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Browsing by Author "von Spakovsky, Michael R."

Now showing 1 - 20 of 78
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    Analysis of Ionomer-coated Carbon Nanofiber for use in PEM Fuel Cell Catalyst Layers
    Garrabrant, Austin Joseph (Virginia Tech, 2019-07-31)
    The typical catalyst layer structure for proton exchange membrane (PEM) fuel cells has changed little over the last two decades. A new electrode design with improved control over factors such as ionic and electrical pathways, porosity, and catalyst placement, could allow the application of less expensive catalyst alternatives. In this work, a novel electrode design based on ionomer-coated carbon nanofibers is proposed and studied. Governing equations for this design were established, and a mathematical model was created and solved using MATLAB to predict the performance of the new electrode design. A parametric study was performed to identify the design variables that had the most significant effect on performance. The best performing catalyst layer design studied with this model produces a current density of 1.1 A cm-2 at 600 mV which is better than state-of-the-art cathode designs. The results offer insight into the performance of ionomer-coated carbon nanofiber catalyst layers and can guide the fabrication and testing of these promising catalyst layer structures.
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    Application of a Decomposition Strategy to the Optimal Synthesis/Design and Operation of a Fuel Cell Based Total Energy System
    Georgopoulos, Nikolaos (Virginia Tech, 2002-04-29)
    A decomposition methodology based on the concept of "thermoeconomic isolation" applied to the synthesis/design and operational optimization of a stationary cogeneration proton exchange membrane fuel cell (PEMFC) based total energy system (TES) for residential/commercial applications is the focus of this work. A number of different configurations for the fuel cell based TES were considered. The most promising set based on an energy integration analysis of candidate configurations was developed and detailed thermodynamic, kinetic, geometric, and economic models at both design and off-design were formulated and implemented. A decomposition strategy called Iterative Local-Global Optimization (ILGO) developed by Muñoz and von Spakovsky was then applied to the synthesis/design and operational optimization of the fuel cell based TES. This decomposition strategy is the first to successfully closely approach the theoretical condition of "thermoeconomic isolation" when applied to highly complex, non-linear systems. This contrasts with past attempts to approach this condition, all of which were applied to very simple systems under very special and restricted conditions such as those requiring linearity in the models and strictly local decision variables. This is a major advance in decomposition and has now been successfully applied to a number of highly complex and dynamic transportation and stationary systems. This thesis work presents the detailed results from one such application.
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    Application of a decomposition strategy to the optimal synthesis/design of a fuel cell sub-system
    Oyarzabal, Borja (Virginia Tech, 2001-08-06)
    The application of a decomposition methodology to the synthesis/design optimization of a stationary cogeneration fuel cell sub-system for residential/commercial applications is the focus of this work. To accomplish this, a number of different configurations for the fuel cell sub-system are presented and discussed. The most promising candidate configuration, which combines features of different configurations found in the literature, is chosen for detailed thermodynamic, geometric, and economic modeling both at design and off-design. The case is then made for the usefulness and need of decomposition in large-scale optimization. The types of decomposition strategies considered are time and physical decomposition. Specific solution approaches to the latter, namely Local-Global Optimization (LGO) and Iterative Local-Global Optimization (ILGO) are outlined in the thesis. Time decomposition and physical decomposition using the LGO approach are applied to the fuel cell sub-system. These techniques prove to be useful tools for simplifying the overall synthesis/design optimization problem of the fuel cell sub-system. Finally, the results of the decomposed synthesis/design optimization of the fuel cell subsystem indicate that this sub-system is more economical for a relatively large cluster of residences (i.e. 50). To achieve a unit cost of power production of less than 10 cents/kWh on an exergy basis requires the manufacture of more than 1500 fuel cell sub-system units per year. In addition, based on the off-design optimization results, the fuel cell subsystem is unable by itself to satisfy the winter heat demands. Thus, the case is made for integrating the fuel cell sub-system with another sub-system, namely, a heat pump.
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    Application of Concurrent Development Practices to Petrochemical Equipment Design
    Lomax, Franklin Delano (Virginia Tech, 2001-03-16)
    Principles of concurrent development are applied to the design of a small-scale device for converting natural gas or liquefied petroleum gas into hydrogen. The small hydrogen generator is intended for serial production for application in the production of industrial hydrogen, fueling stationary fuel cell power systems and refueling hydrogen-fueled fuel cell electric vehicles. The concurrent development process is contrasted with the traditional, linear development process for petrochemical systems and equipment, and the design is benchmarked against existing small hydrogen generators as well as industrial hydrogen production apparatus. A novel system and hardware design are described, and a single cycle of concurrent development is applied in the areas of catalyst development, thermodynamic optimization, and reactor modeling and design. The impact of applying concurrent development techniques is assessed through economic modeling, and directions for future development work are identified.
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    Application of Steepest-Entropy-Ascent Quantum Thermodynamics to Solid-State Phenomena
    Yamada, Ryo (Virginia Tech, 2018-11-16)
    Steepest-entropy-ascent quantum thermodynamics (SEAQT) is a mathematical and theoretical framework for intrinsic quantum thermodynamics (IQT), a unified theory of quantum mechanics and thermodynamics. In the theoretical framework, entropy is viewed as a measure of energy load sharing among available energy eigenlevels, and a unique relaxation path of a system from an initial non-equilibrium state to a stable equilibrium is determined from the greatest entropy generation viewpoint. The SEAQT modeling has seen a great development recently. However, the applications have mainly focused on gas phases, where a simple energy eigenstructure (a set of energy eigenlevels) can be constructed from appropriate quantum models by assuming that gas-particles behave independently. The focus of this research is to extend the applicability to solid phases, where interactions between constituent particles play a definitive role in their properties so that an energy eigenstructure becomes quite complicated and intractable from quantum models. To cope with the problem, a highly simplified energy eigenstructure (so-called ``pseudo-eigenstructure") of a condensed matter is constructed using a reduced-order method, where quantum models are replaced by typical solid-state models. The details of the approach are given and the method is applied to make kinetic predictions in various solid-state phenomena: the thermal expansion of silver, the magnetization of iron, and the continuous/discontinuous phase separation and ordering in binary alloys where a pseudo-eigenstructure is constructed using atomic/spin coupled oscillators or a mean-field approximation. In each application, the reliability of the approach is confirmed and the time-evolution processes are tracked from different initial states under varying conditions (including interactions with a heat reservoir and external magnetic field) using the SEAQT equation of motion derived for each specific application. Specifically, the SEAQT framework with a pseudo-eigenstructure successfully predicts: (i) lattice relaxations in any temperature range while accounting explicitly for anharmonic effects, (ii) low-temperature spin relaxations with fundamental descriptions of non-equilibrium temperature and magnetic field strength, and (iii) continuous and discontinuous mechanisms as well as concurrent ordering and phase separation mechanisms during the decomposition of solid-solutions.
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    Application of the Transient Hot-Wire Technique for Measurement of Effective Thermal Conductivity of Catalyzed Sodium Alanate for Hydrogen Storage
    Christopher, Michael Donald (Virginia Tech, 2006-05-05)
    Sodium alanate, or the Na-Al-H system, has been the focus of intense research over the past decade due to its ability to hold almost 5 wt% of hydrogen. In this research, the effective thermal conductivity, k, of a sample of titanium-doped sodium alanate is studied over a range of operating conditions pertinent to practical on-board hydrogen storage. A transient technique employing a platinum hot-wire is used to make the measurements. A cylindrical experimental apparatus was designed with the aide of a finite element model that was used to quantify the cylinder boundary effects. The apparatus dimensions were optimized based on the finite element results with the goal of minimizing measurement uncertainty and temperature rise during testing. Finite element results were also used to predict test times and current requirements. A sample of sodium alanate was obtained and loaded into the experimental apparatus which was enclosed in a pressure vessel with a controlled atmosphere. Effective thermal conductivity was measured as a function of pressure at the fully-hydrided and fully-dehydrided states. The results from the pressure-dependence investigation were compared to an existing study that utilized an alternate measurement technique. The results matched well qualitatively — the effective thermal conductivity was highly dependent on pressure, and was found to be significantly higher in the fully-dehydrided state. However, the results of this study were 20 to 30% lower than the existing available data. Additionally, an exploratory investigation used the PCI technique to study the effect of varying composition between the fully-hydrided state and the intermediate decomposition step at a relatively constant pressure. Effective thermal conductivity did not vary significantly over this range of compositions.
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    CH4 Adsorption Probability on GaN(0001) and (000−1) during Metalorganic Vapor Phase Epitaxy and Its Relationship to Carbon Contamination in the Films
    Kusaba, Akira; Li, Guanchen; Kempisty, Pawel; von Spakovsky, Michael R.; Kangawa, Yoshihiro (MDPI, 2019-03-23)
    Suppression of carbon contamination in GaN films grown using metalorganic vapor phase epitaxy (MOVPE) is a crucial issue in its application to high power and high frequency electronic devices. To know how to reduce the C concentration in the films, a sequential analysis based on first principles calculations is performed. Thus, surface reconstruction and the adsorption of the CH4 produced by the decomposition of the Ga source, Ga(CH3)3, and its incorporation into the GaN sub-surface layers are investigated. In this sequential analysis, the dataset of the adsorption probability of CH4 on reconstructed surfaces is indispensable, as is the energy of the C impurity in the GaN sub-surface layers. The C adsorption probability is obtained based on steepest-entropy-ascent quantum thermodynamics (SEAQT). SEAQT is a thermodynamic ensemble-based, non-phenomenological framework that can predict the behavior of non-equilibrium processes, even those far from equilibrium. This framework is suitable especially when one studies the adsorption behavior of an impurity molecule because the conventional approach, the chemical potential control method, cannot be applied to a quantitative analysis for such a system. The proposed sequential model successfully explains the influence of the growth orientation, GaN(0001) and (000−1), on the incorporation of C into the film. This model can contribute to the suppression of the C contamination in GaN MOVPE.
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    Cold-start effects on performance and efficiency for vehicle fuel cell systems
    Gurski, Stephen Daniel (Virginia Tech, 2002-12-12)
    In recent years government, academia and industry have been pursuing fuel cell technology as an alternative to current power generating technologies. The automotive industry has targeted fuel cell technology as a potential alternative to internal combustion engines. The goal of this research is to understand and quantify the impact and effects of low temperature operation has on the performance and efficiency of vehicle fuel cell systems through modeling. More specifically, this work addresses issues of the initial thermal transient known to the automotive community as "cold-start" effects. Cold-start effects play a significant role in power limitations in a fuel cell vehicle, and may require hybridization (batteries) to supplement available power. A fuel cell system model developed as part of this work allows users to define the basic thermal fluid relationships in a fuel cell system. The model can be used as a stand-alone version or as part of a complex fuel cell vehicle model. Fuel cells are being considered for transportation primarily because they have the ability to increase vehicle energy efficiency and significantly reduce or eliminate tailpipe emissions. A proton exchange membrane fuel cell is an electrochemical device for which the operational characteristics depend heavily upon temperature. Thus, it is important to know how the thermal design of the system affects the performance of a fuel cell, which governs the efficiency and performance of the system. This work revealed that the impact on efficiency of a cold-start yielded a 5 % increase in fuel use over a regulated drive cycle for the converted sport utility vehicle. The performance of the fuel cell vehicle also suffered due to operation at low temperatures. Operation of the fuel cell at 20 C yielded only 50% of the available power to the vehicle system.
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    Combining In Situ Measurements and Advanced Catalyst Layer Modeling in PEM Fuel Cells
    Regner, Keith Thomas (Virginia Tech, 2011-08-26)
    Catalyst layer modeling can be a useful tool for fuel cell design. By comparing numerical results to experimental results, numerical models can provide a better understanding of the physical processes occurring within the fuel cell catalyst layer. This can lead to design optimization and cost reduction. The purpose of this research was to compare, for the first time, a direct numerical simulation (DNS) model for the cathode catalyst layer of a PEM fuel cell to a newly developed experimental technique that measures the ionic potential through the length of the catalyst layer. A new design for a microstructured electrode scaffold (MES) is proposed and implemented. It was found that there is a 25%-27% difference between the model and the experimental measurements. Case studies were also performed with the DNS to compare the effects of different operating conditions, specifically temperature and relative humidity, and different reconstructed microstructures. Suggested operating parameters are proposed for the best comparison between numerical and experimental results. Recommendations for microstructure reconstruction, MES construction and design, and potential measurement techniques are also given.
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    Comparison of Heat Exchanger Designs for Aircraft Thermal Management  Systems
    Reed, William Cody (Virginia Tech, 2015-09-02)
    Thermal management has become a major concern in the design of current and future more and all electric aircraft (M/AEA). With ever increasing numbers of on-board heat sources, higher heat loads, limited and even decreasing numbers of heat sinks, integration of advanced intelligence, surveillance and reconnaissance (ISR) and directed energy weapons, requirements for survivability, the use of composite materials, etc., existing thermal management systems and their components have been pushed to the limit. To address this issue, more efficient methods of thermal management must be implemented to ensure that these new M/AEA aircraft do not overheat and prematurely abort their missions. Crucial to this effort is the need to consider advanced heat exchanger concepts, comparing their designs and performance with those of the conventional compact exchangers currently used on-board aircraft thermal management systems. As a step in this direction, the work presented in this thesis identifies two promising advanced heat exchanger concepts, namely, microchannel and phase change heat exchangers. Detailed conceptual design and performance models for these as well as for a conventional plate-fin compact heat exchanger are developed and their design and performance optimized relative to the criterion of minimum dry weight. Results for these optimizations are presented, comparisons made, conclusions drawn, and recommendations made for future research. These results and comparisons show potential performance benefits for aircraft thermal management incorporating microchannel and phase change heat exchangers.
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    Computational and Experimental Investigation of the Critical Behavior Observed in Cell Signaling Related to Electrically Perturbed Lipid Systems
    Goswami, Ishan (Virginia Tech, 2018-10-16)
    Problem Statement: The use of pulsed electric fields (PEFs) as a tumor treatment modality is receiving increased traction. A typical clinical procedure involves insertion of a pair of electrodes into the tumor and administration of PEFs (amplitude: ~1 kV/cm; pulse-width: 100 μs). This leaves a zone of complete cell death and a sub-lethal zone where a fraction of the cells survive. There is substantial evidence of an anti-tumor systemic immune profile in animal patients treated with PEFs. However, the mechanism behind such immune profile alterations remains unknown, and the effect of PEFs on cell signaling within sub-lethal zones remains largely unexplored. Moreover, different values of a PEF pulse parameter, for e.g. the pulse-widths of 100 μs and 100 ns, may have different effects on cell signaling. Thus, the challenge of answering the mechanistic questions is compounded by the large PEF parameter space consisting of different combinations of pulse-widths, amplitudes, and exposure times. Intellectual merit: This Ph.D. research provides proof that sub-lethal PEFs can enhance anti-tumor signaling in triple negative breast cancer cells by abrogating thymic stromal lymphopoietin signaling and enhancing stimulatory proteins such as the tumor necrosis factor. Furthermore, experimental evidence produced during this Ph.D. research demonstrates that PEFs may not directly impact the intracellular mitochondrial membrane at clinically relevant field amplitudes. As demonstrated in this work, PEFs may influence the mitochondria via an indirect route such as disruption of the actin cytoskeleton and/or alteration of ionic environment in the cytoplasm due to cell membrane permeabilization. Thus, a reductionist approach to understanding the influence of PEFs on cell signaling is proposed by limiting the study to membrane dynamics. To overcome the problem of investigating the entire PEF parameter space, this Ph.D. research proposes a first-principle thermodynamic approach of scaling the PEF parameter space such that an understanding developed in one regime of PEF pulse parameter values can be used to understand other regimes of the parameter space. Demonstration of the validity of this scaling model is provided by coupling Monte-Carlo methods for density-of-states with the steepest-entropy-ascent quantum thermodynamic framework for the non-equilibrium prediction of the lipid membrane dynamics.
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    Computational Studies of the Mechanical Response of Nano-Structured Materials
    Beets, Nathan James (Virginia Tech, 2020-05-18)
    In this dissertation, simulation techniques are used to understand the role of surfaces, interfaces, and capillary forces on the deformation response of bicontinuous metallic composites and porous materials. This research utilizes atomistic scale modeling to study nanoscale deformation phenomena with time and spatial resolution not available in experimental testing. Molecular dynamics techniques are used to understand plastic deformation of metallic bicontinuous lattices with varying solid volume fraction, connectivity, size, surface stress, loading procedures, and solid density. Strain localization and yield response on nanoporous gold lattices as a function of their solid volume fraction are investigated in axially strained periodic samples with constant average ligament diameter. Simulation stress results revealed that yield response was significantly lower than what can be expected form the Gibson-Ashby formalism for predicting the yield response of macro scale foams. It was found that the number of fully connected ligaments contributing to the overall load bearing structure decreased as a function of solid volume fraction. Correcting for this with a scaling factor that corrects the total volume fraction to "connected, load bearing" solid fraction makes the predictions from the scaling equations more realistic. The effects of ligament diameter in nanoporous lattices on yield and elastic response in both compressive and tensile loading states are reported. Yield response in compression and tension is found to converge for the two deformation modes with increasing ligament diameter, with the samples consistently being stronger in tension, but weaker in compression. The plastic response results are fit to a predictive model that depends on ligament size and surface parameter (f). A modification is made to the model to be in terms of surface area to volume ratio (S/V) rather than ligament diameter (1/d) and the response from capillary forces seems to be more closely modeled with the full surface stress parameter rather than surface energy. Fracture response of a nanoporous gold structure is also studied, using the stress intensity-controlled equations for deformation from linear elastic fracture mechanics in combination with a box of atoms, whose interior is governed by the molecular dynamics formalism. Mechanisms of failure and propagation, propagation rate, and ligament-by-ligament deformation mechanisms such as dislocations and twin boundaries are studied and compared to a corresponding experimental nanoporous gold sample investigated via HRTEM microscopy. Stress state and deformation behavior of individual ligaments are compared to tensile tests of cylinder and hyperboloid nanowires with varying orientations. The information gathered here is used to successfully predict when and how ligaments ahead of the crack tip will fracture. The effects of the addition of silver on the mechanical response of a nanoporous lattice in uniaxial tension and compression is also reported. Samples with identical morphology to the study of the effects of ligament diameter are used, with varying random placement concentrations of silver atoms. A Monte Carlo scheme is used to study the degree of surface segregation after equilibration in a mixed lattice. Dislocation behavior and deformation response for all samples in compression and tension are studied, and yield response specifically is put in the context of a surface effect model. Finally, a novel bicontiuous fully phase separated Cu-Mo structure is investigated, and compared to a morphologically similar experimental sample. Composite interfacial energy and interface orientation structure are studied and compared to corresponding experimental results. The effect of ligament diameter on mechanical response in compressive stress is investigated for a singular morphology, stress distribution by phase is investigated in the context of elastic moduli calculated from the full elastic tensor and pure elemental deformation tests. Dislocation evolution and its effects on strain hardening are put in the context of elastic strain, and plastic response is investigated in the context of a confined layer slip model for emission of a glide loop. The structure is shown to be an excellent, low interface energy model that can arrest slip plane formation while maintaining strength close to the theoretical prediction. Dislocation content in all samples was quantified via the dislocation extraction algorithm. All visualization, phase dependent stress analysis, and structural/property analysis was conducted with the OVITO software package, and its included python editor. All simulations were conducted using the LAMMPS molecular dynamics simulation package. Overall, this dissertation presents insights into plastic deformation phenomena for nano-scale bicontinuous metallic lattices using a combination of experimentation and simulation. A more holistic understanding of the mechanical response of these materials is obtained and an addition to the theory concerning their mechanical response is presented.
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    A Decomposition Strategy Based on Thermoeconomic Isolation Applied to the Optimal Synthesis/Design and Operation of an Advanced Fighter Aircraft System
    Rancruel, Diego Fernando (Virginia Tech, 2003-02-07)
    A decomposition methodology based on the concept of "thermoeconomic isolation" applied to the synthesis/design and operational optimization of an advanced tactical fighter aircraft is the focus of this research. Conceptual, time, and physical decomposition were used to solve the system-level as well as unit-level optimization problems. The total system was decomposed into five sub-systems as follows: propulsion sub-system (PS), environmental control sub-system (ECS), fuel loop sub-system (FLS), vapor compressor and PAO loops sub-system (VC/PAOS), and airframe sub-system (AFS) of which the AFS is a non-energy based sub-system. Configurational optimization was applied. Thus, a number of different configurations for each sub-system were considered. The most promising set of candidate configurations, based on both an energy integration analysis and aerodynamic performance, were developed and detailed thermodynamic, geometric, physical, and aerodynamic models at both design and off-design were formulated and implemented. A decomposition strategy called Iterative Local-Global Optimization (ILGO) developed by Muñoz and von Spakovsky was then applied to the synthesis/design and operational optimization of the advanced tactical fighter aircraft. This decomposition strategy is the first to successfully closely approach the theoretical condition of "thermoeconomic isolation" when applied to highly complex, highly dynamic non-linear systems. This contrasts with past attempts to approach this condition, all of which were applied to very simple systems under very special and restricted conditions such as those requiring linearity in the models and strictly local decision variables. This is a major advance in decomposition and has now been successfully applied to a number of highly complex and dynamic transportation and stationary systems. This thesis work presents the detailed results from one such application, which additionally considers a non-energy based sub-system (AFS).
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    Design and Modeling of a Novel Direct Carbon Molten Carbonate Fuel Cell with Porous Bed Electrodes
    Agarwal, Ritesh (Virginia Tech, 2015-02-03)
    A novel concept has been developed for the direct carbon fuel cell (DCFC) based on molten carbonate recirculating electrolyte. In the cathode, co-current flow of electrolyte with entrained gases carbon dioxide and oxygen is sent in the upward direction through a porous bed grid. In the anode, co-current flow of a slurry of electrolyte entrained with carbon particles is sent in the downward direction through a porous bed grid. The gases carbon dioxide and oxygen in the cathode react on the grid surface to form carbonate ions. The carbonate ions are then transported via conduction to the anode for reaction with carbon to produce carbon dioxide for temperatures under 750 deg C. A mathematical model based on this novel DCFC concept has been developed. The model includes governing equations that describe the transport and electrochemical processes taking place in both the anode and cathode and a methodology for solving these equations. Literature correlations from multi-phase packed-bed chemical reactors were used to estimate phase hold-up and mass transfer coefficients. CO production and axial diffusion were neglected. The results demonstrated that activation and ohmic polarization were important to the cell output. The impact of concentration polarization to the cell output was comparatively small. The bed depths realized were of the order of 10cm which is not large enough to accommodate the economies of scale for a large scale plant, however thousands of smaller cells (10 m^2 area) in series could be built to scale up to a 10 MW industrial plant. Limiting current densities of the order of 1000-1500 A/m^2 were achieved for various operating conditions. Maximum power densities of 200-350 W/m^2 with current densities of 500-750 A/m^2, and cell voltages of 0.4-0.5 V have been achieved at a temperature of 700 deg C. Over temperatures ranging from 700 to 800 deg C, results from the modeled cell are comparable with results seen in the literature for direct carbon fuel cells that are similar in design and construction.
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    Development and Applications of the Modular Automotive Technology Testbed (MATT) to Evaluate Hybrid Electric Powertrain Components and Energy Management Strategies
    Lohse-Busch, Henning (Virginia Tech, 2009-05-11)
    This work describes the design, development and research applications of a Modular Automotive Technology Testbed (MATT). MATT is built to evaluate technology components in a hybrid vehicle system environment. MATT can also be utilized to evaluate energy management and torque split control strategies and to produce physical measured component losses and emissions to monitor emissions behavior. In the automotive world, new technology components are first developed on a test bench and then they are integrated into a prototype vehicle for transient evaluation from the vehicle system perspective. This process is expensive and the prototype vehicles are typically inflexible in hardware and software configuration. MATT provides flexibility in component testing through its component module approach. The flexible combination of modules provides a vehicle environment to test and evaluate new technology components. MATT also has an open control system where any energy management and torque split strategy can be implemented. Therefore, the control's impact on energy consumption and emissions can be measured. MATT can also emulate different types and sizes of vehicles. MATT is a novel, unique, flexible and powerful automotive research tool that provides hardware-based data for specific research topics. Currently, several powertrain modules are available for use on MATT: a gasoline engine module, a hydrogen engine module, a virtual scalable energy storage and virtual scalable motor module, a manual transmission module and an automatic transmission module. The virtual battery and motor module uses some component Hardware-In-the-Loop (HIL) principles by utilizing a physical motor powered from the electric grid in conjunction with a real time simulation of a battery and a motor model. This module enables MATT to emulate a wide variety of vehicles, ranging from a conventional vehicle to a full performance electric vehicle with a battery pack that has virtually unlimited capacity. A select set of PHEV research studies are described in this dissertation. One of these studies had an outcome that influenced the PHEV standard test protocol development by SAE. Another study investigated the impact of the control strategy on emissions of PHEVs. Emissions mitigation routines were integrated in the control strategies, reducing the measured emissions to SULEV limits on a full charge test. A special component evaluation study featured in this dissertation is the transient performance characterization of a supercharged hydrogen internal combustion engine on MATT. Four constant air-fuel ratio combustions are evaluated in a conventional vehicle operation on standard drive cycles. Then, a variable air fuel ratio combustion strategy is developed and the test results show a significant fuel economy gain compared to other combustion strategies, while NOx emissions levels are kept low.
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    Development and Evaluation of a Test Apparatus for Fuel Cells
    Davis, Mark William (Virginia Tech, 2000-05-25)
    The development of a test apparatus for proton exchange membrane fuel cells is presented. The design of the prototype device is provided in detail along with a description of the apparatus. The evaluation of the functionality and effectiveness of the device included measurement of a polarization curve for a 5-cell, 1 kW stack. An effective test apparatus is imperative for stack performance testing, model evaluation, and investigation of new fuel cell technology. This apparatus was designed to measure and control the mass flow rates of the reactant gases, gas pressures, gas temperatures, gas relative humidity, stack temperature, stack current, and the coolant water flow rate. Additionally, the test apparatus can measure the stack voltage, coolant water resistivity, coolant water temperature change across the stack, and the coolant water pressure drop across the stack. The apparatus was shown to provide adequate control of all necessary variables for stack performance evaluation.
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    Development and validation of a computational model for a proton exchange membrane fuel cell
    Siegel, Nathan Phillip (Virginia Tech, 2003-05-01)
    A steady-state computational model for a proton exchange membrane fuel cell (PEMFC) is presented. The model accounts for species transport, electrochemical kinetics, energy transport, current distribution, water uptake and release within the polymer portion of the catalyst layers, and liquid water production and transport. Both two-dimensional and three-dimensional geometries are modeled. For a given geometry, the governing differential equations are solved over a single computational domain. For the two-dimensional model, the solution domain includes a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. For the three-dimensional model the current collectors are also modeled on both the anode and cathode sides of the fuel cell. The model for the catalyst layers is based on an agglomerate geometry, which requires water species to exist in dissolved, gaseous, and liquid forms simultaneously. Data related to catalyst layer morphology that was required by the model was obtained via a physical analysis of both commercially available and in-house membrane electrode assemblies (MEA). Analysis techniques including cyclic voltammetry and electron microscopy were used. The coupled set of partial differential equations is solved sequentially over a single solution domain with the commercial computational fluid dynamics (CFD) solver, CFDesign™ and is readily adaptable with respect to geometry and material property definitions. A fuel cell test stand was designed and built to facilitate experimental validation of the model. The test stand is capable of testing cells up to 50 cm2 under a variety of controlled conditions. Model results for both two and three-dimensional fuel cell geometries are presented. Parametric studies performed with the model are also presented and illustrate how fuel cell performance varies due to changes in parameters associated with the transport of reactants and liquid water produced in the cell. In particular, the transport of oxygen, water within the polymer portions of the catalyst layers and membrane, and liquid water within the porous regions of the cell are shown to have significant impact on cell performance, especially at low cell voltage. Parametric studies also address the sensitivity of the model results to certain physical properties, which illustrates the importance of accurately determining the physical properties of the fuel cell components on which the model is based. The results from the three-dimensional model illustrate the impact of the collector plate shoulders (for a conventional flowfield) on oxygen transport and the distribution of current production within the cell.
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    Dynamic Proton Exchange Membrane Fuel Cell System Synthesis/Design and Operation/Control Optimization under Uncertainty
    Kim, Kihyung (Virginia Tech, 2008-01-10)
    Proton exchange membrane fuel cells (PEMFCs) are one of the leading candidates in alternative energy conversion devices for transportation, stationary, and portable power generation applications. PEMFC systems with their own fuel conversion unit typically consist of several subsystems: a fuel processing subsystem, a fuel cell stack subsystem, a work recovery-air supply subsystem, and a power electronics subsystem. Since these subsystems have different physical characteristics, their integration into a single system/subsystem level unit make the problems of dynamic system synthesis/design and operation/control highly complex. Typically, the synthesis/design optimization of energy systems is based on a single full load condition at steady state. However, a more comprehensive synthesis/design and operation/control optimization requires taking into account part as well as full load conditions for satisfying an optimal efficiency/cost/environmental effect objective. Optimal couple of these various aspects of system development requires dynamic system/subsystem/component modeling and a multi-disciplinary approach which results in an integrated set of diverse types of models and highly effective optimization strategies such as decomposition techniques (e.g., Dynamic Iterative Local-Global Optimization: DILGO). In energy system synthesis/design and operation/control problems, system/ component models are typically treated deterministically, even though input values, which include the specific load profile for which the system or subsystem is developed, can have significant uncertainties that inevitably propagate through the system to the outputs. This deficiency can be overcome by treating the inputs and outputs probabilistically. In this work, various uncertainty analysis methodologies are applied; and among these traditional probabilistic approaches (e.g., Monte Carlo simulation) and the response sensitivity analysis (RSA) method are examined to determine their applicability to energy system development. In particular, these methods are used for the probabilistic (non-deterministic) modeling, analysis, and optimization of a residential 5 kWe PEMFC system, and uncertainty effects on the energy system synthesis/design and operation/control optimization have been assessed by taking the uncertainties into account in the objectives and constraints. Optimization results show that there is little effect on the objective (the operating cost and capital cost), while the constraints (e.g., on the CO concentration) can be significantly affected during the synthesis/design and operation/control optimization.
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    Dynamic Synthesis/Design and Operation/Control Optimization Approach applied to a Solid Oxide Fuel Cell based Auxiliary Power Unit under Transient Conditions
    Rancruel, Diego Fernando (Virginia Tech, 2005-02-11)
    A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, transient regimes may happen quite often and the system response to them is a critical factor in determining the system's feasibility. Therefore, it is important to consider the system dynamics in the creative process of developing the system. A decomposition approach for dynamic optimization developed and applied to the synthesis/design and operation/control optimization of a solid oxide fuel cell (SOFC) based auxiliary power unit (APU) is the focus of this doctoral work. Called DILGO (Dynamic Iterative Local-Global Optimization), this approach allows for the decomposed optimization of the individual units (components, sub-systems or disciplines), while taking into account the intermediate products and feedbacks which couple all of the units which make up the overall system. The approach was developed to support and enhance current engineering synthesis/design practices by making possible dynamic modular concurrent system optimization. In addition, this approach produces improvements in the initial synthesis/design state at all stages of the process and allows any level of complexity in the unit's modeling. DILGO uses dynamic shadow price rates as a basis for measuring the interaction level between units. The dynamic shadow price rate is a representation of the unit's cost rate variation with respect to variations in the unit's coupling functions. The global convergence properties of DILGO are seen to be dependent on the mathematical behavior of the dynamic shadow price rate. The method converges to a "global" (system-level) optimum provided the dynamic shadow price rates are approximately constant or at least monotonic. This is likely to be the case in energy systems where the coupling functions, which represent intermediate products and feedbacks, tend to have a monotonic behavior with respect to the unit's local contribution to the system's overall objective function. Finally, DILGO is a physical decomposition used to solve system-level as well as unit-level optimization problems. The total system considered here is decomposed into three sub-systems as follows: stack sub-system (SS), fuel processing sub-system (FPS), and the work and air recovery sub-system (WRAS). Mixed discrete, continuous, and dynamic operational decision variables are considered. Detailed thermodynamic, kinetic, geometric, physical, and cost models for the dynamic system are formulated and implemented. All of the sub-systems are modeled using advanced state-of-the-art tools. DILGO is then applied to the dynamic synthesis/design and operation/control optimization of the SOFC based APU using the total life cycle cost as objective function. The entire problem has a total of 120 independent variables, some of which are integer valued and dynamic variables. The solution to the problem requires only 6 DILGO iterations.
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    Dynamics of Micro-Particles in Complex Environment
    Yang, Fengchang (Virginia Tech, 2017-07-21)
    Micro-particles are ubiquitous in microsystems. The effective manipulation of micro-particles is often crucial for achieving the desired functionality of microsystems and requires a fundamental understanding of the particle dynamics. In this dissertation, the dynamics of two types of micro-particles, Janus catalytic micromotors (JCMs) and magnetic clusters, in complex environment are studied using numerical simulations. The self-diffusiophoresis of JCMs in a confined environment is studied first. Overall, the translocation of a JCM through a short pore is slowed down by pore walls, although the slowdown is far weaker than the transport of particles through similar pores driven by other mechanisms. A JCM entering a pore with its axis not aligned with the pore axis can execute a self-alignment process and similar phenomenon is found for JCMs already inside the pore. Both hydrodynamic effect and 'chemical effect', i.e., the modification of the concentration of chemical species around JCMs by walls and other JCMs, play a key role in the observed dynamics of JCMs in confined and crowded environment. The dynamics of bubbles and JCMs in liquid films covering solid substrates are studied next. A simple criterion for the formation of bubbles on isolated JCMs is developed and validated. The anomalous bubble growth law (r~t^0.7) is rationalized by considering the relative motion of growing bubbles and their surrounding JCMs. The experimentally observed ultra-fast collapse of bubbles is attributed to the coalescence of the bubble with the liquid film-air interface. It is shown that the collective motion of JCMs toward a bubble growing on a solid substrate is caused by the evaporation-induced Marangoni flow near the bubble. The actuation of magnetic clusters using non-uniform alternating magnetic fields is studied next. It is discovered that the clusters' clockwise, out-of-plane rotation is a synergistic effect of the magnetophoresis force, the externally imposed magnetic torque and the hydrodynamic interactions between the cluster and the substrate. Such a rotation enables the cluster to move as a surface walker and leads to unique dynamics, e.g., the cluster moves away from the magnetic source and its trajectory exhibits a periodic fluctuation with a frequency twice of the field frequency.