bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Teeth Damage on Rotordynamic Instability of Impeller Eye Seals in Centrifugal Compressors
Authors: Xi Jinxiang , Rhode David L.
Published: October 2008

This paper investigates the influence of labyrinth seal teeth damage due to rotor impacting on the performance and the rotordynamic characteristics of impeller eye seals in centrifugal compressors. A well-established CFD-perturbation model was employed to predict the rotordynamic coefficients. The inclusion of at least an approximate shroud leakage path chamber is prefered for accurate prediction of seal-inlet swirl velocity and flow-induced rotordynamic forces. Impeller eye seals with teeth damage: (a) suffer significant leakage increases due to the increased seal clearance and (b) produce higher seal-inlet swirl velocity as well as larger rotordynamic forces, which tend to cause the system to become unstable. It was also found that distorted teeth tip geometries have an insignificant influence on both leakage and rotordynamic coefficients. The leakage path influence on seal-inlet swirl velocity W0 was also explored to thoroughly understand the rotordynamic characteristics of the eye seal subject to various degrees of teeth damage.
Interfacial Dynamics of Two Layer Couette Flow: Gravity Enhanced Kelvin-Helmholtz Instability
Authors: Colistra Joshua H. , Panchagnula Mahesh V. , O¨ztekin Alparslan , Neti Sudhakar , Chen John
Published: October 2008

The Couette flow of two immiscible liquids is examined using flow visualization techniques. The flow dynamics are studied as a function of several independent parameters including gravity. The two fluids are initially separated by a sheet of aluminum sufficient in length to ensure that fully developed flow conditions are achieved for both fluids before they come in contact with each other. The experiments are performed for various flowrates of Canola Oil and Polyethylene Glycol (PEG) corresponding Reynolds numbers for Oil and PEG of 0 to 20 and 0.01 to 0.2 respectively. Photographic images of the flow field are recorded and analyzed with the aid of image analysis software to illustrate interfacial dynamics of the flow. A qualitative and quantitative analysis of the flow instability is performed for various inclinations of the test apparatus, including the extreme cases of upward vertical and downward vertical with the horizontal being the baseline test case. Neutral stability curves are identified for the range of variables studied in the experiments. The long wave instability is observed to be very periodic. At the onset of instability, the flow structure is three-dimensional and exhibits wave growth in the flow direction. The wave growth ultimately results in droplet pinch off from the crest of a folded wave. At a constant relative velocity, the wave length is at a minimum when the flow is oriented in the upward vertical direction, opposing gravity. For a given PEG flowrate, the critical Oil flowrate for the onset of interfacial instability decreases as the angle increases. These results indicate gravity enhanced Kelvin-Helmholtz interfacial instability even for low Reynolds numbers. Through a course of systematic variation of flow angles we have been able to separate the effects of inertia, gravity (buoyancy) and viscous shear forces on the wavelength of instability.
Deformation and Surface Waves Properties of Round Nonturbulent Liquid Jets in Gaseous Crossflow
Authors: Ng C.-L. , Sallam K. A. , Metwally H. M. , Aalburg C.
Published: October 2008

A computational study of the deformation and surface wave properties of nonturbulent round liquid jets in gaseous crossflow is described. The objective of the study was to consider effects of liquid viscosity, liquid/gas density ratio, and crossflow Weber number that are representative of practical sprays. Three-dimensional computations of the deformation of round liquid jets in gaseous crossflow were carried out using FLUENT software utilizing its Volume of Fluid (VOF) module. The computations were evaluated satisfactorily based on earlier measurements of the properties of nonturbulent round liquid jets in crossflow (liquid jet deformation and surface waves) and revealed three-dimensional properties of the surface waves that could not be observed by previous measurements that were taken from the side of the jet.
Ambient Temperature Effects Upon the Flow in Gas Pipelines at Low Speeds
Authors: Morrison Gerald L. , Brar Pardeep
Published: October 2008

The flow field inside gas pipeline meter run is numerically simulated to determine the affects of upstream piping and temperature differences between the meter run pipe and the gas upon the flow. At bulk averaged velocities below 0.6 m/s (2 ft/s) significant changes in the velocity field are present which may alter the response of any flow meter mounted in the meter run. Examples for a bulk average velocity of 0.15 m/s (1/2 ft/s) and temperature differences with magnitudes of 27.7°C (50°F) are presented.
A Dynamic Wall Shear Stress Sensor Based on Ionic Polymers
Authors: Etebari A. , Bennett M. D. , Leo D. J. , Vlachos P. P.
Published: October 2008

This paper presents the first implementation of a novel class of dynamic time-resolved direct skin friction measurements sensor based on active ionic polymer transducers. These ionic polymer sensors have the advantage that they contain no moving parts, perform a direct measurement of shear, and can be mounted directly to the surface of an existing vessel with no modification. During the present effort we characterize the accuracy of the sensors and validate their dynamic measurement response. Using an oscillating Stokes layer calibration procedure we demonstrate measurement accuracy in fluctuating shear on the order of 4.92% over a range of stresses of +/- 3 Pa and signal-to-noise-ratio on the order of 60 dB. The frequency response of the sensor is over 10 kHz however due to experimental limitations we were not able to calibrate for frequencies higher than 140 Hz. These sensors have been shown to be insensitive to vibration or pressure. Also, an automatic change of impedance compensation approach is proposed that allows in-situ recalibration of the sensors and accounts for environmental effects such as changes of temperature on the sensors performance. The results demonstrate the potential for using ionic polymer sensors to perform accurate, high frequency measurements of shear in turbulent boundary layers.
A New Unsteady Model for Dense Cloud Cavitation
Authors: Hosangadi A. , Ahuja V.
Published: October 2008

A new unsteady, cavitation model for dense cloud cavitation is presented wherein the phase change process (bubble growth/collapse) is coupled to the acoustic propagation in a multi-phase fluid. This cavitation model predicts the number density and radius of bubbles in vapor clouds by tracking both the aggregate surface area and volume fraction of the cloud. Hence, formulations for the dynamics of individual bubbles (e.g. Rayleigh-Plesset equation) may be integrated within the macroscopic context of a dense vapor cloud i.e. a cloud that occupies a significant fraction of available volume and contains numerous bubbles. This formulation has been implemented within the CRUNCH CFD, which has a compressible “real” fluid formulation, a multi-element, unstructured grid framework, and has been validated extensively for liquid rocket turbopump inducers. Rigorous validation of the formulation is presented for various cases including unsteady simulations of a cavitating NACA0015 airfoil where the frequency of pressure fluctuations and time-averaged mean cavity lengths were compared with experimental data. The model also provides the spatial and temporal history of the bubble size distribution in the vapor clouds that are shed, an important physical parameter that is difficult to measure experimentally and is a significant advancement in the modeling of dense cloud cavitation.
Innovative Flow Field Combination Design on Direct Methanol Fuel Cell Performance
Authors: Jung Guo-Bin , Su Ay , Tu Cheng-Hsin , Weng Fang-Bor , Chan Shih-Hung
Published: October 2008

The flow-field design of Direct Methanol Fuel Cells (DMFCs) is an important subject about the DMFCs performance. Flow-fields play an important role on ability to transport fuel and drive out the products (H2O, CO2). In general, most of fuel cells utilize the same structure of flow-field for both anode and cathode. The popular flow-fields used for DMFCs are parallel and grid designs. Nevertheless, the characteristics of reactants and products are entirely different in anode and cathode of DMFCs. Therefore; the influences of the flow-fields designs on the cell performance were investigated due to the same logic for catalyst used for cathode and anode differently. To get the better and more stable performance of DMFC, three flow-fields (Parallel, Grid and Serpentine) are utilized with different combination were studied in this research. As a consequence, by using parallel flow-field in anode side and serpentine flow -field in cathode, the most and highest power output was obtained.
Modeling of Various Transport Processes in a Natural Gas Reformer
Authors: Yuan Jinliang , Ren Fuan , Sunde´n Bengt , Sun Peiting
Published: October 2008

In this study, a three-dimensional calculation method has been developed to simulate and analyze steam reforming of natural gas, and the effects on various transport processes in a duct from a compact steam reformer. The reformer conditions such as the combined thermal boundary conditions on solid walls, mass balances associated with the reforming reactions and gas permeation to/from the porous catalyst layer are applied in the analysis. Momentum and heat transport together with fuel gas species equations have been solved by coupled source terms and variable thermo-physical properties (such as density, viscosity, specific heat, etc.) of the fuel gas mixture. The predicted results are presented and discussed for a composite duct consisting of a porous catalyst reaction area, the flow duct and solid layers. Parametric studies are conducted and the results show that the variables, such as fuel reformer temperatures and duct configuration, have significant effects on the transport processes and reformer performance.
Effects of Various Reactions on the Gas Flow and Heat Transfer in an Anode Duct of ITSOFCs
Authors: Yuan Jinliang , Sunde´n Bengt
Published: October 2008

Recently, there has been considerable interest in the internal reforming reactions of solid oxide fuel cells (SOFCs) using methane or natural gas via. The internal reforming and electrochemical reactions appear in the porous anode layer, and may lead to inhomogeneous temperature and gas species distributions according to the reaction kinetics. A three-dimensional calculation method has been further developed to simulate and analyze the internal reforming and the electrochemical reactions, and the effects on various transport processes in a thick anode duct. In this study, the composite duct consists of a porous anode, fuel flow duct and solid current connector. Momentum, heat transport and gas species equations have been solved by coupled source terms and variable physical properties (density, viscosity, specific heat, etc.) of the fuel gas mixture. The combined thermal boundary conditions on solid walls, mass balances (generation and consumption) associated with the various reactions and gas permeation to/from the porous electrode are applied in the analysis. Simulation results show that the internal reforming and the electrochemical reactions, and operating conditions are significant for fuel gas transport and heat transfer in the anode.
The Influence of a Novel Bio-Cell Flow Field Pattern on the Performance and Operating Conditions for a PEM Fuel Cell
Authors: Weng Fang-Bor , Su Ay , Lo Kai-Fan , Tu Cheng-Hsin
Published: October 2008

A novel bio-cell flow field pattern is experimentally investigated by determining fuel cell performance and optimal operating conditions. The cell performance is analyzed by the polarization curve and the long-term stability. The bio-cell flow channel structure has a main feed track, a secondary branch track, and repeats to promote water removal from gas diffusion layer. The performance of the bio-cell flow field pattern is optimal performance when the cell is operated with low humidity gases and low cell temperature. In addition, the bio-cell flow field exhibits stable performance for non-humidified air. The fuel cell with the novel bio-cell flow field has advantages for low relative humidity operations. The results of the bio-cell flow field could potentially simplify fuel cell system design without humidifiers.
Transient Analysis of PEM Fuel Cell for Hybrid Vehicle Application
Authors: Arsie Ivan , Di Domenico Alfonso , Pianese Cesare , Sorrentino Marco
Published: October 2008

The paper focuses on the simulation of a hybrid vehicle with proton exchange membrane fuel cell as the main energy conversion system. A modeling structure has been developed to perform accurate analysis for powertrain and control system design. The models simulate the dynamics of the main powertrain elements and fuel cell system to give a sufficient description of the complex interaction between each component under real operating conditions. A control system based on a multi-level scheme has also been introduced and the complexity of control issues for hybrid powertrains have been discussed. Such a study has been performed to analyze the energy flows among the powertrain components. The results highlight that optimizing these systems is not a trivial task and the use of precise models can improve the powertrain development process. Furthermore, the behavior of system state variables and the influence of control actions on fuel cell operation have also been analyzed. Particularly, the effects of the introduction of a rate limiter on the stack power have been investigated, evidencing that a 2 kW/s rate limiter increased the system efficiency by 10% while reducing the dynamic performances of the powertrain in terms of speed error (i.e. 25 %).
PEM Fuel Cell Research Direction for Automotive Application
Authors: Fagley John , Conley Jason , Masten David
Published: October 2008

In recent years, there has been an increasing amount of PEM (proton exchange membrane) fuel cell-related research conducted and subsequently published by universities and public institutions. While a good deal of this research has been useful for understanding the underlying fundamental aspects of fuel cell components and operation, much of it is not as useful for a group working on automotive applications as it could be. The reason for this is that in order to be put to practical use in an automotive application, the system being studied must meet certain constraints; satisfying targets for projected system costs, system efficiency, volumetric and gravimetric power densities (packaging), and operating conditions. For example, numerous recent publications show studies with PEM fuel cells designed and built such that limiting current density is achieved at 0.9 A/cm2 or lower, and voltages of 600 mV can only be achieved at current densities less than 0.6 A/cm2. This type of performance is sufficiently below what is required for commercial application, that any conclusions drawn from these works are difficult to extrapolate to a system of commercial automotive interest. The purpose of this article is to show, through use of engineering calculations and cost projections, what operating conditions and performance are required in a commercial automotive fuel cell application. In addition, best known (public domain) performance and corresponding conditions are given, along with Department of Energy Freedom Car targets, which can be used for state-of-the-art benchmarking. Also, reference is made to a university publication where performance (500 mV at 1.5 A/cm2) close to automotive application targets was achieved, and important aspects of their components and flow field geometry are highlighted. It is our hope that through this publication, further PEM fuel-cell related research can be directed toward the region of greatest interest for commercial, automotive application.
A Matlab-Simulink Analysis of Hybrid SOFC Dynamic Behavior
Authors: Desideri Umberto , Lazaroiu Gheorghe , Zaninelli Dario , Lazaroiu Cristian
Published: October 2008

Fuel cells are more and more used in hybrid systems with micro-turbines for electricity and heat delivery. Two types of fuel cells are mostly used for cogeneration: solid-oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC). Solid oxide fuel cells (SOFC) are operating at high temperatures that make them well suited for cogeneration process, but influence the entire system due to their dynamic character. The hybrid system requires a pre-reforming process in order to convert the fuels, such as natural gas, in a gas with a high hydrogen content for the electrochemically oxidation with air. In this paper the dynamic model of the SOFC and of the hybrid system is developed in Matlab-Simulink environment. The comprehensive model is based on the electrochemical and thermal equations and on the temperature dynamics. The response of the SOFC and of the hybrid system to the variation of fuel flow and other parameters is investigated.
Water Balance Experiments in PEM Fuel Cell for Measurements of Water Transport Properties Through Membrane
Authors: Yan Qiangu , Liu Qingyun , Wu Junxiao
Published: October 2008

Water balance in a polymer electrolyte membrane fuel cell (PEMFC) was investigated by measurements of the net drag coefficient under various conditions. The effects of water balance in the PEM fuel cell on the cell performance were also investigated at different operating conditions. Experimental results reveal that the net drag coefficient of water through the membrane depends on current density and humidification of feed gases. It is found that the net drag coefficient (net number of water molecules transported per proton) has values between 0.93 and −0.015 depending on operating condition, current loading and level of humidification. It was also found that the humidity of both anode and cathode inlet gases had significant effect on fuel cell performance. The results will be used to define conditions of optimal hydration of the membrane. Based on the performance and resistance measurements, optimal humidification can be achieved. The resistance of working fuel cell shows that the membrane resistance increases with the feed gas relative humidity (RH) decreased. Data obtained will be used to evaluate the transport parameters such as net flux of water through the membrane and the effective drag under various operating conditions, and further provide validation data for the fuel cell modeling and simulation efforts.
Modeling PEMFC With FLUENT: Numerical Performance and Validations With Experimental Data
Authors: Li Shaoping , Cao Jing , Wangard William , Becker Ulrich
Published: October 2008

A 3-dimensional, two-phase Computational Fluid Dynamics (CFD) model for PEMFC simulations has been developed and implemented in FLUENT, a general-purpose commercial software package with multi-physics capabilities. The model formulation was given in details in the previous ASME fuel cell conference, together with in-situ distributions of current densities and species concentrations computed for a simple geometry. In this paper, numerical performance of this model in terms of computing time and parallel efficiency are assessed through the computation of a relatively larger-size fuel cell (50 cm2) with serpentine channels. The convergence history and parallel performance data show that the Fluent’s PEMFC model is numerically robust and efficient. In addition to the numerical performance, the physical validity of the model is tested through comparisons with experimental data of polarization curves and local current density distributions from the most recent work of Mench et al [1]. Comparisons with the data show good agreement in the overall polarization curves and reasonably good agreement in local current density distributions too. The comma-shaped local polarization curves seen in the experiments are qualitatively correctly captured. Moreover, our computations show that hydrogen mass fraction and molar concentration can both increase along the anode flow channel, despite that hydrogen is being depleted in the anodic electrochemical reaction. The reason for this to happen is that the osmotic drag moves the water from anode to cathode at a much faster rate than the hydrogen depletion rate. An analytical derivation that reveals the relationship between species molar concentration and mass fraction is also given.
A Comparison Between the LCA of a PEMFC and an MCFC System for the Production of Electric Energy, and Traditional Energy Conversion Systems
Authors: Desideri Umberto , Zepparelli Francesco
Published: October 2008

This paper investigates the environmental impact of electric energy generation by using a PEMFC and an MCFC system. Fuel cells are considered to be ultra-clean energy conversion devices, since pollutants emissions during operation have a very low concentration, compared to those of traditional energy systems. In order to understand the real environmental impact of fuel cells, this is not enough and it is necessary to study their “cradle-to-grave” life, starting from the construction phase, to the operational life and eventually to its disposal. In fact, it is not really correct to say that fuel cells are almost zero-emission systems, because they produce not-negligible emissions during manufacturing and to produce hydrogen. The method used in this paper is the Life Cycle Assessment (LCA), which has been calculated with the software SimaPro 5.0. The functional unit chosen in this study is the production of 1 kWh of electric energy by a PEMFC and an MCFC. Thanks to this approach, the critical process related to the production of energy by the previous fuel cell systems, (i.e. the production of hydrogen by natural gas steam reforming), has been determined. After a separated LCA of the PEMFC and the MCFC, a comparison was made between the two systems, considering the environmental impact of electric energy generation. Finally, the production of electric energy by a PEMFC and an MCFC systems is compared to that by conventional energy conversion systems.
Ignition Bifurcations in a Stirred Tank PEM Fuel Cell
Authors: Promislow Keith , Nazarov Igor
Published: October 2008

A mathematical model of ignition dynamics and bistable operation of a Stirred Tank Reactor (STR) Polymer Electrolyte Membrane (PEM) fuel cell is developed. In dry inlet gas operation the positive feedback between current, water production, and membrane resistance within the model leads to two stable “ignited” states. Within the framework of the model these correspond to a uniform current distribution or a partially extinguished cell with localized current production. The switching between the two regimes is accompanied by hysteresis and transient behavior on the order of 2–4 hours in a small 1 cm2 cell. We compare the numerical results to experimental data gathered by Jay Benziger and show that the steady-state distribution of membrane water is very sensitive to several parameters and is an excellent diagnostic for determining material parameters.
Structural Properties and Ionic Conductivity of Ce0.8Gd0.2O2 and Ce0.8Sm0.2O2 Thin Films Grown by Pulsed Laser Deposition
Authors: Sze Boscope M. K. , Wong C. N. , Wong K. H.
Published: October 2008

Thin films of Ce0.8Gd0.2O2 and Ce0.8Sm0.2O2 oxide electrolytes have been fabricated by pulsed laser deposition on (100)LaAlO3 substrates at temperature from 300 °C to 700 °C and under 100 mTorr oxygen ambient pressure. The crystal structure, crystallinity and lattice parameters of the as-deposited films are investigated by X-ray diffraction. High quality epitaxial and polycrystalline films are obtained at different growth conditions. We have made impedance measurements on these films in the temperature range from 300 °C to 850 °C. Our results reveal a mark increase in the ionic conductivity of these films in comparison with those of the corresponding bulk materials. The observed enhancements are closely related to the crystallinity of the films. Conductivities of 0.1 S/cm or higher for Ce0.8Gd0.2O2 and Ce0.8Sm0.2O2 are obtained at 500 °C. We have demonstrated that in utilizing these thin films solid oxide fuel cells operating at below 500 °C are possible.
Universal Mass-Transport Limited Fuel Cell Current Curves
Authors: Wetton Brian , Kim Gwang-Soo , Promislow Keith , St-Pierre Jean , Stockie John
Published: October 2008

A unit fuel cell with straight gas channels operating in low voltage conditions is considered. Anode electrochemical losses are neglected and a particular, simple form for mass transport limitations to the cathode oxygen reduction electrochemistry is taken and validated with experimental results. With experimentally fitted parameters, it is found that the mass transport limitations dominate the behaviour of the local current density at low voltages. Combined with a simple channel model for oxygen flux, this leads to a description of local current densities independent of cell voltage, assuming only that it is low. The work is motivated by Polymer Electrolyte Membrane fuel cells but the ideas are general enough to be applied to other fuel cell types.
Off-Design Performance Analysis of a Solid-Oxide Fuel Cell/Gas Turbine Hybrid for Auxiliary Aerospace Power
Authors: Freeh Joshua E. , Steffen Christopher J. , Larosiliere Louis M.
Published: October 2008

A solid-oxide fuel cell/gas turbine hybrid system for auxiliary aerospace power is analyzed using 0-D and 1-D system-level models. The system is designed to produce 440kW of net electrical power, sized for a typical long-range 300-passenger civil airplane, at both sea level and cruise flight level (12,500m). In addition, a part power level of 250kW is analyzed at the cruise condition, a requirement of the operating power profile. The challenge of creating a balanced system for the three distinct conditions is presented, along with the compromises necessary for each case. A parametric analysis is described for the cruise part power operating point, in which the system efficiency is maximized by varying the air flow rate. The system is compared to an earlier version that was designed solely for cruise operation. The results show that it is necessary to size the turbomachinery, fuel cell, and heat exchangers at sea level full power rather than cruise full power. The resulting estimated mass of the system is 1912 kg, which is significantly higher than the original cruise design point mass, 1396 kg. The net thermal efficiencies with respect to the fuel LHV are calculated to be 42.4% at sea level full power, 72.6% at cruise full power, and 72.8% at cruise part power. The cruise conditions take advantage of pre-compressed air from the on-board Environmental Control System, which accounts for a portion of the unusually high thermal efficiency at those conditions. These results show that it is necessary to include several operating points in the overall assessment of an aircraft power system due to the variations throughout the operating profile.
Experimental Analysis of an Autothermal Gasoline Reformer for Automotive Purposes
Authors: Caners Chris G. , Peppley Brant A. , Harrison Steven J. , Oosthuizen Patrick H. , McIntyre Craig S.
Published: October 2008

One of the main uses of fossil fuels is in the transportation sector, leading to environmental consequences such as climate change and smog. In order to move towards a more sustainable energy infrastructure, a transition must begin between fossil fuels and renewable fuels, such as biogas and hydrogen. One possibility to drive this transition is through the application of reforming technology to the automotive sector. The objectives of this project were to experimentally validate a computational fluid dynamics model, while at the same time analyze the data and model in order to improve the design of the bench-scale reformer for use in automotive applications. The model was validated through the experimental data generated through the use of a series of thermocouples and gas chromatography. The highest lower heating value efficiency and dry molar percentage output of hydrogen were 58% and 43% respectively, with conversion percentages approaching 100%.
Overview of Field Performance by UTC Fuel Cells’ Transportation Fuel-Cell Power Plants
Authors: Narasimhamurthy Praveen , Kabir Zakiul
Published: October 2008

UTC Fuel Cells (UTCFC) over the last few years has partnered with leading automotive and bus companies and developed Polymer Electrolyte Membrane (PEM) fuel-cell power plants for various transportation applications, for instance, automotive, buses, and auxiliary power units (APUs). These units are deployed in various parts of the globe and have been gaining field experience under both real world and laboratory environments. The longest running UTC PEM fuel cell stack in a public transport bus has accumulated over 1350 operating hours and 400 start-stop cycles. The longest running APU fuel cell stack has accrued over 3000 operating hours with more than 3200 start-stop cycles. UTCFC PEM fuel-cell systems are low noise and demonstrate excellent steady state, cyclic, and transient capabilities. These near ambient pressure, PEMFC systems operate at high electrical efficiencies at both low and rated power conditions.
Steady State and Transient Modeling of a PEM Fuel Cell Power Plant for Transportation Applications
Authors: Seshadri Parthasarathy , Kabir Zakiul
Published: October 2008

UTC Fuel Cells recently developed a freeze capable fuel cell power plant for automotive applications. Steady state and dynamic system models were developed for design and performance characterization. The results of the power plant performance testing indicate very good agreement with these models. Testing showed that the power plant achieves stable performance at all power levels including low power holds. UTC Fuel Cells’ cell stack technology enables operation of the power plant at near-ambient pressure. Additionally the low system pressure drop allows the power plant to achieve very high electrical efficiencies at all power levels. The peak efficiency is about 58% at approximately 20% of rated power. Since the power plant does not require compressors, the auxiliary power requirement and acoustic noise level for the system are also low. The cell stack’s capability of internal water management and ability to operate at low reactant stoichiometric ratio result in a very stable and predictable transient capability to ramp up from low power to rated power in < 2 sec and also to step down from rated power to low power levels instantaneously in a stable manner.
Experimental Studies of Liquid Water Droplet Growth and Instability at the Gas Diffusion Layer/Gas Flow Channel Interface
Authors: Hickner Michael A. , Chen Ken S.
Published: October 2008

Experimental investigations were carried out to visualize the dynamic behavior (contact angle hysteresis and droplet shape) of liquid water droplets on carbon paper gas diffusion layers that are typically employed in proton exchange membrane fuel cells (PEMFCs). The experimental technique mimicks the generation of liquid water and formation of droplets in an air shear flow at the gas diffusion layer – gas flow channel interface of a simulated PEMFC cathode. Images obtained of growing liquid water droplets yield information on the contact angle hysteresis and droplet height, which were subsequently used to map droplet “instability” diagrams. These instability diagrams provide quantitative guidance on liquid water droplet removal at the gas diffusion layer/gas flow channel interface under the conditions of interest. The experimentally mapped droplet diagrams are compared with those predicted using a simplified model based on a macroscopic force balance and reasonably good agreement is obtained.
Tear Resistance of Proton Exchange Membranes
Authors: Dillard David A. , Lai Yeh-Hung , Budinski Michael , Gittleman Craig
Published: October 2008

Through the thickness flaws or “pinholes” in proton exchange membranes (PEM) allow gas crossover that can lead to fuel cell failure. The formation of these flaws is not fully understood, but one possible mechanism is that small flaws could grow through crack propagation in the fracture mechanics sense. Although relatively brittle features are sometimes observed in failures resulting under simulated fuel cell conditions, the stress strain plots of the membranes themselves exhibit considerable ductility. In an effort to use fracture mechanics principles to characterize PEMs, fracture parameters associated with the essential work of fracture from double edge notch tensile (DENT) specimens; the tear energy obtained from the trouser tear test; and cutting energies associate with knife slitting were measured and compared. Presumably through reducing crack tip blunting, the knife slitting test is able to measure fracture energies as low as 200J/m2, two orders of magnitude smaller than measured in the other tests. The results are sensitive to rate, temperature, and moisture level. Although the implications of these properties to fuel cell durability are not yet understood, they may have applicability in the more brittle features that are sometimes observed.
Thermo-Kinetic Representation and Transient Simulation of a Molten Carbonate Fuel Cell
Authors: Carroll Brian C. , Kiehne Thomas M. , Lukas Michael D.
Published: October 2008

There are a growing number of models in the literature dealing with the transient behavior of fuel cells. However, few, if any, employ fundamental kinetic theory to model the fuel reformation process while simultaneously simulating fuel cell behavior from a transient, system-level perspective. Thus a comprehensive, transient fuel cell model has been developed that includes all the relevant thermodynamics, chemistry, and electrical characteristics of actual fuel cell operation. The model tracks the transient temperature response of a fuel cell stack, chemical specie concentrations of exhaust gases, efficiency of the fuel reformation equipment, and electrical output characteristics. Model results provide a concise, parametric evaluation of the influence of operating conditions and user-controlled parameters on fuel cell performance. The model is validated against transient Molten Carbonate fuel cell (MCFC) data from a subscale stack.
PEMFC Operation With Methanol Reforming Process
Authors: Park Jong-Man , Kim Hyun-Jong , Shul Yong-Gun , Han Haksoo , Kim Hasuck , Kim Dong Hyun , Yoo Seung-Eul
Published: October 2008

Most of studies for on-board hydrogen production for fuel cells are based on two types of carbon compounds. One is oxygen-containing compounds, methanol, ethanol and etc. The others are hydrocarbons such as ethers (dimethylether, etc), natural gas, propane gas, gasoline, jet fuel and diesel fuel. Automotive Polymer Electrolyte Membrane Fuel Cell (PEMFC) requires hydrogen gas to operate. The most convenient way to obtain the gas would be to use an on-board fuel processor to convert or reform commonly available liquid fuels, such as gasoline, methanol, and ethanol, into hydrogen. In this study, Methanol is used as hydrogen source which is also convenient for production, transportation and storage. PEMFC with methanol fuel process system, which is mainly composed of two parts, methanol reforming reaction and preferential oxidation (PROX), has been evaluated to study the enhancing stability of the system.
Modeling of a Fuel Cell Hybrid Electric Vehicle
Authors: Van Tiem Ryan A. , Hoff Craig J. , Berry K. Joel
Published: October 2008

This paper explains the method by which a small “Neighborhood” fuel cell hybrid electric vehicle (FCHEV) was modeled using Matlab/Simulink and validated using National Instruments Data Acquisition equipment and a chassis dynamometer. The vehicle is a modified four-passenger Global Electric Motorcar (GEM), which was designed for city or neighborhood operation where maximum speed limits do not exceed 35 mph. The stock 72-Volt shunt wound GE motor is powered by six 12-Volt Trojan lead-acid batteries. The vehicle was converted to a FCHEV by integrating a Ballard Nexa™ fuel cell module and a Spectrodyne Systems DC-DC converter. These devices act as a constant current range extending supplemental power source. The goals of the creation of the model include predicting motor and battery performance while driving under transient conditions and predicting the range of the vehicle while using one or several fuel cell stacks.
Study of the Flow Field in Channels and Internal Manifolds on the Interconnect of a Planar Solid Oxide Fuel Cell
Authors: Chung T.-D. , Chyou Y.-P. , Yu D.-D.
Published: October 2008

This research is focused on the flow characteristics in channels and internal manifolds embedded on the interconnect of a planar solid oxide fuel cell (SOFC), by implementing both numerical simulations and optical experiments. The selected commercial computational fluid dynamics (CFD) code solves the three-dimensional fluid problems with a finite volume method, while laser Doppler velocimetry (LDV) is utilized to measure the two-dimensional flow field in the channels on a hydraulic half-model. It is shown that the trend of measured velocity distribution is quite consistent with the counterpart predicted by the concurrent simulation, which serves as the mutual benchmarking process for the both approaches. Furthermore, parametric studies are carried out numerically, to reveal the insights of flow characteristics in the flow passages. The results show that pressure balance mechanisms among various channels and in-/outlet plenums turn out to be a critical measure, as far as the velocity distribution is concerned.
Fuel Cell Hybrids, Their Thermodynamics and Sustainable Development
Authors: Winkler Wolfgang
Published: October 2008

The increasing demand on primary energy and the increasing concern on climatic change demand immediately a sustainable development, but still there remain open questions regarding its technical realisation. The second law of thermodynamics is a very simple but efficient way to define the principle design rules of sustainable technologies in minimising the irreversible entropy production. The ideal, but real process chain is defined by a still reversible structure or logic of the process chain—the reversible reference process chain—but consisting of real components with an irreversible entropy production on a certain level. It can easily be shown for energy conversion and for transportation that hybridisation in general can be indeed a measure to meet the reversible process chain and to minimise the entropy flow to the environment. Fuel cells are principal reversible converters of chemical energy and thus a key element within hybridisation. Depending on application, CHP may be a hybridisation step or only a slight improvement. There is a fundamental difference in heating a house or in supplying an endothermic chemical reaction with reaction entropy. The use of heat recovery and isolation is a necessary measure to minimise the entropy flow to the environment and can be described by a reversible reference process as well. The application of reversible reference process chains shows that hybrid systems with fuel cells are a technical feasibility to approach very closely the thermodynamic potential. This development differs from the past where the technical possibilities of materials and manufacturing limited the technology to meet reversibility and thus sustainability.
The Optimal Shape for a Unit PEM Fuel Cell
Authors: Vargas J. V. C. , Ordonez J. C. , Bejan A. , Amico S. C.
Published: October 2008

This paper develops a mathematical model and a structured procedure to optimize the internal structure (relative sizes, spacings) and external shape (aspect ratios) of a unit PEM fuel cell so that net power is maximized. The optimization of flow geometry is conducted for the smallest (elemental) level of a fuel cell stack, i.e., the unit PEM fuel cell, which is modeled as a unidirectional flow system. The polarization curve, total and net power, and efficiency are obtained as functions of temperature, pressure, geometry and operating parameters. The optimization is subjected to fixed total volume. There are two levels of optimization: (i) the internal structure, which basically accounts for the relative thicknesses of two reaction and diffusion layers and the membrane space, and (ii) the external shape, which accounts for the external aspect ratios of a square section plate that contains all unit PEM fuel cell components. The available volume is distributed optimally through the system so that the net power is maximized. Temperature and pressure gradients play important roles, especially as the fuel and oxidant flow paths increase. Numerical results show that the optimized internal structure is “robust” with respect to changes in external shape. The optimized internal structure and external shape are results of the optimal balance between electrical power output and pumping power required to supply fuel and oxidant to the fuel cell through the gas channels. Directions for future improvements at the PEM fuel cell stack level in constructal geometric optimization are discussed.
Modeling of a Methane Fuelled Direct Carbon Fuel Cell System
Authors: Woudstra N. , van der Stelt T. P. , Hemmes K.
Published: October 2008

Energy conversion today is subject to high thermodynamic losses. About 50 to 90 % of the exergy of primary fuels is lost during conversion into power or heat. The fast increasing world energy demand makes a further increase of conversion efficiencies inevitable. The substantial thermodynamic losses (exergy losses of 20 to 30 %) of thermal fuel conversion will limit future improvements of power plant efficiencies. Electrochemical conversion of fuel enables fuel conversion with minimum losses. Various fuel cell systems have been investigated at the Delft University of Technology during the past twenty years. It appeared that exergy analyses can be very helpful in understanding the extent and causes of thermodynamic losses in fuel cell systems. More than 50 % of the losses in high temperature fuel cell (MCFC and SOFC) systems can be caused by heat transfer. Therefore system optimisation must focus on reducing the need for heat transfer as well as improving the conditions for the unavoidable heat transfer. Various options for reducing the need for heat transfer are discussed in this paper. High temperature fuel cells, eventually integrated into gas turbine processes, can replace the combustion process in future power plants. High temperature fuel cells will be necessary to obtain conversion efficiencies up to 80 % in case of large scale electricity production in the future. The introduction of fuel cells is considered to be a first step in the integration of electrochemical conversion in future energy conversion systems.
Direct Carbon Fuel Cell With Hydroxide Electrolyte: Cell Performance During Initial Stage of a Long Term Operation
Authors: Zecevic S. , Patton E. M. , Parhami P.
Published: October 2008

This paper describes a Direct Carbon-Air Fuel Cell (DCFC) which uses a molten hydroxide electrolyte. In DCFCs, carbon is electrochemically directly oxidized to generate the power without a reforming process. Despite its compelling cost and performance advantages, the use of molten metal hydroxide electrolytes has been ignored by DCFC researches, primarily due to the potential lack of invariance of the molten hydroxide electrolyte caused by its reaction with carbon dioxide. This paper describes the electrochemistry of DCFC based on molten hydroxide electrolyte and discusses means to overcome the historical carbonate formation. Furthermore, it describes the cell performance during the initial stage of a long term operation and discusses the causes for the initial cell performance degradation. To date, five 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 basic feature of the cell is a simple design in which 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. The cell successfully demonstrated delivering more than 50 A at 0.3 V with the current density exceeding 100 mA/cm2. Main feature of DCFC with hydroxide electrolyte is that the cell performance decreases over time mainly due to oxygen cathode polarization. There are three possible causes for this performance decay: Carbonate formation, electrolyte evaporation due to air bubbling, and corrosion products build up. In order to determine the right cause for the performance decay a series of experiments was carried out investigating various parameters involving cell temperature, water content in the melt, current density, carbonate content in the melt, melt level in the cell, air flow rate and intermittent on-off operation. DCFC was operating at constant current while cell voltage and electrode potentials were recorded over time. Results obtained indicated that the performance of DCFC with hydroxide electrolyte during initial 200 h is governed by the oxygen cathode performance that is mainly affected by corrosion products. The corrosion products catalyze decomposition of peroxide ions which are reacting species at the cathode resulting in an increase of cathode polarization over time. Effect of carbonate ions on the initial cell performance decay is insignificant as compared to the effect of corrosion product. Means to overcome the corrosion products issue were discussed.
Numerical Simulation of Liquid Water Behavior in Separator-Channels in PEMFC Using LBM
Authors: Tabe Yutaka , Ochi Takamichi , Kikuta Kazushige , Chikahisa Takemi , Shinohara Hideki
Published: October 2008

In a polymer electrolyte membrane fuel cell, the condensed water in the separator-channel prevents the supply of reactants to electrodes, which deteriorates the cell performance. The Lattice Boltzmann simulation has been conducted to understand the behavior of condensed water in the separator-channels. The scheme for the two-phase flow with large density difference was applied and the boundary condition for wettability at the corner inside the channel was examined. The present simulation demonstrates the effects of the cross-sectional shape, the wettability of channel and the volume of condensed water on the liquid water behavior. In the hydrophilic separator-channels, the liquid water spreads along the channel wall to form film and, in a specific condition, the water draws away from the gas diffusion layer, which suppresses the flooding. On the other hand, the liquid water forms sphere, covering larger area of the surface of gas diffusion layer in the hydrophobic separator-channels, but the drain performance of liquid water is superior.
Hydrogen Production From Methane by Using Oxygen Permeable Ceramics
Authors: Takamura Hitoshi , Aizumi Yusuke , Kamegawa Atsunori , Okada Masuo
Published: October 2008

Oxygen permeable ceramics based on mixed conductors are attracting much attention for use in partial oxidation of hydrocarbons as a novel technique for syngas and pure hydrogen production. This paper describes the preparation and oxygen permeation properties including methane reforming property of a novel member of oxygen permeable ceramics. The materials used are solid solutions of (La0.5Ba0.3Sr0.2)(FexIn1−x)O3−δ. The single phase of perovskite-type (La0.5Ba0.3Sr0.2)(FexIn1−x)O3−δ is obtained in the range of x = 0.4 to 0.9. The highest oxygen flux densities of 2.2 and 11 μmol/cm 2 s are attained for (La0.5Ba0.3Sr0.2)(FexIn1−x)O3−δ (x = 0.6) at 1000 °C under He / air and CH4 / air gradients, respectively. The electrical conductivity of (La0.5Ba0.3Sr0.2)(Fe0.6 In0.4)O3−δ is dominated by p-type conduction having a slope of 1/4 under the high P(O2) region. The oxide-ion conductivity of the same sample is estimated to be 0.05 S/cm at 800°C. Even though the oxygen flux density slightly decreases with increasing time, high CO selectivity of 90 % is kept for 100 h. The oxygen flux density of the solid solution is also discussed in the context of surface exchange kinetics.