A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item EFFECTS OF EXTERNAL PRESSURE ON SOLID STATE DIFFUSION OF LITHIUM IN LITHIUM-ION BATTERIES(2016) Williard, Nicholas Dane; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electrochemical-mechanical effects in lithium-ion batteries refer to the phenomena that give way to the piezo-electrochemical properties observed during intercalation of lithium into lithium-ion battery electrodes. By applying perturbations to the external pressure of a lithium-ion battery, the dynamics of lithium intercalation, in particular the diffusion rate of lithium-ions onto and out of battery electrodes, can be studied with respect to the open-circuit potential and the applied hydrostatic pressure. In this study, commercial thin film batteries were subjected to tests in a low-pressure chamber and in a dynamic materials analyzer simulating hydrostatic pressures between 0 and 115 KPa. Under each hydrostatic pressure condition, galvanostatic intermittent titration technique (GITT) was performed to measure and correlate lithium diffusivity to battery strain, open-circuit potential, and applied hydrostatic force. From the data a model was developed for lithium diffusivity as a function of open circuit potential and hydrostatic pressure. The implications of this work extend from the use of lithiated graphite for energy harvesting and actuation to policy and regulations for how batteries should be safely transported. To provide some insight into how this work can be applied to policy actions, current international regulations regarding the air transport of lithium-ion batteries are critically reviewed. The pre-shipping tests are outlined and evaluated to assess their ability to fully mitigate risks during battery transport. In particular, the guidelines for shipping second-use batteries are considered. Because the electrochemical state of previously used batteries is inherently different from that of new batteries, additional considerations must be made to evaluate these types of cells. Additional tests are suggested that evaluate the risks of second-use batteries, which may or may not contain incipient faults. Finally, this work is extended to supercapacitors through the development of a model to predict the oxidation of functional groups on the surface of graphite electrodes with respect to operational temperature and voltage. This model is used to predict the operational life of supercapacitors and validates the model on accelerated testing data. The final results are compared to previous models proposed in literature.Item Diffusion Dynamics in Interconnected Communities(2015) Wei, Xiaoya; Abed, Eyad H.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, multi-community-based Susceptible-Infected-Recovered (SIR) and Susceptible-Infected-Susceptible (SIS) models of infection/innovation diffusion are introduced for heterogeneous social networks in which agents are viewed as belonging to one of a finite number of communities. Agents are assumed to have well-mixed interactions within and between communities. The communities are connected through a backbone graph which defines an overall network structure for the models. The models are used to determine conditions for outbreak of an initial infection. The role of the strengths of the connections between communities in the development of an outbreak as well as long-term behavior of the diffusion is also studied. Percolation theory is brought to bear on these questions as an independent approach separate from the main dynamic multi-community modeling approach of the dissertation. Results obtained using both approaches are compared and found to be in agreement in the limit of infinitely large populations in all communities. Based on the proposed models, three classes of marketing problems are formulated and studied: referral marketing, seeding marketing and dynamic marketing. It is found that referral marketing can be optimized relatively easily because the associated optimization problem can be formulated as a convex optimization. Also, both seeding marketing and dynamic marketing are shown to enjoy a useful property, namely ``continuous monotone submodularity." Based on this property, a greedy heuristic is proposed which yields solutions with approximation ratio no less than 1-1/e. Also, dynamic marketing for SIS models is reformulated into an equivalent convex optimization to obtain an optimal solution. Both cost minimization and trade-off of cost and profit are analyzed. Next, the proposed modeling framework is applied to study competition of multiple companies in marketing of similar products. Marketing of two classes of such products are considered, namely marketing of durable consumer goods (DCG) and fast-moving consumer goods (FMCG). It is shown that an epsilon-equilibrium exists in the DCG marketing game and a pure Nash equilibrium exists in the FMCG marketing game. The Price of Anarchy (PoA) in both marketing games is found to be bounded by 2. Also, it is shown that any two Nash equilibria for the FMCG marketing game agree almost everywhere, and a distributed algorithm converging to the Nash equilibrium is designed for the FMCG marketing game. Finally, a preliminary investigation is carried out to explore possible concepts of network centrality for diffusions. In a diffusion process, the centrality of a node should reflect the influence that the node has on the network over time. Among the preliminary observations in this work, it is found that when an infection does not break out, diffusion centrality is closely related to Katz centrality; when an infection does break out, diffusion centrality is closely related to eigenvector centrality.Item Gasification and Combustion of Large Char Particles and Tar(2015) Molintas, Henry; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although diffusion is known to play an important role for gasification and combustion of large char particles, their effects on conversion rates, kinetic parameters and other relevant factors have not been thoroughly analyzed. Similarly, tar reduction is not yet well understood. Central to these challenges is the shortage of experimental data for reduction of tar and large char particles. Likewise, analytical models for reduction processes have not been systematically examined. In this study, large char particles between 1.5 to 7 mm are gasified and combusted non-isothermally with initial temperatures up to 1000 degree celcius using various oxidants. Tar is also reduced with steam and vitiated air continuously and non-isothermally. In the absence of mathematical tools for large particle reduction analysis, models are proposed and derived in this study. Carbon and large near-spherically or irregularly shaped particles are modeled as large disk-shaped and spherically-shaped particles, respectively. One-film ash segregated core and random pore models are explored to analyze char reduction data and these are found to provide consistent and inconsistent results, respectively. Thiele analysis is also used and it indicates that less porous particles are consumed more externally at the surface than internally. For C + O2⇒ CO2 reductions, disk-shaped particles ignite when reactor temperature reaches 584 degree and these processes are purely kinetic controlled for 1.5 mm thick samples. Reduction of spherically-shaped particles shows that O2 enrichment as compared to a 50 degree celcius rise in reactor temperature substantially improves conversion. Oxygen enrichment with steam also significantly increases conversion of 5.5 mm thick disk-shaped particle up to 600 % under identical reactor conditions. For C + CO2⇒2CO reductions, conversion rates increased five-fold when reactor temperature is increased from 850 to 1000 degree Celsius. Increasing initial reactor temperatures and O2 enrichment provide an increase in char reactivity, diffusional rate, conversion, reduction rate and surface temperature. Most of the large particle reductions investigated here operate near kinetic-diffusion controlled regime. Calculated total energy released during combustion is within the range of Dulong’s empirical formula. At higher tar concentrations, CO and H2 production moderately increase between 814 to 875 degree celsius.Item SMOKE POINTS OF MICROGRAVITY AND NORMAL GRAVITY COFLOW DIFFUSION FLAMES(2009) Dotson, Keenan Thomas; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Smoke points were measured in microgravity aboard the International Space Station (ISS) as part of the Smoke Points in Coflow Experiment (SPICE), and in normal gravity conditions. In microgravity conditions increasing the coflow velocity or decreasing the burner diameter increased the smoke point flame length. A simplified prediction of centerline jet velocity did not yield residence-time-based criticalities or data collapse. Simulation of non-reacting flows showed that the simplified centerline velocity prediction was able to predict velocity decay for only relatively weak coflows. An improved model may yield different results. In normal earth gravity coflow velocity exhibited mixed effects. For burner diameters of 0.41, 0.76, and 1.6 mm, smoke points increased with increases of coflow velocity. For an unconfined coflow burner with a burner diameter of 13.7 mm smoke point length decreased with increasing coflow velocity for ethylene and propylene, while increasing for propane flames.Item Numerical Simulation of Low-Pressure Explosive Combustion in Compartment Fires(2008-11-19) Hu, Zhixin (Victor); Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A filtered progress variable approach is adopted for large eddy simulations (LES) of turbulent deflagrations. The deflagration model is coupled with a non-premixed combustion model, either an equilibrium-chemistry, mixture-fraction based model, or an eddy dissipation model. The coupling interface uses a LES-resolved flame index formulation and provides partially-premixed combustion (PPC) modeling capability. The PPC sub-model is implemented into the Fire Dynamic Simulator (FDS) developed by the National Institute of Standards and Technology, which is then applied to the study of explosive combustion in confined fuel vapor clouds. Current limitations of the PPC model are identified first in two separate series of simulations: 1) a series of simulation corresponding to laminar flame propagation across homogeneous mixtures in open or closed tunnel-like configurations; and 2) a grid refinement study corresponding to laminar flame propagation across a vertically-stratified layer. An experimental database previously developed by FM Global Research, featuring controlled ignition followed by explosive combustion in an enclosure filled with vertically-stratified mixtures of propane in air, is used as a test configuration for model validation. Sealed and vented configurations are both considered, with and without obstacles in the chamber. These pressurized combustion cases present a particular challenge to the bulk pressure algorithm in FDS, which has robustness, accuracy and stability issues, in particular in vented configurations. Two modified bulk pressure models are proposed and evaluated by comparison between measured and simulated pressure data in the Factory Mutual Global (FMG) test configuration. The first model is based on a modified bulk pressure algorithm and uses a simplified expression for pressure valid in a vented compartment under quasi-steady conditions. The second model is based on solving an ordinary differential equation for bulk pressure (including a relaxation term proposed to stabilize possible Helmholtz oscillations) and modified vent flow velocity boundary conditions that are made bulk-pressure-sensitive. Comparisons with experiments are encouraging and demonstrate the potential of the new modeling capability for simulations of low pressure explosions in stratified fuel vapor clouds.Item An Experimental and Theoretical Investigation of the Low Temperature Creep Deformation Behavior of Single Phase Titanium Alloys(2006-10-26) Oberson, Paul Gregory; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Titanium alloys are used for many applications due to their desirable properties, including high strength-to-weight ratio, corrosion resistance, and biocompatibility. They are used for aerospace, chemical, nuclear, industrial, biomedical, and consumer applications. Often, titanium components are subject to stresses for an extended time. It is known that single-phase hexagonally close-packed (HCP) alpha and body-centered cubic (BCC) beta-titanium alloys deform over time, or creep, at low temperatures (<0.25*Tm). However, factors that affect creep behavior including microstructure and alloy chemistry are not well understood. The aim of this investigation is to experimentally and theoretically study the creep deformation behavior of single-phase alpha and beta-titanium alloys. The first part of the investigation concerns alpha-Ti alloys. The low temperature creep behavior was studied experimentally, using alpha-Ti-1.6wt.%V as the model alloy. Creep testing was performed at a range of temperatures and slip and twinning were identified as creep deformation mechanisms. The activation energy for creep was measured for the first time for an alpha-Ti than deforms by twinning. A change in activation energy during creep is explained by a model for twin nucleation caused by dislocation pileups. The theoretical aspect of the investigation concerns the phenomenon of slow twin growth (time-dependent twinning) during low temperature creep of alpha and beta-Ti alloys. This phenomenon is unusual and poorly understood as twins in bulk metals are expected to grow very fast. It was suggested that interstitial atoms, particularly oxygen could be responsible for time-dependent twinning but there were no models to explain this. As such, crystallographic models were developed for the HCP lattice of alpha-Ti and the BCC lattice of beta-Ti to show how the octahedral interstitial sites where atoms such as oxygen can reside are eliminated by the atomic movements associated with twinning. As such, the rate of twin growth, and in turn the creep strain rate is controlled by the diffusion of oxygen away from these eliminated sites. The results of these findings are valuable when designing Ti alloys for improved creep resistance and mechanical reliability. This work was supported by the National Science Foundation under Grant Number DMR-0513751.