UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item ENABLING CO2 ISOTHERMAL COMPRESSION USING LIQUID PISTON AND INTEGRATED GAS COOLER(2022) Kim, Timothy; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New avenues of decreasing environmental impacts and increasing the efficiency of HVAC systems are constantly being explored in the race to reduce carbon emissions and global warming. These new avenues have led to the exploration of the use of carbon dioxide as a refrigerant in refrigeration applications. Many researchers have also investigated ways to reduce the power consumption of compressors, which is typically the main source of power draw for HVAC systems. One theoretical process to achieve this is through isothermal compression. This thesis explores the idea of isothermally compressing CO2 by using a liquid piston and integrated gas cooler to achieve higher efficiencies with this transcritical cycle. A test facility was designed, sized, constructed, and calibrated to emulate the suction and discharge conditions of a typical CO2 system for air conditioning applications. A prototype of the liquid piston and integrated gas cooler chamber was designed and constructed as well. A simulation model was built in Engineering Equation Solver in order to properly design the gas cooler chamber. Other critical components have been carefully chosen to ensure smooth operation of the system. Results show isothermal efficiencies of up to 82.7% during steady-state operation and an isothermal efficiency of 91.2% during steady-state operation with the additional help of evaporative cooling. Comparing this to other conventional compressors give up to 34.2% absolute improvement in the isothermal compressor efficiency. These results show sufficient performance to warrant the design of a fully working prototype despite efficiency/capacity tradeoffs in the system. Challenges had been encountered such as the loss of refrigerant through the liquid piston, which will be accounted for in the next prototype. Discussion of the next prototype includes the use of a double-acting piston and a smaller tubed fractal heat exchanger design.Item VENTILATION IMPACT ON AIRBORNE TRANSMISSION OF RESPIRATORY ILLNESS IN STUDENT DORMITORIES(2018) Jenkins, Sara T; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents a study of the effect of ventilation rates on the bioaerosols that cause upper respiratory illness. A network of 147 sensors was placed in a pair of dormitories on a college campus to measure carbon dioxide concentrations over two semesters. The concentration results served as input into multi-zone ventilation models of the two buildings, which had different heating, ventilation, and air conditioning (HVAC) systems. The dormitory with a central mechanical ventilation system had, as expected, a higher turnover of fresh air compared to the other, which relied on exhaust fans and infiltration. This well-ventilated building also contained far fewer occupants with recorded upper respiratory illness incidence in comparison to the poorly ventilated building. The central ventilation system increased dorm room ventilation rates by 500%, while decreasing respiratory illness incidence by over 85%. Comparative studies have shown similar findings with increased ventilation reducing incidence of upper respiratory illness by an order of magnitude.Item The Effects of CO2 and Temperature on the Soil Microbial Carbon and Nitrogen of Urban and Rural Forests(2015) Kulka, Elizabeth; McIntosh, Marla S; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigated and compared the effects of elevated temperature and elevated CO2 on the microbial biomass carbon (MBC) and nitrogen (MBN) of urban and rural forest soils. Soils analyzed from Baltimore Long-Term Ecological Research forests in June and October, 2014 had greater MBC and MBN quantities in rural than urban forests. A controlled environmental chamber study was conducted where June-collected soils were planted with hybrid poplars and exposed to ambient and elevated temperature and CO2 levels. After exposure for 49 days, MBC and MBN quantities were again greater in rural than urban soils. Soil MBC was greater under elevated than ambient CO2, while soil MBN was greater under elevated than ambient CO2 and temperature. Results suggest that if temperature and CO2 levels increase in the Baltimore area as predicted, microbial C and N pools in the studied forests will increase, and will remain greater in rural than urban soils.Item Enhanced Gas-Liquid Absorption Utilizing Micro-Structured Surfaces and Fluid Delivery Systems(2014) Ganapathy, Harish; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite intensive research and development efforts in renewable energy in recent years, more than 80% of the energy supply in the year 2040 is expected to come from fossil fuel-based sources. Increasing anthropogenic greenhouse gas emissions led the United States to legislatively limit domestic CO2 emissions to between 1000-1100 lb/MWh for new fossil fuel-fired power plants, thus creating an urgent need for efficient gas separation (capture) processes. Meanwhile, the gradual replacement of coal with cleaner burning natural gas will introduce additional challenges of its own since nearly 40% of the world's gas reserves are sour due to high concentrations of corrosive and toxic H2S and CO2 gases, both of which are to be separated. Next-generation micro-structured reactors for industrial mass and heat transfer processes are a disruptive technology that could yield substantial process intensification, size reduction, increased process control and safety. This dissertation proposes a transformative gas separation solution utilizing advanced micro-structured surfaces and gas delivery manifolds that serves to enhance gas separation processes. Experimental and numerical approaches have been used to achieve aggressive enhancements for a solvent-based CO2 absorption process. A laboratory-scale microreactor was investigated to fundamentally understand the physics of multiphase fluid flow with chemical reactions at the length scales under consideration. Reactor design parameters that promote rapid gas separation were studied. Computational fluid dynamics was used to develop inexpensive stationary (fixed) interface models for incorporation with optimization engines, as well as high fidelity unsteady (deforming) interface models featuring universal flow regime predictive capabilities. Scalability was investigated by developing a multiport microreactor and a stacked multiport microreactor that represented one and two orders magnitude increase in throughput, respectively. The present reactors achieved mass transfer coefficients as high as 400 1/s, which is between 2-4 orders of magnitude higher than conventional gas separation technologies and can be attributed to the impressive interfacial contact areas as high as 15,000 m2/m3 realized in this study through innovative design of the system. The substantial enhancement in performance achieved is indicative of the high level of process intensification that can be attained using the proposed micro-structured reactors for gas separation processes for diverse energy engineering applications. This dissertation is the first comprehensive work on the application of micro-structured surfaces and fluid delivery systems for gas separation and gas sweetening applications. More than ten refereed technical publications have resulted from this work, part of which has already been widely received by the community.Item The Effect of CO2 on Copper Partitioning in Sulfur- Free and Sulfur-Bearing Felsic Melt-Vapor-Brine Assemblages(2012) Tattitch, Brian Christopher; Candela, Philip; Piccoli, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Analysis of fluid inclusions from porphyry copper deposits (PCD) reveals that magmatic vapor and/or brine are vital for the removal of copper from arc magmas and its transport to the site of ore formation. Experiments in melt-vapor-brine systems allow for investigating the partitioning of copper between silicate melts and volatile phases under magmatic conditions. The presence of CO2 affects both the pressure of vapor saturation and the composition of exsolving volatile phases. However, PCD are primarily sulfide ore deposits, and the role of sulfur must also be examined as part of magmatic-hydrothermal experiments. Therefore, the partitioning of copper in CO2 ± S-bearing experiments was examined in an attempt to provide insights into copper partitioning and the generation of PCD. I present the results from experiments performed at 800 °C and 100 MPa in CO2-bearing melt-vapor-brine systems with XCO2 = 0.10 and 0.38. The compositions of vapor and brine inclusions and run-product glasses were used to determine the compositions of the magmatic phases. The partitioning of copper between brine and vapor (DCu b/v ±2σ) increases from 25(±6) to 100 (±30) for sulfur-free experiments and increases from 11(±3) to 95(±23) for sulfur-bearing experiments as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between vapor and melt (DCu v/m ±2σ) decreases from 9.6(±3.3) (sulfur-free, HCl-bearing), 18(±8) (sulfur-bearing, HCl-free), and 30(±11) (sulfur-bearing, HCl-bearing) at XCO2 = 0.10, to 2(±0.8)(HCl-free) at XCO2 = 0.38, sulfur-free or sulfur-bearing. These data demonstrate that copper partitioning in sulfur-free, CO2-bearing systems is controlled by the changes in the salinity of the vapor and brine corresponding to changes in XCO2. Sulfur-bearing experiments demonstrate that magmatic vapors are enriched in copper in the presence of sulfur at low XCO2. However, the enrichment of copper in the magmatic vapor is suppressed for sulfur-bearing systems at high XCO2. The MVPart model presented by Candela and Piccoli (1998) was modified to incorporate CO2 to predict trends in efficiency of removal of copper into exsolving CO2-bearing magmatic volatile phases. The CO2-MVPart model predicts two to three times lower efficiency for CO2-rich (XCO2 = 0.38) magmatic volatile phases compared to low-CO2 (XCO2 ≤ 0.10) systems.Item Performance and Oil Retention Characteristics of a CO2 Heat Pump Water Heater(2008) Fernandez, Nicholas Edward Peter; Radermacher, Reinhard K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A CO2 heat pump water heater (HPWH) was investigated experimentally and analytically. In the first stage of the study, a baseline performance was measured, investigating the effect of operating parameters on the system performance under typical tank heating scenarios. In the second, the CO2 HPWH was modeled to investigate the effect of optimizing key components. In the third, the oil retention mass, the increase in pressure drop, and the COP degradation were measured as a function of oil mass fraction. In the fourth, two alternative system configurations were investigated for potential performance enhancement; a two-stage compression cycle with internal heat exchanger and a system with a suction line heat exchanger. Overall, the CO2 cycle seems uniquely suited for water heating. CO2 HPWHs have enormous energy savings potential if the cooling from the evaporator can be harnessed during the summer months, and rejected to the environment during the colder months.Item Variability of terrestrial carbon cycle and its interaction with climate under global warming(2008-08-04) qian, haifeng; Zeng, Ning; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Land-atmosphere carbon exchange makes a significant contribution to the variability of atmospheric CO2 concentration on time scales of seasons to centuries. In this thesis, a terrestrial vegetation and carbon model, VEgetation-Global-Atmosphere-Soil (VEGAS), is used to study the interactions between the terrestrial carbon cycle and climate over a wide-range of temporal and spatial scales. The VEGAS model was first evaluated by comparison with FLUXNET observations. One primary focus of the thesis was to investigate the interannual variability of terrestrial carbon cycle related to climate variations, in particular to El Niño-Southern Oscillation (ENSO). Our analysis indicates that VEGAS can properly capture the response of terrestrial carbon cycle to ENSO: suppression of vegetative activity coupled with enhancement of soil decomposition, due to predominant warmer and drier climate patterns over tropical land associated with El Niño. The combined affect of these forcings causes substantial carbon flux into the atmosphere. A unique aspect of this work is to quantify the direct and indirect effects of soil wetness vegetation activities and consequently on land-atmosphere carbon fluxes. Besides this canonic dominance of the tropical response to ENSO, our modeling study simulated a large carbon flux from the northern mid-latitudes, triggered by the 1998-2002 drought and warming in the region. Our modeling indicates that this drought could be responsible for the abnormally high increase in atmospheric CO2 growth rate (2 ppm/yr) during 2002-2003. We then investigated the carbon cycle-climate feedback in the 21st century. A modest feedback was identified, and the result was incorporated into the Coupled Carbon Cycle Climate Model Inter-comparison Project (C4MIP). Using the fully coupled carbon cycle-climate simulations from C4MIP, we examined the carbon uptake in the Northern High Latitudes poleward of 60˚N (NHL) in the 21st century. C4MIP model results project that the NHL will be a carbon sink by 2100, as CO2 fertilization and warming stimulate vegetation growth, canceling the effect of enhancement of soil decomposition by warming. However, such competing mechanisms may lead to a switch of NHL from a net carbon sink to source after 2100. All these effects are enhanced as a result of positive carbon cycle-climate feedbacks.