Influence Of Channel Geometry Research Articles

Enhancing multi-channel electrode structures in ceramic fuel cells: Influence of channel geometry on mass transport and electrochemical performance

Solid oxide fuel cell (SOFC) is a promising electrochemical power generation technology that contributes to the decarbonization of the energy field. The electrode-supported design enables the use of thinner electrolytes, but inevitably brings additional gas diffusion resistance, which in turn leads to higher anodic concentration polarization. Therefore, additional structural design of the electrodes is necessary to ensure efficient operation and mass transport resistance minimization. In this study, anode-supported micro-tubular SOFCs with different multi-channel geometries have been attempted, during which samples with 4 channels exhibit unique elliptical structure due to meticulous manipulation. In addition, the 4-channel samples have been compared to the single-channel and 7-channel counterparts, exploring the influential mechanism of the anode micro-structure on gas transfer and electrochemical performances. The study found that the 4-channel cell has the best geometric controllability, with its minimum gas diffusion path reduced to below 50 μm, such thin electrochemical active region (EAR) accounts for >50 % of the entire external surface. Efficient utilization of the geometric surface area considerably facilitates the fuel transport to reach anode/electrolyte interface. By enhancing the mass transport efficiency, the maximum power density of the 4-channel cell reached 1.40 W cm−2 (750 °C), which is 22.8 % and 102.9 % higher than that of the 7-channel and single-channel cells under the same conditions. The highly efficient anode design additionally exhibited exceptional structural integrity and mechanical robustness, thereby ensuring the steady operation of the cell.

Boost ionic selectivity by coating bullet-shaped nanochannels with dense polyelectrolyte brushes

The influence of channel geometry on the ionic selectivity and ionic current rectification of soft nanochannels was numerically investigated. The nanochannels coated with polyelectrolyte layers (PELs) are termed as soft nanochannels. The asymmetric category of nanochannels, i.e., bullet-shaped, was considered in this study. When PEL is dense, the ionic partitioning effect cannot be ignored. To this end, through adopting a numerical approach using the finite element method, Poisson–Nernst–Planck and Navier–Stokes equations were solved at steady-state conditions by considering different values of permittivity, diffusivity, and dynamic viscosity for the PEL and the electrolyte. The results show that the PEL–electrolyte property difference leads to a significant improvement of the rectification behavior, especially at low and moderate salt concentrations. This not only highlights the importance of considering different properties for the PEL and the electrolyte but also implies that the rectification behavior of soft nanochannels/nanopores may be improved considerably by utilizing denser PELs. Considering a charge density of 80 mol/m3 and a bulk concentration of 20 mM, we demonstrate that the rectification factors for the bullet nanochannels, from 3.35 by ignoring the ion partitioning effect, can reach the values of 4.88 by considering the ion partitioning effect, respectively.

Impacts of the shape of soft nanochannels on their ion selectivity and current rectification

The influence of channel geometry on the ion selectivity and ionic current rectification (ICR) of soft nanochannels was numerically investigated. The nanochannels coated with polyelectrolyte layers (PELs) are termed as soft nanochannels. Two categories of channels, asymmetric (trumpet, conical, bullet, and cigar-like) and symmetric (cylindrical), were considered in this study. When PEL is dense, the ionic partitioning effect can not be ignored. To this end, through adopting a numerical approach using the finite element method (FEM), Poisson-Nernst-Planck and Navier-Stokes equations were solved at steady-state conditions by considering different values of permittivity, diffusivity, and dynamic viscosity for the PEL and the electrolyte. The model findings were verified by comparing them with the existing simulation data. For the cylindrical type nanochannels, the results indicate that the rectification factor is approximately equal to unity, and the ion selectivity at positive voltages is approximately equal to that at negative voltages. For other geometries, the ion selectivity factor of the channel was STrumpet>SConical>SBullet>SCigar. On the other hand, at low bulk concentrations, the rectification factor was RfCigar>RfBullet>RfConical>RfTrumpet, while at high bulk concentrations, it was RfBullet>RfCigar>RfConical>RfTrumpet. Considering a charge density of 80mol/m3 and a bulk concentration of 20mM, we demonstrate that the rectification factors for the bullet and trumpet types nanochannels, from 3.35and0.56 by ignoring the ion partitioning effect, can reach the values of 5.78 and 0.83 by considering the ion partitioning effect, respectively.

Red blood cell hitchhiking enhances the accumulation of nano- and micro-particles in the constriction of a stenosed microvessel.

We investigate the circulation of nano- and micro-particles, including spherical particles and filamentous nanoworms, with red blood cells (RBCs) suspension in a constricted channel that mimics a stenosed microvessel. Through three-dimensional simulations using the immersed boundary-based Lattice Boltzmann method, the influence of channel geometries, such as the length and ratio of the constriction, on the accumulation of particles is systematically studied. Firstly, we find that the accumulation of spherical particles with 1 μm diameter in the constriction increases with the increases of both the length and ratio of the constriction. This is attributed to the interaction between spheres and RBCs. The RBCs "carry" the spheres and they accumulate inside the constriction together, due to the altered local hydrodynamics induced by the existence of the constriction. Secondly, nanoworms demonstrate higher accumulation than that of spheres inside the constriction, which is associated with the escape of nanoworms from RBC clusters and their accumulation near the wall of main channel. The accumulated near-wall nanoworms will eventually enter the constriction, thus enhancing their concentration inside the constriction. However, an exceptional case occurs in the case of constrictions with large ratio and long length. In such circumstances, the RBCs aggregate together tightly and concentrate at the center of the channel, which makes the nanoworms hardly able to escape from RBC clusters, leading to a similar accumulation of nanoworms and spheres inside the constriction. This study may provide theoretical guidance for the design of nano- and micro-particles for biomedical engineering applications, such as drug delivery systems for patients with stenosed microvessels.

Numerical simulation study on the influence of channel geometry on discharge characteristics of low-power magnetically shielded Hall thrusters

By adjusting the relative position of the magnetic screen and the discharge channel, two types of 200 W low-power magnetically shielded Hall thrusters with three different Brexit/Brmax ratios (the ratio of the radial magnetic field magnitude at the channel exit to the maximum radial magnetic field magnitude, taken at the channel mean diameter) using straight wall channel and chamfered wall channel geometries are designed. The discharge characteristics of the plasma under the two types of Hall thrusters are studied using the particle-in-cell (PIC) simulation, and the influence laws of the channel geometries on the thrust, specific impulse, and anode efficiency of the 200 W Hall thrusters are verified by the corresponding experimental testing. Both the results indicate that compared with the straight wall channel geometry of 200 W magnetically shielded Hall thrusters, the chamfered wall channel geometry will reduce the neutral gas density in the discharge channel, causing the propellant utilization efficiency to be lower, thus decreasing the ionization rate, ultimately reducing the performance of thruster. It explains and clarifies the reason for the dark gap phenomenon and the lower performance of the low-power magnetically shielded Hall thrusters.

The influence of channel geometry on the pressure drop in automotive catalytic converters: Model development and validation

This paper presents a detailed physics based model for the pressure drop through a honeycomb substrate for several channel shapes and void fractions. A CFD-based computational model of a single channel is used to study the pressure drop when flow is entering, passing through and leaving the substrate. An extensive set of 3D computational experiments covering square, hexagonal and triangular channel cross-sections, void fractions from 0.39 to 0.84 and channel Re from 95 to 1284 is used. It is shown that altering the void fraction changes the pressure drop at the inlet and outlet of the substrate, however, its effect on the friction factor inside the substrate is minor. The resulting model can be used either as a semi-empirical lumped model for pressure drop and in 3D full-scale simulations with a porous medium representing the substrate. A validation for the velocity profile in a full scale monolith with experimental data available in the literature is carried out and an excellent agreement is observed. The proposed model significantly improves the prediction of the flow distribution across the substrate, which has remained unaddressed historically by existing models and is the most important effect required to make accurate predictions of heat distribution, conversion efficiency and others in full-scale simulations.

Particle-laden thin-film flow in helical channels with arbitrary shallow cross-sectional shape

Particle-laden flows in helical channels are of interest for their applications in spiral particle separators used in the mining and mineral processing industries. In this paper, we extend the previous work of Lee, Stokes, and Bertozzi [“Behaviour of a particle-laden flow in a spiral channel,” Phys. Fluids 26, 043302 (2014)] by studying thin-film flows of monodisperse particle-laden fluid in helically wound channels of arbitrary centerline curvature and torsion and arbitrary cross-sectional shape. In the case where the particles are uniformly distributed through the depth of the film, significant analytic progress can be made yielding insight into the influence of channel geometry on particle distribution across the channel cross section: the governing equations reduce to a single nonlinear ordinary differential equation, which is readily integrated numerically to obtain the solution subject to appropriate boundary conditions. Motivated by possible application to the design of spiral separators, we consider the effects of changing the channel centerline geometry, the cross-sectional shape and the particle density on the resulting flows, and the radial distribution of particles. Our results support the findings in the work of Arnold, Stokes, and Green [“Thin-film flow in helically wound rectangular channels of arbitrary torsion and curvature,” J. Fluid Mech. 764, 76–94 (2015)] regarding the effect of channel centerline geometry and cross-sectional shape on flows in particle-free regions. In particle-rich regions, similar effects are seen although the primary velocity is lower due to increased effective mixture viscosity. Of key interest is the effect of channel geometry on the focusing of the particles for given fluxes of fluid and particles. We find that introducing a trench into the channel cross section, a feature often used in commercial spiral particle separators, leads to a smaller radial width of the particle-rich region, i.e., sharper focusing of the particles, which is consistent with experiments showing that channel geometry influences particle separation in a spiral separator.

Influence of Channel Geometry on Micro Flow Mixing with a Dilute Polymer Solution

Influence of Channel Geometry on Micro Flow Mixing with a Dilute Polymer Solution

Influence of channel geometry on Fischer-Tropsch synthesis in microstructured reactors

The discussion on renewable sources of synthesis gas and the discussion about flaring associated gas initiated new attraction on Fischer-Tropsch (FT) synthesis with a specific focus on smaller scale due to the distributed nature of the feedstock. Microstructured reactors have often been discussed as solution for process intensification at isothermal conditions and are thus supposed to be suitable for small scale applications in FT synthesis. Catalyst integration into these reactors can be done by packing of particles, but hydrodynamics and heat transfer in the case of gas-liquid flow under FT conditions with unusual ratios of particle diameter to channel or slit width are not accessible by calculation methods without many assumptions and uncertainties. An analysis of the product distribution in two different microstructured reactors with different slit width at otherwise constant conditions revealed that slight temperature gradients in the catalyst bed over the slit width may be the reason for ca. 3% lower chain growth probability at channel width 1.5mm compared to 0.8mm. By applying predictions from kinetic parameters from literature, the estimated peak ΔT was 6K in the larger microstructure. When this peak ΔT was used to calculate the change in methane selectivity and conversion, however, the methane selectivity increase with slit width was over-predicted and a conversion change was assumed, which did not occur in the experiments. Nevertheless, the experiments and the comparison with calculated chain growth probability are indicators for isothermal conditions in channel widths <1mm.

The influence of channel geometry on tidal energy extraction in estuaries

A significant number of estuaries in the UK not only have a high tidal range but also exhibit strong tidal currents, where tidal farms could be used for energy extraction. The purpose of this study is to examine the influence that the geometry of the channel has on the impact of a tidal farm deployed in an estuary. A hydrodynamic model (Mike21) has been used to model several idealised estuaries with dimensions based on a group of locations suitable for tidal energy extraction in the UK. The maximum changes in low and high water levels with the tidal farm have been identified for each case and located within the estuary. The changes in the time for the low and high tides in a point inside the estuary have also been analysed. It is noted that the maximum changes in water levels due to the farm range from the order of mm to a few dm and the locations of these changes are strongly dependent on the geometry of each case. In addition, the effect is generally more noticeable in the increase of low water levels and the decrease of high water levels than vice-versa. This would be associated with the loss of intertidal areas and the reduction of flood risk levels. In terms of the changes in the time of low and high water levels, the effects of the farm are negligible in all cases.

Influence of Channel Geometry and Flow Variables on Cyclone Cooling of Turbine Blades

A study examining the internal cooling of turbine blades by swirling flow is presented. The sensitivity of swirling flow is investigated with regard to Reynolds number, swirl intensity, and the common geometric features of blade-cooling ducts. The flow system consists of a straight and round channel that is attached to a swirl generator with tangential inlets. Different orifices and 180-deg bends are employed as channel outlets. The experiments were carried out with magnetic resonance velocimetry (MRV) for which water was used as flow medium. As the main outcome, it was found that the investigated flows are highly sensitive to the conditions at the channel outlet. However, it was also discovered that for some outlet geometries the flow field remains the same. The associated flow features a favorable topology for heat transfer; the majority of mass is transported in the annular region close to the channel walls. Together with its high robustness, it is regarded as an applicable flow type for the internal cooling of turbine blades. A large eddy simulation (LES) was conducted to analyze the heat transfer characteristic of the associated flow for S0=3 and Re=20,000. The simulation showed an averaged Nusselt number increase of factor 4.7 compared to fully developed flow. However, a pressure loss increase of factor 43 must be considered as well.

The influence of hydrodynamic factors, membrane surface properties and channel geometries on membrane performance and fouling mechanisms

Modern theoretical understanding of colloidal and suspended matter membrane fouling mechanisms are presented and discussed. State-of-the-art simulation models of concentration polarization calculations for different channel conditions are described and influence of the fouling layers on the flux and rejection decrease are evaluated. Results of experimental investigations are presented that suggest a quantitative evaluation of fouling rates and membrane flux prognosis due to colloidal fouling with time. The influence of channel geometry on fouling is demonstrated and discussed. The main disadvantage of spiral wounded membrane modules which is attributed to the presence of a separation spacer mesh in the feed channel is discussed.

The Fabrication of a Micro-Channel for Metallic Bipolar Plates Using a Rubber Pad Forming Process

A bipolar plate is one the most crucial and costliest of the various components of a proton exchange membrane fuel cell (PEMFC). It is important to reduce the cost of bipolar plate, not only in terms of material, but also in terms of the manufacturing process, to allow the commercialization of PEMFC’s. The performance of PEMFC’s is also of importance. Metallic bipolar plates have been the subject of much attention recently, because of their low material cost, formability and excellent thermal and mechanical prosperities. Therefore, this study uses a rubber pad forming process for stainless 316L steel to fabricate a bipolar plate with serpentine channels. A computational fluid dynamics (CFD) analysis is performed, in order to determine the influence of channel geometries, such as channel width, channel height and rib width, on the flow distribution of the reactant. Using the CFD results, finite element analysis models are then constructed and the formability of the micro-flow channel is studied. Finality, experiments are conducted to determine the channel height and thickness distribution of the bipolar plate. The numerical results are verified by the experimental results.

CFD modelling of profiled-membrane channels for reverse electrodialysis

Reverse electrodialysis (RE) is a promising technology for electric power generation from controlled mixing of two differently concentrated salt solutions, where ion-exchange membranes are adopted for the generation of ionic currents within the system. Channel geometry strongly influences fluid flow and thus crucial phenomena such as pressure drop and concentration polarization. Profiled membranes are an alternative to the more commonly adopted net spacers and offer a number of advantages: avoiding the use of non-conductive and relatively expensive materials, reducing hydraulic losses and increasing the active membrane area. In this work, Computational Fluid Dynamic simulations were performed to predict the fluid flow and mass transfer behaviour in channels with profiled membranes for RE applications. In particular, channels equipped with pillars were simulated. The influence of channel geometry on fluid flow and concentration polarization was assessed by means of a parametric analysis for different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

Influence of channel geometry on water levels above siphonic roof outlets

This paper describes a full-scale experimental investigation into the effects of box gutter geometry on the open channel flow conditions above siphonic roofwater outlets. In particular, the effects of channel width (300, 400, 480 and 600 mm) and length (14.86 and 32.00 m) were investigated through measurements of flow rate, water depth and longitudinal velocity in the box gutter. The experimental results showed that for the same outlet flow rate, the depth of water in the gutter varied by up to 211% for the two different gutter lengths tested. Generally, the greatest water depths for the different flow rates were recorded in the 400-mm wide gutter and the lowest water depths were recorded in the 300-mm wide gutter. It was also found that the maximum flow rate through the single 110-mm diameter outlet varied depending on the width of the gutter. Practical application: The depth of water in the open channel box gutters above the outlets of siphonic drainage systems is an important design variable and it is imperative to be able to accurately estimate these water depths during all phases of operation to reduce flood damage risk. This research study has found that for the same flow rate, varying the length of gutter on either side of a siphonic outlet strongly influences the depth of water along the gutter and above the outlet. The results suggest that there may be an optimum gutter width and length for which different siphonic outlets may perform more efficiently.

Performance of a Conceptual Counter-flow MCHE operated on CuO-Water Nanofluid: Influence of Geometry

Microchannel heat exchangers (MCHE) can be designed with different channel geometry such as circular, triangular, rectangular, square, trapezoidal and hexagonal. In this article, the influence of channel geometry on thermal, hydraulic and overall performance of counter flow MCHE operated on CuO-water nanofluid flow has been highlighted. Herein, Nusselt and Poiseuille number for different channel shapes and thermo-physical properties of CuO-water nanofluid (viz., thermal conductivity, viscosity…) have been designed either empirically or adopted from literature. Results of pumping power with trapezoidal geometry MCHE is found to be highly sensitive with nanofluid flow. Rectangular shape counter flow MCHE gives highest thermal effectiveness, while iso-triangular geometry gives best overall performance by converting the pressure energy into thermal energy.

Surface landing of microtubule nanotracks influenced by lithographically patterned channels

Microtubules, which serve as cellular structural components in nature, can be placed within a lithographically patterned channel as engineered nanoscale tracks for bionanotechnology applications. We study the landing behavior of microtubules upon their diffusion onto a kinesin-coated glass surface in the presence of the channel. The influence of channel geometry on the landing rate of microtubules is experimentally characterized using channels with varying width. Additionally, we develop a theoretical model to quantitatively analyze our data by accounting for geometrical constraints due to both the width and height of the channels against the diffusion of the landing microtubules.

Influence of channel geometry on the performance of a counter flow microchannel heat exchanger

Microchannel heat exchangers (MCHE) can be made with channels of various geometries. Their size and shape may have considerable effect on the thermal and hydraulic performance of a heat exchanger. In this paper numerical simulation is carried out to solve 3D developing flow and 3D conjugate heat transfer of a balanced counter flow microchannel heat exchanger (CFMCHE) to evaluate the effect of size and shape of channels on the performance of CFMCHE for the same volume of heat exchanger. The effect of shape of the channels on its performance is studied for different channel cross-sections such as circular, square, rectangular, iso-triangular and trapezoidal. Results show that for the same volume of a heat exchanger, increasing the number of channels lead to increase in both effectiveness and pressure drop. Moreover circular channels give the best overall performance (thermal and hydraulic) among various channel shapes. New correlations are developed to predict the value of heat exchanger effectiveness and performance index as a function of relative size of channels with overall heat exchanger volume, Reynolds number and thermal conductivity ratio.

The Influence of Channel Geometry on the Refrigeration Rate of Ball Bearings of High-Speed Turbocharger with the Help of Waste Pack Lubrication

The paper examines the influence of channel geometry on hydrodynamics of an oil-air mix for the purpose of increasing the intensity of heat exchange from a ball-bearing support of high-speed turbocharger. The parametrical analysis of channel geometry is carried out and the optimum variant of cooling which would raise concentration of oil on the internal ring of the bearing is chosen.

Investigation on the influence of channel geometries on PEMFC performance

The performance of PEM fuel cells depends significantly on an appropriate flow field design. In this work, an investigation on the influence of channel geometries on stack performance has been carried out. A parallel flow field design was developed to widely eliminate the influence of other flow field design parameters, e.g. degree of parallelization and gas crossover effects at the turning points of the gas channels. Starting from a basic design, channel as well as rib dimensions were varied and their influence on stack performance were studied. An optimum between 0.7 and 1 mm was found for either channel or for rib widths. For wider dimensions, the influence on mass transport (ribs) or lateral conductivity (channels) has been found to become significant. For very small dimensions, the manufacturing effort becomes unacceptably high (ribs) and the probability of channel clogging by formation of water droplets increases. In general, narrow channel dimensions are preferred for high current densities, whereas wider dimensions are better at low current densities.