Now showing items 1-20 of 214

• #### Computational modeling of barometric pressure fluctuation effects on explosive methane-air mixtures in a longwall mine gob

The coal mining industry remains a major player in electrical power generation for the United States. The U.S. Energy Information Administration (EIA) reported that coal made up the second largest share after natural gas, approximately 30%, of the total electricity generated in 2016, further, it projected to remain significant for the next several decades together with natural gas and renewable sources (U.S. Energy Information Administration, 2017). Longwall mining has become more frequently used for extracting coal seams underground due to increased safety and higher productivity (EMFI, 2010) through improved mechanization and mining system automation. In the longwall method, the overlying strata are allowed to cave behind the working face after the coal is extracted, creating a porous zone known as gob. Due to its caving nature, the gob is inaccessible and presents a safety concern because of the potential methane and explosive mixture accumulation occurring in this zone. There have been a great number of mine fires and explosions in the United States and other countries that suggest the existence of an explosive mixture or Explosive Gas Zone (EGZ) in bleeder-ventilated longwall gob (Loane et al., 1975; Lynn et al., 1986; McKinney et al., 2001; Dziurzynski and Wasilewski, 2012; Brune, 2013). These events indicated that EGZs must have existed inside and around the perimeter of gobs. The risk of mine explosion will increase if these EGZs flow out from the gob to the surrounding mine entries. As mine pressure is influenced by atmospheric pressure, the fluctuation of outside pressure can disturb the pressure differential between the internal and external gob which consequently can either escalate or lessen the EGZ outgassing from the gob. A number of studies have related major mine explosions to abrupt barometric pressure changes in the United States, South Africa, Australia, and Poland (Hosler, 1948; Boyer, 1964; Kissell et al., 1973; Fauconnier, 1992; Hemp, 1994; Wasilewski, 2014; Belle, 2014; and Lolon et al., 2015). These disastrous mine explosions seem to happen frequently in stormy weather, which occur during late fall and winter seasons in the United States. Numerical modeling using computational fluid dynamics (CFD) is considered the best suited approach for simulating methane air flow and mixture in the gob due to its inaccessibility, which limits the availability of gob data. CFD modeling has been widely used for mine ventilation analysis around the globe (Brunner et al., 1995; Wala et al., 1997; Balusu et al., 2002; Calizaya et al., 2004; Lowndes et al., 2005; Yuan et al., 2006; and Ren, 2010). CFD models were developed in this research to analyze the formation of EGZ in the bleeder-ventilated gob, the effects of various magnitudes and rates of barometric pressure change on the EGZ and methane outgassing, and the potential location of the outgassing. The model was built based on an actual supercritical longwall mine in the western United States, characterized by the condition of no overlying strata hanging above the shields. The ventilation parameters and gob permeability characteristics were obtained from cooperating mines which had been studied by earlier researchers in this project (Worrall, 2012; Wachel, 2012; Marts, 2015; Gilmore, 2015). Other parameters not available in the cooperating mine such as location and operating pressure of methane source, and pressure drops of regulators and other ventilation controls were obtained from reliable references or similar studies, or calculated to be deemed reasonable. The outputs of CFD modeling indicated fringes of EGZ within the gob and several back-end crosscuts on the tailgate return under static atmospheric pressure condition, confirming an observation reported by several explosion investigations and studies of EGZ existence within the gob (Elkins et al., 2001; McKinney at al., 2001; Cario, 2006; Woodward and Sheffield, 2008; Page et al., 2011; Brune, 2013). The CFD models show that, under normal bleeder ventilation systems, it is very likely that EGZ is formed within the gob as a result of ventilation air ingressing into the gob and mixing with methane. The CFD models further identified that the methane and EGZ outgassing occurs in barometric pressure fall scenarios. The rise of atmospheric pressures is a preferable situation with respect to EGZ outgassing; CFD models demonstrated decreasing EGZ volumes within the gob and tailgate return crosscuts associated with barometric or external pressure increases. Therefore, this research focused more on the barometric fall scenarios than the cases where it increased. The hypotheses of gob breathing and outgassing under various scenarios of pressure changes were also proved by analyzing the modeling outputs. The amount of EGZ outflow depended on the magnitude of the pressure drop. The rate of outgassing was controlled by the rate of pressure fall. The location of EGZ outgassing was mainly in the tailgate and bleeder returns. Gob permeability is major assumption in this modeling, thus a parametric study is presented to discuss the effect of permeability on the base EGZ cases. The study shows larger EGZ volume within the gob with higher permeability. Higher permeable gob allowed more air ingression into the gob which, mixed with methane, resulted in larger EGZ. At the end, the results were validated against data provided by cooperating mines and also compared with published data. The main objective of this research is to provide the mining industry, its stakeholders, and the body of knowledge with a better understanding of gob breathing, EGZ outgassing and the methane explosion risk associated with barometric pressure changes and, ultimately, recommendations of best practices to prevent outgassing from resulting in a mine explosion. The best practices recommended include the risk matrix, a continuous monitoring system, and Gob Ventilation Borehole (GVB) application.

• #### Fe-Zn phase evolution and cracking behavior in Zn-coated press-hardened steel

Zinc coated press-hardened steel (PHS) sheet used for the production of strong, corrosion resistant parts is of interest to the automotive community, but concerns about liquid metal embrittlement (LME) remain. Specifically, mitigation of the cracking associated with LME, via better understanding of the alloyed coating microstructural features and interaction with the substrate sheet during deformation, is desired. The objective of this work was to relate the microstructural evolution in the Fe Zn coatings heat treated under single or two step thermal processing to the cracking response in specimens deformed in uniaxial tension at elevated temperature. A Gleeble® 3500 was used to heat treat galvanized 22MnB5 samples at hold times and temperatures relevant to new processes recently implemented in some hot stamping lines. The alloying achieved during heating and isothermal holding of specimens was assessed, and used to interpret the cracking response in the coating (and substrate) of specimens deformed at high temperature. Heat treatments using systematic heating rates were conducted to investigate the Fe Zn phase development during heating to elevated temperatures relevant to press-hardening. The specimen heated at the slowest rate (2 ºC/s) was comprised of the most Fe rich phases (Γ and αFe(Zn)). Comparison of the compositions of these phases to the Fe Zn phase diagram at elevated temperature (775 ºC), suggested that this coating was mostly solid at elevated temperature, and thus may also have had little Zn rich liquid upon reaching target temperature. The minimized Zn-rich liquid at elevated temperature indicated that minimal soak time would have been necessary to eliminate Zn rich liquid in the coating prior to deformation at this temperature. Specimens were also heat treated via single or two step isothermal profiles (with and without deformation), and the phases were identified and quantified to understand the state of the coating microstructure at each room and elevated temperature. Fractions of the phases developed in the alloyed coating were measured using quantitative image analysis. The fraction of δ (most Zn rich phase identified in the coatings) appeared to decrease, while the fraction of Γ and αFe(Zn) appeared to increase, with increased soak time and temperature. The δ phase was not observed in specimens heat treated at the highest time/temperature combinations (850 °C for 60-120 s), suggesting that the greatest degree of alloying occurred in these specimens. The fraction of the δ+Γ1 phases present in each specimen are suggested to contribute to the amount of Zn rich liquid present at the highest deformation temperature (700 °C), and thereby used to estimate the amount of Zn rich liquid available during elevated temperature tensile tests. The thickness of the αFe(Zn) layer also increased with increased hold time and temperature. The αFe(Zn) layer thickness in undeformed versus deformed specimens and composition gradient across the layer were measured and compared to cracking behavior of the layer. At the highest deformation temperature (700 °C), the αFe(Zn) layer exhibited cracks after intermediate and extended hold times (60 and 120 s); these specimens also exhibited the higher αFe(Zn) layer thicknesses and slightly narrower composition gradients compared to specimens deformed at the same temperature after shorter soak times. The cracking behavior observed in the deformed specimens was related to the microstructural evolution of the coating based on the inferred amount of liquid present at elevated temperature and the relative plasticity of the αFe(Zn) layer. In most cases, deep cracking into the substrate sheet associated with LME was avoided. Crack mitigation during deformation was proposed to be a result of: reduction of Zn rich liquid via increased alloying or reduced deformation temperature, or retention of a continuous αFe(Zn) layer between the alloyed coating and substrate, that served as a barrier layer to prevent contact between the Zn rich liquid and Fe-substrate.
• #### Open pit mine planning with stockpiling

This dissertation consists of three papers; the first is published in European Journal of Operational Research, the second is nearing submission to it Optimization and Engineering, and the third is nearing submission to International Journal of Mining, Reclamation and Environment. These papers apply operations research techniques to open pit mine production scheduling with stockpiling (OPMPS+S). The first paper, "Linear Models for Stockpiling in Open-pit Mine Production Scheduling Problems," reviews existing models to solve OPMPS+S and shows that a nonlinear-integer model provides an exact solution but is intractable even for medium-size data sets. Then, we present an approximation to that nonlinear-integer model, solve the nonlinear-integer and proposed models for multiple data sets and show that the latter model provides solutions very close to those provided by the nonlinear-integer model. By pairing this novel formulation with recently developed linear programming algorithms and heuristics, we demonstrate a dramatic decrease in solution time. The second paper, "Comparison of an Improved Open Pit Mine Stockpiling Model with Commercial Software," introduces an extension to the model presented in the first paper to blend material with multiple grades and a contaminant in the stockpile. Existing state-of-the-art algorithms which exploit the problem structure produce integer solutions with associated net present value at least 18.7% higher than that provided by commercial of-the-shelf software. The third paper, "Open Pit Mine Planning with Degradation Due to Stockpiling," presents three new models that consider degradation due to stockpiling, compare them with models without degradation, and show that degradation has a major impact on the value a stockpile provides.
• #### Experimental measurement and numerical modeling of multiphase flow in Middle Bakken

Relative permeability is an extremely useful measure of complex multiphase fluid flow in a porous media, and much effort has been devoted by researchers to determine the factors that control relative permeability. Nevertheless, detailed experimental data especially for unconventional cores is scant in literature, which motivated this research. Specifically, this dissertation addresses and presents the relative permeability end points measured in two Middle Bakken formation cores for three different processes: oil-displacing-water, water-displacing-oil, and gas-displacing-oil using a steady-state method. The Middle Bakken petrophysical properties measured by Weatherford Labs and by the author are also presented. Finally, two conceptual models were proposed to explain the measured end-point relative permeability, and a 2-D lattice Boltzmann simulator was developed to obtain relative permeability curves of a two-dimensional pore geometry generated using Voronoi grid. The contributions of this work include measurements of relative permeability in a tight shale, interpretation of experimental results using pore-scale features of the cores, and lattice Boltzmann simulation.
• #### Joint interpretation of time-lapse gravity data and production data for a gas reservoir

Time-lapse gravity is commonly used to monitor fluid movement and is especially useful when monitoring water encroachment in a gas reservoir. Although time-lapse gravity has considerable resolution, it is beneficial to integrate different types of data to complement the time-lapse gravity interpretation. When monitoring water-influx in a reservoir, an increase in water yield in some wells may indicate increasing saturation values and suggest areas of fluid movement and density contrast change with time. These data can complement the time-lapse gravity analysis, the question to address is how to integrate these data appropriately to enhance the monitoring of the edgewater in a reservoir. The objective of this thesis is to develop a workflow to invert a time-lapse gravity data set and production data in the Sebei Gas field located in Western China and monitor the edgewater encroachment. Three time-lapse gravity surveys were conducted between 2011 and 2013 and were processed to estimate the error associated with the data set. Consequently, production data collected from 286 wells, was evaluated and plotted to determine its feasibility as complementary data for the time-lapse gravity inversion. To jointly interpret these two independent data sets, I converted the sparse production data into a time-lapse reference model that is subsequently incorporated as a guide into a generalized density inversion. To constrain the inversion result to a similar lithologic setting of the reservoir, I imposed spatially varying upper and lower density bound constraints for different depths of the model. Through this approach, I construct a set of density contrast models that are guided by the measured changes in gas/water yield, and whose distribution fits the time-lapse gravity data. The results of this work demonstrate that integrating the production data and setting lithology dependent bound constraints produced an improved definition of density changes. By doing a first order estimation of the porosity in the reservoir, I verified that the porosity estimated using the recovered density contrast of the improved model are more representative of the known porosity of the gas reservoir.
• #### Understanding structure-property relations in β-eucryptite under pressure and at elevated temperature

β-eucryptite (LiAlSiO4) has received widespread attention from both industry and academia due to its negative coefficient of thermal expansion (CTE) and one-dimensional Li ionic conductivity. Additionally, β-eucryptite undergoes a pressure-induced phase transformation at relatively low pressures. These various behaviors arise because the crystal structure is open and highly anisotropic. The present study uses several experimental methods to better understand the relation between the structure and the electrical and mechanical behavior of β-eucryptite. Synthesis and processing methods were developed to make pure β-eucryptite and β-eucryptite doped with Mg of varying particle sizes. In-situ diamond anvil cell - x-ray diffraction was performed to study the pressure induced phase transformation from β-eucryptite to the high pressure phase β-eucrypite. With the assistance of Rietveld refinement and atomistic modeling, the crystal structure of the β-eucrypite was determined to be an orthorhombic with space group Pna21. This is the first time that both space group and atomic positions of the high pressure phase have been reported. It is also observed that Mg-doped β-eucryptite undergoes the pressure induced transformation at slightly higher pressures than pure β-eucryptite (2.47 GPa compared with 1.8 GPa hydrostatic stress), implying that Mg stabilizes β-eucryptite. Furthermore, the presence of Mg leads to a state in which two high pressure phases coexist. It was observed that the critical pressure for transformation to the high pressure phase decreases with increasing β-eucryptite grain size, up to a critical grain size beyond which grain size does not play a role. The experimental results were described by a nucleation and growth model for transformation. The effect of structural changes in β-eucryptite from low to high temperature was examined by measuring the Li ionic conductivity with electrochemical impedance spectroscopy (EIS). The conductivity is strongly influenced by the Li order-disorder transition at ~500 ºC where the activation energy dramatically increases. It is shown that the conductivity depends on whether or not the Li ions motion is correlated or uncorrelated, leading to three activation energies: at low temperatures it is correlated, at ~500 ºC it is uncorrelated and at high temperatures it is correlated again. The present study focuses on measuring the material properties under pressure and at high temperature, separately. These measurements provide the first step towards elucidating the relation between mechanical and electrical properties. The order-disorder transition is caused by the rearrangement of Li ions at evaluated temperature, an event which is expected to influence the pressure-induced phase transformation. Specifically, a change in the size of the Li channels resulting from Li redistribution, is expected to modify the phase transformation pressure.
• #### Central Laser Facility at the Pierre Auger Observatory. Studies of the atmospheric vertical aerosol optical depth and other applications to cosmic ray measurements, The

The two largest observatories in the world dedicated to the study of Ultra High Energy Cosmic Rays (UHECR) are the Pierre Auger Observatory (Auger) in Mendoza, Argentina and the Telescope Array (TA) in Utah, USA. The measurements of the cosmic ray flux by Auger and TA present a discrepancy at the highest part of the energy spectrum. In this thesis, I study if this discrepancy can be attributed to instrumental effects related to the measurements of the atmospheric aerosol contents in Auger. The Auger Fluorescence Detector (FD) measures the scattered light from laser tracks generated by the Central Laser Facility (CLF) and the eXtreme Laser Facility (XLF) located near the center of Auger, to estimate the vertical aerosol optical depth (τ (z,t)). A good knowledge of τ (z,t) is needed to obtain unbiased and reliable FD measurements of the energy of the UHECR primary particle. The CLF was upgraded substantially in 2013 to improve laser reliability. A substantial part of my Ph.D work is dedicated to building, maintaining and analyzing data from this upgraded facility. The upgraded CLF includes a backscatter Raman LIDAR which independently measures τ (z,t). For the first time in a cosmic ray experiment, two years of measurements of τ (z,t) obtained with the Raman LIDAR are compared with the measurements obtained with the FD. Based on these comparisons, an alternative atmospheric database was created to study its effects on the measurements of the flux as a function of energy. The resulting energy spectrum plot is found to be more compatible with the energy spectrum plot released by TA.