bluuebunny/crossref_metadata_2025 huggingface dataset:
High Pressure Reforming and Hydrogen Purification for Military Fuel Cell Use
Authors: Oster Benjamin G. , Hurley John P. , Patel Nikhil , Aulich Ted R. , Collings Michael E. , Timpe Ronald C. , Holcomb Franklin H.
Published: June 2009

A high pressure water reforming (HPWR) reactor system was designed and constructed to operate at pressures up to 82,737kPa and temperatures up to 913K. Initial HPWR hydrogen production tests have been conducted with two feedstocks: an aromatics- and sulfur-free natural gas-derived fuel (S-8) and methanol. Feedstock and water were continuously pumped into a reactor vessel where they were catalytically reformed to a hydrogen-rich reformate product. Reactor temperature, pressure, and space velocity were varied to determine their effect on reformate hydrogen concentration. Online, Raman spectroscopy gas composition measurements were verified by gas chromatography. Experiments conducted to date have resulted in a S-8-derived 33,784kPa reformate containing 58 mol% hydrogen at 898K, and a methanol-derived 33,784kPa reformate containing 76 mol% hydrogen at 638K. To enable high-pressure purification of HPWR-generated reformate to proton exchange membrane (PEM) fuel cell-quality hydrogen, EERC researchers have been working to tailor a low-pressure Oak Ridge National Laboratory-developed electrical swing adsorption (ESA) technology for high-pressure separation of hydrogen from other reformate constituents. Reformate purification experiments conducted to date have achieved quantitative separation of hydrogen from a bottled gas mixture of carbon monoxide, carbon dioxide, and methane at 1,379kPa. Near-term future experimental work will focus on 1) optimization of the HPWR process using sulfur- and aromatics-free jet fuel, and then JP-8 fuel, at pressures of up to 34,474kPa and 2) optimization of the ESA process for hydrogen purification at pressures of up to 34,474kPa. Results of these experiments will be presented.
Quasi-3-D Dynamic Model of an Internally Reforming Planar Solid Oxide Fuel Cell for Hydrogen Co-Production
Authors: Shaffer Brendan , Hunsuck Michael , Brouwer Jacob
Published: June 2009

A simplified quasi-3-dimensional model of a solid oxide fuel cell (SOFC) is developed to investigate the dynamics of internal reformation in an SOFC. The dynamic model solves dynamic equations that govern relevant physical and chemical processes in a simplified geometric representation of a planar SOFC. This makes the model complex enough to resolve major performance characteristics and simple enough to be used in dynamic analyses and controls development at the system level. The model solves dynamic mass, momentum and energy conservation equations to provide local temperature, species concentrations, and current density distributions. These distributions are resolved in two dimensions across the cell, but each 2-D distribution resolves 5 separate control volumes through the nodal unit cell: the PEN; anode and cathode gas compartments; and interconnects. Internal reforming chemical kinetic expressions are included in the model formulation. Simulations show that extent of internal reformation impacts the dynamic temperature difference across the cell. Steady state maximum temperature differential across the cell can be reduced to about 100 K with 100% internal reformation and a cross-flow configuration. A full hydrogen co-production system was then modeled by integrating the SOFC model with heat exchangers, combustor, blower, and hydrogen collector. For conditions of a constant cathode exhaust temperature of 1273 K and lower fuel utilization (60%–70%), the dominant thermal influence on the cell temperature was cooling by the endothermic reformation reactions. But at higher fuel utilization conditions, the dominant thermal influence was the convective cooling of the cathode gases. System simulations showed no tradeoff between power and H2 production if the cathode exhaust temperature is held constant at 1273 K. High power and high H2 production conditions were found to be synergistic: high hydrogen production leads to high electrochemical efficiency and lower air flow rate leading to fewer parasitic losses. Dynamic SOFC responses to manipulation of fuel flow rate within the range of fuel utilization between 60 and 85% indicate that the system can be adequately controlled to produce various amounts of hydrogen and electricity.
Design, Simulation, and Control of a 100 Megawatt-Class Solid Oxide Fuel Cell Gas Turbine Hybrid System
Authors: Mueller Fabian , Tarroja Brian , Maclay James , Jabbari Faryar , Brouwer Jacob , Samuelsen Scott
Published: June 2009

A 100 MW-class planar solid oxide fuel cell, synchronous gas turbine hybrid system has been designed, modeled and controlled. The system is built of 70 functional fuel cell modules each containing 10 fuel cell stacks, a blower to recirculate depleted cathode air, a depleted fuel oxidizer and a cathode inlet air recuperator with bypass. The recuperator bypass serves to control the cathode inlet air temperature while the variable speed cathode blower recirculates air to control the cathode air inlet temperature. This allows for excellent fuel cell thermal management without independent control of the gas turbine, which at this scale will most likely be a synchronous generator. In concept the demonstrated modular design makes it possible to vary the number of cells controlled by each fuel valve, power electronics module, and recirculation blower, so that actuators can adjust to variations in the hundreds of thousands of fuel cells contained within the 100 megawatt hybrid system for improved control and reliability. In addition, the modular design makes it possible to take individual fuel cell modules offline for maintenance while the overall system continues to operate. Parametric steady state design analyses conducted on the system reveal that the overall fuel-to-electricity conversion efficiency of the current system increases with increased cathode exhaust recirculation. To evaluate and demonstrate the conceptualized design, the fully integrated system was modeled dynamically in Matlab–Simulink®. Simple proportional feedback with steady state feed-forward controls for power tracking, thermal management, and stable gas turbine operation were developed for the system. Simulations of the fully controlled system indicate that the system has a high efficiency over a large range of operating conditions, decent transient load following capability, fuel and ambient temperature disturbance rejection as well as the capability to operate with a varying number of fuel cell modules. The efforts here build upon prior work and combine the efforts of system design, system operation, component performance characterization and control to demonstrate hybrid transient capability in large-scale coal synthesis gas-based applications through simulation. Furthermore, the use of a modular fuel cell system design, the use of blower recirculation, and the need for integrated system controls are verified.
The Impact of Rib/Channel, Water and Heat Transport on Limiting Current Density
Authors: Tabuchi Yuichiro , Kubo Norio
Published: June 2009

Proton exchange membrane fuel cells (PEMFCs) are regarded as a promising alternative clean power source for automotive applications. Key to the acceptance of PEMFCs for automobiles are cost reduction and power density for compactness. In order to meet these requirements, further improvement of cell performance is required. In particular, under higher current density operation, water and heat transport in PEMFC have strong effects on cell performance. In this study, the impact of Rib/Channel dimensions, heat and water transport on cell performance under high current density is investigated using the multiphase mixture model (M2 model), and the limiting current density is evaluated using a uniform test cell with 10cm2 active area and 24 straight channels. Limiting current densities were measured under different oxygen concentrations at 70°C and 70% relative humidity at both sides. In order to neglect the effect of liquid water in channels and the distribution of oxygen and hydrogen concentrations along the flow channel, large flow rates were introduced at both sides. Experimental results show a nonlinear relation between oxygen concentration in the channel and limiting current density. Numerically it is found that this nonlinear trend is caused by liquid water in the Rib region. In addition, it is also found that not only liquid water, but also heat transport and water transport through the membrane significantly affect the limiting current density. Finally, it is concluded that the combination analysis using limiting current experiments of uniform cell system and M2 model is very useful for fundamental understanding and for fuel cell design optimization.
A Study on the Bypass Flow in a Gas Diffusion Layer of a PEMFC With Serpentine Flow Channels
Authors: Kim Kyoungyoun , Sohn Young-Jun , Kim Minjin , Cho Choong-Won , Lee Won-Yong
Published: June 2009

The serpentine channel widely employed in fuel cells has a strong pressure gradient between adjacent channels in specific regions. Due to the pressure gradient, some amount of reactant gas can be delivered to a catalyst layer by convection as well as diffusion processes in a gas diffusion layer (GDL). The enhancement of the convective flow, so-called bypass flow affects performance of a PEMFC since the bypass flow can lead to significant concentration loss at the downstream of the gas channel. The purpose of the present paper is to examine the effect of the bypass flow in the GDL on performance of a PEMFC with serpentine channels. By performing three-dimensional numerical simulation, we have investigated the bypass flow in the GDL and its effects on the distribution of reactant concentration and local current density. Three-dimensional Navier-Stokes equations are solved with the source and sink terms associated with the electrochemical reactions in the fuel cell. Several values of GDL permeability have been tested to see the effects of GDL permeability on the bypass flow for different cathode stoichiometry rate. The present numerical results show that, with increasing GDL permeability, local current density distribution changes significantly and becomes more inhomogeneous.
Electrode Reaction and Cell Performances of IT-SOFC Using BaZrO3 Proton Conductors
Authors: Iguchi Fumitada , Sata Noriko , Hiroo Yugami
Published: June 2009

This paper evaluates cell performances and electrode reaction of an electrolyte supported type cell using 15mol% Y-doped BaZrO3 perovskite type proton conductors. BaZrO3 based proton conductors show high proton conductivity and chemical stability in IT-SOFC operating atmospheres. But, because of low sintering properties, there is little knowledge about electrolyte performance, e.g., open circuit voltage, electrode over potential. Through SOFC generation test, it is confirmed that cell performance of the cell is determined by electrode over potential. Electrode over potential is significantly higher than similar configuration cell using BaCeO3 based proton conductors. Those results suggests that study of differences in electrode reaction between BaCeO3 based proton conductors and BaZrO3 based proton conductors will be necessary.
The Dynamic Modeling of PEMFC System for Automotive Application
Authors: Kang Sanggyu , Min Kyoungdoug
Published: June 2009

Water and thermal management are crucial factors in determining the performance of PEMFC for automotive application. In order to investigate the effect of cell humidity and temperature on the performance of PEMFC, a dynamic model of a PEMFC system for automotive application has been developed by using Matlab/Simulink®. The model is composed of a PEM unit cell, membrane humidifier, and thermal management system (TMS). At first, fuel and air are well hydrated by the shell and tube humidifier, then humidified fuel and air flow into the PEMFC for electrochemical reaction. PEMFC temperature was maintained at a constant level by the thermal management system. The active area of PEM model is 240 cm2. The cell was discretized into several control volumes in the through-plane to resolve energy balance and species diffusion. The membrane humidifier model is also discretized into three control volumes in the through-plane to resolve the mass conservation and energy balance. Fuel and air are hydrated by the diffusion of the water through the membrane. The thermal management system consists of radiator, fan and pump. De-ionized water cools down the temperature of PEMFC. In order to validate the model, the model was compared with a corresponding experiment. Comparison shows that simulation results are in good agreement with experiments. And the dynamic response of PEMFC with regard to the change of current was also investigated. The model is useful to elucidate the relationships between operating conditions such as air relative humidity, temperature, etc. It is expected that this dynamic modeling of PEMFC system can contribute to the design optimization of PEM fuel cell system for vehicle application.
Determination of Local Conditions in PEFCs by Combining Spatially Resolved Current Density Measurements With Real-Time Modelling
Authors: Kno¨ri T. , Schulze M. , Friedrich K. A.
Published: June 2009

In this contribution a simplified, isothermal, two-phase, one-dimensional model for the calculation of the cathodic gas flow along the flow field channels of a polymer electrolyte fuel cell (PEFC) are presented. The composition of the humidified oxidant gas, average gas velocity, pressure drop, and other quantities can be calculated for different gas distributor structures. Thereby, the model requires several input parameters determined solely by experiment and operation conditions, e.g. the water content of the feed gas, local current densities, and gas flow rates. In contrast to other models, the cross-section reduction has been taken into account which results from the penetration of the gas diffusion layer (GDL) into the flow field channels due to the mounting pressure. Beyond this, the model needs no fit-parameters for further adjustment. For investigating the factors limiting the performance of a PEFC, the DLR has developed several techniques for measuring the spatially resolved current density distribution [1–5]. In order to investigate the origin of the corresponding effects, one of these techniques has been improved by implementing the model of the cathodic gas flow as an on-line feature. The combination of a spatially resolved measurement technique with a real-time simulation gives a better understanding of the local processes within the cell and represents a helpful tool for the development of fuel cell components as well as for the optimization of the operating conditions. In the presentation the results for a 25 cm2 serpentine flow field at different operation modes are shown.
On Modeling Multi-Component Diffusion Inside the Porous Anode of Solid Oxide Fuel Cells Using Fick’s Model
Authors: Cayan Fatma N. , Pakalapati Suryanarayana R. , Elizalde-Blancas Francisco , Celik Ismail
Published: June 2009

Stefan-Maxwell Model (SMM) and simple Fick’s Model (FM) type of relations both including Knudsen diffusion for the calculation of species mole fraction distribution inside the porous anode of a solid oxide fuel cell (SOFC) were compared and it was found that at low current densities the models agree well but as current increases the differences also increase. Based on the findings an empirical correction is proposed for the effective diffusivity used in Fick’s Model. The corrected diffusivity coefficient gave better agreement with the Stefan-Maxwell model and even at higher current densities the error is less than 5%. This correction was implemented via a three-dimensional, in-house SOFC simulation code (DREAM-SOFC) which uses Fick’s Model type relations for diffusion flux calculations. The code also takes into account methane steam reforming (MSR) and water gas shift (WGS) reactions and the electrochemical oxidation of both H2 and CO. As an application, a SOFC button cell which is being tested at West Virginia University was simulated. The results with and without the proposed correction for effective diffusivity are compared.
Design and Demonstration of a Multi-Fuel Automotive Fuel Processor With Novel Catalyst Integration and Rapid Control System
Authors: O’Brien Christopher , Leshchiner Michael , Ryan Todd M.
Published: June 2009

A complete automotive onboard multi-fuel fuel processing system (FPS) that addresses some of the key issues of catalyst utilization, fuel cell integration, and control has been developed and tested, producing up to 1.2 grams per second of hydrogen flow in a total package volume of 150 liters. The reforming reaction train includes an autothermal reforming (ATR) zone, water-gas shift (WGS), and preferential oxidation (PrOx), all integrated with heat exchange to preheat reactants and generate process steam. The WGS and PrOx catalysts are integrated on metallic supports within a novel modular catalyst coated heat exchanger design that incorporates catalytic reaction, air preheat, and steam generation in a single device. The use of the catalyst coated heat exchangers eliminates the need for separate high-temperature shift and low-temperature shift zones; these are replaced by a continuously cooled single WGS zone. Due to the novel integration of catalytic heat exchangers, this FPS uses approximately half the catalyst per unit hydrogen produced than the previous generation, allowing for significant cost reduction.
Experimental Evaluation of CO Poisoning of an Air-Breathing High Temperature PEM Fuel Cell Stack
Authors: Das Susanta K. , Reis Antonio , Ubong Etim U. , Berry K. Joel
Published: June 2009

In this paper, we experimentally studied an air breathing high temperature PEM fuel cell at steady operating conditions to investigate the effects of CO poisoning at different temperatures ranges between 120°C∼180°C. The effects of changes in temperatures with different amount of CO poisoning on the current-voltage characteristics of the fuel cell are investigated. Experimental data of this type would be very useful to develop design parameters of fuel processor based on reformate hydrocarbons. The high CO tolerance of high temperature PEM fuel cells makes it possible to use the reformate gas directly from the reformer without further CO removal. Here we considered the fact that a steam reformer is a consumer of heat and water, and fuel cell stacks are a producer of heat and water. Thus, integration of the fuel cell stack and the reformer is expected to improve the system performance. The results obtained from the experiments showed variations in current-voltage characteristics at different temperatures with different CO poisoning rates. The results will help to understand the overall system performance development strategy of high temperature PEM fuel cell in terms of current-voltage characteristics, when fed with on-site reformate hydrogen gas with variable CO concentrations.
Waste-to-Energy at Military Installations Through Pyrolysis and Direct Carbon Fuel Cells
Authors: Lux Scott M. , Holcomb Franklin H. , Wolk Ronald H.
Published: June 2009

This study gathered information on the quantities of waste material available at U.S. Army installations from installation reports and from the U.S. Army Solid Waste Annual Reporting System (SWARS) database, then estimated the amount of electricity that Direct Carbon Fuel Cell (DCFC) technologies could generate from those wastes, and finally compared those amounts with the current average annual electrical loads at the ten largest Army installations. The analysis finds that the use of a pyrolysis/DCFC system could produce an average of 12% of the entire installation’s demand at nine of the top 10 largest Army installations. Such a system could offset demand at the other installation in the top 10 by 33%.
Experimental Approaches for Distribution and Behavior of Water in Gas Diffusion Layer Using Neutron and X-Ray Imaging Techniques
Authors: Kim TaeJoo , Kim JongRok , Sim CheulMuu , Kaviany Massound , Son SangYoung , Kim MooHwan
Published: June 2009

In this investigation, we visualized water distribution and behavior of water in GDL (Gas Diffusion Layer) which is one of components of PEMFC using both Neutron and X-ray imaging. The experimental facility had a single channel on both faces of GDL. One was the channel to supply water and the other was the channel to supply air. These images were captured for water was supplied on different pressures. This investigation was performed at HANARO, KAERI, Korea (Neutron imaging) and Pohang Accelerator Laboratory, Korea (X-ray imaging). Neutron imaging showed good contrast and wide spatial range and X-ray imaging showed high spatial resolution. And they had 1∼2 sec exposure times which is enough time for unsteady behavior of water. These showed that neutron and X-ray imaging techniques are a powerful tool to visualize the water distribution and a behavior of porous media like as components of PEMFC. And they can give more useful data if they cooperate complementary.
A Membrane-Type Humidifier for Fuel Cell Applications: Controller Design, Analysis and Implementation
Authors: McKay Denise A. , Stefanopoulou Anna G. , Cook Jeffrey
Published: June 2009

A membrane-based gas humidification apparatus was employed to actively manage the water vapor entrained in the reactant gas supplied to a fuel cell stack. The humidification system utilizes a gas bypass and a series of heaters to achieve accurate and fast humidity and temperature control. A change in fuel cell load induces a reactant mass flow rate disturbance to this humidification system. If not well regulated, this disturbance creates undesirable condensation and evaporation dynamics, both to the humidification system and the fuel cell stack. Therefore, controllers were devised, tuned and employed for temperature reference tracking and disturbance rejection. The coordination of the heaters and the bypass valve is challenging during fast transients due to the different time scales, the actuator constraints, and the sensor responsiveness. Two heater controller types were explored: on-off (thermostatic) and variable (proportional integral), to examine the ability of the feedback system to achieve the control objectives with minimal hardware and software complexity. This controller tuning methodology is useful for optimizing response time versus heater parasitic losses.
Modeling and Simulations of Deformation and Transport in PEM Fuel Cells
Authors: Yesilyurt Serhat
Published: June 2009

Performance degradation and durability of PEM fuel cells depend strongly upon transport and deformation characteristics of their components especially the polymer membrane. Physical properties of the membrane, such as its ionic conductivity and Young’s modulus depend on its water content, which varies significantly with operating conditions and during transients. Recent studies indicate that cyclic transients may induce hygrothermal fatigue that leads to the ultimate failure of the membrane shortening its lifetime, and thus, hindering the reliable use PEM fuel cells for automotive applications. In this work, we present two-dimensional simulations and analysis of coupled deformation and transport in PEM fuel cells. A two-dimensional cross-section of anode and cathode gas diffusion layers, and the membrane sandwiched between them is modeled using Maxwell-Stefan equations in the gas diffusion layers, Biot’s poroelasticity and Darcy’s law for deformation and water transport in the membrane and Ohm’s law for ionic currents in the membrane and electric currents in the gas diffusion electrodes. Steady-state deformation and transport of water in the membrane, transient responses to step changes in load and relative humidity of the anode and cathode are obtained from simulation experiments, which are conducted by means of a commercial finite-element package, COMSOL Multiphysics.
Economic Viability of a Molten Carbonate Fuel Cell Working With Biogas
Authors: Castell Albert , Margalef Pere , Medrano Marc , Cabeza Luisa F. , Samuelsen Scott G.
Published: June 2009

Catalonia (Spain) has a significant potential of biogas production from agricultural activities and municipal waste. In addition, there are plenty of industrial cogeneration plants, but most of them use conventional fuels such as natural gas, and conventional energy conversion devices, such as internal combustion engines. Molten carbonate fuel cells are ultra-clean and highly efficient power generator devices capable of converting biogas into electricity and heat. Located in Lleida (Catalonia), Nufri is a fruit processing company with a long tradition on biogas production and cogeneration, with an installed capacity bigger than 4.5 MW. This study analyzes the economic viability of a fuel cell operating on biogas in Spain, on a real case basis (Nufri). Different fuel cell capacities are analyzed (from 300 kW to 1200 kW). A parametric study of different fuel cell prices ($/kW installed) is performed. Additional biogas cleanup requirements are taken into account. The results are based on the Spanish legislation, which establishes a special legal framework that grants favorable, technology-dependent feed-in premiums for renewable energy and cogeneration. Results show that the payback period ranges from 5 to 8 years depending on the fuel cell capacity and installation price.
Characterization of Electrochemical Properties of Micro-Tubular SOFCs
Authors: Galloway Kevin V. , Sammes Nigel M. , Song Jung-Hoon
Published: June 2009

Micro-tubular Solid Oxide Fuel Cells (SOFCs) are fabricated and tested for analysis of electrochemical properties. The impedance spectra from individual 1.8 mm diameter, 1.2 cm length micro-tubular SOFCs are obtained for the intermediate temperature range of 450–550°C. The micro-tubular SOFC used is anode supported, consisting of a NiO and Gd0.2Ce0.8O2−x (GDC) cermet anode, thin GDC electrolyte, and a La0.6Sr0.4Co0.2Fe0.8O3−y (LSCF) and GDC composite cathode. The impedance information gained at cell operating temperatures of 450, 500, and 550°C yielded power density losses of 0.8, 2.3, and 4.6% respectively. Within the operating temperature range of 450–550 °C, the ceria based 1.8 mm micro-tubular SOFCs were found to have a power densities ranging between 180–740 mW/cm2.
Mechanical Properties of Micro-Tubular SOFCs
Authors: Roy Brycen R. , Sammes Nigel M. , Suzuki Toshio
Published: June 2009

A fundamental issue with micro-tubular Solid Oxide Fuel Cells (SOFCs) is improvement of the mechanical strength of the cell. Fabricated using extrusion and co-firing techniques, the approximately 1.7 mm diameter tubes are composed of NiO-Gd-doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and La0.8Sr0.2Co0.6Fe0.4O3 (LSCF)-GDC as a cathode. The mechanical properties of SOFCs are analyzed through burst testing, c-ring testing, and micro- and nano-indentation testing; the burst test is an especially important parameter because of improved power efficiency at increased fuel pressures. Results from micro- and nano-indentation tests performed on electrolytecoated Ni-GDC anode pellets indicate that the hardness of GDC is comparable or greater than that of YSZ. To develop a trend for the mechanical behavior of micro-tubes in relation to variations in fabrication techniques, several parameters were varied. The standard tubes, used as a baseline for variations had four key design parameters as follows: they were not reduced, contained 40% pore former, were sintered at 1400 °C and had a wall thickness of approximately 0.64 mm. A variation on each of the four parameters was performed. The four variations were 1) to reduce the standard tube, 2) to increase the percent pore former to 60%, 3) to decrease sintering temperature to 1350 °C, and 4) to decrease the wall thickness to approximately 0.56 mm. An average burst strength of 9.6 ± 0.6 MPa was observed for the standard tubes, 14.6 ± 7.0 MPa for the reduced tubes, 5.4 ± 3.2 MPa for the increased pore former, 12.9 ± 4.1 MPa for the decreased sintering temperature and 11.7 ± 2.4 MPa for the thinner tubes.
No New Physics in Single-Phase Fluid Flow and Heat Transfer in Mini- and Micro-Channels: Is It a Conclusion?
Authors: Tao Wen-Quan , He Ya-Ling , Tang Gui-Hua , Li Zhuo
Published: June 2009

First the flow friction characteristics of nitrogen and helium in stainless steel microtubes, glass microtubes, square glass microchannels, and rectangular silicon microchannels are tested. The data in glass microtubes with diameters from 50 to 201 microns and in square glass channels with characteristic diameters from 52 to 100 microns show that the friction factors are in good agreement with the conventional predictions. The friction factors in stainless-steel tubes with diameters from 119 to 300 microns are much higher than the conventional ones. The results for two of the four silicon microchannels with characteristic diameters from 26–60 microns are in good agreement while those of the other two channels are larger. This discrepancy is resulted from the large relative surface roughness. Smaller friction factors in glass microtubes with diameters from 10 to 20 microns are obtained due to the rarefaction effect. Second the flow friction experimental data for deionized water flow in glass microtubes with diameters from 50 to 530 microns show that friction factors and transition Reynolds numbers are in good agreement with the conventional predictions. However, the friction factors in stainless steel microtubes with diameters from 50–1570 microns are much higher than the conventional predictions. This discrepancy is attributed to the large surface relative roughness or denser roughness distribution. Numerical simulations considering electroviscous effect are carried out. The simulation results show that the electroviscous effect does not play a significant role in the friction factor for channel dimensions of the order of microns though it does affect the velocity profile and hence it could be neglected in engineering applications for channel dimensions of the order of microns. Third the measured local Nusselt number distribution of deionized water along the axial direction of the stainless steel tubes of 373–1570 microns with uniform heat flux do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. Numerical study reveals that the large ratio of wall thickness to tube diameter at low Reynolds number causes significant axial heat conduction in the tube wall, leading to a non-linear streamwise distribution of the fluid temperature. The axial wall heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness. In conclusion, the conventional fluid flow and heat transfer theories should still be applied for single-phase flow in smooth microchannels. Nevertheless, micro-channels do raise some issues to be paid special attention to when being applied.
Thermal and Electrical Conductivities of Polycrystalline Metallic Nanofilms Based on the Kinetic Theory
Authors: Feng Bo , Li Zhixin , Zhang Xing
Published: June 2009

A model is developed for in-plane thermal conductivity of nanostructured metallic films based on the kinetic theory, which attributes the reduced thermal conductivity to the reduced mean free path of electrons. The partially inelastic electron-surface scattering and grain-boundary impedance by quantum mechanical treatment are elaborately included. Meanwhile, the mean free path of electrons is also used to study in-plane electrical conductivity of nanofilms. Both electrical conductivity and thermal conductivity, varying with film thickness and temperature, are observed to be lower than corresponding bulk values, agreeing well with the experimental data. The grain-boundary scattering is theoretically found to dominate over surface scattering to enhance the size effect on electrical and thermal conductivities. In addition, the size effect in low temperature appears more dramatic due to larger electron Knudsen number. We further examine the Lorenz number of nanofilms and find the Wiedemann-Franz law is seriously violated. The Coulomb blockade and the neutral excitation of electron-hole pair are used to offer a more detailed picture. Excessive thermal conductivity is also evaluated resorting to concepts in granular metals to show the validity of this account.
Laminar Flow Characteristics in Entry Region in Microchannels
Authors: Zhang Tiantian , Jia Li
Published: June 2009

A three-dimensional incompressible model of microchannel is proposed. Flow characteristics of nitrogen flow in different microchannels (hydraulic diameter ranging 100–500μm, the ratio of length-to-diameter ranging 60–150, the ratio of height-to-width ranging 0.2–1) have been investigated numerically. It is found that the velocity distribution in microchannels is obviously different from that in conventional channels, and the maximum velocity occurs not in the channel core as conventional theory expected but near the walls due to the surface effect. These phenomena result in the reduction of the thickness of hydrodynamic boundary layer. So the hydrodynamic entry length in microchannels is much larger than that in conventional channels. Theoretical analysis was given to explain these phenomena. The effects of Reynolds number, hydrodynamic diameter, length-to-diameter ratio and height-to-width ratio on hydrodynamic entry length were analyzed. The correlation between L/D and Re and height-to-width ratio, which is useful for designing and optimizing the microchannel heat sinks and other microfluidic devices, was suggested.
Capturing Nongradient Transport in Nonequilibrium Gas Flow
Authors: Gu X. J. , Emerson D. R.
Published: June 2009

A higher order moment method is employed to construct the transport model for nonequilibrium gas flow in microscale geometries. The one dimensional planar Couette flow was chosen to demonstrate the significance of capturing the nongradient transport phenomena in the prediction of velocity and temperature fields. For planar Couette flow in the transition regime, the velocity profile is nonlinear and the induced temperature field is no longer parabolic. These features are attributed to the nongradient transport mechanism in a nonequilibrium gas. Furthermore, it is revealed that, for a given temperature field, the gradient transport model overestimates the heat transfer significantly. This, again, can be compensated by the nongradient transport mechanism.
Study of Molecular Dynamic Simulation of Poiseuille Flow in a Microchannel
Authors: Liu Chao , Liu Binwu , Zhang Hualing
Published: June 2009

A flow wall collision model between two parallel plane walls was established for argon fluid through a microchannel under the condition of action of periodical external force. Based on this model, two kinds of wall (hydrophilic and hydrophobic) were applied on the flow simulations from non-equilibrium molecular dynamics (NEMD). There are 864 fluid particles and wall particles separately in the simulated system. The non-dimension height of microchannel is 9.667. The velocity profile and temperature profile of argon fluid in hydrophilic microchannel predicted by molecular dynamics simulation are in good agreement with the analytical solution based on the Navier–Stokes and energy equations. The velocity profile and the temperature profile experience a large jump in the layers close to the hydrophobic wall.
Vortex Flow Driven by Electroosmosis in Microchannels
Authors: Zhong Wu , Chen Yunfei
Published: June 2009

The governing equations of electroosmotic flow, including the Navier-Stokes (N-S) equations, Laplace equation and Poisson-Boltzmann equation, are set up in a straight microchannel. The meshless method is employed as a discrete scheme for the solution domain. The semi-implicit multistep (SIMS) method is used to solve the Navier-Stokes equations. The simulation results demonstrated that different patterns of the zeta potential over the channel surface could induce different flow profiles for the vortex. The rotational direction of the vortex is determined by the electroosmotic driving force.
Thermal Conductivity and Operating Temperature Effect on the Interline Region in a Micro/Miniature Heat Pipe
Authors: Sait Hani H. , Demsky Steve M. , Ma HongBin
Published: June 2009

An analytical model describing thin film evaporation is developed that includes the effects of surface tension, frictional shear stress, wetting characteristics and disjoining pressure. The effects of thermal conductivity of working fluids and operating temperature on the evaporating thin film region are also studied. The results indicate that when the thermal conductivity of the working fluid increases, a high heat flux can be removed from the evaporating thin film region. The operating temperature affects the thin film evaporation. The higher the operating temperature, the more heat flux can be removed from the region. The information of thin film evaporation presented in the paper results in a better understanding of heat transfer mechanism occurring in micro heat pipes.
Inertance Effects to Diffuser Micropumps Flow Rate Spectrum
Authors: Le Ngoc-Bich , Lin Mau-Sheng , Hsu Yi-Chu , Jang Ling-Sheng
Published: June 2009

This study presents a diffuser micropump and characterizes its output flow rates, like the parabola shape on the frequency domain and the effecting factors. First, equivalent circuit using fluid-electric analogy was built up; then, the flow rate analysis results were compared to experiment results to verify the applicability of the circuit simulation. The operation frequency was 800 Hz for both cases and the maximum flow rates were 0.078 and 0.075 μl/s for simulation and experiment result, respectively. The maximum flow rate difference was 3.7%. The circuit then was used to analyze the inertial effects of transferred fluid as well as system components to the output flow rates. This work also explains why the flow rate spectrum has the shape of parabola. The analysis results showed that without inertial effects, the micropump flow rates are linearly proportional to the operation frequency; otherwise it has parabola shape. The natural frequency of the actuator-membrane structure was recognized using finite element method to verify if this parameter affects the characteristics of the flow rates. The experiment and simulation results demonstrated 800 Hz and 91.4 kHz for the frequency of the maximum pumping flow rate and the first mode natural frequency of actuator-membrane structure, respectively. It indicates that the structure natural frequencies of the actuator-membrane structure do not play any role to operate the micropumps.
Universal Force Diagrams of a Microbubble in Static Fluid on a Surface
Authors: Hsiao C. C. , Wei P. S. , Chen K. Y.
Published: June 2009

Universal dimensionless three-dimensional universal lift force diagrams for a microbubble (or pendant drop) in a static liquid on a solid surface (or orifice) are presented in this work. Studying microbubble dynamics is important due to its controlling mass, momentum, energy and concentration transfer rates in many micro- and nano-sciences and technologies. In this study, dimensionless force diagrams are introduced by applying the solutions for microbubble shape for small Bond number in literature. A parametric study reveals that two dimensionless independent parameters, Bond number and contact angle (or base radius), are required to specify force diagrams. Each diagram can be divided into three regions, depending on if the microbubble surface contains the inflection or neck point. The total life force includes hydrostatic buoyancy, difference in gas and hydrostatic pressures at the base, capillary pressure, as well as surface tension induced by the variation of circumference which has not been treated in literature. In the absence of viscous stress, the total lift force equals surface tension induced by the variation of circumference.
A Molecular Dynamics Study on Energy Accommodation Coefficients in Microchannels
Authors: Sun Jun , Li Zhixin
Published: June 2009

Energy accommodation coefficient (EAC), used in thermal boundary condition in micro gas flow and heat transfer, is reported to be always less than unity and greatly influenced by the wall characters. According to EAC’s definition, the statistical algorithm was described and EAC for argon gas was studied by two dimensional NEMD simulations with heat conduction between two smooth platinum plates at different temperatures. With one wall’s temperature fixed, the non-equilibrium EACs were calculated by changing the other wall’s temperature. Meanwhile, the equilibrium EAC at one temperature can be extrapolated from a series of non-equilibrium EACs as the temperature difference approaches to zero. The effects of wall temperature, wall temperature difference, and Kn on EAC were investigated. Non-equilibrium EAC increases with wall temperature difference decreased, and becomes larger with increased Kn. And equilibrium EAC is larger for lower temperature and larger Kn.
Two-Fluid Electroosmotic Flow in Microchannels
Authors: Wong T. N. , Gao Y. , Wang C. , Yang C. , Nguyen N. T. , Chai J. C. , Ooi K. T.
Published: June 2009

This paper presents theoretical and experimental investigations of the pressure-driven two-liquid flow in microchannels with the electroosmosis effect. For a fully developed, steady state, laminar flow of two liquids combined the pressure gradient, electroosmosis and surface charges at the liquid-liquid interface, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field. It was shown that adjusting the externally applied electric field could control the fluid interface position precisely. The prediction from the proposed model compares very well with measured data.
Visualization of Fluid Mixing in a Microchamber on a Rotating Disk
Authors: Chiu H.-C. , Chen J.-Y. , Chen Jerry M.
Published: June 2009

This paper reports flow visualization experiments of fluid mixing in a microchamber on a rotating disk. The two centrifuge-driven sample fluids were brought in contact at the Y-junction microchannel and then flowed to a circular mixing chamber where the main course of mixing took place. The flow images were acquired using a micro-image-capturing unit in synchronization with the rotational motion to allow only one shot of the targeted object on the rotating disk per revolution. Both the visualization and quantification of flow images show that the mixing efficiency of the two fluids depends not only on the rotational speed but also on the depth of the channels. It is found that the mixing efficiency generally decrease with increasing rotational speed in the lower speed range (≤ 420 rpm). Beyond this lower speed range, the mixer with a larger channel depth h = 300 μm shows an increase of mixing efficiency with increasing rotational speed to reach as much as 83% at 1200 rpm. For the mixer with a smaller channel depth h = 200 μm, however, the mixing efficiency continues deceasing or becomes flat with increasing rotational speed. It is also found that the counter-clockwise rotation produces a better mixing efficiency than the clockwise rotation in the high speed range.
Effect of Orifice Jet Configuration on Heat Transfer in a Channel With Inclined Target Surface Cooled by Single Array of Impinging Jets With Outflow in Both Directions
Authors: Al-Mubarak Ali A. , Shaahid S. M. , Al-Hadhrami Luai M.
Published: June 2009

An experimental work has been carried out to investigate the effect of orifice-jet plate configuration on heat transfer behavior in a channel with inclined heated target plate cooled by single array of equally spaced impinging jets. Air ejected from an array of orifices is impinges on the heated target surface. The target plate forms the leading edge of a gas turbine blade. The study includes the effect of various orifice-jet plate configurations, feed channel aspect ratios H/d = 5, 7, and 9, and Reynolds number Re = 9300, 14400, and 18880 on the heat transfer characteristics for a given outflow orientation (outflow passing out in both the directions). Three orifice-jet plate configurations (centered, staggered, and tangential holes) have been examined. It has been noticed that Nusselt number (Nu) is high for higher aspect ratios. For a given plate-1 with single array of equally spaced centered jets and for Re = 18800 (outflow passing out in both directions), the local Nu for H/d = 9 has been found to be greater than Nu of H/d = 7 by 5%. The percentage increase in average Nu has been found to be about 11% with centered holes as compared staggered orifice-jet plate. The percentage increase in average Nu has been found to be about 11% with staggered jet-plate as compared to tangential orifice-jet plate configuration.
Hydrodynamic Flows in Electrowetting
Authors: Ko Sung Hee , Lee Horim , Kang Kwan Hyoung
Published: June 2009

In this work, we found experimentally that there exist fairly strong fluid flows in AC electrowetting, which can be utilized as a means to mix the fluids in EWOD-based micro-devices. We visualized the internal flow. There may exist two distinct flow-generation mechanisms; one is the droplet oscillation, and the other is the electrohydrodynamic flow. The flow pattern is significantly dependent on the applied AC frequency. At low frequencies (represented here by 1 kHz), the center of the vortices is located somewhat randomly and the flow directs upward near the symmetric axis. At high frequencies (represented by 128 kHz), however, a pair of vortices having quite a regular structure is clearly visible and the flow directs downward near the symmetric axis. The flow patterns are strongly dependent on the position of the point electrode. The droplet surface undergoes a periodic oscillation (visualized by a high-speed camera) with a frequency exactly twice the frequency of applied electrical signal. The oscillating interface can generate a steady streaming. However the numerical results show that there exists no electric field at low AC frequencies. On the contrary, there exists quite a strong electric field inside the droplet at high frequencies. It means the electrohydrodynamic flow cannot be generated at the low frequency region, and the droplet oscillation might cause the flow generation at low frequencies. We also demonstrated the flow can be beneficially utilized as a mixing method.
Improved Capillary Electrophoresis Separation Using a Capillary Bundle
Authors: Nguyen Nam-Trung , Sun Yi , Kwok Yien-Chian
Published: June 2009

Joule heating is an undesirable effect in capillary electrophoresis (CE). The heat generated by the electrical current leads to a temperature gradient along the separation channel and consequently affects the separation quality. Since the heat is inversely proportional to the electric resistance of the separation column, increasing the electric resistance can reduce the effect of Joule heating. Currently, due to the limit of fabrication technique and detection apparatus, the typical dimensions of CE microchannels are in the range of 50 μm to 200 μm. In this paper, we describe the method of reducing the cross-sectional area of the separation channel and increasing the channel’s surface for better heat dissipation. A photonic crystal fiber (PCF) is a bundle of extremely narrow channels, which ideally work as separation columns. The PCF was simply encapsulated in a polymethylmethacrylate (PMMA) microchannel right after a T-shaped injector. CE was simultaneously but independently carried out in 54 narrow capillaries, each capillary with diameter of 3.7 μm. The capillary bundle could sustain high electric field strength up to 1000 V/cm due to efficient heat dissipation, thus faster and enhanced separation was attained.