bluuebunny/crossref_metadata_2025 huggingface dataset:
The Application of Synthetic Jet Actuators for the Modification of the Characteristics of Separated Shear Layers
Authors: Watson Mark , Jaworski Artur J. , Wood Norman J.
Published: November 2008

This paper presents an experimental investigation related to controlling the unsteady characteristics of the separated shear layers occurring over highly swept wings, and in particular focuses on application of synthetic jet actuators for modification of unsteady dynamic loading on the wing surface due to the phenomenon referred to as vortex breakdown (burst). In the post burst flow region the surface pressure measurements reveal the presence of certain characteristic spectral peaks that are thought to represent the presence of a spiralling filament of vorticity inside the expanded vortex that is known to be present in the burst flow over swept wings. This paper details an investigation into how the use of an array of 18 discrete synthetic jet actuators, distributed along the leading edge of a delta wing with a 60° sweep angle, can be used to alter the spectral content of this unsteadiness and reduce the level of unsteady pressure found in the post-burst region toward the wing trailing edge by up to 40%. Measurements of the surface pressure spectral distributions over the wing are presented together with PIV measurements of the vortex cross-section, conducted in the successive planes parallel to the wing trailing edge. Additional surface flow visualisation indicates that the effect of the actuators on the leading edge boundary layer is to induce local separation delays close to each actuator orifice, which introduce “ripples” into the shear layer as it separates. The results obtained are used to formulate an interpretative hypothesis attempting to explain the mechanisms responsible for modification of the spectral content and the level of excitation measured on the wing surface.
The PIV Measurements on the Flow Fields in an Unshrouded Centrifugal Pump
Authors: Liu Chao , Tang Fangping , Sun Sun , Cheng Li , Zhou Jiren
Published: November 2008

PIV was applied to the measurements of flow field in an unshrouded centrifugal pump impeller. Three windows were selected for the measurements. Three operation points of the pump were taken during the measuring. The ratios (Q/QBEP) of the flow rate for measuring are 0.6, 1.0, and 1.4, respectively. The velocity distributions in blade-to-blade passages obtained at different windows give the evidence that the velocity distributions are asymmetric even under the design operation point. A lower velocity zone existed at middle of blade-to-blades passages near the pressure-side of the blade.
Molecular-Level Tribology Under Transient Conditions
Authors: Al-Samieh M. F. , Rahnejat H. , Dowson D.
Published: November 2008

The paper investigates the mechanisms that contribute to the formation of a lubricant film formed between contiguous bodies in concentrated contacts in conjunctions of order of few nanometers. It has been observed that in such vanishing gaps a lubricant film is formed due to the combined effects of Newtonian slow viscous action and molecular and structural forces in the intervening fluid and with surfaces in contact. The mechanism of solvation is not in accord with classical physics, but is little understood in the structureless environment that ensues beyond the continuum that is usually promoted by viscous action. The paper strives to explain the behaviour of this narrow conjunction, particularly the role of solvation effect under stop-start motion.
Investigation of Heterogeneous Mixtures With Dual-Modality Transducers Fabricated Using Thick-Film Technology
Authors: Meng Guangtian , Jaworski Artur J. , Dyakowski T. , Hale J. M. , White N. M.
Published: November 2008

The work presented in this paper focuses on development of a dual modality sensor, for deployment within an oil and gas extraction plant to measure the composition of oil-water mixtures. The sensors combine ultrasonic and electrical measurement techniques. These are of course non-destructive, rapid, and can potentially provide an on-line industrial measurement. In addition, the combination of two techniques could potentially be reliable in a wider range of process conditions and could contain self-calibration features. The sensors used in the current study were manufactured using thick-film technology, which enables construction of multilayered structures of both conductive and non-conductive layers, some of which may exhibit piezoelectric properties for ultrasonic measurement purposes. These are later fired on a ceramic substrate to provide rugged sensors, capable of working in aggressive industrial environments. Experiments were conducted for mixtures of vegetable oil and saline water to investigate the feasibility of such dual dual-modality sensors. The time of flight of ultrasonic wave in pure liquids and heterogeneous mixtures was measured. It has been shown that the signal obtained from the transducers is sufficiently strong to warrant the measurement of the speed of sound in heterogeneous mixtures of oil and water. A study of the effects of oil concentration and temperature on the speed of sound has been conducted. A mathematical model has been tested, which relates the speed of sound to the volume fraction taking into account the reflection and refraction on the droplet interfaces. The experimental results subjected to linear regression agree very well with the theoretical predictions. The electrical measurement was conducted at three different frequencies. In general, the values of capacitance and conductance decrease with increasing oil percentage. In the middle oil percentages a discontinuity occurs in the decreasing trend. In the high oil percentages, the experimental results agree very well with theoretical predictions.
Humans Jumping on Flexible Structures: Effect of Structural Properties
Authors: Yao ShiPing , Harrison Robert E. , Wright Jan R. , Pavic Aleksandar , Reynolds Paul
Published: November 2008

The behaviour of humans jumping on flexible structures has become a matter of some importance for both structural integrity and also human tolerance. The issue is of great interest for stadia, footbridge and floor structures. A test rig has been developed for exploring the forces, accelerations and displacements that occur when a human subject jumps on a flexible structure where motion can be perceived. In tests reported earlier, it was found that the human is able to generate near resonant response of the structure but it was extremely difficult, if not impossible, to jump at or very near to the natural frequency of the structure when the structural vertical motion is significant. Also, the force developed by the subject was found to drop significantly near resonance. In this paper, the effect of the subject-to-structure mass ratio and the damping ratio of the structure on the ability of the subject to jump near resonance, and on the force drop out, is presented. It is shown that as the structure becomes more massive and more highly damped it moves less for nominally the same jumping excitation. In this situation, it becomes easier to jump near resonance and the degree of force drop out reduces, though it is still significant.
Peri-Implant Bone Adaptation Under Dynamic Mechanical Stimulation: The Guinea Pig Model
Authors: Jaecques Siegfried V. N. , De Smet Els , Muraru Luiza , Jansen John A. , Wevers Martine , Vander Sloten Jos , Naert Ignace E.
Published: November 2008

The present work is part of a larger project to analyse adaptive bone remodelling around implants that receive controlled mechanical stimulation immediately post-operatively. Percutaneous implants in the tibiae of guinea pigs are used as an implant model [1]. For evaluation, microfocus computed tomography (μCT) can be used to complement or partially substitute conventional histology [2]. In the studied model implant system, μCT-based histomorphometry can be used as a substitute for histology in regions at a distance of more than 1000 μm from the titanium implant. Within this limitation, a significant effect of mechanical stimulation can be observed also under in vivo μCT conditions. The optimally osteogenic stimulus in the studied model should cause a strain rate amplitude of 1600 microstrain/s or less in the cortical bone at a distance of 2.3 mm from the implant centre. Future work will include a detailed study of strains in the peri-implant bone with in vivo micro CT-based finite element models.
Mass Finishing of Parts Produced by Direct Metal Laser Sintering
Authors: Boschetto A. , Veniali F. , Miani F.
Published: November 2008

This paper presents some practical considerations on finishing of parts made by direct metal laser sintering (DMLS). The main process capabilities limitations of this promising rapid tooling technique are in fact in the surface roughness of the produced parts. This fact hinders the introduction of DMLS as a widely employed industrial process, especially for what concerns the production of moulds and inserts and allows their use only as preseries tools in injection moulding of plastics, since the requirements for preseries tools are worse than those needed during the process. Barrel finishing, in turn, is a well established technique to improve the roughness of parts of complicated shape by means of a soft mechanical action over the surface. The results herewith presented show that it is possible to achieve roughness of the order of 1 μm Ra even when starting from initial roughness of the order of 15 μm Ra, i.e. those typically attained by DMLS.
A Mechanical Model Representation of the In Vivo Behaviour of Bulk Tissue
Authors: Aritan Serdar , Oyadiji S. Olutunde , Bartlett Roger M.
Published: November 2008

The aim of this study was to characterise the bulk modulus properties of the upper arm under relaxed and controlled contraction which is defined as 25% of the maximum voluntary contraction. A new testing machine was designed to generate constant load on the upper arm and measure the deformation over time. The machine consists of a device which is effectively a cuff that applies controllable pressure on a 47 mm wide band of the upper arm. Six different loads (10, 20, 30, 40, 50 and 60 kgf) were applied over a period of time of up to a maximum of 120 seconds. The deflection-time curves obtained show strongly non-linear response of the bulk tissue. The non-linearity manifested by these deflection-time curves is in terms of both time- and load-dependency. For each load, the creep behaviour follows an exponential law typical of viscoelastic materials. At low loads (below 30kgf), the creep response increases fairly linearly as the load is increased from 10 kgf to 30 kgf. But at high loads (above 30 kgf), the creep response increases only slightly as the load is increased from 30 kgf to 60 kgf. Beyond a load of 60 kgf, the deflection or creep becomes negligible. This implies that the upper arm has reached the state of incompressibility. The creep behaviour of the upper arm was simulated using four Voigt viscoelastic models in series. The three obvious soft tissues of the upper arm, namely skin, fat and muscle, were modelled in series. The effects of blood vessels and connective tissue were also modelled in series with the other tissues.
Simulation of Wave and Jet Formations in Explosive/Impact Welding
Authors: Akbari Mousavi A. A. , Al-Hassani S. T. S.
Published: November 2008

Explosively driven impact welding is a process to produce bi-metallic plates and tubes. Whilst well established it has been essentially an empirical process. Recent work to numerically analyse part of the process is described. Using a finite difference engineering package, the oblique impact of a thin flyer plate on a relatively thick base was modelled. The results were validated by data from carefully controlled experiments using a pneumatic gun. Straight and wavy interfaces and jetting phenomena were modelled, and the magnitude of the waves and the velocity of jet predicted. The numerical analysis predicted a hump ahead of the collision point. Wave formation appears to be the result of variations in the velocity distribution at the collision point and periodic disturbances of the materials. Higher values of plastic strain were predicted in wavy interfaces. Bonding was found to be a solid state welding process. Phase changes which occur may be due to high temperatures (but less than the melting temperature) at the collision point.
Dynamic Behaviour of Rotary Vane Engines With One, Two and Four Work Units
Authors: Librovich B. V. , Nowakowski A. F.
Published: November 2008

A mathematical model to analyse the dynamic behaviour of a novel rotary vane engine (RVE) is introduced in this paper. The RVE have a number of advantages compare to a majority of other reciprocating engine types. The main advantages are a simple structure and the small number of moving parts. The geometrical structure and dynamical behaviour of engines with a different number of work units is considered in detail. This has been examined through a study of torque transmission with a particular reference to how this is affected by the non-circular geometry of gear pitch curves. A possible method for balancing of asymmetric cogwheels have been proposed as well. The analysis concludes that by using an appropriate design and arrangement of cogwheels and all moving parts, vibration due to impulsive gas torque can be attenuated.
Optimisation of Ceramic/Steel Composite Armour of a Constant Thickness
Authors: Cakir Tanju , Yildirim R. Orhan , Ogel Bilgehan
Published: November 2008

Impact resistance of ceramic/steel composite armour against 7.62 mm AP (armour piercing) projectile is examined analytically, numerically and experimentally. Total armour thickness is taken to be constant. Ceramic tile thickness and steel plate thickness are changed to observe the effect of the variation of the thickness ratio on the impact behaviour of ceramic/steel armour. Results show that the impact behaviour of ceramics is related to the ceramic tile thickness and back plate thickness. It is found that there is an optimum ceramic/steel thickness ratio which provides the best protection against a specified threat for a constant total armour thickness.
Characterization of Water Vapor Transmission Rate Through Fuel Cell Components
Authors: Jena Akshaya , Gupta Krishna , Connolly Matthew
Published: November 2008

An instrument capable of measuring vapor transmission rates under any desired gradient of concentration and pressure is described. The test temperature, pressure, humidity and flow rates are precisely controlled and accurately measured. The instrument is completely automated to obtain objective and accurate results and minimal involvement of the operator. Using this instrument a variety of materials, including a fuel cell component, have been successfully investigated.
Cell-to-Cell Variations With Increasing SOFC Stack Size
Authors: Burt A. C. , Celik I. B. , Gemmen R. S. , Smirnov A. V. , Rogers W. A.
Published: November 2008

This numerical study considers the influence of stack size on cell-to-cell performance variations within a stack of planar solid oxide fuel cells. In order to model large stacks (>10 cells) a pseudo 2-D scheme was implemented using the parallel technique of domain decomposition and was solved on a Beowulf cluster. Results were obtained for stacks of 5, 10, and 20 cells. The results indicate that although significant variations in temperature were observed the voltage variations were practically negligible for cases with uniform flow distribution. Non-uniform fuel flow distribution resulted in more significant cell voltage variations. With the assumption of adiabatic boundary conditions, increasing stack size resulted in slightly lower average cell temperatures but increased outlet temperature of the fuel and air channel in the top cell.
Improvement of the Performance for the Anode-Supported SOFCs
Authors: Amaha S. , Baba Y. , Yakabe H. , Sakurai T.
Published: November 2008

Recently performance tests for the 10-cell stack with 13 cm × 13 cm cells were carried out. Although the cell size was 5 cm × 5 cm in early stage of the developments, we have succeeded in scaling up to 13 cm × 13 cm. A high performance for the 5 cm × 5 cm single cell was obtained. However, the performance of the single stack with the 13 cm × 13 cm cell was extremely low. For a high performance of the 13 cm × 13 cm cell stack, an improvement of the gas seal, an even distribution of the fuel and a mitigation of contact resistances were attempted. For a gas seal, we used mica sheets for the seal between the interconnector and the separator, the interconnector and the frame. For an even distribution of the fuel, we designed the channel of the gas flow by using the computational calculation on the thermo-fluid dynamics. For the mitigation of the contact resistances, we analyzed a property of a warp of the cell in detail. Using the findings of the analyses we weighted the cell stack appropriately. On the basis of the result, the performance of the 10-cell stack with 13 cm × 13 cm cell was improved.
Performance of Small Stack for Intermediate Temperature-Operating Solid Oxide Fuel Cells Using Stainless Steel Interconnects
Authors: Bae Joongmyeon , Park Jin Woo , Lim Hee Chun , Ahn Kyo-Sang , Yoo Young-Sung
Published: November 2008

Recently we have started a project to develop 1 kW-class SOFC system for Residential Power Generation (RPG) application supported by Korean Government. For a 1 kW-class SOFC stack that can be operated at intermediate temperatures, we started to develop anode-supported, planar type SOFC to have advantages for commercialization of SOFCs considering mass production and using cost-effective interconnects such as ferritic stainless steels. Anode-supported single cells with thin electrolyte layer of YSZ were fabricated and their small stacks were built and evaluated. The size of anode-supported single cells after final sintering was about 5 × 5 cm2, and the thickness of electrolyte and the cathode layer were about 20 μm and 30 μm, respectively. I-V and AC impedance characteristics of these single cells and small stacks were evaluated at intermediate temperature (650 ∼ 750°C) by using hydrogen gas as a fuel. We have already carried out long-term performance test for YSZ thin electrolyte single cell for above 26,000 h (3 years) at 750°C, applying 0.76 V with power density of 200 mW/cm2. Using these YSZ thin electrolyte 5 × 5 cm2 cells and Inconel interconnect plates coated by silver paste, the 15-cell and 60-cell short stack were prepared. The initial stack (15 cell) voltage at 150 mW/cm2 was 12.5 V in hydrogen as fuel of 120 sccm/cell at 750°C and decreased to about 10.9 V at 500 h of operation time. It was then stabilized and kept until 4,000 h with a degradation rate of 10 mV/(1000 h, 1 cell). AC impedance of this small stack and microstructure of cell components were analyzed during and after the operation. Furthermore thin electrolyte cells and ferritic stainless steel interconnects were built into a 4-cell stack and the small stack was operated at 650°C for cost-effective planar SOFC RPG system. I-V and AC impedance characteristics of the small stack were evaluated at 650°C by using hydrogen as a fuel.
Fuel Cell Dogmas Revisited
Authors: Hemmes Kas
Published: November 2008

In spite of the fact that the development of fuel cells is a scientific and technological process, the process, itself seems to be largely determined by the human factor. By a number of examples it will be shown that this development and the creative process necessary for its progress is sometimes hindered by strong beliefs and frequently repeated statements that within themselves hold some truth but do not represent the whole truth. They can be regarded as ‘fuel cell dogmas’. Often implicit assumptions or particular boundary conditions lie behind the dogma. These assumptions or conditions may be altered in the course of the developments or for specific applications. Sometimes the dogma is essentially a rumor that is conveniently accepted by newcomers in the field. An example of the latter is the ban on the use of LiNa carbonate electrolyte in a MCFC instead of LiK because of its supposed higher corrosiveness. Nowadays LiNa is accepted as the new standard electrolyte in a MCFC. The most famous dogma is that fuel cells are more efficient than heat engines because they are not limited by the Carnot efficiency. Yet it is not always true, not even in the reversible limit. For a long time polymer fuel cells were considered to be inherently too expensive because of the membrane and the Pt catalyst. They were considered only suitable for niche markets such as space applications. As we know now General Motors believes differently. Other dogmas are: - Fuel cells have a higher efficiency than heat engines. - A fuel cell converts Hydrogen into power and heat. - Nernst loss is always proportional to utilization and inevitable. - To be economically feasible the fuel gas utilization should be as high as possible. - To be economically feasible the fuel cell should be operated at the highest possible power density. - A fuel cell always has two inlets and two outlets. - In order to use solid fuels in a fuel cell they must be gasified first. - Only low temperature fuel cells are suitable for automotive applications. These and other dogmas will be critically analyzed in terms of the underlying assumptions and boundary conditions. New options that are revealed by breaking through the dogmas are briefly sketched.
Fuel Cells Constructal Optimization and Research Perspectives
Authors: Vargas Jose´ V. C. , Ordonez Juan C. , Bejan A.
Published: November 2008

The hydrogen economy is a possible alternative to the current oil based global economy. The technology to build and operate fuel cells is well advanced. However, cost is the reason why fuel cells are not being installed wherever there is a need for more power. Therefore, optimization is a natural alternative to reduce cost and make fuel cells increasingly more attractive for power generation. This paper discusses the process of determining the internal geometric configuration of a unit fuel cell for maximum power. The optimization of construction (architecture) starts at the smallest (elemental) fuel cell level. The optimization of system architecture must be subjected to a fixed volume constraint. There are several degrees of freedom in the fuel cell configuration, i.e., the thickness of two gas channels (fuel and oxidant), two diffusion layers and two reaction layers (anode and cathode) and the electrolyte solution space. Research perspectives for fuel cells are presented and discussed.
Transient Response Analysis of Polymer Electrolyte Fuel Cell Considering Equivalent Circuit, Mass and Heat Conservation
Authors: Ito K. , Koori H. , Onda K. , Masuda H. , Masuoka T.
Published: November 2008

Elucidating the transient characteristics of a polymer electrolyte fuel cell in consideration of its applications, such as electric vehicles, is an important step in its development. In this study, we created a mathematical model to predict the transient behavior of a unit-PEFC, analyzed the model numerically, and compared the results with experimental results. Consequently, we elucidated that the transient behavior of a PEFC is summarized by two physical phenomena: (1) water redistribution in the polymer electrolyte membrane, (2) the discharge/charge of electric double layer capacitance.
Mass Transport Inside a Flowing Electrolyte Direct Methanol Fuel Cell
Authors: Golriz M. R. , Gu J. , James D.
Published: November 2008

In this work, analytical mass transfer models are developed for two different types of Direct Methanol Fuel Cells (DMFCs). One type is a conventional assembly with a proton exchange membrane (PEM) and the other utilizes a flowing electrolyte in addition to a PEM to reduce methanol crossover. These models are used to predict methanol crossover behaviour that is a major issue affecting the efficiencies of PEM-DMFCs. It is shown that using flowing electrolyte DMFCs can lead to a significant decrease in methanol crossover with a corresponding increase in electrical efficiency of the cell. Combined with the experiments carried out in a previous work, the simulation showed significant efficiency improvements when using a flowing electrolyte DMFC compared to a traditional PEM assembly.
A Study of CO2 Gas Bubble Behavior in a Direct Methanol Fuel Cell
Authors: Yang H. , Zhao T. S. , Ye Q.
Published: November 2008

CO2 bubble behavior in the anode flow field of a transparent Direct Methanol Fuel Cell (DMFC) was investigated visually. The DMFC consisted of a membrane electrode assembly (MEA) with an active area of 4.0×4.0 cm2, two transparent enclosures made of Lucite material, and two bipolar plates made of 316 stainless steel plate, in which a single 2.0×2.0 mm2 serpentine channel having a total length of 420 mm was formed. An image recording system was employed to capture images of the two-phase flow behavior in the anode flow field. It is found that at low current densities, small discrete bubbles appeared in the anode flow field. At moderate current densities, a number of gas slugs formed in addition to small discrete bubbles. At high current densities, the flow field was predominated by rather long gas slugs. The effects of methanol solution flow rate, cell operating temperature, and methanol concentration on the CO2 behavior and the cell performance have also been investigated. Furthermore, for the present flow field consisting of a single serpentine channel, the phenomenon of big CO2 gas slugs blocking the channel was never found for all the test conditions in this work.
Microfabrication Procedures Applicable to Micro-Fuel Cell Design and Production
Authors: Raffaelle R. P. , Ruf H. J. , Piscani E. C.
Published: November 2008

The potential of the extremely high energy densities that may be possible with a hydrogenic fuel cell makes it an attractive alternative as a future microsystem power source. Additionally, the potential for integration of such devices could be enhanced if they were manufactured in a manner that is compatible with standard microelectronic and MEMS applications. A combination of silicon micromaching and direct writing was used to produce a microelectronic PEM fuel cell. The fuel cell was fabricated on a silicon wafer containing inter-digitated microchannels which were synthesized using an STS Systems Inc. deep reactive ion etcher. The anode and cathode composite materials were deposited using the MicroPen™. The MicroPen™ system is a fluid dispensing system that is capable of “writing” lines of materials with virtually any viscosity onto a wide range of potential substrates. Final assembly was accomplished by the application of a Nafion™ cover plate. These techniques were successfully employed to produce a working microelectronic fuel cell in silicon. The electrical characterization of the device demonstrated an open circuit voltage of 250 mV with a short circuit of 10 μA measured at room temperature when the cell was provided with hydrogen and oxygen at 1 ATM.
An Electroanalytical Study of Methanol Crossover Through Nafion 117 Membrane
Authors: Kadirgan F. , Savadogo O.
Published: November 2008

The proton exchange membrane direct methanol fuel cells (PEMDMFC) shows considerably lower performance than the hydrogen fuel cell because of inefficient methanol oxidation and the crossover of the methanol through the membrane that separates the anode from the cathode. This paper describes an electrochemical method to measure the crossovered methanol for Nafion® 117 membrane in function of the time. The permeation rate is calculated both by electroanalytical and gas chromatography methods at 25°C. The results are discussed.
A Low-Cost Solid Oxide Fuel Cell (SOFC) Power Conditioner for Residential Utility
Authors: Burra R. K. , Mazumder Sudip K.
Published: November 2008

We describe a low-cost power-electronics system (PES) for a solid-oxide fuel cell (SOFC) power system which achieves an efficiency η > 93%. The PES comprises of a DC/DC boost converter followed by a soft-switched multi-level high-frequency (HF) inverter and an AC/AC converter. The DC/DC converter comprises zero-ripple boost inductor, which reduces the switching current ripple of the SOFC-stack output current, thereby, possibly improving the durability of the SOFC. Additionally, the design of the inductor yields higher power at a reduced cost. Finally, preliminary experimental results and efficiency estimates for individual subsystems of the PES are demonstrated.
A Three Dimensional CFD Model for PEMFC
Authors: Li Shaoping , Becker Ulrich
Published: November 2008

A 3-D computational model for PEMFC simulations has been developed and implemented in FLUENT, a general purpose commercial software package with multi-physics capabilities. In this modeling approach, transport equations for flow, energy, species and electro-potentials (both solid and membrane phases) are solved in the entire computational domain that includes current collectors, gas channels, porous gas diffusion layers, reacting catalyst layers and the membrane. Various physical processes such as multi-component diffusion, electro-chemical reactions, membrane water transport, liquid water formation and vaporization are considered. The numerical model is also capable of simulating other physical effects encountered in PEMFC operations such as contact resistance, joule heating, time-varying operating conditions etc. The details of the modeling approach are first presented in this paper. Results of a parametric study are then given, which sheds lights on the dependence of in-situ distributions, such as species concentrations, water content, temperature and current density, on various operating conditions.
Fitting a Three Dimensional PEM Fuel Cell Model to Measurements by Tuning the Porosity and Conductivity of the Catalyst Layer
Authors: Bang Mads , Odgaard Madeleine , Condra Thomas Joseph , Kær So̸ren Knudsen
Published: November 2008

A three-dimensional, computational fluid dynamics (CFD) model of a PEM fuel cell is presented. The model consists of straight channels, porous gas diffusion layers, porous catalyst layers and a membrane. In this computational domain, most of the transport phenomena which govern the performance of the PEM fuel cell are dealt with in detail. The model solves the convective and diffusive transport of the gaseous phase in the fuel cell and allows prediction of the concentration of the species present. A special feature of the model is a method that allows detailed modelling and prediction of electrode kinetics. The transport of electrons in the gas diffusion layer and catalyst layer is accounted for, as well as the transport of protons in the membrane and catalyst layer. This provides the possibility of predicting the three-dimensional distribution of the activation overpotential in the catalyst layer. The current density dependency on the gas concentration and activation overpotential can thereby be addressed. The proposed model makes it possible to predict the effect of geometrical and material properties on the fuel cell’s performance. It is shown how the ionic conductivity and porosity of the catalyst layer affects the distribution of current density and further how this affects the polarization curve. The porosity and conductivity of the catalyst layer are some of the most difficult parameters to measure, estimate and especially control. Yet the proposed model shows how these two parameters can have significant influence on the performance of the fuel cell. The two parameters are shown to be key elements in adjusting the three-dimensional model to fit measured polarization curves. Results from the proposed model are compared to single cell measurements on a test MEA from IRD Fuel Cells.
Modeling of a Fuel Cell Power System for Automotive Applications
Authors: Kolavennu Panini K. , Telotte John C. , Palanki Srinivas
Published: November 2008

In this paper, the primary components of a PEM fuel cell for automotive propulsion using methane as a fuel are analyzed. Basic chemical engineering principles are utilized to assess the role of thermodynamics, heat transport, and reaction kinetics. The amount of methane required is calculated as function of hydrogen produced as well as the power produced. The heat duty for thermal control of the system is analyzed.
Advances in the Research and Development of Fuel Cells in China
Authors: Ma Chong-Fang , Guo Hang , Ye Fang , Yu Jian
Published: November 2008

As a clean, high efficiency power generation technology, fuel cell is a promising choice of next generation power device. Widely application of fuel cells will make a contribution to save fuels and reduce atmospheric pollution. In recent years, fuel cells science, technology and engineering have attracted great interest in China. There are more and more Chinese scientists and engineers embark upon fuel cell projects. The government also encourages academic institutions and companies to enter into this area. Research and development of fuel cells are growing rapidly in China. There are many chances and challenges in fuel cells’ research and development. The state of the art of research and development of fuel cells in China was overviewed in this paper. The types of fuel cells addressed in this paper included alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, proton exchange membrane fuel cells and direct methanol fuel cells.
Oxygen Permeation and Methane Conversion Properties of CERIA-Based Composite Membranes Prepared by Tape-Casting Technique
Authors: Takamura H. , Kobayashi T. , Kamegawa A. , Okada M.
Published: November 2008

The preparation of ceria-based composite membranes by means of tape-casting technique and their oxygen permeation and methane reforming properties have been investigated. Oxide powders comprising of Ce1−xRExO2−δ (RE = Pr and Sm) and spinel-type ferrites of MFe2O4 (M = Mn and Co) were prepared by the citrate-based liquid-mix technique. As a result of the optimization of ceramics slurry composition, a crack-free dense membrane with dimensions of 5 cm × 5 cm and 140 μm in thickness was successfully prepared by means of doctor-blade technique. For Ce0.9Sm0.1O2−δ - 15 vol% MnFe2O4 membrane with a thickness of 133 μm, an oxygen flux density of 9.5 μmol/cm2 s was attained at 1000 °C under Ar-10% CH4 (100 sccm). The methane conversion and CO selectivity were 22 and 93%, respectively. Laminated membranes comprising of Ce0.9Sm0.1O2−δ - 15 vol% MnFe2O4 and Ce0.9Sm0.1O2−δ - 30 vol% CoFe2O4 were also prepared. The laminated membrane with one Ce0.9Sm0.1O2−δ - 30 vol% CoFe2O4 layer showed a higher oxygen flux density by a factor of 40% than that without Ce0.9Sm0.1O2−δ - 30 vol% CoFe2O4 layer.
The Analysis of the Operating Characteristics in a 600W Air Cooling Portable PEMFC
Authors: Sohn Young-Jun , Park Gu-Gon , Yang Tae-Hyun , Yoon Young-Gi , Lee Won-Yong , Yim Sung-Dae , Kim Chang-Soo
Published: November 2008

To maintain proper operating conditions is important to get optimal output power of a polymer electrolyte membrane fuel cell (PEMFC) stack. The air cooled fuel cell stack is widely used in sub kW PEMFC systems. The higher the power density of a stack, however, the more difficult it is to get well balanced operating conditions for the system such as the relative humidity, the temperature of stack, the rate of usage of reactant and so on. A 600W air cooled PEMFC stack was experimentally investigated to evaluate the design performance and to get optimal operating conditions for the portable application. The relationship between the operating conditions and the performance was analyzed. The results can be used as design criteria for portable PEMFC under various conditions.
Influence of Porous Electrode Structure on PEM Fuel Cells Design and Performance
Authors: Asinari Pietro , Coppo Marco
Published: November 2008

Gas flow in fuel cell porous electrodes is usually modelled with Darcy’s law, which requires the definition of a resistance constant for the material. This can be done directly via experimentation or indirectly via numerical tuning to fit experimental data on cell behaviour. Both methods lack generality, as they do not take into account the particular porous structure of each electrode. In the present work, a numerical procedure for calculating the resistance constant for a given porous structure is presented. This procedure is based on Lattice Boltzmann models, which treat the problem from a microscopic point of view, reproducing collisions between fluid molecules and solid particles. It can be demonstrated that under certain hypotheses, these models yield Navier-Stokes equations on a macroscopic scale, hence obeying fluid mechanics laws. Here the flow in a set of thirty randomly generated porous structures was analyzed, thus obtaining a distribution of values for Darcy’s constant. The analysis was repeated for ten different pressure gradients applied to a portion of the electrode and for three different volume porosities. The results showed that, for a given volume porosity, the value of Darcy’s constant is strongly affected by the material porous structure. On the other hand, the mean value of resistance remained almost constant while varying the applied load, thus correctly reproducing the linear dependence between velocity and pressure gradient, as stated by Darcy’s law. As fuel cell models are a great help in designing and predicting component operation, a further analysis was carried out in order to study the influence of the electrode resistance constant on cell performance prediction. The Lattice Boltzmann model was used to obtain resistance data characteristic of fuel cell electrodes, and the results were implemented in a one dimensional fuel cell model. The simulations showed that the variation of Darcy’s constant does not significantly affect the prediction of the cell polarization curve, while a significant effect was found on the prediction of the exact operating point on the polarization curve. In conclusion, if accurate modelling of a fuel cell is required, great care must be taken in evaluating the electrodes resistance constant. The procedure presented here, coupled with a non destructive tomography scan of the electrode structure could greatly help in refining existent fuel cell models.
High Performance Polymer Electrolyte Membrane Fuel Cell Electrodes
Authors: Rajalakshmi N. , Rajini R. , Dhathathreyan K. S.
Published: November 2008

Several methods are being attempted to improve the performance of PEM Fuel cell electrodes so that the cost of the overall system can be brought down. The performance can be improved if the utilization of the catalyst in the electrode increases. One of the early successful method was to add a proton conducting polymer, such as NafionR to the catalyst layer. However there is a limit to the amount of NafionR that can be added as too much NafionR affect the gas diffusion. The other method is to increase the surface area of the catalyst used which has also been adequately demonstrated. Alternative methods for providing increased proton conductivity and catalyst utilization are thus of great interest, and a number of them have been investigated in the literature. One method that is being extensively investigated is to introduce the catalyst onto the polymer electrolyte membrane followed by lamination with gas diffusion electrode. In the present work, we have carried out two methods i) screen print the catalyst ink on the NafionR membrane ii) catalyze the NafionR membranes by reducing a suitable platinum salt on the membrane. Standard gas diffusion electrodes were then laminated onto this membrane. The performances of Membrane Electrode Assemblies (MEAs) prepared by these routes have been compared with the commercially available Gore catalysed membrane. It was observed that catalysed NafionR membranes show a better performance compared to the catalyst ink screen printed on the NafionR membrane and commercial Gore membrane under identical operating conditions. However MEAs with Gore membrane give a better performance in the iR region compared to the other MEAs prepared using NafionR membrane. The lesser performance with Gore membrane is probably due to the limitations in the lamination method employed.
Prediction of Flow Field in a MCFC Stack With Channels of Complex Shape
Authors: Ryu D. N. , Choi D. H.
Published: November 2008

The flow passage in a molten-carbonate-fuel-cell (MCFC) stack consists of the cathode/anode channels of complex geometry and the inlet/outlet manifolds that supply and collect the gas to and from the channels. It is one of the major design concerns that the reacting fluid be uniformly distributed to each cell and to various regions of the electrode surface in the channel. The paper outlines the method to predict the flow characteristics in the flow passage of a multiple cell MCFC system. Rather than trying to obtain the finite volume solution over the entire channel directly, the channel with uniformly distributed trapezoidal shape supports is approximated by the equivalent porous medium. The effective permeability and the inertial resistance factor are estimated from a rigorous 3D finite volume calculation for a single periodic module of the channel. The pressure in the manifold is determined iteratively from the empirical head-loss relation in a tube. The volume change of the fluid due to chemical reaction is taken into consideration in the analysis. It is shown in the paper that the flow field in both cathode and anode channels is successfully computed and the mass flux to each cell can also be predicted. The pressure drop versus the flow rate for different stack-manifold arrangements is also discussed in the paper.
Coupled CFD and Process Simulation of a Fuel Cell Auxiliary Power Unit
Authors: Zitney Stephen E. , Prinkey Michael T. , Shahnam Mehrdad , Rogers William A.
Published: November 2008

A high-temperature auxiliary power unit (APU) based on solid oxide fuel cell (SOFC) technology is analyzed in this study using coupled computational fluid dynamics (CFD) and process simulation. The tightly integrated process flowsheet consists of a reformer, desulfurizer, SOFC stack, combustor, and various heat exchange and rotating equipment items. A detailed three-dimensional CFD model is used to represent the cross-flow, planar SOFC. Process simulations are used to calculate the overall material and energy balances. Coupled CFD and process simulations are performed over a range of fuel cell currents to generate a voltage current curve and analyze the effect of current on fuel utilization, power density, and overall system efficiency. The fuel cell APU system considered here generated 4.3 kW of power and yielded a maximum fuel-to-electricity conversion efficiency of 45.4% at a current of 18 amperes. Integrated CFD and process simulations provide a better understanding of the fluid mechanics that drive overall performance and efficiency of fuel cell systems. In addition, the analysis of the fuel cell using CFD is not done in isolation but within the context of the whole APU process.
Numerical Investigation on the Strategies for Reducing the Cell Temperature Gradient of an Indirect Internal Reforming Tubular SOFC
Authors: Nishino Takafumi , Iwai Hiroshi , Suzuki Kenjiro
Published: November 2008

Strategies to reduce the temperature gradient of the cell have been numerically examined by using a comprehensive analytical model of an indirect internal reforming tubular SOFC, the first generation of which was presented at the last conference in 2003 (1st ICFCSET). In particular, how the air flow rate, gas inlet temperature and density distribution of reforming catalyst affect the thermal field in the cell has been examined. Based on the calculated results, it has been confirmed that larger air flow rate reduces the maximum temperature and accordingly the temperature gradient of the cell, while lower inlet temperatures of gases reduce only the average temperature of the cell. For the reforming catalyst distribution, it has been determined that the temperature gradient of the cell can be fairly reduced by adjusting the amount and allocation of the catalyst. In addition, it has been revealed that the distribution pattern of the catalyst has little effect on the average temperature, so that the power generation performance of the cell is not affected by the adjustment of the catalyst distribution pattern substantially.
Numerical Simulation of Heat Transfer and Fluid Flow of a Flat-Tube High Power Density Solid Oxide Fuel Cell
Authors: Lu Yixin , Schaefer Laura , Li Peiwen
Published: November 2008

In this paper, a three dimensional numerical model to simulate the steady state heat transfer and fluid flow of a flat-tube high power density solid oxide fuel cell (HPD-SOFC) is developed. A computer code is programmed using the FORTRAN language to solve the governing equations for continuity, momentum and energy conservation. The highly coupled temperature and flow fields of the air stream and the fuel stream inside and outside a typical channel of a flat-tube HPD-SOFC are investigated. The variation of the temperature and flow fields with the current output is studied. The simulation also predicts pressure drop behavior of the both the air and fuel streams. This heat transfer and fluid flow modeling of the computer code will be used to simulate the overall performance of a flat-tube HPD-SOFC in the near future, and to help optimize the design and operation of a SOFC stack in practical applications.
Direct Carbon Fuel Cell With Molten Hydroxide Electrolyte
Authors: Zecevic Strahinja , Patton Edward M. , Parhami Parviz
Published: November 2008

Historically, despite its compelling cost and performance advantages, the use of molten hydroxide electrolytes has been ignored by DCFC researches, primarily due to the potential for formation of carbonate salt in the cell. This paper describes the electrochemistry of a patented medium-temperature DCFC based on molten hydroxide electrolyte, which overcomes the historical carbonate formation. An important technique discovered for significantly reducing carbonate formation is to ensure high water content of the electrolyte. Water helps hydrolysis of the carbonates and reduces formation of peroxide and superoxide ions that may react with carbon dioxide producing carbonate ions. High water content can be achieved by maintaining a humid atmosphere above the melt. To date, four successive generations of medium temperature DCFC prototypes have been built and tested at SARA Inc. to demonstrate the technology, all using graphite rods as their fuel source. The cells all used a simple design in which the cell containers served as the air cathodes and successfully demonstrated delivering more than 40 A at 0.3 V with the current density exceeding 200 mA/cm2. The basic feature of this simple cell design is that the cathode is not traditional gas fed electrode type. It is a non-porous electrode structure made of an inexpensive Fe-Ti alloy and gaseous oxygen is introduced into the cell by bubbling humid air through the electrolyte. Results obtained indicated that the cell operation was under a mixed activation-Ohmic-mass transfer control. The activation control is mainly due to slow anode oxidation of carbon, the Ohmic control is mainly due to a large electrode spacing whereas the mass transfer control is most likely because of slow diffusion of oxygen species (O2, O22−, O2−, and H2O) to the cathode surface. Cell performances are improved in the new generation cell design, which has been recently built, and which enables faster mass transfer of the reaction species and a lower voltage drop across the electrolyte. In the new design, the cathode is a separate perforated component of the cell that allows the use of a larger surface area electrode and for the electrode spacing to be varied.