Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item MICROBIAL BIOFILMS ON MICROPLASTICS: A LOOK INTO THE ESTUARINE PLASTISPHERE OF THE CHESAPEAKE BAY(2021) Sosa , Ana Paula; Chen, Feng; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microplastics are plastic particles that are smaller than 5 millimeters and are often found as pollution in our waterways. These polymer particles are globally distributed and are a direct result of human activity. Because of their rigidity and durability, microplastics are an ideal substrate for enhanced microbial growth and biofilm development. While microplastics have been studied in various contexts, only few studies have characterized the microbial communities on different types of plastic particles, but no study has been done in the estuarine water. In this study, we exposed three different types of plastics (polypropylene, polystyrene, and polylactic acid) to the water of Baltimore’s Inner Harbor, along with a non-plastic glass control. We used both in situ and in vitro incubations to understand the development of biofilm communities on microplastics. Microbial communities were analyzed based on the 16S rRNA gene sequences. We found that microbial composition on biofilm is distinct from that in the surrounding water, and different microplastic types have a minor impact on the composition of biofilm communities. The similarity between microbial communities on plastic and non-plastic particles suggests that surface supports rather than material types could be more critical for biofilm formation. Succession of microbial communities on the microplastics and interesting bacterial groups were described. Isolation and microscopic observations were also applied in this study. The presence of phototrophic organisms like filamentous cyanobacteria and Auxenochlorella on microplastic biofilms is interesting, and little is known about their contribution to carbon fixation in the ocean. Biofilms formed on microplastic surfaces could potentially affect the ecosystems via different mechanisms, including local nutrient cycling and the transportation of invasive or harmful species. As plastic production and mismanagement continues to be pervasive in our society, it is paramount that we include biofilm development into the framework of general ecology in order to truly understand the impact of plastic pollution and safeguard our ecosystems.Item INVESTIGATION INTO PYROLYSIS AND GASIFICATION OF SOLID WASTE COMPONENTS AND THEIR MIXTURES(2021) Burra, Kiran Raj Goud; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unsustainable dependence on fossil fuel reserves for energy and material demands is leading to growing amounts of CO2 concentration in the atmosphere and irreversible climate changes. Carbon neutral sources such as abundant biomass reserves and landfill-destined high energy density wastes such as plastics, and tire-wastes can be utilized together for energy and material production for a sustainable future. Pyrolysis and gasification can convert these variable feedstocks into valuable and uniform synthetic gas (syngas) with versatile downstream applicability to energy, liquid fuels, and other value-added chemicals production. But seasonal availability, high moisture and ash content, and relatively low energy density of biomass can result in significant energy and economic losses during gasification. Furthermore, gasification of plastic wastes separately was found to result in feeding issues due to melt-phase, coking, and agglomerative behavior leading to operational issues. To resolve these issues, co-processing of biomass with these plastics and rubber wastes was found to be promising in addition to providing synergistic interaction leading to enhanced syngas yield and inhibitive behavior in some cases and thus motivating this work. This dissertation provides a deconvoluted understanding and quantification of the source and impact of these interactions for better process performance and alleviation of inhibitive interaction needed to develop reliable co-gasification of feedstock mixtures. To achieve this, plastic and tire wastes were investigated separately and mixed with different biomass species using a series of feedstock arrangements to understand synergistic influence on the syngas yield and kinetics in comparison to mono-conversion. Influence of operating conditions such as feedstock composition, temperature and gasifying agent was also examined for desirable conditions of energy recovery and high-quality syngas yield. Lab-scale semi-batch reactor studies equipped with online product gas analysis, along with thermogravimetric studies were utilized to obtain insight into the products yield, kinetics, and energy conversion. These results provided a better understanding of the influence of feedstocks and their interaction on the syngas and process behavior. They address the knowledge gap in versatile feedstock-flexible gasifier development for efficient and reliable syngas production from varying solid waste and biomass component mixtures with minimal changes to the operating conditions.Item QUASIPARTICLES IN SUPERFLUIDS AND SUPERCONDUCTORS(2020) Curtis, Jonathan; Galitski, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quasiparticle descriptions are a powerful tool in condensed matter physics as they provide an analytical treatment of interacting systems. In this thesis we will apply this tool to theoretically describe two systems: a superconductor interacting with cavity photons and a flowing Bose-Einstein condensate forming a sonic black hole. First we will consider a two-dimensional s-wave BCS superconductor coupled to microwave cavity photons. We show how a nonequilibrium occupation of the photons can induce a nonequilibrium distribution of superconductor Bogoliubov quasiparticles, yielding an enhancement of the superconducting gap. The analytic dependence of this enhancement is provided in terms of the photon spectral and occupation functions, offering a large parameter space over which enhancement exists. Next, we analyze the equilibrium properties of a similar superconductor-cavity structure which has strong sub-dominant d-wave pairing interaction. In this case there is a collective mode known as the Bardasis-Schrieffer mode, which is essentially an uncondensed d-wave Cooper pair. We show that by driving an external supercurrent through the sample the Bardasis-Schrieffer mode can be hybridized with cavity photons, forming exotic Bardasis-Schrieffer-polaritons. We then turn to consider a flowing Bose-Einstein condensate. In the presence of inhomogeneous flow, the long-wavelength motion of quasiparticles can be mapped onto the kinematics of matter fields in a curved spacetime. This mapping allows for the simulation of a black hole and its interactions with quantum fields via analogy. We show that in the case of a step-like jump in the condensate flow the emission of analogue Hawking radiation is accompanied by evanescent modes which are pinned to the event horizon. Finally, we generalize this setup to include pseudo-spin half spinor Bose condensates. In this case, we show that the analogue spacetime the quasiparticles experience can be of the exotic Newton-Cartan type. Newton-Cartan gravity -- the geometric formulation of Newtonian gravity -- is realized when the Goldstone mode disperses quadratically as opposed to linearly. The nature of the analogue spacetime is controlled by the presence or absence of an easy-axis anisotropy in the boson spin-exchange interaction. We conclude by arguing that this Newton-Cartan spacetime can be experimentally realized in current platforms.Item Investigation of the Effect of Oxygen Concentration on the Pyrolytic Decomposition of Polypropylene(2018) Turner, Brent Allen; Stoliarov, Stanislav; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Limited research exists on the effect of oxygen on the species production during the controlled surface area pyrolytic decomposition of polypropylene. In this study, the pyrolytic decomposition of polypropylene was conducted in 0% O2, 5% O2, and 15% O2. The pyrolyzate produced during the experiments was analyzed using three methods. First, a custom tube-furnace reactor, auto-sampling system, and unique sample boat were developed to pyrolyze, collect, and deliver pyrolyzate to a GC-BID/MS for species identification and quantification. Data collected were converted to rates of production and mass evolved for individual species identified. Second, using the same tube-furnace reactor pyrolyzate was sent directly to a stack of IR and FID analyzers to measure O2, CO, CO2, and total hydrocarbon production. This data was converted and used to compare with and verify the data from the GC-BID analysis. Thermogravimetric analysis was used as a third technique to measure the mass loss of the polypropylene under the three O2 scenarios. For all three analytical methods, the effect of O2 was studied and was found to have a profound effect on species evolution and the temperature at which the reactions initiated.Item Prediction of Upward Flame Spread over Polymers(2016) Leventon, Isaac Tibor; Stoliarov, Stanislav I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, the existing understanding of flame spread dynamics is enhanced through an extensive study of the heat transfer from flames spreading vertically upwards across 5 cm wide, 20 cm tall samples of extruded Poly (Methyl Methacrylate) (PMMA). These experiments have provided highly spatially resolved measurements of flame to surface heat flux and material burning rate at the critical length scale of interest, with a level of accuracy and detail unmatched by previous empirical or computational studies. Using these measurements, a wall flame model was developed that describes a flame’s heat feedback profile (both in the continuous flame region and the thermal plume above) solely as a function of material burning rate. Additional experiments were conducted to measure flame heat flux and sample mass loss rate as flames spread vertically upwards over the surface of seven other commonly used polymers, two of which are glass reinforced composite materials. Using these measurements, our wall flame model has been generalized such that it can predict heat feedback from flames supported by a wide range of materials. For the seven materials tested here – which present a varied range of burning behaviors including dripping, polymer melt flow, sample burnout, and heavy soot formation – model-predicted flame heat flux has been shown to match experimental measurements (taken across the full length of the flame) with an average accuracy of 3.9 kW m-2 (approximately 10 – 15 % of peak measured flame heat flux). This flame model has since been coupled with a powerful solid phase pyrolysis solver, ThermaKin2D, which computes the transient rate of gaseous fuel production of constituents of a pyrolyzing solid in response to an external heat flux, based on fundamental physical and chemical properties. Together, this unified model captures the two fundamental controlling mechanisms of upward flame spread – gas phase flame heat transfer and solid phase material degradation. This has enabled simulations of flame spread dynamics with a reasonable computational cost and accuracy beyond that of current models. This unified model of material degradation provides the framework to quantitatively study material burning behavior in response to a wide range of common fire scenarios.Item A Generalized Model for Wall Flame Heat Flux During Upward Flame Spread on Polymers(2015) Korver, Kevin; Stoliarov, Stanislav; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A current model accurately predicts flame to surface heat flux during upward flame spread on PMMA based on a single input parameter, the mass loss rate. In this study, the model was generalized to predict the heat flux for a broad range of polymers by adding the heat of combustion as a second input parameter. Experimental measurements were conducted to determine mass loss rate during upward flame spread and heat of combustion for seven different polymers. Four types of heat of combustion values were compared to determine which generated the most accurate model predictions. The complete heat of combustion yielded the most accurate predictions (± 4 kW/m2 on average) in the generalized model when compared to experimental heat flux measurements collected in this study. Flame heat flux predictions from FDS direct numerical simulations were also compared to the generalized model predictions in an exploratory manner and found to be similar.Item A Non-Intrusive Method for Temperature Measurements in Flames Produced by Milligram-Sized Solid Samples(2014) Frances, Colleen; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fires are responsible for the loss of thousands of lives and billions of dollars in property damage each year in the United States. Flame retardants can assist in the prevention of fires through mechanisms which either prevent or greatly inhibit flame spread and development. In this study samples of both brominated and non-brominated polystyrene were tested in the Milligram-scale Flaming Calorimeter and images captured with two DSL-R cameras were analyzed to determine flame temperatures through use of a non-intrusive method. Based on the flame temperature measurement results, a better understanding of the gas phase mechanisms of flame retardants may result, as temperature is an important diagnostic in the study of fire and combustion. Measurements taken at 70% of the total flame height resulted in average maximum temperatures of about 1656 K for polystyrene and about 1614 K for brominated polystyrene, suggesting that the polymer flame retardant may reduce flame temperatures.Item A MULTISCALE APPROACH TO PARAMETERIZATION OF BURNING MODELS FOR POLYMERIC MATERIALS.(2014) Li, Jing; Stoliarov, Stanislav I.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A quantitative understanding of the processes that occur in the condensed phase of burning materials is critical for the prediction of ignition and growth of fires. A number of models have been developed to simulate these condensed phase processes. The main issue that remains to be resolved is the determination of parameters to be input into these models, which are formulated in terms of fundamental physical and chemical properties. This work is focused on developing and applying a systematic methodology for the characterization of polymeric materials based on milligram-scale and bench-scale tests to isolate specific chemical and/or physical processes in each scale level. The entire study is divided into two parts corresponding to two different scale tests and analysis. The first part is concentrated on the measurement of kinetics and thermodynamics of the thermal degradation of polymeric materials at milligram-scale. It employs a simultaneous thermal analysis instrument capable of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A numerical model is utilized to fit TGA data and obtain thermal degradation kinetics to a continuum pyrolysis model. This model is subsequently employed to analyze DSC heat flow and extract sensible, melting and degradation reaction heats. The extracted set of kinetic and thermodynamic parameters is shown to simultaneously reproduce TGA and DSC curves for a set of 15 widely used commercial polymers. Then the first part of this study was extended to bench-scale gasification experiments that were carried out in a controlled atmosphere pyrolysis apparatus (CAPA) which has been recently developed in our group. The CAPA is used to measure material gravimetric and thermal changes during thermal decomposition in an anaerobic atmosphere with a capability of analyzing material thermal transport properties. These properties, combined with material kinetics and thermodynamics from the first part of this study, were used as inputs for a pyrolysis model to simulate one-dimensional polymer gasification under wide range of external heat fluxes. The predictive power of this model and validity of its parameters are verified against the results of gasification experiments. 7 out of 15 polymers were validated in bench-scale and the parameterized simulations are in reasonable agreement with experimental data under wide range of conditions.Item EVOLUTION OF FLAME TO SURFACE HEAT FLUX DURING UPWARD FLAME SPREAD ON POLYMETHYL METHACRYLATE (PMMA)(2011) Leventon, Isaac Tibor; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The heat feedback profile across 5cm wide, 15cm tall samples of PMMA is measured as a flame spreads vertically across its surface. Incident heat flux to a water cooled gauge is determined with peak values averaging to 36kW/m^2 across the height of the sample. This heat flux has been separated into its convective and radiative components and, at this scale, radiative heat transfer is shown to account for between 5 and 15% of total flame to surface heat flux. Based on these measurements, net heat flux into the pyrolyzing material can be determined. Correlations, expressed solely as a function of sample burning rate, predicting net heat feedback to the material's surface are developed.Item HETEROGENEOUS POLYMERIZATION OF METHYL METHACRYLATE AT LOW TEMPERATURE IN DISPERSED SYSTEMS(2011) EMDADI, LALEH; CHOI, KYU YONG; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ABSTRACT Title of thesis HETEROGENEOUS POLYMERIZATION OF METHYL METHACRYLATE AT LOW TEMPERATURE IN DISPERSED SYSTEMS Laleh Emdadi, Master of Science, 2011 Directed by: Professor, Dr. Kyu Yong Choi, Chemical and Biomolecular Engineering Department Dispersion polymerization is a unique method to prepare monodisperse polymer particles of 1-10 µm in a single step process. This process is usually carried out at high temperatures that are not cost effective and suitable for special applications such as encapsulation of bio materials. Production of uniform polymer particles at low temperatures via dispersion polymerization has not been studied widely yet. In this research, dispersion polymerization of methyl methacrylate (MMA) in a nonpolar solvent, n-hexane, using N,N-dimethylaniline (DMA) and lauroyl peroxide (LPO) as redox initiators at low temperature has been studied. The evolutions of monomer conversion, polymer molecular weight distribution (MWD), and particle morphology were determined. Under specific reaction conditions, monodisperse micron-sized polymer particles were produced. The same technique was applied in the confined reaction space of a monomer droplet. Using this new process, called micro dispersive suspension polymerization, polymer particles with different internal morphologies produced with various potential applications.