Parallel Flow Channels Research Articles

Performance improvement in direct borohydride/peroxide fuel cells

Direct borohydride/peroxide fuel cells (DBPFCs) show progressively deteriorating performance during operation for various reasons such as decreasing reactant concentrations, gas evolution and uneven distribution of liquids. The present study aims to emphasize the importance of certain design parameters, such as bipolar plate materials, flow fields and manifold design, in determining the DBPFC performance. Bipolar materials and flow channel design have been investigated. A power density of 67 mW cm−2 has been obtained with composite graphite and parallel flow channel bipolar plates. It has increased to 87 mW cm−2 using sintered graphite and then to 93.3 mW cm−2 using sintered graphite with serpentine flow fields. The stacking of DBPFCs results in a loss of performance and unstable output. The performance has remained nearly unchanged as the cell number was increased by applying an independent cell liquid distribution network (ICLDN). Using an ICLDN, power densities of 98.3, 83.3 and 82 mW cm−2 have been obtained for single-cell, 3-cell and 6-cell stacks, respectively. Finally, a controlled oxidant feeding system (COFS) has been developed to provide stable output power, and it has demonstrated a stable output power of 6 W for 2.5 h.

An investigation of the PEM fuel cells performance with partially restricted cathode flow channels and metal foam as a flow distributor

In the present work, the performance of proton exchange membrane fuel cells is studied for three cases; A fuel cell with two parallel flow channels (model A), locally baffle restricted flow channels (model B), and metal foam as a flow distributor (model C). The fully coupled thermal-electrochemical equations are numerically solved in three dimensions, based on the macroscopic, single-domain, and finite-volume approaches. While having no significant effect on temperature distribution, the existence of baffles inside flow channels results in more oxygen penetration into gas diffusion and catalyst layers at the cathode side of the cell. This improves the chemical reaction rate, current density and cell performance. Using metal foam increases oxygen concentration and current density at the cathode catalyst surface, and improves the uniformity of their distributions. Furthermore, a more uniform temperature distribution is achieved, when compared with the other cases. For the considered dimensions, it is observed that decreasing the flow channel depth results to an increase in current density and also in pressure drop along channels (models A and C). Moreover, increasing metal foam porosity can increase the current density value and decrease pressure drop in model C, while it has nearly no effects on temperature distribution.

Experimental investigation of two-phase slug flow distribution in horizontal multi-parallel micro-channels

A visualization experimental study was conducted to investigate the phase distribution of two-phase slug flow in horizontal multi-parallel micro-channels, which was composed of a header with hydraulic diameter of 0.667mm and three parallel channels with hydraulic diameter of 0.500mm, all with rectangle cross section. Nitrogen and 0.03wt% sodium dodecyl sulfate (SDS) solution were used as the test working fluids. The inlet superficial velocities of gas and liquid varied from 0.130 to 1.47m/s and from 0.128 to 0.900m/s, respectively. The high speed recording technique was utilized to elucidate the fluid dynamics of two-phase flow in parallel micro-channels. It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depended on the inlet gas slug length and the inlet real velocity, and the channels in the front of the header can influence the phase distribution of the adjacent channel in the rear. Moreover, a uniform phase distribution was achieved at high real velocity with short gas slugs.

In-Situ Analysis of the in-Plane Current Distributions in an Electrolyte-Supported Planar Solid Oxide Fuel Cell By Segmented Electrodes

Solid oxide fuel cells have been of practical concern as a high efficiency power generation device. Problems of the SOFCs include current distribution that decays the total cell performance and efficiency, and causes electrode degradation chemically and thermo-­mechanically. In the planar SOFCs, the fuel and oxidant distributions cause current and temperature distributions over the electrodes under the separator ribs and flow channels. Optimized design of the separator is hence required to improve the power generation characteristics and durability of practical fuel cell stacks. Although there have been a number of numerical analyses, very few experimental investigations confirming in-­situ current distributions to reveal the influence of the separator structure have been reported. We have therefore addressed measurements of in­-plane spatial current variations of an electrolyte-­supported planar SOFC having segmented cathodes under the rib and the flow channels so far1. In the present study, we investigate the effect of the rib width on the current distribution. We used the planar cell having three segmented cathodes assembled with segmented cathode separators for electrical insulation. The segmented cathodes were employed opposing to a rib and a set of parallel flow channels of the anode separator. The cell was composed of NiO­-10Sc1CeSZ anode, 10Sc1CeSZ electrolyte, GDC interlayer, and LSCF-­GDC cathode. The electrode area for the channel was 1.3 cm2 (2.5 x 0.5 cm) each while those for the ribs were 0.63 cm2 (2.5 x 0.25 cm), 1.3 cm2 (2.5 x 0.5 cm), 1.9 cm2 (2.5 x 0.75 cm). The anode and cathode separator made of SUS430 had the flow channels with a width of 3 mm, a depth of 1 mm, and a length of 2.5 cm. Silver mesh was employed for the current collection of both sides. Current voltage (I­V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell2. The anode and cathode were electrically connected with the four-­terminal method. The anode NiO was reduced to Ni by feeding H2/N2 mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed upward with mixtures of H2/N2 and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by a tubular electric furnace at open circuit voltage (OCV). We prepared cathode separators with flow channels facing those of the anode. Several flow channels were added opposing to the anode rib to minimize the cathode overpotential by poor oxygen transport under the cathode rib indicated in our previous report1 since we focus on the current distributions ascribed to the anode overpotential. Larger rib width results in larger anodic overpotentials under the anode rib compared with the anode flow channel according to a decrease in the inlet hydrogen flow rate. We also model the current and hydrogen partial pressure distributions by finite element simulation (COMSOL Multiphysics) so that the model agrees with the in­-situ current distributions derived by the segmented cathodes from setting the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. This modeling is useful to design separators to improve the performance and durability of practical stacks. 1. Takahiro Koshiyama, Hironori Nakajima, Takahiro Karimata, Tatsumi Kitahara, Kohei Ito, Soichiro Masuda, Yusuke Ogura, Jun Shimano, Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes, ECS Trans., Vol. 68, 1, 2217-2226 (2015). 2. Özgür Aydin, Takahiro Koshiyama, Hironori Nakajima, Tatsumi Kitahara, In-­situ Diagnosis and Assessment of Longitudinal Current Variation by Electrode­-Segmentation Method in Anode­-Supported Microtubular Solid Oxide Fuel Cells, J. Power Sources, Vol. 279, 218–223 (2015).

Phase distribution of nitrogen–water two-phase flow in parallel micro channels

The present work experimentally investigated the phase splitting characteristics of gas–liquid two-phase flow passing through a horizontal-oriented micro-channel device with three parallel micro-channels. The hydraulic diameters of the header and the branch channels were 0.6 and 0.4 mm, respectively. Five different liquids, including de-ionized water and sodium dodecyl sulfate (SDS) solution with different concentration were employed. Different from water, the surface tension of SDS solution applied in this work decreased with the increment of mass concentration. Through series of visual experiments, it was found that the added SDS surfactant could obviously facilitate the two-phase flow through the parallel micro channels while SDS solution with low concentration would lead to an inevitable blockage of partial outlet branches. Experimental results revealed that the two phase distribution characteristics depended highly on the inlet flow patterns and the outlet branch numbers. To be specific, at the inlet of slug flow, a large amount of gas preferred flowing into the middle branch channel while the first branch was filled with liquid. However, when the inlet flow pattern was shifted to annular flow, all of the gas passed through the second and the last branches, with a little proportion of liquid flowing into the first channel. By comparison with the experimental results obtained from a microchannel device with five parallel micro-T channels, uneven distribution of the two phase can be markedly noticed in our present work.

In situ investigation of proton exchange membrane fuel cell performance with novel segmented cell design and a two-phase flow model

A novel segmented fuel cell device based on the multi-layered printed circuit board (PCB) flow field plates is designed to study the localized fuel cell performance with various operation conditions. With embedded sensors, distributions of current density, relative humidity (RH) and temperature for both anode and cathode are measured simultaneously along the direction of straight parallel flow channels. Meanwhile, a stationary two-phase flow fuel cell model is developed to study the internal reaction parameter distributions and the results are compared with the in situ experimental measurements. In the co-flow operation mode of hydrogen and air, current density and reactants' RH distributions are sensitive to the stoichiometry of air but the effect from hydrogen is minor. Water transfer behavior, local reactants' RH status, temperature gradients and their impacts on current distributions are analyzed based on the in situ measurements and the coupled model analysis. The segmented cell device discussed in this paper, as well as the experimental and modeling results can be employed to optimize stack design and operating parameters with “visible” internal distributions of water, RH and temperature inside membrane electrode assembly (MEA). Further investigation on fuel cell performance and lifetime with different reactant flow directions is also suggested.

A study on scaled up proton exchange membrane fuel cell with various flow channels for optimizing power output by effective water management using numerical technique

Water management in a Proton Exchange Membrane Fuel Cell (PEMFC) is of great concern for effective cell performance. Existing studies for scaled up model of PEMFC are limited only to experimental work where effective water management with respect to flow channels are very few, time consuming and less economical. In this connection, an extensive numerical study is carried out for optimizing the flow channels with respect to optimal pressure drop and water management for the scaled up model (225 cm2) of PEMFC which is a novel approach in the field of PEMFC. This numerical study involves four different configurations of flow channels. In the first configuration, predominantly existing serpentine parallel flow channel for 25 cm2 is extended to 225 cm2 cross-sectional area. Serpentine zig-zag, straight parallel and straight zig-zag are the other three flow channels considered respectively in this study. The three dimensional flow through the PEMFC is simulated by solving the governing principles namely mass, momentum, energy, species and electro-chemical equations. It is found that the power density developed by straight flow channel with zig-zag flow path is 0.3758 W/cm2 and is the maximum of the configurations considered and this is due to effective water management with minimal pressure drop.

Experimental and Numerical Study on the Improvement of Uniformity Flow in a Parallel Flow Channel

Experimental and Numerical Study on the Improvement of Uniformity Flow in a Parallel Flow Channel

Effect of humidity content and direction of the flow of reactant gases on water management in the 4-serpentine and 1-serpentine flow channel in a PEM (proton exchange membrane) fuel cell

The performance of a PEM (proton exchange membrane) fuel cell depends on design and operating parameters such as relative humidity, operation pressure, and number of channels and direction of the flow of reactant gases. In this study, a three-dimensional, two-phase model has been established to investigate the water management and performance of PEM fuel cell with rectangular geometry and 1-serpentine and 4-serpentine with parallel flow, counter flow and cross flow for hydrogen and oxygen. The numerical simulation was realized with a PEM fuel cell model based on the FLUENT. The active area of each cell is 24.8 cm2 that its weight is 1300 gr. The material of the gas diffusion layer is carbon clothes, the membrane is nafion117 and the catalyst layer is a plane with 0.004 g cm−2 platinum. Pure hydrogen is used on the anode side and oxygen on the cathode side. Simulation results are obtained for voltage as a function of current density at different humidity. The simulation results are compared with the experimental data, and the agreement is found to be good. The results show that the cell performance at lower voltages increases with increasing humidity in cell with 4-Serpentine flow channel and also in cell with 1-Serpentine flow channel, cell performance at all voltages increases with increasing humidity. In cell with 4-Serpentine and parallel flow channel cell performance is better than counter and cross flow in low voltage and in cell with 1-Serpentine and parallel flow, performance is better than counter and cross flow in high voltage.

Non-linear stability analysis of uniformly heated parallel channels for different inclinations

The linear stability for two phase flow in parallel channels has been studied quite extensively. However, the analysis is valid only for infinitesimally small perturbations. Therefore, there is a need to carry out non-linear stability analysis for small finite sized perturbations. Moreover, earlier studies do not consider inclination of these channels, which exist for various applications. The focus of the present work is to carry out linear as well as non-linear stability analysis of parallel channels for various inclinations (including vertical and horizontal). The bifurcation analysis is carried out to capture the non-linear dynamics of the system and to identify regions in the parameter space for which subcritical and supercritical bifurcations exist. The study is carried out for different inclination angles in order to characterize the effect of inclination on the stability of the system. The analysis shows that, at all inclinations, a GH point and BT point exist. The subcritical and supercritical bifurcations are confirmed by numerical simulation of the time-dependent, nonlinear ODEs for the selected points in the operating parameter space using MATLAB ODE solver. The identification of these points is important because the stability characteristics of the system for finite (though small) perturbations are dependent on them.

Mechanism of Efficacy of Microvascular Decompression for Trigeminal Neuralgia

The linear stability for two phase flow in parallel channels has been studied quite extensively. However, the analysis is valid only for infinitesimally small perturbations. Therefore, there is a need to carry out non-linear stability analysis for small finite sized perturbations. Moreover, earlier studies do not consider inclination of these channels, which exist for various applications. The focus of the present work is to carry out linear as well as non-linear stability analysis of parallel channels for various inclinations (including vertical and horizontal). The bifurcation analysis is carried out to capture the non-linear dynamics of the system and to identify regions in the parameter space for which subcritical and supercritical bifurcations exist. The study is carried out for different inclination angles in order to characterize the effect of inclination on the stability of the system. The analysis shows that, at all inclinations, a GH point and BT point exist. The subcritical and supercritical bifurcations are confirmed by numerical simulation of the time-dependent, nonlinear ODEs for the selected points in the operating parameter space using MATLAB ODE solver. The identification of these points is important because the stability characteristics of the system for finite (though small) perturbations are dependent on them.

Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes

Solid oxide fuel cells have been of practical concern as a high efficiency power generation device. Problems of the SOFCs include current distribution that decays the total cell performance and efficiency, and causes electrode degradation chemically and thermo-mechanically. In the case of the planar SOFCs, the fuel/oxidant distributions and current collecting resistance cause current and temperature distributions over the electrodes under the separator ribs and flow channels. Optimized design of the separator is hence required to improve the power generation characteristics and durability of practical fuel cell stacks. Although there have been a number of numerical analyses, very few experimental investigations confirming in-situ current distributions to reveal the influence of the separator structure have been reported. The aim of the present study is thus to measure in-plane spatial current variations of an electrolyte-supported planar SOFC with segmented cathodes under the rib and the flow channel. We used the planar cell having three segmented cathodes (Fig. 1) assembled with segmented cathode separators for electrical insulation. The segmented cathodes were placed opposing to a rib and a set of parallel flow channels of the anode separator. The cell was composed of NiO-10Sc1CeSZ anode, 10Sc1CeSZ electrolyte, GDC interlayer, and LSCF-GDC cathode. The electrode area was 1.4 cm2(2.8 x 0.5 cm) each. The anode and cathode separator made of SUS430 had the flow channels with a width of 3 mm, a depth of 1 mm, and a length of 2.8 cm. Silver mesh was employed for the current collection of both sides. Current voltage (I-V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell[1]. The anode and cathode were electrically connected with the four-terminal method. The anode NiO was reduced to Ni by feeding H2/N2 mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed upward with mixtures of H2/N2and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by an electric furnace at open circuit voltage (OCV) We prepared two types of the cathode separators. One of them had the same flow channel configuration as the anode separator where the rib and flow channels faces each other. The other had additional flow channels opposing to the anode rib. With the former cathode separator, the overpotential at the electrodes under the anode rib was significantly large compared with the flow channel part, whereas the latter case exhibited smaller overpotentials under the rib. This indicates that oxygen transport becomes poor under the wider rib in the cathode side. The latter case also indicated that the electrode under the rib exhibited smaller overpotential than the flow channel part when fuel is sufficient, owing to the smaller contact resistance under the rib by the compression. We also calculated current and hydrogen/oxygen partial pressure distributions by finite element simulation (COMSOL Multiphysics). The current distributions can be calculated so that they agree with the in-situ current distributions derived by the segmented cathodes from setting the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. Thereby oxygen starvation under the wider cathode rib and larger electron conduction resistance for the flow channel part were numerically evaluated to explain the in-situ currents for the segmented cathodes. We will present the results for the case of H2 starvation. [1] Ö. Aydin, T. Koshiyama, H. Nakajima, T. Kitahara, In-situ Diagnosis and Assessment of Longitudinal Current Variation by Electrode-Segmentation Method in Anode-Supported Microtubular Solid Oxide Fuel Cells, J. Power Sources, Vol. 279, 218–223 (2015).Fig. 1 Segmented cathodes on the planar SOFC. Figure 1

The Effect of Blockages in a PEM Electrolyzers Flow-Field on Performance and Temperature Distribution

The ever growing demand for renewable energies is currently focused primarily on wind and solar energy. The technologies that capture these renewable energies have developed greatly in the past few decades with an ever increasing installed capacity all around the world. This capacity is however limited by grid demand and intermittency of the source1. In order to fully utilize the installed capacity of sustainable and renewable energy harnessed in wind and solar farms the energy collected at off-peak hours needs to be stored. By coupling a water electrolyzer to the wind and solar power farms the normally unutilized wind and solar energy can then be stored as hydrogen2. Hydrogen has a distinct advantage over other storage methods such as batteries, compressed air or pumped water as it can be stored and then employed into a variety of applications. The stored hydrogen could be used to fuel transportation, process chemicals, produce fertilizers and regenerate electricity1. Hydrogen also shares the ability to be produced, stored and distributed locally or centralized making it a versatile solution that can be brought very quickly into real, already existing commercial markets1. The major hurdles facing PEM electrolysis before it can truly and competitively enter commercial markets, is the cost of production and the operational cost of the system. The production costs have been a major focus of research since 2010, and a 41% reduction in cost has been shown at the stack level alone in the past 5 years3. The reduction of operational costs and increase in life expectancy and performance of the stacks also plays a critical role in the overall cost of hydrogen production. Some external factors such as poor water supply quality, hot spots from areas of high contact resistance or uneven loading of the catalyst layer, or even blockages in the flow-fields may limit a PEM electrolysis systems lifetime as well as its performance. In this article the effect of a blockage, as well as multiple blockages in a parallel flow channel were investigated using a segmented cell capable of reading temperature and current distribution over the entire active area of the test cell. A blockage in the flow-field could arise in an electrolysis cell through a large variety of sources, broken pieces of sealing material, poor water quality, large particles from corroded supply tubing and errors made during assembly. This could cause increases in mass transport losses and increased degradation rates on the rest of the cell. The cells operating parameters were also varied to explore and determine how to mitigate the effects a flow blockage has on cell performance. Figure 1

Pretreatment of Human Polymorphonuclear Leukocytes (PMN) with a New Carbon Monoxide (CO)‐Releasing Molecule (CORM401) Inhibits PMN Migration across Vascular Endothelial Cells

BackgroundWe have demonstrated that carbon monoxide (CO)‐releasing molecules (CORMs) reduce septic inflammation by interfering with PMN recruitment to the inflamed organs. In this study, we assessed the effects/mechanisms of a newly synthesized Mn2+‐based water‐soluble CORM (CORM401) on PMN adhesion(A)/transendothelial migration(TM) in an experimental model of endotoxemia in vitro.MethodsHuman umbilical vein endothelial cells (HUVEC) grown on parallel flow channels were stimulated with LPS (1μg/ml; 6 hrs) and interacted with freshly isolated PMN (pretreated for 30 min with CORM401 or its inactive compound, iCORM401; 100μM) for 5 min in the presence of 1.0 dyn/cm2 shear stress. Subsequently, HUVEC:PMN co‐cultures were additionally perfused for 20 min with medium containing CORM/iCORM401. Video and confocal microscopy were used to analyze PMN A/TM.ResultsStimulation of HUVEC with LPS significantly increased PMN A (0.6+/‐0.1 vs 30.9+/‐7.6 cells/mm2; p<0.05) and TM (0.0 vs 41.7+/‐6.8%; p<0.001). Pretreatment of PMN with CORM401 had no effect on PMN A, however significantly reduced PMN TM (41.7+/‐6.8% vs 15.2+/‐4.3; p<0.01). The above findings were confirmed employing confocal microscopy.ConclusionCORM401 interferes with PMN recruitment by specific suppression of PMN migratory potential. (HSFO 393, IRF‐25‐12).

Spanwise correlations in the wake of a circular cylinder and a trapezoid placed inside a circular pipe

The spanwise correlation of a circular cylinder and a trapezoidal bluff body placed inside a circular pipe in fully developed turbulent regime is studied using hotwire anemometer. The present configuration possesses complex fluid structure interaction owing to the following features: high blockage effect; low aspect ratio of the body; upstream turbulence and interaction of axisymmetric flow with a two dimensional bluff body. The spatial correlation of such configuration is seldom reported in the literature. Results are presented for Reynolds number of ReD=1×105. Three different blockage ratios (0.14, 0.19 and 0.28) are considered in the present study. Correlation coefficient is observed to improve with increase in blockage ratio. Compared to a circular cylinder, a trapezoidal bluff body possesses high correlation length. The near wall effects tend to increase the phase drift, which is reflected in low correlation coefficients close to the pipe wall. The results show that the simultaneous effect of curvature, low aspect ratio and upstream turbulence reduces the correlation coefficients significantly as compared to unconfined and confined (parallel channel) flows. The low frequency modulations with a circular cylinder are higher for lower blockage ratios. The three-dimensionality of vortex shedding for trapezoid with a blockage ratio of 0.28 was observed to be lower compared to circular cylinder and all other blockage ratios. Low frequency modulations were found to be responsible for weak vortex shedding from a circular cylinder compared to a trapezoidal bluff body. The vortex shedding is observed to be nearly two dimensional in case of a trapezoidal bluff body of blockage ratio 0.28.

Redox speciation analysis of dissolved iron in estuarine and coastal waters with on-line solid phase extraction and graphite furnace atomic absorption spectrometry detection

An automatic on-line solid phase extraction (SPE) system employing the flow injection (FI) technique directly coupled to a graphite furnace atomic absorption spectrometer (GFAAS) was established for speciation and determination of dissolved iron in estuarine and coastal waters. Fe(II) was mixed with ferrozine solution in a sample stream to form the Fe(II)–ferrozine complex which was extracted onto a C18 SPE cartridge, eluted with eluent and detected with GFAAS. In a parallel flow channel, Fe(III) was reduced to Fe(II) with ascorbic acid and then detected in the same way as Fe(II). The home-made interface between FI–SPE and GFAAS efficiently realized the sample introduction to the furnace in a semi-automated way. Parameters of the FI–SPE system and graphite furnace program were optimized based on a univariate experimental design and an orthogonal array design. The salinity effect on the method sensitivity was investigated. The proposed method provided a detection limit of 1.38nmolL−1 for Fe(II) and 1.87nmolL−1 for Fe(II+III). With variation of the sample loading volume, a broadened determination range of 2.5–200nmolL−1 iron could be obtained. The proposed method was successfully applied to analyze iron species in samples collected from the Jiulongjiang Estuary, Fujian, China. With the 2-cartridge FI–SPE system developed, on-line simultaneous determination of Fe species with GFAAS was achieved for the first time.

0223 バースト波励起プラズマアクチュエータの時空間構造の解明

A plasma actuator has been extensively investigated as a flow control device for separated flow in recent years. A burst wave induced plasma actuator (BWPA) is highly expected for the separation control in low Reynolds number flow. In this study, spatiotemporal flow characteristics induced by BWPA in a parallel plate laminar channel flow was investigated by particle image velocimetry. Furthermore, the BWPA was applied to a backward facing step flow to control the flow separation and reattachment. The experimental result shows that the velocity amplitude defined as the magnitude of velocity oscillation generated by BWPA strongly influences the decrease of reattachment length.

일체형원자로 인쇄기판형 증기발생기 열수력학적 설계

The vessel of integral reactor contains its major primary components such as the fuel and core, pumps, steam generators, and a pressurizer, so its size is proportional to the required space for the installation of each component. The steam generators take up the largest volume of internal space of reactor vessel and their volumes is substantial for the overall size of reactor vessel. Reduction of installation space for steam generators can lead to much smaller reactor vessel with resultant decrease of overall cost for the components and related facilities. A printed circuit heat exchanger is one of the compact types of heat exchangers available as an alternative to conventional shell and tube heat exchangers. Its name is derived from the procedure used to manufacture the flat metal plates that form the core of the heat exchanger, which is done by chemical milling. These plates are then stacked and diffusion bonded, converting the plates into a solid metal block containing precisely engineered fluid flow passages. The overall heat transfer area and pressure drops are evaluated for the steam generator based on the concept of the printed circuit heat exchanger in this study. As the printed circuit heat exchanger is known to have much larger heat transfer area density per unit volume, we can expect significantly reduced steam generator compared to former shell and tube type of steam generator. For the introduction of new steam generator, two design requirements are considered: flow area ratio between primary and secondary flow paths, and secondary side parallel channel flow oscillation. The results show that the overall volume of the steam generator can be significantly reduced with printed circuit type of steam generator.

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

Water slugs form in the gas flow channels of polymer electrolyte membrane fuel cells (PEMFCs) which hinder reactant transport to the catalyst layer. We report a study correlating video images of slug formation and motion with pressure/flow measurements in parallel gas flow channels. Slugs move when the differential gas pressure exceeds the force to advance the contact lines of the slug with the channel walls. Water slugs can divert the gas flow through the gas diffusion layer (GDL) beneath the ribs to adjacent channels. The flow diversion can cause slugs to stop moving. Slug size and motion has been correlated with in situ GDL permeabilities as functions of GDL compression. Compression reduces the GDL permeability under the ribs much more than the GDL permeability under the channel. A model is presented to describe the spatio‐temporal location of slugs in a PEMFC flow field. © 2014 American Institute of Chemical Engineers AIChE J, 61: 355–367, 2015

Thermal management of high temperature polymer electrolyte membrane fuel cell stacks in the power range of 1–10 kWe

Maintaining optimal temperature of the stack is critical for efficient operation of high temperature polymer electrolyte membrane fuel cells. While a number of possibilities of thermal management exist for small stacks, the problem becomes more complicated for larger stacks. In the present study, the thermal management of stacks in the power range of 1–10 kWe is considered through computational fluid dynamics simulations. It is shown that large stacks need to have dedicated cooling plates through which a coolant is circulated. Further, stacks of the size of 10 kWe can have reasonably low cell temperature variations (∼20 K) only by passing pre-heated liquid coolant through the coolant plates. Estimates show that the concomitant increase in the coolant flow rate induces large pressure drops, of the order of 30 bar, if a four-parallel serpentine is used on an active cell area of 30 cm × 30 cm. It is therefore necessary to use parallel channel flow fields with carefully designed feeder manifolds to maintain optimal cell temperatures and reasonably low coolant pressure drops in large stacks.