The sound absorption coefficient of the hybrid materials is measured using an impedance tube.FindingsThe type of microchannels (i.e. one, two or four layers), the number of rotations and the number of tubes are key parameters affecting the acoustic performance. For instance, in the 500 Hz octave band (α500), sound absorption of a 30 mm thick hybrid material can reach 0.52 which is 5.7 times higher than the α500 of a typical periodic porous material with the same thickness. Moreover, the broadband sound absorption for mid-frequencies is reasonably high with and α1000 > 0.7. The ratio of first absorption peak wavelength to structure thickness λ/T can reach 17, which is characteristic of deep-subwavelength behaviour.Originality/valueThe concept and experimental validation of a compact hybrid material combining a periodic porous structure such as microchannels and long helical tubes are original. The ability to increase low-frequency sound absorption at constant depth is an asset for applications where volume and weight are constraints.

Liquid metal infiltration of silicon based alloys into porous carbonaceous materials. Part II: Experimental verification of modelling approaches by infiltration of Si-Zr alloy into idealized microchannels

In this work, the mechanisms leading to the pore closure in reactive melt infiltration (RMI) of carbon by pure silicon and a near eutectic Si-8 at-pct Zr alloy at 1500 and 1700 °C under vacuum were studied. Various geometrical configurations of microchannels were fabricated via laser ablation of glassy carbon plates. The micron size capillary channels allowed simplifying the complicated porosity distribution in the infiltration of powder or fibres based porous preform while keeping the physical dimensions in the range of where the physical phenomenon of pore closure takes place. The extent of infiltration was analysed by means of X-ray radiography. For RMI of pure Si, the widely accepted decrease in capillary radius by the formation of a solid state SiC layer by the reaction of liquid Si and C was observed, but did not lead to closure and it is hence not the infiltration limiting step in channels as small as 10 μm. However, in the case of the Si-Zr alloy infiltration, another mechanism of pore closure was observed, namely the precipitation of zirconium silicides at the infiltration front, due to Zr enrichment in the alloy by the continuous consumption of Si for the formation of SiC.

Flow boiling characteristics in different configurations of stepped microchannels

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

Numerical Simulation of Flow in a Wavy Wall Microchannel Using Immersed Boundary Method

Background: Fluid flow in microchannels is restricted to low Reynolds number regimes and hence inducing chaotic mixing in such devices is a major challenge. Over the years, the Immersed Boundary Method (IBM) has proved its ability in handling complex fluid-structure interaction problems. Objectives: Inspired by recent patents in microchannel mixing devices, we study passive mixing effects by performing two-dimensional numerical simulations of wavy wall in channel flow using IBM. Methods: The continuity and Navier-Stokes equations governing the flow are solved by fractional step based finite volume method on a staggered Cartesian grid system. Fluid variables are described by Eulerian coordinates and solid boundary by Lagrangian coordinates. A four-point Dirac delta function is used to couple both the coordinate variables. A momentum forcing term is added to the governing equation in order to impose the no-slip boundary condition between the wavy wall and fluid interface. Results: Parametric study is carried out to analyze the fluid flow characteristics by varying amplitude and wavelength of wavy wall configurations for different Reynolds number. Conclusion: Configurations of wavy wall microchannels having a higher amplitude and lower wavelengths show optimum results for mixing applications.

Thermal–Hydraulic Performance in a Microchannel Heat Sink Equipped with Longitudinal Vortex Generators (LVGs) and Nanofluid

In this study, the numerical conjugate heat transfer and hydraulic performance of nanofluids flow in a rectangular microchannel heat sink (RMCHS) with longitudinal vortex generators (LVGs) was investigated at different Reynolds numbers (200–1200). Three-dimensional simulations are performed on a microchannel heated by a constant temperature with five different configurations with different angles of attack for the LVGs under laminar flow conditions. The study uses five different nanofluid combinations of Al2O3 or CuO, containing low volume fractions in the range of 0.5% to 3.0% with various nanoparticle sizes that are dispersed in pure water, PAO (Polyalphaolefin) or ethylene glycol. The results show that for Reynolds number ranging from 100 to 1100, Al2O3–water has the best performance compared with CuO nanofluid with Nusselt number values between 7.67 and 14.7, with an associated increase in Fanning friction factor by values of 0.0219–0.095. For the case of different base fluids, the results show that CuO–PAO has the best performance with Nusselt number values between 9.57 and 15.88, with an associated increase in Fanning friction factor by 0.022–0.096. The overall performance of all configurations of microchannels equipped with LVGs and nanofluid showed higher values than the ones without LVG and water as a working fluid.

Numerical Investigation on Multiphase Flows in Various Configurations of Microchannels Using Computational Fluid Dynamics

A two-dimensional domain of multiphase flow analyses in this study using the Volume of Fluid (VOF) model was carried out in order to simulate and predict the fluid flows and mixing performance of two miscible liquids in various microchannel configurations. The various microchannels configurations were designed accordingly and the simulation was carried out based on the justified conditions, assumptions and considerations by using the commercial computational fluid dynamics (CFD) software, FLUENT. The grid type and size of the computational domain were verified in terms of stability by performing the grid independence analysis. The result showed that static mixing would be possible to achieve in various configurations of microchannels, however, the simulation results predicted that it appeared to be more efficient in complex and retrofitted microchannels. It showed the potential to promote and enhance chaotic advection, compositions distribution, and diffusivity as compared to basic microchannels that are mostly dependent only on the injection focus. Furthermore, the Reynolds number appeared to be a significant factor to enhance the mixing performance in microchannel beside the configurations.

Fabrication of 3D Microfluidic Channels and In‐Channel Features Using 3D Printed, Water‐Soluble Sacrificial Mold

AbstractRecent advent of additive manufacturing potentiates the fabrication of microchannels, albeit with limitations in resolution of printed structures, freedom of geometry, and choice of printable materials. Herein, a method is developed by sacrificial molding to fabricate microchannels in various polymer matrices and geometries. This method allows for rapid fabrication of 3D microchannels and channels harboring intricate in‐channel features. The method uses commercially available fused deposition modeling 3D printer and filament made of polyvinyl alcohol (PVA). Mechanically stable molds are fabricated for 3D microchannels that can be completely removed in water. Importantly, the PVA mold is stable and resilient in hydrogels despite being hygroscopic. Perfusion channels are fabricated in biocompatible substrates such as gelatin and poly(ethylene glycol) diacrylate. Fabrication of the network of 3D multilayer microchannels is demonstrated by preassembling sacrificial molds from modular pieces of molds. Intricate staggered‐herringbones grooves (SHGs) are also fabricated within microchannels to produce micromixers. The versatility and resilience of the method developed here is advantageous for biological and chemical applications that require 3D configurations of microchannels in various matrices, which would not be compatible with fabrication by direct 3D printing and softlithography.

Laminar Heat Transfer Behavior of a Phase Change Material Fluid in Microchannels With Staggered Pins

Microchannels have been studied extensively for a variety of heat transfer applications including electronic cooling. Many configurations of microchannels have been studied and compared for their effectiveness in terms of heat removal. Recently, the use of staggered pins in microchannels has gained considerable traction, since they can promote internal flow fluctuations that enhance internal heat transfer. Furthermore, staggered pins in microchannels have shown higher heat removal characteristics because of the continuous breaking and formation of the heat transfer fluid boundary layer. However, they also exhibit higher pressure drop because the pins act as flow obstructions. This paper presents numerical results of two characteristic staggered 100-μm pins (square and circular) in microchannels. The heat transfer performance of a single phase fluid (SPF) in microchannels with staggered pins, and the corresponding pressure drop characteristics are presented. Furthermore, a phase change material (PCM, n-eicosane) fluid was also considered by implementing the effective specific heat capacity model approach to account for the corresponding phase change process of PCM fluid. Comparisons of the heat transfer characteristics of single phase fluid and PCM fluid are presented for two different pin geometries and three different Reynolds numbers. Circular pins were found to be more effective in terms of heat transfer by exhibiting higher Nusselt number. Microchannels with circular pins were also found to have lower pressure drop compared to the square-pin microchannels.

Performance Analysis of a Microchannel Heat Sink with Various Rib Configurations

Microchannel heat sinks with ribbed channels in various configurations have been analyzed numerically using the three-dimensional Navier–Stokes equations. Six configurations of ribbed microchannels (i.e., rectangular, semicircular, triangular, rectangular–triangular, rectangular–semicircular, and triangular–circular) were tested with a fixed width and length of ribs, and compared to smooth channels in a Reynolds number range of 100–500. The results indicate that thermal resistance of microchannels is greatly reduced by introducing ribs and that triangular ribs show the lowest thermal resistance throughout the Reynolds number range. However, the pressure drop is also increased greatly because of the ribs, and rectangular ribs induced the largest pressure drop.

A comparative study of flow boiling heat transfer in three different configurations of microchannels

In this work, an experimental investigation has been made to compare the flow boiling characteristics of deionized water in three different configurations of microchannels. The investigated channel configurations are uniform cross-section, diverging cross-section and segmented finned microchannels. In each configuration, an array consisting of twelve numbers of microchannels with rectangular cross-section has been fabricated on a copper block with footprint area of 25.7×12.02mm2. Experiments have been conducted with subcooled liquid state at the entry with coolant mass and heat fluxes vary in the range 100–350kg/m2s and 10–350kW/m2, respectively. Depending upon the heat flux and coolant flow rate different regimes of two phase boiling have been observed. The comparison of three configurations has been made in terms of heat transfer coefficient, pressure drop characteristics and affinity towards backflow or flow reversal in the channels. Bubbles dynamics and their role in flow reversal and flow instability have been discussed for all three types of channels. For entire operating conditions, segmented finned channels demonstrate the highest heat transfer coefficient with negligible higher pressure drop compared to other two configurations of channels. The performance of diverging cross-section channels is better than the uniform cross-section channels. However, they underperform compared with segmented finned channels. At higher heat flux, bubble clogging and flow reversal problem is worst in uniform cross-section channels. The problem is partially solved in diverging cross-section channels. Segmented channels completely relieve the problem of bubble clogging allowing smooth and easy passage of growing bubbles. Moreover flow reversal is not observed in segmented channels for entire operating range of heat flux and coolant mass flux.

Numerical study of liquid flow and heat transfer in rectangular microchannel with longitudinal vortex generators

The liquid flow and conjugated heat transfer performance of single phase laminar flow in rectangular microchannels equipped with longitudinal vortex generators (LVGs) are numerically investigated. Deionized-water with temperature-dependent thermo-physical properties is employed to conduct the simulations. Three dimensional simulations are performed using an open-source flow solver based on finite volume approach and SIMPLEC algorithm. Five different configurations of the microchannel with different angles of attack of the LVGs are considered. Simulation results are compared with available experimental data and a deviation below 10% is achieved. The results show that for Reynolds number ranged from 100 to 1100, there is a 2–25% increase in the Nusselt number for microchannels with LVGs, while the friction factor increased by 4–30%. Except one at Re = 100, the overall performance of the all configurations of microchannels with LVGs is higher than ones without LVG's.

Electrokinetic Trapping and Patterning of Colloidal Particles in Asymmetric Ratchet Microchannels

Insulator-based dielectrophoresis offers advantages over electrode-based dielectrophoresis in many ways, and has been extensively used to manipulate, sort and selectively trap particles and biological species in Lab-on-a-chip devices. This work exploits insulator-based dielectrophoresis in two configurations of asymmetric ratchet microchannels to trap microscale particles using DC biased AC voltages. It demonstrates for that dynamics of the DEP trapping zones changes in response to voltages higher than the threshold trapping voltages, and also with longer experimental runtimes. Numerical modeling is used to support and validate the results, and the physical interpretations of the changing DEP are explained using a dimensionless trapping number. With the help of the microchannel configurations, it is revealed that the orientation of the asymmetric ratchet relative to the direction of DC electric field plays a key role in determining the trapping patterns

Electrokinetic Trapping and Patterning of Colloidal Particles in Asymmetric Ratchet Microchannels

Insulator-based dielectrophoresis offers advantages over electrode-based dielectrophoresis in many ways, and has been extensively used to manipulate particles and biological species in lab-on-a-chip devices. This work exploits insulator-based dielectrophoresis in two configurations of asymmetric ratchet microchannels to trap and concentrate microscale particles using DC-biased AC voltages. It demonstrates that the dynamics of the trapping pattern in ratchet microchannels changes in response to voltages higher than the threshold trapping voltages, and also changes with progression of the trapping experiment. Numerical modeling is used to support and validate the results, and the physical interpretations of the changing dielectrophoresis are explained using a defined dimensionless trapping number. It is revealed that the orientation of the asymmetric ratchet relative to the direction of the applied DC electric field plays a key role in determining the particle trapping patterns in the ratchets.

Study of Convective Heat Transfer through Micro Channels with Different Configurations

The micro channel heat sinks play a very important role in the functioning of the micro-electronic components. In this paper the focus is on the convective heat transfer performance of the micro channels with different geometric configurations. The heat transfer performance is a function of surface area available for the heat transfer and the heat transfer coefficient of the heat carrying medium. An experimental heat transfer analysis is carried out with different configurations of micro channels. Micro channels with four different configurations are fabricated using Photo Chemical Machining Process. The performance analysis is done in the Reynolds Number range of 15 to 50, with different inlet flow velocities and the experimental analysis emphasis the fact that the geometrical configuration of the channel has a considerable effect on the micro channel heat transfer performance and in turn on the performance of the micro-electronic components.

Facile One-Step Fabrication of an Undercut Structure by Solution Dewetting on a Water/Ice Mold

This paper reports a facile one-step method for making undercut structures by dewetting of poly(methyl methacrylate) (PMMA) chloroform solution on an ice/water mold. The ice mold is prepared by filling the microchannels with water (micromolding in capillaries (MIMIC) method) and then freezing. The curving surface of water mold stripes and the trapezoidal structure of ice mold stripes work as complementary structures devoted to the formation of undercut structures. The undercut structures with angles of 60° and 40° are obtained, which is determined by the states of ice or water mold. Meanwhile, if the solution concentration is increased, patterned polymer films with different configurations of undercut structures or microchannels would be gained.