Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
7 results
Search Results
Item LIVE, LEARN, WORK, WALK: CREATING RESILIENT MULTI-FAMILY HOUSING IN DETROIT, MICHIGAN(2023) Edwards, Joseph Chase; Kelly, Brian P.; Architecture; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Detroit, Michigan, and its residents have suffered through economic, social, and environmental hardships from the fall of industrialization since 1950. Some of the largest issues within the city of Detroit are high vacancy rates, high unemployment rates, poverty, and overall lack of acknowledgement to its residents. However, in recent years, organizations within the city have begun to implement various outreach programs to beautify Detroit, improve its current housing situation, and promote community engagement. This thesis proposition looks to help aid these efforts through the introduction of a vertical smart growth architectural hybrid typology used as a catalyst human-centric, resilient urban housing. This is accomplished through the introduction of a community-focused and supportive building program. Overall, creating a self-sufficient, live-work micro-ecosystem to bring life back into the city center.Item Solid Oxide Fuel Cell and Gas Turbine Hybrid Cycles for Aerospace Power and Propulsion(2022) Pratt, Lucas Merritt; Cadou, Christopher P.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid propulsion systems combining gas turbine and solid oxide fuel cells (GT/SOFCs) have the potential to substantially reduce carbon emissions from 737-class aircraft. Many turbine/fuel cell hybrid cycles have been proposed for ground-based energy conversion at the utility scale, and some work has investigated small-scale (<500 kW) fuel cell-based energy conversion systems for aircraft (mostly auxiliary power units). However there is relatively little known about large hybridengine/fuel cell systems capable of providing main propulsive power in large (i.e. 737-class) aircraft. This work takes several important steps toward filling this gap. First, it develops an analytical model of a GT/SOFC system that provides insight into the trends and tradeoffs associated with varying design parameters across a wide design space. Key insights that emerged from this modeling effort are: a)Increasing the fraction of fuel processed by the fuel cell always increases effciency. b) A tradeoff between fuel cell effciency and specific power determines the optimum range of the vehicle. This tradeoff is heavily influenced by the polarization curveof the SOFC. This optimum operating point is different from the maximum power point. c) The GT/SOFC could be used to increase the cycle’s flow specific work, enabling a smaller core to drive the same size fan. This premise is investigated in more detail later in the thesis. d) The fraction of fuel processed by the fuel cell is limited by the ability to cool it. An analytical expression for this limit is derived but in general the maximum power output of the fuel cell is limited to less than half of the total system power output for most hybridization schemes. Second, this work develops an improved thermodynamic model of the hybrid turbine and fuel cell system. The model accounts for off-design performance of the turbomachinery as well as suffcient details of the transport and electrochemistryin the fuel cell to predict the effect of specific design changes (physical dimensions, flow rates, pressure, temperature, etc.) and operating conditions on power output, energy conversion effciency, and system mass. The model is implemented using a NASA-developed tool called Numerical Propulsion System Simulation (NPSS) that is emerging as a standard in modern engine development. While third-party NPSS fuel cell modules are available, they are not suitable for fuel cell design because key performance parameters like utilization, effciency, and specific power are inputs. Our module predicts fuel cell performance from its geometric attributes (channel length, width, height, number) and electrochemical attributes (i.e. temperature, pressure and composition effects on the polarization curve). Such capability is computationally expensive but essential for predicting GT/SOFC performance over varying flight conditions. This work implemented a) ’guardrails’ to prevent solver divergence due to self-reinforcing high or low temperatures, b) an adaptive Newtonsolver damping scheme to improve convergence, c) an electrochemical performance map to find close initial conditions, and d) the option for methane as an additional fuel, amongst other alterations. Taken together, these changes reduced execution time from weeks to hours and greatly improved stability making the thermodynamic model a much more useful tool for design and analysis. Third, the NPSS system model is used to assess the viability of two possible hybridization schemes. The first is a ‘parallel’ hybrid system where an SOFC powers an electric motor that assists the turbine in driving the main fan. The second is a ‘turboelectric’ hybrid system where all of the propulsive power is provided electrically by a fuel cell working in tandem with a mechanical generator attached to the gas turbine. The results show that a parallel hybrid can reduce fuel consumption by 27%, but requires a reformer/fuel cell that achieves > 1kW/kg to achieve range parity with a conventionally-powered B737. This occurs because the thermodynamic effciency of the system increases by 10% and the propulsive effciency increases by 10% due to the higher bypass ratio made possible by the increase in flow specific work associated with hybridization. The turboelectric system reduces fuel consumption by 12% when 25% of power is generated by the SOFC, but requires a reformer/fuel cell that achieves > 1.2kW/kg to achieve range parity with a conventionally-powered B737. This higher specific power requirement occurs because the gas turbine operates at a lower OPR = 15 vs. OPR = 24 to enable recuperation via a heat exchanger. The heat exchanger also improves the thermodynamic performance of both the Brayton cycle and the SOFC (by reducing preheating requirements) even at 30% effectiveness, but adds mass and complexity. Fourth, this work investigates the potential impacts of introducing the fuel cell exhaust—which is hot and contains large amounts of water and combustible reformate—on the Brayton cycle. The system modeling efforts show that the fuelcell exhaust can constitute up to 70% of the total mass flow rate through the system and up to 50% of the total net heat release. Therefore, the effect of the fuel cell exhaust on the operation of the main combustor is expected to be substantial both for integration with traditionally injected fuels, and influencing trades for the SOFC subsystem design choices that affect that exhaust (e.g. fuel utilization). Subsequent chemical kinetic simulations implemented in Cantera show that SOFC exhaust adiabatic flame temperatures can reach as high as 2200K, laminar flame speeds may vary by as much as 500% across a range of fuel utilization targets, ignition delay times with hydrocarbon/air mixtures can reach the millisecond range, and mixed SOFC exhaust can achieve extinction strain rates of over 300,000/s in pressures reasonable for gas turbines. These results suggest that aircraft GT/SOFCs may also require new combustor designs for effective hybridization.Item Development of Hybrid Air-Water Rotor Transition Thrust Prediction and Control(2020) Semenov, Ilya Yevgeniyevich; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid vehicles are able to function in some combination of aerial, underwater, and terrestrial environments, which greatly expands the scope of missions a vehicle can perform. Hybrid aerial-water (HAW) vehicles are a promising subcategory that are designed to operate in two vastly different fluid mediums. Multirotor HAW vehicles configurations have advantages in maneuverability, but pose a challenge in the water entry or water exit transitions. The interaction of a powered rotor with the air-water interface and its performance in a mixed air-water medium are poorly understood. Previous HAW vehicle strategies avoid a powered rotor with additional propulsion and buoyancy systems, constraining the design space. A custom test stand was constructed to better understand rotor performance during the air-water transition. By recording powered rotor performance during controlled water entries and exits in a large tank, several novel observations were made. Previously unrecorded phenomenon such as the gradual height and RPM dependent transition and the underwater ceiling effect are determined. These observations inform the development of the Transition Index TI, a novel metric that indicates the transition state of the rotor, without the need for specialized sensors or computationally intensive modeling. TI is applied to experimental data to make further observations, and is also used in a novel thrust prediction formulation. The first known low-order prediction of thrust through the transition is validated against experimental data, and allows for the development of a TI based controller. A preliminary controller implementation shows promising results in maintaining constant thrust through the air-water transition. Finally, a HAW vehicle to apply this controller is built. Careful consideration to the waterproofing and motor choice is shown and preliminary flight tests are demonstrated. Future expansion on the application of the novel TI and thrust prediction has great potential to advance the capabilities of hybrid aerial-water vehicles.Item Gas Turbine / Solid Oxide Fuel Cell Hybrids: Investigation of Aerodynamic Challenges and Progress Towards a Bench-Scale Demonstrator(2019) Pratt, Lucas Merritt; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern aircraft are becoming more electric making the efficiency of on-board electric power generation more important than ever before. Previous work has shown that integrated gas turbine and solid oxide fuel cell systems (GT-SOFCs) can be more efficient alternatives to shaft-driven mechanical generators. This work advances the GT-SOFC concept in three areas: 1) It develops an improved model of additional aerodynamic losses in nacelle-based installations and shows that external aerodynamic drag is an important factor that must be accounted for in those scenarios. Additionally, this work furthers the development of a lab-scale prototype GT-SOFC demonstrator system by 2) characterizing the performance of a commercial off-the-shelf (COTS) SOFC auxiliary power unit that will become part of the prototype; and 3) combining a scaled-down SOFC subsystem model with an existing thermodynamic model of a small COTS gas turbine to create an initial design for the prototype.Item A NEW CLASS OF HYBRID HYDROGELS(2013) Fernandes, Neville Justine; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid hydrogels are a novel way of combining materials with different properties and retaining their individual functionalities within the same composite gel. Here we attempt to demonstrate how this approach can be used to create hydrogels whose morphologies can be altered depending on external stimuli. First we report the creation of hollow hybrid gels which are similar to the previously created solid hybrid gels but have the advantage of a faster and enhanced response to external stimuli. Two stimuli that we have specifically investigated are temperature and solvent composition. We show how to modify the type and extent of response of the gels, i.e. make them shrink or swell, by changing the composition of the polymer as well as the crosslinker within the gel. Thereafter, we also demonstrate how the responses can be manipulated to change the morphology of the hybrid gel itself.Item AN EVALUATION OF HYBRID VARIATIONAL-ENSEMBLE DATA ASSIMILATION FOR THE NCEP GFS(2012) Kleist, Daryl Timothy; Ide, Kayo; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Several variants of hybrid data assimilation algorithms have been developed and tested within recent years, particularly for numerical weather prediction (NWP). The hybrid algorithms are designed to combine the strengths of variational and ensemble-based techniques while at the same time attempting to mitigate their weaknesses. One such variational-based algorithm is under development for use with the National Centers for Environmental Prediction's (NCEP) global forecast system (GFS) model. In this work, we attempt to better understand the impact of utilizing a hybrid scheme on the quality of analyses and subsequent forecasts, as well as explore alternative extensions to make better use of the ensemble information within the variational solver. A series of Observing System Simulation Experiments (OSSEs) are carried out. It is demonstrated that analysis and subsequent forecast errors are generally reduced in a 3D-hybrid scheme relative to 3DVAR. Several variational-based 4D extensions are proposed and tested, including the use of a variety of dynamic constraints. A simple approach for hybridizing the 4D-ensemble with a time-invariant contribution is proposed and tested. The 4D variants are shown to be superior to the 3D-hybrid, with positive contributions from static B as well as the dynamic constraint formulations. It is clear from both the 3D and 4D experiments that more sophisticated methods for dealing with inflation and localization in the ensemble update are needed even within the hybrid paradigm. Lastly, a method for applying piecewise scale-dependent weights is proposed and successfully tested. The 3D OSSE-based results are also compared with results from an experiment using real observations to corroborate the findings. It is found that in general, most of the results are comparable, though the positive impact in the real system is more consistent and impressive.Item DEVELOPMENT OF A SIMPLIFIED, MASS PRODUCIBLE HYBRIDIZED AMBIENT, LOW FREQUENCY, LOW INTENSITY VIBRATION ENERGY SCAVENGER (HALF-LIVES)(2010) Khbeis, Michael Tawfik; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Scavenging energy from environmental sources is an active area of research to enable remote sensing and microsystems applications. Furthermore, as energy demands soar, there is a significant need to explore new sources and curb waste. Vibration energy scavenging is one environmental source for remote applications and a candidate for recouping energy wasted by mechanical sources that can be harnessed to monitor and optimize operation of critical infrastructure (e.g. Smart Grid). Current vibration scavengers are limited by volume and ancillary requirements for operation such as control circuitry overhead and battery sources. This dissertation, for the first time, reports a mass producible hybrid energy scavenger system that employs both piezoelectric and electrostatic transduction on a common MEMS device. The piezoelectric component provides an inherent feedback signal and pre-charge source that enables electrostatic scavenging operation while the electrostatic device provides the proof mass that enables low frequency operation. The piezoelectric beam forms the spring of the resonant mass-spring transducer for converting vibration excitation into an AC electrical output. A serially poled, composite shim, piezoelectric bimorph produces the highest output rectified voltage of over 3.3V and power output of 145uW using ¼ g vibration acceleration at 120Hz. Considering solely the volume of the piezoelectric beam and tungsten proof mass, the volume is 0.054cm3, resulting in a power density of 2.68mW/cm3. Incorporation of a simple parallel plate structure that provides the proof mass for low frequency resonant operation in addition to cogeneration via electrostatic energy scavenging provides a 19.82 to 35.29 percent increase in voltage beyond the piezoelectric generated DC rails. This corresponds to approximately 2.1nW additional power from the electrostatic scavenger component and demonstrates the first instance of hybrid energy scavenging using both piezoelectric and synchronous electrostatic transduction. Furthermore, it provides a complete system architecture and development platform for additional enhancements that will enable in excess of 100uW additional power from the electrostatic scavenger.