Mechanical Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2263
Browse
3 results
Search Results
Item TOPOLOGICAL ANALYSIS OF DISTANCE WEIGHTED NORTH AMERICAN RAILROAD NETWORK: EFFICIENCY, ECCENTRICITY, AND RELATED ATTRIBUTES(2023) Elsibaie, Sherief; Ayyub, Bilal M.; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The North American railroad system can be well represented by a network with 302,943 links (track segments) and 250,388 nodes (stations, junctions, and waypoints), and other points of interest based on publicly accessible geographical information obtained from the Bureau of Transportation Statistics (BTS) and the Federal Railroad Administration (FRA). From this large network a slightly more consolidated subnetwork representing the major freight railroads and Amtrak was selected for analysis. Recent improvements in network and graph theory and improvements in all-pairs shortest path algorithms make it more feasible to process certain characteristics on large networks with reduced computation time and resources. The characteristics of networks at issue to support network-level risk and resilience studies include node efficiency, node eccentricity, and other attributes derived from those measures, such as network arithmetic efficiency, network geometric central node, radius, and diameter, and some distribution measures of the node characteristics. Rail distance weighting factors, representing the length of each rail line derived from BTS data, are mapped to corresponding links, and are used as link weights for the purpose of computing all pair shortest paths and subsequent characteristics. This study also compares the characteristics of North American railroad infrastructure subnetworks divided by Class I carriers, which are the largest railroad carriers classified by the Surface Transportation Board (STB) by annual operating revenue, and which together comprise most of the North American railroad network. These network characteristics can be used to inform placement of resources and plan for natural hazard and disaster scenarios. They relate to many practical applications such as network efficiency to distribute traffic and a network’s ability to recover from disruptions. The primary contribution of this thesis is the novel characterization of a detailed network representation of the North American railroad network and Class I carrier subnetworks, with established as well novel network characteristics.Item HEAT AND MASS TRANSFER ANALYSIS AND PERFORMANCE IMPROVEMENT FOR AIR GAP MEMBRANE DISTILLATION(2022) Kim, Gyeong Sung; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Seawater desalination method can be largely divided into evaporation- and membrane-based techniques. From decades ago, the global installation capacity of reverse-osmosis membrane-based seawater desalination (SWRO) started outgrowing that of the evaporative desalination plant due to its higher energy efficiency and it became the mainstream technology in the 20th century. However, small-scale SWRO facilities installed on South Korean islands are not competitive compared to the thermally driven evaporation method as their specific energy consumption (SEC) values are highly ranging in 9 – 19 kWh∙m^(-3) and there have been frequent maintenance events.By taking the advantages of direct utilization of renewable and thermal energy, air gap membrane distillation (AGMD) is investigated in this study as an improved approach. From the preliminary experimental study, it was found that the lower air-gap pressure of AGMD helps to increase its water productivity. However, most of the heat and mass transfer models in AGMD used the constant atmospheric pressure for the air gap. Therefore, new models considering the pressure effect of the air gap is needed. Since maintaining a vacuum pressure in the gap requires additional energy, a vacuum technique consuming less energy is also needed. In addition to controlling the total pressure of the gap, condensation augmentation on the cooling surface on one side of the gap is critical since the vapor flux is dependent on the vapor pressure in the gap. As the preliminary experimental study showed that the dropwise condensation mode dominates the condensation of AGMD, the effect of gap size between the condensation surface and hydrophobic membrane is needed to be investigated. Therefore, this research was performed with the following objectives: (i) experimental investigation and mass transfer model development for vacuum applied AGMD (V-AGMD), (ii) development of a wave-powered desalination system using V-AGMD, (iii) experimental investigation of condensation in AGMD, and (iv) development of condensation enhancement technology for AGMD. From the modeling and experimental research, this study made the following major research outcomes and observations. First, a straightforward mass transfer model was developed by using the concept of Kinetic Theory of Evaporation and temperature fraction value between the fluid temperatures of feed and coolant, based on the AGMD experimental results. This model was evaluated experimentally and showed an excellent prediction of water flux in various air-gap pressures without measuring each temperature of the interface of the feed-membrane-air-cooling surface-coolant. Second, considering that the air gap of AGMD can be operated in a vacuum state using wave power, a novel wave-powered AGMD desalination device was proposed and evaluated for the island’s dwellers. Third, during the whole AGMD tests, only dropwise condensation (DWC) modes were observed on the stainless-steel condensing wall. Therefore, experiments were conducted to understand the physical pattern of DWC from nucleation to departure. After testing under various temperature and humidity conditions, it was confirmed that the average size of the water droplets followed the power law for each case. Fourth, as the periodic cleaning of the condensate wall of AGMD could improve the production of condensate, an experimental study was subsequently performed for the condensation augmentation using an electrohydrodynamic (EHD) method. By both cleaning periodically and applying 2.5 kV and 5.0 kV fields on the condensing surface in a thermos-hygrostat chamber, the water production rate was increased by 32% and 88%, respectively. This study concluded that the performance of an AGMD desalination system can be improved by applying a vacuum or an EHD device in its air gap. Therefore, pilot-scale experiments will be conducted as future studies to evaluate the commercial viability of the improved system.Item A BAYESIAN NETWORK PERSPECTIVE ON THE ELEMENTS OF A NUCLEAR POWER PLANT MULTI-UNIT SEISMIC PROBABILISTIC RISK ASSESSMENT(2021) DeJesus Segarra, Jonathan; Bensi, Michelle T.; Modarres, Mohammad; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nuclear power plants (NPPs) generated about 10% of the world’s electricity in 2020 and about 1/3 of the world’s low-carbon electricity production. Probabilistic risk assessments (PRAs) are used to estimate the risk posed by NPPs, generate insights related to strengths and vulnerabilities, and support risk-informed decisionmaking related to safety and reliability. While PRAs are typically carried out on a reactor-by-reactor basis, the Fukushima Dai-ichi accident highlighted the need to also consider multi-unit accidents. To properly characterize the risks of reactor core damage and subsequent radiation release at a multi-unit site, it is necessary to account for dependencies among reactors arising from the possibility that adverse conditions affect multiple units concurrently. For instance, the seismic hazard is one of the most critical threats to NPP structures, systems, and components (SSCs) because it affects their redundancy. Seismic PRAs are comprised of three elements: seismic hazard analysis, fragility evaluation, and systems analysis. This dissertation presents a Bayesian network (BN) perspective on the elements of a multi-unit seismic PRA (MUSPRA) by outlining a MUSPRA approach that accounts for the dependencies across NPP reactor units. BNs offer the following advantages: graphical representation that enables transparency and facilitates communicating modeling assumptions; efficiency in modeling complex dependencies; ability to accommodate differing probability distribution assumptions; and facilitating multi-directional inference, which allows for the efficient calculation of joint and conditional probability distributions for all random variables in the BN. The proposed MUSPRA approach considers the spatial variability of the ground motions (hazard analysis), dependent seismic performance of SSCs (fragility evaluation), and efficient BN modeling of systems (systems analysis). Considering the spatial variability of ground motions represents an improvement over the typical assumption that ground motions across a NPP site are perfectly correlated. The method to model dependent seismic performance of SSCs presented is an improvement over the current “perfectly dependent or independent” approach for dependent seismic performance and provides system failure probability results that comply with theoretical bounds. Accounting for these dependencies in a systematic manner makes the MUSPRA more realistic and, therefore, should provide confidence in its results (calculated metrics) and risk insights.