Parallel Channel Configurations Research Articles

Design and performance of parallel-channel nanocryotrons in magnetic fields

We introduce a design modification to conventional geometry of the cryogenic three-terminal switch, the nanocryotron (nTron). The conventional geometry of nTrons is modified by including parallel current-carrying channels, an approach aimed at enhancing the device's performance in magnetic field environments. The common challenge in nTron technology is to maintain efficient operation under varying magnetic field conditions. Here, we show that the adaptation of parallel channel configurations leads to an enhanced gate signal sensitivity, an increase in operational gain, and a reduction in the impact of superconducting vortices on nTron operation within magnetic fields up to 1 T. Contrary to traditional designs that are constrained by their effective channel width, the parallel nanowire channels permits larger nTron cross sections, further bolstering the device's magnetic field resilience while improving electro-thermal recovery times due to reduced local inductance. This advancement in nTron design not only augments its functionality in magnetic fields but also broadens its applicability in technological environments, offering a simple design alternative to existing nTron devices.

Effects of bipolar plate flow channel configuration on thermal-electric performance of direct ammonia solid oxide fuel cell: Part Ⅱ- Promoting in-cell ammonia endothermic decomposition via a novel parallel S-type channel arrangement

This study explores impact of novel parallel S-type flow channels on ammonia decomposition and thermal-electric performance of direct ammonia solid oxide fuel cells (DA-SOFC). Four channel arrangements were numerically studied using single cell models: conventional unbent channels (UC–SOFC) and proposed single S-type channels (SSC–SOFC), double S-type channels (DSC-SOFC), triple S-type channels (TSC-SOFC). Cells with parallel S-type channel configurations outperformed unbent channel. Particularly, TSC-SOFC demonstrated the highest electrical performance, followed by DSC-SOFC, SSC-SOFC, and UC-SOFC, with power density improvements of 33.6%, 28.9%, and 28.9%, respectively, compared to UC-SOFC. Further results show that S-type channels exhibit self-heating effects, namely heat released from electrochemical reactions in the rear supplies ammonia decomposition in the front part of channel, enhancing H2 concentration and electrochemical reactions. Quantitatively, back-heat supplied to primary ammonia decomposition region increases by 33.3% in TSC-SOFC compared to UC-SOFC. Moreover, higher velocities in S-type channels enhance species diffusion towards rib underside, finally reducing concentration polarization and, improving power density. Besides of electrical performance, recycling waste heat from exhaust gases also improve the temperature uniformity cell, potentially benefiting the stability and lifetime of cell.

Planar water disinfection reactor with parallel-channel configurations

A novel planar water disinfection reactor is proposed to treat potable water with UV-C LED. Parallel-channel configurations are employed to increase the flow uniformity, microbial exposure time and UV fluence for microorganisms. The effects of flow rate, baffle width and number of flow channels (n) are examined to elevate the efficiency of sterilizing Escherichia coli (E. coli). Experimental results indicate that the four-channel reactor exhibited the highest inactivation efficiency owing to high flow uniformity, sufficient exposure time and circulation elimination in the flow channel. The reactor achieved 4.0 log at a flow rate of 29 mL/min and a total radiative power of 80 mW. The disinfection flow rate was 45% higher than that of the reactor without channel configuration. Increasing the number of channels helps eliminate circulations and achieve uniform flow distribution. However, microbial exposure time and UV fluence decrease with increased number of channels. Additionally, when the baffle width was less than 6 mm, the log inactivation increased with baffle width due to the increase in the degree of flow uniformity. The flow reactor achieves optimal disinfection efficiency at baffle width 6 mm due to an optimal combination of flow variation and water velocity.

Design and Modeling of Metallic Bipolar Plates for a Fuel Cell Range Extender

Fuel cells, designed for mobile applications, should feature compact and low-weight designs. This study describes a design process that fulfills the specific needs of target applications and the production process. The key challenge for this type of metallic bipolar plate is that the combination of two plates creates three flow fields, namely an anode side, a cathode side, and a coolant. This illustrates the fact that each cell constitutes an electrochemical converter with an integrated heat exchanger. The final arrangement is comprised of plates with parallel and separate serpentine channel configurations. The anode and cathode sides are optimized for operation under dry conditions. The final plate offers an almost perfect distribution of coolant flow over the active area. The high quality of this distribution is almost independent of the coolant mass flow, even if one of the six inlet channels is blocked. The software employed (OpenFOAM and SALOME) is freely available and can be used with templates.

Enhanced gas flow uniformity across parallel channel cathode flow field of Proton Exchange Membrane fuel cells

Distributing reactant gases uniformly within a cell to ensure proper delivery of reactants is one the crucial requirements for better performance of a Proton Exchange Membrane fuel cell (PEMFC). This investigation was focused on enhancing flow uniformity and distribution of cathode air in parallel channel flow field of a fuel cell. 2D Computational Fluid Dynamic (CFD) simulations (i.e. using ANSYS) were performed to solve continuity and momentum equations across the cathode flow field. Two main designs with modified headers that provide (i) Z-type, and (ii) combination of Z and U-types parallel channel configurations, were investigated for uniform distribution of gases. Our presented designs were able to allow 60–70% of total flow through frontal 80% of total gas flow field area which was just reversed case compared to conventional design where nearly 80% of flow passes through 20% of total area at the end portion of the gas flow field. Further improvement in flow distributions was observed by segmenting the flow field.

Role of network geometry on fluid displacement in microfluidic color-changing windows

We have previously demonstrated a microfluidic elastomer, which changes apparent color and could have potential applications in smart windows. The practical use of such functional microfluidic systems requires rapid and uniform fluid displacement throughout the channel network with minimal amount of liquid supply. The goal of this simulation study is to design various microfluidic networks for similar applications including, but not limited to, the color-switching windows and compare the liquid displacement speed and efficiency of the designs. We numerically simulate and analyze the liquid displacement in the microfluidic networks with serpentine, parallel and lattice channel configurations, as well as their modified versions with wide or tapered distributor and collector channels. The data are analyzed on the basis of numerical criteria defined to evaluate the performance of the corresponding functional systems. We found that the lattice channel network geometry with the tapered distributors and collectors provides most rapid and uniform fluid displacement with minimum liquid waste. The simulation results could give an important guideline for efficient liquid supply/displacement in emerging functional systems with embedded microfluidic networks.

Numerical modeling of flow boiling instabilities using TRACE

Dynamic flow instabilities in two-phase systems are a vitally important area of study due to their effects on a great number of industrial applications, including heat exchangers in nuclear power plants. Several next generation nuclear reactor designs incorporate once through steam generators which will exhibit boiling flow instabilities if not properly designed or when operated outside design limits. A number of numerical thermal hydraulic codes attempt to model instabilities for initial design and for use in accident analysis.TRACE, the Nuclear Regulatory Commission’s newest thermal hydraulic code is used in this study to investigate flow instabilities in both single and dual parallel channel configurations. The model parameters are selected as to replicate other investigators’ experimental and numerical work in order to provide easy comparison. Particular attention is paid to the similarities between analysis using TRACE Version 5.0 and RELAP5/MOD3.3. Comparison of results is accomplished via flow stability maps non-dimensionalized via the phase change and subcooling numbers.Results of this study show that TRACE does indeed model two phase flow instabilities, with the transient response closely mimicking that seen in experimental studies. When compared to flow stability maps generated using RELAP, TRACE shows similar results with differences likely due to the somewhat qualitative criteria used by various authors to determine when the flow is truly unstable.

Discrete geometry optimization for reducing flow non-uniformity, asymmetry, and parasitic minor loss pressure drops in Z-type configurations of fuel cells

Parallel channel configurations, such as Z-type, used to distribute reagents in planar fuel cells provide lower overall pressure drop as compared to other channel designs. However, due to their inherent characteristics, flow maldistribution in parallel configurations is commonly observed and leads to starvation of reagents in middle channels. In addition, the Reynolds number dependent minor losses at branching tee junctions may cause asymmetric flow non-uniformity and reagent imbalance between the cathode and anode. Herein, we present a universal and simple optimization method to simultaneously reduce flow maldistribution, asymmetry, and parasitic pressure in Z-type parallel configurations of fuel cells or fuel cell stacks that has improved scalability relative to previous methods. A discrete model's governing equations were reduced to yield geometric ratios between headers. Increasing header widths to satisfy these ratios reduced flow maldistribution without modifying parallel channel geometry as validated by computation fluid dynamics (CFD) simulations. Furthermore, decreased Reynolds numbers throughout the headers reduced minor pressure drops and flow distribution asymmetry. We offer several methods to reduce the optimized geometry's footprint, including an adaptation of the discontinuous design.

A verification problem for thermal hydraulics systems codes dealing with twin, parallel-boiling and inverted U-tubes condensing channels

In the analysis of the nuclear safety of complex nuclear systems, almost one-dimensional system thermal–hydraulics codes will be used perhaps for a couple of decades from now. Computational Fluid Dynamics (CFD) tools are accepted at present to be a support of such analyses and they are used coupled to systems codes or as separate analysis tools for isolated components with boundary conditions obtained from systems codes. The restricted acceptance of “pure” CFD codes is due to many reasons but two of them are relevant, namely (a) the apparent lack of CFD grade experimental data and (b) the need for a complete verification and validation (V&V) and the uncertainty quantification for the codes currently available. There is plenty of experimental data related to integral test facilities (ITFs) that constitute macroscopic systems behavior information and a consolidated data base for such purposes. Despite of this, additional verification cases may be added to the above mentioned consolidated data. In the present paper, flow oscillations in parallel channel configurations with system codes are studied in diverse configurations. Different models, calculation options and, in particular, in-phase or out-of-phase oscillations were studied, both in heated and cooled parallel channels. The emphasis is on the effects of concentrated irreversible pressure losses coefficients at the inlet and at the outlet of the channels. In the case of cooled steam generator channels, the results of the Semiscale Integral Test Facility operating in natural circulation conditions are revisited. The results presented in this paper, show how a validation case lead to find a not still reported (in the Authors knowledge) verification case. The problem is related to twin-parallel-boiling and condensing, inverted U-tubes channels and connected through common plena. This is, of course, a problem that deserved many tens of papers in the last four decades. Flow splitting without reversal was computationally found and to explain this behavior a theoretical model limited in scope was developed that was a posteriori verified using a particular systems code (RELAP5) commonly applied to perform safety analyses of nuclear power plants. The rationale followed, the theoretical analysis performed and the confirmatory computational results found are summarized in this paper.

Discrete approach for flow field designs of parallel channel configurations in fuel cells

It is the major challenge to transform a laboratory scale production of fuel cells to an industrial scale one and to meet the requirements of throughput, operating life, low cost, reliability and high efficiency in R&D of fuel cells. Designs of uniform flow distribution are central to upscale fuel cells as well as to tackle critical issues of water, thermal and current management. However, in spite of our growing appreciation of designs of uniform flow distribution, there is little or no practical solution to ensure a uniform flow distribution across channels of a cell and cells of a stack in designs of flow fields. The purpose of this paper was to develop a discrete approach to find a design that met requirements of flow distribution uniformity and pressure drop in parallel channel configurations with Z-type arrangement through adjustments of configurations and normalised structural parameters. Variation of the frictional and the momentum coefficients with flow velocities was incorporated into the flow distribution equation to improve modelling accuracy. We also developed procedure, measures and guideline for the designs of flow distribution and pressure drop to bridge knowledge gap between the generalised theory and industrial applications. The results showed that the present approach could provide the practical guideline to evaluate quantitatively performance of different layout configurations, structures, and flow conditions.

Investigation of the Effect of CO2 bubbles and Slugs on the Performance of a DMFC by Means of Laser-optical Flow Measurements

A single-channel DMFC is constructed that allows for flow measurements at the anode side as well as detailed time–resolved cell-voltage measurement. The coherence between flow and bubble clogging and slug movement can be investigated without parasitic effects like flow shortcuts through the gas diffusion layer (GDL) between neighbouring channels, as in serpentine or parallel-channel configurations. Optical access is granted to the anode side by a transparent foil, which is necessary for the application of the laser-optical velocity measurement technique (micro–particle image velocimetry, μPIV). Small fluorescent particles are added to the fluid, which are illuminated by a laser. The particle movement can be optically detected using a microscope, and transferred to a planar velocity field. Hence, the appearance and evolution of CO2 bubbles can be qualitatively and quantitatively investigated. The analysis of the velocity structure around a CO2 bubble or a moving slug allows a deeper understanding of the coherence of fluid motion, channel blockage, and cell performance. In addition to the flow analysis, a time-resolved measurement of the cell voltage is performed. The results clearly indicate that the cell power increases when huge bubbles reduce the free cross-section area of the channel. Methanol is forced into the GDL, i.e. methanol is continuously convected to the catalyst layer and is oxidised to CO2. Hence, the fuel consumption increases and the cell performance rises. When the huge bubble is released from the GDL and forms a moving slug, the moving slug effectively cleans the channel from CO2 bubbles on its way downstream. Since the channel cross-section is not severely diminished by the bubbles at this stage, the methanol flow is no longer forced into the GDL. The remaining amount of methanol in the GDL is oxidised. The cell power decreases until enough CO2 is produced to eventually form bubbles again that significantly reduce the free cross-section of the channel, and the process starts again.

Pressure drop and flow distribution in parallel-channel configurations of fuel cells: Z-type arrangement

A general theoretical model based on mass and momentum conservation has been developed to solve the flow distribution and the pressure drop in Z-type configurations of fuel cells. While existing models neglected either friction term or inertial term, the present model takes both of them into account. The governing equation of the Z-type arrangement was formulated to an inhomogeneous version of the U-type one. Thus, main existing models have been unified to one theoretical framework. The analytical solutions are fully explicit that they are easily used to predict pressure drop and flow distribution for Z-type layers or stacks and provide easy-to-use design guidance under a wide variety of combination of flow conditions and geometrical parameters to investigate the interactions among structures, operating conditions and manufacturing tolerance and to minimize the impact on stack operability. The results can also be used for the design guidance of flow distribution and pressure drop in other manifold systems, such as plate heat exchanges, plate solar collectors, distributors of fluidised bed and boiler headers.

Analysis and optimization of flow distribution in parallel-channel configurations for proton exchange membrane fuel cells

Parallel channels have many advantages, such as low pressure drop and easy fabrication, but they may cause flow maldistribution which would result in low reaction efficiency. This study presents an analytical model to calculate the flow distribution of the parallel channels based on the assumption of the analogy between fluid flow and electrical network. The model, which ultimately releases from the solution of a set of nonlinear equations, is validated by comparing with the results obtained from three-dimensional computational fluid dynamics (CFD) simulations. Consequently, the model is used to optimize the geometric dimension of a parallel plate to obtain a uniform flow field distribution.

Pressure drop and flow distribution in parallel-channel configurations of fuel cells: U-type arrangement

An analytical model based on mass and momentum conservation has been developed to solve the flow and pressure distribution in fuel cell stacks. While existing models neglected either friction effect or inertial effect, the present model takes both of them into account. The analytical solutions are fully explicit so that the velocity and pressure distribution in fuel cell stacks are directly correlated with the geometrical parameters of fuel cell stacks. Parameter Sensitivity is also analysed to determine the influence of geometrical structures and parameters on flow performance of fuel cell stacks. It is found that friction and momentum effects work in opposite directions, the former tending to produce a pressure drop and the latter a pressure rise. The proper balance of the two effects can result in less non-uniformity and an optimal design. Furthermore, the existing solution by Bassiouny and Martin [Flow distribution and pressure drop in plate heat exchanges. Part I. U-type arrangement. Chem Eng Sci 1984;39(4):693–700] is a special case of the present solutions without the friction effect and those by Kee et al. [A generalized model of the flow distribution in channel networks of planar fuel cells. J Power Sources 2002;109:148–59] and Maharudrayya et al. [Flow distribution and pressure drop in parallel-channel configurations of planar fuel cells. J Power Sources 2005;144:94–106] are another special case without inertial effect.

Performance evaluation of passive direct methanol fuel cell with methanol vapour supplied through a flow channel

This work examines the effect of fuel delivery configuration on the performance of a passive air-breathing direct methanol fuel cell (DMFC). The performance of a single cell is evaluated while the methanol vapour is supplied through a flow channel from a methanol reservoir connected to the anode. The oxygen is supplied from the ambient air to the cathode via natural convection. The fuel cell employs parallel channel configurations or open chamber configurations for methanol vapour feeding. The opening ratio of the flow channel and the flow channel configuration is changed. The opening ratio is defined as that between the area of the inlet port and the area of the outlet port. The chamber configuration is preferred for optimum fuel feeding. The best performance of the fuel cell is obtained when the opening ratio is 0.8 in the chamber configuration. Under these conditions, the peak power is 10.2 mW cm −2 at room temperature and ambient pressure. Consequently, passive DMFCs using methanol vapour require sufficient methanol vapour feeding through the flow channel at the anode for best performance. The mediocre performance of a passive DMFC with a channel configuration is attributed to the low differential pressure and insufficient supply of methanol vapour.

Pressure drop and flow distribution in multiple parallel-channel configurations used in proton-exchange membrane fuel cell stacks

Single U- and Z-type parallel-channel configurations for gas distributor plates in planar fuel cells reduce the pressure drop but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. In this paper, previous analytical solutions obtained for single U- and Z-type flow configurations are extended to multiple U- and multiple Z-type flow configurations of interest to fuel cell applications. Algorithms to calculate flow distribution and pressure drop in multiple U- and Z-type flow configurations are developed. The results are validated by comparison with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. It is found that there is a significant improvement in the flow distribution in some configurations without paying for extra pressure drop. The possibility of unmatched distribution on the cathode and the anodes sides is also highlighted. Careful design of the flow configuration is therefore necessary for optimum performance.

Flow distribution and pressure drop in parallel-channel configurations of planar fuel cells

Parallel-channel configurations for gas-distributor plates of planar fuel cells reduce the pressure drop, but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. This study presents an analysis of the flow distribution through parallel-channel configurations. One-dimensional models based on mass and momentum balance equations in the inlet and exhaust gas headers are developed for Z- and U-type parallel-channel configurations. The resulting coupled ordinary differential equations are solved analytically to obtain closed-form solutions for the flow distribution in the individual channels and for the pressure drop over the entire distributor plate. The models have been validated by comparing the results with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. Application of the models to typical fuel-cell distributor plates shows that severe maldistribution of flow may arise in certain cases and that this can be avoided by careful choice of the dimensions of the headers and the channels.

BWR Regional Instability Analysis by TRAC/BF1-ENTRÉE—I: Application to Density-Wave Oscillation

In this study, applicability of the TRAC/BF1-ENTRÉE code to regional instability was demonstrated in two parts. In Part I, fidelity of numerical models was studied with regard to the density-wave oscillation. Based on the FRIGG-4 loop test, predictability of the code has been demonstrated. An appropriate time integration scheme was explored, and it was found that the numerical viscosity can be minimized by applying the semi-implicit method and specifying the material Courant number close to unity. Applicability of the code was studied for simulating parallel channel configurations. The bypass channel effect was quantified as a function of its flow area and power level. The influence of flow mixing at the upper and lower plena was studied.