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 DEVELOPMENT OF A GENERAL-PURPOSE STEADY-STATE SIMULATION FRAMEWORK FOR VAPOR COMPRESSION SYSTEMS(2020) Huang, Ransisi; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The vapor compression system is the dominating technology in heat pumping, air conditioning and refrigeration. Vapor compression is associated with significant energy consumption and high global warming potential. Steady-state simulation of vapor compression system is a crucial numerical technology that helps to assess and mitigate the energy and environmental impact of these systems. This dissertation aims to advance the steady-state modeling and simulation technologies for vapor compression systems toward higher level of flexibility, computational efficiency, and robustness, improving designs and reducing time to market.First, the dissertation proposes a generalized solution methodology for the steady-state analysis of arbitrary systems. A tripartite-graph based tearing algorithm is proposed to generically formulate the residual equations. The methodology was extensively validated by five test systems with capacities from 10 to 100 kW. The maximum simulation energy imbalance was 0.91%, and the maximum system performance deviation was 8.94%. The methodology was also applied to analyze two advanced vapor compression systems, presenting strong capability to contribute to the acceleration of their R&D stage. Second, the dissertation develops an approximation-assisted modeling methodology to speed up the steady-state system simulation. Three approximation-assisted heat exchanger models were compared in terms of accuracy and computational efficiency. Kriging metamodel presented the highest accuracy among the three. For heat exchanger performance approximation, its overall ∆P and ∆h mean absolute error (MAE) were 4.46% and 0.9%, respectively. For system simulations, the maximum COP and capacity errors with Kriging metamodel were 2.54% and 1.45%, respectively. System simulation was sped up by 10X - 600X, depending on the test conditions. Third, the dissertation proposes two convergence improvement approaches on the basis of nonlinear equation fundamentals, and assessed them on a standard vapor compression system as a first step, allowing for later application to more complex cycles. The assessment results show that a large initial Jacobian condition number presents low convergence probability at the current initial guess point. The results also indicate a correlation between component nonlinearity and simulation convergence. It was found that by changing the characterization methods in the heat exchanger models, 47 out of 51 originally non-converged cases were able to reach convergence.Item The Contributions of Temperament Traits to Regulatory Flexibility in Preschoolers: The Importance of Context(2019) Shoplik, Helena; Teglasi, Hedwig; Counseling and Personnel Services; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Regulatory flexibility is the ability to meet demands of a situation by modulating behavior, cognitions, or emotions. While flexibility has been studied in early childhood, the role of reactive and regulatory temperamental building blocks of flexibility to meet requirements of different contexts has not been investigated in preschoolers. Routine and novel contexts require very different skills to navigate appropriately. For example, different aspects of inhibition and modulation of behavior may come into play in familiar/routine and novel contexts such those that depart from expectations. The current study utilized a parent-report measure of temperament and of regulatory flexibility to address the unique contributions of selected temperamental attributes—including attention, sensitivity, and activity level—to regulatory flexibility. Because of its multi-faceted nature (Kashdan & Rottenberg, 2010), an in-depth examination of regulatory flexibility could provide a better understanding of how preschoolers meet the challenges that are introduced to them during this developmental period.