Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item New Methodology for Predicting Ultimate Capacity of One-Sided Composite Patch Repaired Aluminum Plate(2019) Hart, Daniel C; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Composite patch repairs are an alternative to traditional weld repair methods to address cracking in aluminum plates. Analytical and numerical design methods use linear elastic fracture mechanics (LEFM) and do not account for elastic-plastic crack tip behavior demonstrated in static tests of one-sided patch repaired ductile panels. This research used digital image correlation (DIC) and three-dimensional finite element analysis (FEA) with first order elements to study crack tip effects due to the one-sided composite patch applied to center crack tension (CCT) specimens loaded monotonically to failure. The measurable effects on crack tip behavior due to the composite patch were ultimate tensile load increase of more than 100% and a total achieved crack opening displacement (COD) increase of 20% over the unpatched behavior. Crack tip fracture behavior was found to be an intrinsic property of the aluminum and directly related to the COD independent of the one-sided composite patch. Increased capacity was related to accumulation of large-strain free surface area and through thickness volume ahead of the crack tip. Test data and numerical predictions correlated with measured load, strain, displacement fields, and J-integral behavior. Correlation of displacement fields with HRR and K fields established a state of small scale yielding prior to failure. Data and predictions indicated critical COD occurs when unpatched and patched large strain area is equivalent, which occurs before crack tip behavior transitions from small scale to large scale yielding and crack growth. Identifying a critical COD for both small and large scale one-sided patch repaired cracked ductile panels results in a predicted failure closer to the ultimate tensile load and 80% greater than predicted with LEFM methods. Observations and predictions demonstrated in this research resulted in three scientific contributions: (1) development of criteria to determine crack growth in cracked ductile panels repaired with a one-sided composite patch using a critical COD, (2) development of a three-dimensional FEA to study development of the plastic zone and evolution of the large-strain region ahead of the crack tip, and (3) development of a numerical methodology to predict ultimate tensile load capacity of cracked ductile panels repaired with a one-sided composite patch.Item EIT BASED PIEZORESISTIVE TACTILE SENSORS: A SIMULATION STUDY(2019) Nankani, Ayush; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electrical impedance tomography (EIT) is an imaging technique that uses voltage measurements to map the internal conductivity distribution of a body by applying current on electrodes attached to the boundary of that body. EIT has many applications, ranging from medical imaging to 3D printing. This imaging method is also being used for tactile sensing using stretchable piezoresistive sensors, mainly for robotic applications. Although prior research has focused on qualitative illustrations of tactile sensing, this thesis focuses on quantitative evaluation. In this thesis different current injection patterns are quantitatively analyzed using performance metrics to understand their effect on the resulting EIT images.Item Simulation and Analysis of Energy Consumption for Two Complex and Energy intensive Buildings on UMD Campus(2019) Kelly, Jason; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Physical Sciences Complex and Eppley Recreational Center are multi-purpose buildings which are complex in functionality and are among the highest consumers of energy on the UMD campus. Building energy analyses used to identify energy efficiency measures to optimize energy efficiency in the buildings. Detailed building energy models were developed in EnergyPlus and OpenStudio that sought to mimic current operations of the buildings. PSC model results deviated respectively -1.05%, 1.19%, and 5.27% for electricity, steam, and chilled water. ERC model results deviated respectively 0.47%, 5.3%, and 2.2% from annual electricity, hot water, and gas. Four energy efficiency measures for the Physcial Sciences Complex provided energy model predicted energy savings of 3,757 MMBtu or 7.5% of the building’s energy consumption. Four efficiency measures were identified for the Eppley Recreation Center with energy model predicted energy savings of 3,390 MMBtu or 8.4% of the building’s energy consumption.Item Investigation of Swirl Assisted Colorless Distributed Combustion (CDC) for Gas Turbine Application(2019) Feser, Joseph Samuel; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Colorless Distributed Combustion (CDC) is a novel method to enhance flame stability and thermal field uniformity, increase combustion efficiency and reduce pollutants emission, including noise. The focus of this thesis is to investigate swirl-assisted distributed combustion at high thermal intensity for gas turbine application. This thesis investigates the impact of fuel enrichment on CDC conditions by using naphthalene as a fuel additive in ethanol to increase the heating value without compromising ultra-low emissions, in addition to investigating how CDC fuel flexibility can mitigate instability associate with hydrogen enriched alternate fuels. To better predict and implement CDC design in future gas turbine combustors a distributed combustion index (DCI) will be developed to determine the impact of heat release intensity, equivalence ratio, preheat temperature and entrainment gas on distributed conditions. Lastly, the impact of flowfield interaction on achieving CDC condition will be examined for enhanced understanding of mixing required for CDC.Item DESIGN AND DEVELOPMENT OF POTASSIUM FORMATE BASED ATMOSPHERIC WATER HARVESTER(2019) Ayyagari, Veeresh; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the deteriorating climate and rapid depletion of natural resources, the problem of water scarcity is on the rise and there is a pressing need for sustainable technologies to address this problem. The extraction of water from air has been identified as a potential solution to address the water scarcity problem in arid regions that do not have ready access to seawater. Much of the research in the literature in tackling this problem has been focused on the use of heat pumps and the use of Atmospheric Water Harvesters (AWH) via solid desiccant technologies. Very little work has been done on utilizing the potential of liquid desiccants for the extraction of water using potassium formate. Through this thesis, we have designed and developed a novel and low-cost AWH utilizing (i) an aqueous solution of potassium formate as the medium for absorption of moisture during the night when the relative humidity is high and (ii) solar energy for removal of absorbed moisture from the desiccant solution during the day. A prototype was built and a performance of 1.9 kg/m2/day with a moisture uptake of 0.34 kg of water/kg of salt was recordedItem Moisture Transport through Housing Materials Enclosing Critical Automotive Electronics(2019) Roman, Artur; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In automotive electronics, humidity-sensitive electronics are encapsulated by protective housings that are attached to the car body. Typical housing materials are comprised of polymer composites, through which moisture transport occurs. The objective of this paper is to provide a predictive capability for moisture transport through automotive housings enclosing a cavity with electronic modules. The temperature-dependent moisture properties including moisture diffusivity, solubility, and saturated concentration of three housing material candidates are characterized first. Then, the analogy between heat transfer and the mass transfer is implemented to model the moisture transport into the cavity enclosed by the housing materials. To cope with the transient boundary condition at the housing material and the cavity interface, the effective volume scheme is used, treating the cavity as an imaginary polymer with an extremely large diffusivity and “equivalent solubility.” The prediction is subsequently validated through an experimental setup designed to monitor the in-situ humidity condition inside the cavity sealed by the housing materials. The prediction and experimental results agree well with each other, which corroborates the validity of the FEA modeling and the measured moisture properties.Item HIGH-FORCE ELECTROSTATIC INCHWORM MOTORS FOR MILLIROBOTICS APPLICATIONS(2019) Penskiy, Ivan; Bergbreiter, Sarah; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to scaling laws and ease of fabrication, electrostatic actuation offers a promising opportunity for actuation in small-scale robotics. This dissertation presents several novel actuator and motor designs as well as new techniques by which to characterize electrostatic gap closing actuators. A new motor architecture that uses in-plane electrostatic gap-closing actuators along with a flexible driving arm mechanism to improve motor force density is introduced, optimized, manufactured, and tested. This motor operates similarly to other inchworm-based microactuators by accumulating small displacements from the actuators into much larger displacements in the motor. Using an analytical model of the inchworm motor based on the static force equilibrium condition, optimizations of a full motor design were performed to maximize motor force density. In addition, force losses from supporting flexures were included to calculate the theoretical motor efficiency for different motor designs. This force density optimization analysis of the gap-closing actuators and supporting motor structures provided the basis for designing and manufacturing inchworm motors with flexible driving arms and gap-closing actuators. The motor required only a single-mask fabrication and demonstrated robust performance, a maximum speed of 4.8mm/s , and a maximum force on the shuttle of 1.88mN at 110V which corresponds to area force density of 1.38mN/mm2. In addition, instead of estimating motor force based on drawn or measured dimensions which often overestimates force, the demonstrated maximum motor force was measured using calibrated springs. The efficiency of the manufactured motor was measured at 8.75% using capacitance measurements and useful work output. To further increase force output from these motors, several new designs were proposed, analyzed, and tested. Thick film actuators that take advantage of a through-wafer etch offered a promising opportunity to increase force given the linear increase in force with actuator thickness. However, fabrication challenges made this particular approach inoperable with current manufacturing capabilities. New actuator designs with compliant and zipping electrodes did demonstrate significant increases in force, but not the order of magnitude increase promised by modeling and analysis. In order to study and understand this discrepancy, several new techniques were developed to electrically and electromechanically characterize the force output of these new actuator designs. The first technique identifies parameters in an equivalent circuit model of the actuator, including actuator capacitance. By monitoring change in capacitance along the travel range of the motor, electrostatic force in equilibrium can be estimated. Charge transferred to and from the actuator can also provide an estimate of actuator efficiency. The second technique uses a constant rate spike to more thoroughly explore the rapid dynamics of actuator pull-in and zipping. New characterization methods allowed for collecting large amounts of data describing performance of motors with zipping and compliant electrodes. The data was used to back up the main hypothesis of force output discrepancy between theory and practice. Also, it was used to highlight extreme sensitivity of proposed motors toward manufacturing process and its tolerances.Item ADVANCED MODELING AND REFRIGERANT FLOW PATH OPTIMIZATION FOR AIR-TO-REFRIGERANT HEAT EXCHANGERS WITH GENERALIZED GEOMETRIES(2019) Li, Zhenning; Radermacher, Reinhard K; Aute, Vikrant C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air-to-refrigerant heat exchangers are key components of the heating, ventilation, air-conditioning and refrigeration systems. The evolving simulation and manufacturing capabilities have given engineers new opportunities in pursuing complex and cost-efficient heat exchanger designs. Advanced heat exchanger modeling tools are desired to adapt to the industrial transition from conventional refrigerants to low Global Warming Potential (low-GWP) refrigerants. This research presents an advanced heat exchanger performance prediction model which distinguishes itself as a cutting-edge simulation tool in the literature to have capabilities, such as to (i) model heat exchangers with variable tube shape and topology, (ii) improved numerical stability, (iv) multiple dehumidification models to improve evaporator prediction, and (v) CFD-based predictions for airflow maldistribution. Meanwhile, HX performance is significantly influenced by the refrigerant flow path arrangements. The refrigerant flow path is optimized for various reasons such as to (i) mitigate the impact of airflow maldistribution, (ii) reduce material/cost, (iii) balance refrigerant state at the outlet of each circuit, and (iv) ensure overall stable performance under a variety of operating conditions. This problem is particularly challenging due to the large design space which increases faster than n factorial with the increase in the number of tubes. This research presents an integer permutation based Genetic Algorithm (GA) to optimize the refrigerant flow path of air-to-refrigerant heat exchangers. The algorithm has novel features such as to (i) integrate with hybrid initialization approaches to maintain the diversity and feasibility of initial individuals, (ii) use effective chromosome representations and GA operators to guarantee the chromosome (genotype) can be mapped to valid heat exchanger designs (phenotype), and (iii) incorporate real-world manufacturability constraints to ensure the optimal designs are manufacturable with the available tooling. Case studies have demonstrated that the optimal designs obtained from this algorithm can improve performance of heat exchangers under airflow maldistribution, reduce defrost energy and assure stable heat exchanger performance under cooling and heating modes in reversible heat pump applications. Comparison with other algorithms in literature shows that the proposed algorithm exhibits higher quality optimal solutions than other algorithms.Item MAXIMIZING THE FINANCIAL RETURNS OF USING LIDAR SYSTEMS IN WIND FARMS FOR YAW ERROR CORRECTION APPLICATIONS(2019) Bakhshi, Roozbeh; Sandborn, Peter A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wind energy is an important source of renewable energy with significant untapped potential around the world. However, the cost of wind energy production is high and efforts to lower the cost of energy generation will help enable more widespread use of wind energy. Ideally, wind turbines have to be aligned with wind flow at all times. However, this is not the case and there exists and angle between a wind turbine nacelle’s central axis and the wind flow. This angle is called yaw error. Yaw error lowers the efficiency of turbines as well as lowers the reliability of key components in turbines. LIDAR devices can correct the yaw error; however, they are expensive and there is a trade-off between their costs and benefits. In this dissertation, a stochastic discrete-event simulation is developed that models the operation of a wind farm. By maximizing the Net Present Value (NPV) changes associated with using LIDAR devices in a wind farm, the optimum number of LIDAR devices and their associated turbine stay time will be determined. These optimum values are a function of number of turbines in the wind farm for specific turbine sizes. The outcome of this dissertation will help wind farm owners and operators to make informed decisions about purchasing LIDAR devices for their wind farms.Item WATER, ION, AND GRAPHENE: AN ODYSSEY THROUGH THE MOLECULAR SIMULATIONS(2019) Wang, Yanbin; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Water is known as the most common and complicated liquid on earth. Meanwhile, graphene, defined as single/few layer graphite, is the first member in the 2-dimensional materials family and has emerged as a magic material. Interactions between water and graphene generate many interesting phenomena and applications. This thesis focuses on applying molecular dynamics (MD), a powerful computational tool, for investigating the graphene-water interactions associated with various energetic and environmental applications, ranging from the wettability modification, species adsorption, and nanofluidic transport to seawater desalination. A key component of one domain of applications involves a third component, namely salt ions. This thesis attempts that and discovers a fundamentally new way in which the behavior of ions with the air-water interfaces should be probed. In Chapter 1, we introduce the motivation and methods and the overall structure of this thesis. Chapter 2 focuses on how MD simulations connect the statistical mechanics theory with the experimental observations. Chapter 3 discusses the simulation results revealing that the spreading of a droplet on a nanopillared graphene surface is driven by a pinned contact line and bending liquid-surface dynamics. Chapter 4 probes the interactions between a water drop and a holey graphene membrane, which is prepared by removing carbon atoms in a circular shape and which can serve as catalyst carriers. Accordingly, chapter 5 studies the effects of various terminations on water-holey graphene interactions, showing that water flows faster and more thoroughly through the membrane with hydrophobic terminations, compared to that with hydrophilic terminations. In chapter 6, simulations describe the generation of enhanced water-graphene surface area during the water-holey-graphene interactions in presence of an applied time-varying force on the water drop. In chapter 7, we focus on the ion-water interaction at the water-air interface to fully understand the fluidic dynamics during any seawater desalination. Our research revisits the energetic change while ion approaches water-air interface and shows that the presence of ion at the interface enhances capillary-wave fluctuation. Finally, in chapter 8 we summarize the main findings of the thesis and provide the scope of future research.