Effect Of Channel Geometry Research Articles

Channel Geometry Effect for Proton Exchange Membrane Fuel Cell With Serpentine Flow Field Using a Three-Dimensional Two-Phase Model

This study presents a complete three-dimensional, two-phase transport model for proton exchange membrane fuel cells based on the two-fluid method, which couples the mass, momentum, species, and electrical potential equations. The different liquid water transport mechanisms in the flow channels, gas diffusion layers, catalyst layers, and membrane are modeled using two different liquid water transport equations. In the flow channels, gas diffusion layers, and catalyst layers, the generalized Richards equation is used to describe the liquid water transport including the effect of the pressure gradient, capillary diffusion, evaporation and condensation, and electro-osmotic, while in the membrane, the liquid water transport equation only takes into account the effect of back diffusion and electro-osmotic. Springer’s model is utilized on the catalyst layer-membrane interface to maintain continuum of the liquid water distribution. The model is used to investigate the effect of flow channel aspect ratio on the performance of fuel cells with single and triple serpentine flow fields. The predictions show that for both flow fields, the cell performance improves with decreasing aspect ratio. The aspect ratio has less effect on the cell performance for the triple serpentine flow field than for the single serpentine flow field due to the weaker under-rib convection.

Numerical Analysis of Flow Bifurcations in a Closed-Off Channel

This article describes the results of numerical simulations of fluid flow in a closed-off channel with a plain nozzle. Simulations were performed with the application of the Finite Volume Procedure in the Multi Flower 2D non-commercial application. The main objective of the study was to determine the effect of channel geometry on flow structure. A total of 27 system configurations were analyzed for nine geometry variants and three pressure variants in the feed nozzle. Based on the obtained results, five main flow types were determined, including Hopf bifurcation structures. This paper presents simulation results and it analyzes the observed flow types in view of general bifurcation conditions. The final section of this paper describes the attempts at repeating numerical simulations with the use of the FLUENT commercial application. This simulation makes a reference to an earlier experiment carried out by DYBAN et al. (1971), and it is a continuation of previous research described

Enhancement of electrokinetically driven microfluidic T‐mixer using frequency modulated electric field and channel geometry effects

This study reports improved electrokinetically driven microfluidic T-mixers to enhance their mixing efficiency. Enhancement of electrokinetic microfluidic T-mixers is achieved using (i) an active approach of utilizing a pulsating EOF, and (ii) a passive approach of using the channel geometry effect with patterned blocks. PDMS-based electrokinetic T-mixers of different designs were fabricated. Experimental measurements were carried out using Rhodamine B to examine the mixing performance and the micro-particle image velocimetry technique to characterize the electrokinetic flow velocity field. Scaling analysis provides an effective frequency range of applied AC electric field. Results show that for a T-mixer of 10 mm mixing length, utilizing frequency modulated electric field and channel geometry effects can increase the mixing efficiency from 50 to 90%. In addition, numerical simulations were performed to analyze the mixing process in the electrokinetic T-mixers with various designs. The simulation results were compared with the experimental data, and reasonable agreement was found.

Numerical investigation of water droplet dynamics in a low-temperature fuel cell microchannel: Effect of channel geometry

The sensitivity of liquid water to geometry of cathode gas microchannel in low-temperature fuel cells is investigated numerically. The two-phase flow is resolved using 3D CFD simulations with the volume-of-fluid (VOF) method. Simulations for microchannels with different cross-sections, including rectangle with aspect ratios in a range of 0.1–2, trapezoid, upside-down trapezoid, triangle, rectangle with a curved bottom wall, and semicircle are compared. The 0.5 aspect ratio rectangle yields the longest detachment time and the largest detachment diameter, whereas the longest removal time occurs for the 0.25 aspect ratio case. With decreasing aspect ratio for the rectangle the pressure drop increases and the coverage ratio decreases. The 0.1 and 2 aspect ratios rectangles have the largest water saturation. For microchannels with different cross-sections, the detachment time, detachment diameter, and removal time of the water droplet are found to be in this order: triangle < trapezoid < rectangle with a curved bottom wall < rectangle < upside-down trapezoid. The friction coefficient increases by a factor of 2–4 in the presence of water. The upside-down trapezoid yields the maximum coverage ratio and water saturation, while the rectangle with a curved bottom wall results in the minimum values.

Dynamics of Single Polymer Chain in Channel Flow: Effect of Channel Geometry

Dynamics of Single Polymer Chain in Channel Flow: Effect of Channel Geometry

Development of a Thermal Switch Using the Channel Geometry Effect for Electronic Packages

This paper presents development of a thermal switch using the channel geometry effect for electronic packages operated via liquid conduction channel formation. The developed switch panel is composed of several conduction channels, breathing grooves, reservoirs, and actuators. On demand, a vacant conduction channel is filled with liquid supplied by an actuator and conducts heat from hot spots to cold. This result in increased heat dissipation. Through this switch operation, the temperature of the designated hot spot can be controlled locally. In order to realize the desired switch operation of liquid contact and separation properly, with liquid supplied from a reservoir and returning to the reservoir, the liquid column should be precisely controlled based on the channel design. The conduction channel has a burst area to achieve the precise liquid control. The burst area was geometrically designed to provide a hydrophilic surface by taking into account the burst pressure. In a series of experiments, the newly designed switch was shown in controlling the liquid column well. The results also showed that this thermal switch was able to regulate the heat flow and the temperature distribution locally through the achieved liquid column control.

Effect of water distributions on performances of JARI standard PEFC by using neutron radiography

In order to clarify the effects of water on performances of a Polymer Electrolyte Fuel Cell (PEFC), measurements of water distribution in a Japan Automobile Research Institute (JARI) standard PEFC were carried out by means of neutron radiography. Furthermore, the cell voltage and the pressure drop between inlet and outlet of the air were simultaneously measured. The effects of channel geometry, i.e. single-serpentine and three-serpentine, and relative humidity of air were investigated. The experimental conditions were current density of 200 mA/cm 2, hydrogen utilization of 9.5% and oxygen utilization of 40%. The membrane electrode assembly (MEA) area was 5×5 cm 2. The pictures were taken every 15 s with an exposure time of 12 s. The spatial resolution was 108 μm. From the measurements, the relation between cell voltage, pressure drop and movement of the water plug in the channel was confirmed. The PEFC performances were recovered as the water plug evacuated. Condensed water in the channel decreased with the relative humidity of air. However, it may affect the MEA permanence. Therefore, the most appropriate operating conditions should be decided for taking into account both the cell performances and the permanencies.

Effect of channel geometry on cell adhesion in microfluidic devices

Microfluidic channels coated with ligands are a versatile platform for the separation or enrichment of cells from small sample volumes. This adhesion-based mode of separation is mediated by ligand-receptor bonds between the cells and channel surface and also by fluid shear stress. This paper demonstrates how aspects of microchannel geometry can play an additional role in controlling cell adhesion. With a combination of computational fluid dynamics modeling and cell adhesion experiments, channels with sharp turns are shown to have regions with near-zero velocity at the turn regions where large numbers of cells adhere or become collected. The lack of uniform adhesion in the turn regions compared to other regions of these channels, together with the large variability in observed cell adhesion indicates that channels with sharp turns are not optimal for cell-capture applications where predictable cell adhesion is desired. Channels with curved turns, on the other hand are shown to provide more uniform and predictable cell adhesion provided the gap between parallel arms of the channels is sufficiently wide. The magnitude of cell adhesion in these curved channels is comparable to that in straight channels with no turns.

High-throughput flow alignment of barcoded hydrogel microparticles.

Suspension (particle-based) arrays offer several advantages over conventional planar arrays in the detection and quantification of biomolecules, including the use of smaller sample volumes, more favorable probe-target binding kinetics, and rapid probe-set modification. We present a microfluidic system for the rapid alignment of multifunctional hydrogel microparticles designed to bear one or several biomolecule probe regions, as well as a graphical code to identify the embedded probes. Using high-speed imaging, we have developed and optimized a flow-through system that (1) allows for a high particle throughput, (2) ensures proper particle alignment for decoding and target quantification, and (3) can be reliably operated continuously without clogging. A tapered channel flanked by side focusing streams is used to orient the flexible, tablet-shaped particles into a well-ordered flow in the center of the channel. The effects of channel geometry, particle geometry, particle composition, particle loading density, and barcode design are explored to determine the best combination for eventual use in biological assays. Particles in the optimized system move at velocities of approximately 50 cm s(-1) and with throughputs of approximately 40 particles s(-1). Simple physical models and CFD simulations have been used to investigate flow behavior in the device.

DNA molecule dynamics in converging–diverging microchannels

The conformation and dynamics of double-stranded DNA molecules in a pressure-driven flow-through sharp/gradual converging (contraction half)-diverging (expansion half) square microchannels with a contraction/expansion ratio of 4:1/1:4 were examined using fluorescently labelled lambda-phage DNA at 2.2< or =De< or =30.7 (De is Deborah number, a dimensionless number used in rheology to characterize how fluid a material is). Both the diffusion and stretching of individual DNA molecules and their local flow velocity were visualized, recorded and measured with using MPIV (microparticle image velocimetry) and CLSM (confocal laser-scanning microscopy). The onset of the coil-stretch transition in DNA molecules and their time-dependent velocity in the elongational flow were determined at the micro scale. Velocity distribution was measured via MPIV through orange fluorescent particles and carboylate-modified microspheres of 1 mum diameter. The channel geometry effect in terms of the channel contraction/expansion angle, the mass flow rate and the solution viscosity on the conformation and diffusion were also studied and discussed.

Numerical and experimental evaluation of microfluidic sorting devices

The development of lab-on-a-chip devices calls for the isolation or separation of specific bioparticles or cells. The design of a miniaturized cell-sorting device for handheld operation must follow the strict parameters associated with lab-on-a-chip technology. The limitations include applied voltage, high efficiency of cell-separation, reliability, size, flow control, and cost, among others. Currently used designs have achieved successful levels of cell isolation; however, further improvements in the microfluidic chip design are important to incorporate into larger systems. This study evaluates specific design modifications that contribute to the reduction of required applied potential aiming for developing portable devices, improved operation reliability by minimizing induced pressure disturbance when electrokinetic pumping is employed, and improved flow control by incorporating directing streams achieving dynamic sorting and counting. The chip designs fabricated in glass and polymeric materials include asymmetric channel widths for sample focusing, nonuniform channel depth for minimizing induced pressure disturbance, directing streams to assist particle flow control, and online filters for reducing channel blockage. Fluorescence-based visualization experimental results of electrokinetic focusing, flow field phenomena, and dynamic sorting demonstrate the advantages of the chip design. Numerical simulations in COMSOL are validated by the experimental data and used to investigate the effects of channel geometry and fluid properties on the flow field.

Effect of channel geometry in regular ceramic packings on the hydrodynamics of heat and mass exchange

It is noted that regular structured packings, which permit maximum development of the contact surface, are coming into increasingly greater use in the chemical, petroleum-refining, petrochemical, and other branches of industry for implementation of heat-and mass-exchange processes in liquid-vapor (gas) systems. It is demonstrated that for equal surface areas of the elements of irregular and regular granular beds and equal Reynolds numbers, the coefficients of mass exchange are approximately equal, while the coefficient of hydraulic resistance of the regular packing is three orders lower. A simple packing in the form of a block with parallel through channels (passages) is investigated on a structural and production basis. A relationship is presented for the specific surface of the channel and the size of the cell, and it is established that the specific surface, and of course, the active surface decreases by a factor of 30 as the size of the cell increases from 0.5 to 15 mm. The influence exerted by the cross section of the channel on head loss is also demonstrated.

Effects of channel geometry on buoyancy-driven mixing

The evolution of the concentration and flow fields resulting from the gravitational mixing of two interpenetrating miscible fluids placed in a tilted tube or channel is studied by using direct numerical simulation. Three-dimensional (3D) geometries, including a cylindrical tube and a square channel, are considered as well as a purely two-dimensional (2D) channel. Striking differences between the 2D and 3D geometries are observed during the long-time evolution of the flow. We show that these differences are due to those existing between the 2D and 3D dynamics of the vorticity field. More precisely, in two dimensions, the strong coherence and long persistence of vortices enable them to periodically cut the channels of pure fluid that feed the front. In contrast, in 3D geometries, the weaker coherence of the vortical motions makes the segregational effect due to the transverse component of buoyancy strong enough to preserve a fluid channel near the front of each current. This results in three different regimes for the front velocity (depending on the tilt angle), which is in agreement with the results of a recent experimental investigation. The evolution of the front topology and the relation between the front velocity and the concentration jump across the front are investigated in planar and cylindrical geometries and highlight the differences between 2D and 3D mixing dynamics.

327 Y字型バルブレスマイクロポンプ特性に及ぼす形状の影響(T06-2 マイクロ・ナノ熟流体システム(2) マイクロフルイディクス,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

327 Y字型バルブレスマイクロポンプ特性に及ぼす形状の影響(T06-2 マイクロ・ナノ熟流体システム(2) マイクロフルイディクス,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

21014 ディスク型SOFCの流路形状に関する流体力学的検討(流体工学(2),一般講演,学術講演)

The velocity fields between two parallel disks which simulate the disk-type solid oxide fuel cell (SOFC), were measured by using particle image velocimetry (PIV). The experiments have been performed for two kinds of disk channels to investigate the effects of channel geometries on flow behavior. In the fuel channel with four linear-shape current collectors, a working gas was decelerated along the flow direction and hence optimum flow conditions were restricted. However, by adopting the involute curve as a current collector shape, uniform velocity distributions were realized in the fuel channel except for near its outer rim. On the other hands, in the air channel having four injection holes, non-uniform flow distribution is enlarged with increase of flow rate and local high velocity regions were observed at a high flow rate. We confirmed that the increase of the hole cross section area and the number of injection holes led to the uniform velocity distributions.

Boiling heat transfer and two-phase flow of water in a single shallow microchannel with a uniform or diverging cross section

The present work investigates and compares boiling heat transfer and two-phase flow of water in a single shallow uniform-cross-section microchannel and a diverging one with a diverging angle of 0.183°. Both types of microchannel are with a depth of about 20 µm and a mean hydraulic diameter of about 33.3 µm. Experiments are conducted to study the effect of channel geometry on boiling heat transfer in the microchannel. It is found that the slug bubbles tend to grow exponentially in the present shallow microchannels, uniform-cross section or diverging. The results reveal that the diverging microchannel presents better performance in boiling heat transfer than that of the uniform-cross-section one, primarily due to more stable two-phase flow in the diverging microchannel. Empirical correlations based on convective boiling are developed, respectively, for both types of microchannel. For the same mass flow rate, the diverging microchannel presents a higher single-phase flow pressure drop, while the two-phase flow in both types of channel shows approximately the same pressure drop for boiling at the same heat flux.

Study of flow dynamics in complex vessels using Doppler optical coherence tomography

Doppler optical coherence tomography (DOCT) was applied to non-invasive monitoring of multi-dimensional velocity profiles within complex geometry vessels. The effect of channel geometry on the flow dynamics at different inlet flow rates was studied. We investigated the flow dynamic regime arising at the junction of T- and Y-shaped samples and vessels with aneurism. We show that at constant input volume flow rate, the distribution of flow velocities measured along a cross-sectional plane orthogonal to the inlet arm, located at 20 mm off the junction, exhibited stationary and laminar behaviour. A non-homogeneous distribution of flow velocity along a junction cross-section plane is presented. Finally, we demonstrate the feasibility of DOCT for mapping the velocity profiles along the vessels' junction with a spatial resolution of about 10 × 10 × 10 µm3 and a minimum detectable velocity of about 2 mm s−1.

Dynamics of DNA molecules in a cross-slot microchannel

The conformation and dynamics of double-stranded DNA molecules in cross-slots with equal arm length are examined with the use of fluorescent labeled λ-DNA at 1.2 ⩽ De ⩽ 4.1. The stretching and diffusing of individual DNA molecules in the flow was visualized/recorded with confocal laser scanning microscopy (CLSM). The onset of coil–stretch transition of DNA molecules in the elongational flow was determined at the microscale. Time-dependent velocity distribution with a mean velocity of 50 µm s−1 for DNA molecules was observed by using FluoSpheres, carboylate-modified microspheres over a time period of 1.12 s. The effect of channel geometry (corner and outlet regions) and the solution viscosity (1.9 cP and 9.5 cP) on the conformation and diffusion are also studied and discussed. We report the maximum and the distribution of the fractional extension of DNA molecules, and show that the maximum fractional extension may be up to 0.64 at De = 4.1, and at a strain of 1.8 there is a corresponding stretching force of about 200 pN.

Electrochemical reaction and performance of proton exchange membrane fuel cells with a novel cathode flow channel shape

This study focuses on the investigation of the electrochemical reaction along a novel cathode flow channel of PEM fuel cells with various shoulder/channel (S/C) ratios at the outlet port. A three-dimensional mathematical model, considering conservation principles of mass, momentum, species and electric current is employed. Local variations of important model variables such as reactant concentration and local current density are presented by contour plots to elucidate the effects of channel geometry on transport process, catalyst reaction and cell performance. The potential fields of solid and membrane phases are also resolved in the cell domain and the driving force of the electrochemical reactions – the catalyst activation overpotential – is harnessed in modeling. Numerical calculations reveal the influence of the cathode channel configuration on the local distributions of various model variables. The results also show the dependence between optimal channel configuration and cell operating condition. At a medium reaction rate, the reaction sites underneath the shoulder region generate more current than the channel region. Therefore, a convergent channel configuration with a larger S/C ratio at the outlet port develops more current because such a design facilitates the electron transport and enhances local activation overpotential. However, as the cell voltage decreases and the reaction rate increases, such configuration loses its merit gradually as the requirement for a higher reactant concentration is more important and the reaction sites underneath the channel region have a higher reaction rate. Consequently, the divergent channel configuration with a lower S/C ratio of 0.67 performs better at a cell voltage of 0.22 V.

Hydraulic control of two-layer flow in “quadratic”-type channels

Previous treatments of two-layer exchange flow in a rectangular channel have been extended to consider the effects of channel geometry upon the hydraulics of such flows. Attention has been focussed on exchange flows in “quadratic” channels, defined as flow cross-sections having a single width maximum w0 at the surface and a single depth maximum D (not necessarily located at the centre of the channel cross section). The analysis shows that the effects of channel cross sectional geometry can be quantified satisfactorily in terms of a shape parameter ξ representing the ratio of the cross-sectional area of the channel to that of the equivalent rectangular channel having the same values of w0 and D. Specifically, the analysis shows that suitable definitions of the Froude numbers of the two fluid layers in terms of the shape parameter ξ enables the familiar general criticality condition G2 = 1 to be formulated for the composite Froude number. Furthermore, the application of the so-called functional approach to this type of channel problem is demonstrated to show excellent quantitative consistency with the results of the hydraulic model for two reference examples (the horizontal constriction and the sill) already studied intensively for rectangular channels. Finally, an analysis is presented to show the effects upon the exchange flow of changes in ξ in the along-channel direction.