A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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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 SYSTEM MODELING AND MATERIAL DEVELOPMENT FOR STANDALONE THERMOELECTRIC POWER GENERATORS(2014) Huang, Dale Hsien-Yi; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation addresses the need to develop a scalable and standalone power generator for personal, commercial, and military transportation and communication systems. The standalone thermoelectric power generator (TPG) converts heat to electrical power in a unique way that does not draw on conventional power sources like batteries. A TPG is comprised of four main components: a heat source, thermoelectric modules, a heat sink, and thermal insulation. For system modeling and materials development purposes, the dissertation invented the first pyrophoric heated standalone TPG, solid-state renewable heat source, and two-component nanocomposite thermoelectric power generation material. In this work, the first pyrophoric heated standalone thermoelectric power generator was designed, fabricated, and tested. The bases of the system were four porous silicon carbide combustors for the exothermic reaction of pyrophoric iron powder with oxygen. These combustors provided a heat source of 2,800 to 5,600 W to the heat sinks (through TE modules) at conditions suitable for a standalone, pyrophoric iron fueled TE power generator. The system integrated with 16 commercial bismuth telluride thermoelectric modules to produce 140 to 280 W of electrical power with a TE power conversion efficiency of ~5%. This demonstration represents an order-of-magnitude improvement in portable electrical power from thermoelectrics and hydrocarbon fuel, and a notable increase in the conversion efficiency compared with other published works. To optimize the TE heat-to-power conversion performance of the TPG, numerical simulations were performed with computational fluid dynamics (CFD) using FLUENT. The temperature dependent material properties of bismuth telluride, effects of air flow rate (6 – 14 m/s) at 300 K, and effects of thermoelectric element thickness (4 – 8 mm) on temperature gradient generated across the module are investigated under constant power input (7.5 W). The obtained results reveal that all geometric parameters have important effect on the thermal performance of thermoelectric power generation module. The optimized single TE element thickness is 7 mm for electrical power generation of 0.47 W at temperature difference of 138 K. The TE heat-to-power conversion efficiency is 6.3%. The first solid-state renewable heat source (without the use of hydrocarbons) were created with porous silicon carbide combustors coated with pyrophoric 1-3 micron-sized iron particles mixture. The thermal behavior and ignition characteristics of iron particles and mixtures were investigated. The mixture include activate carbon and sodium chloride, in which iron is the main ingredient used as fuel. The final mixture composition is determined to consist of iron powder, activate carbon, and sodium chloride with a weight ratio of approximately 5/1/1. The mixture generated two-peak DSC curves featured higher ignition temperatures of 431.53°C and 554.85°C with a higher heat generation of 9366 J/g than single iron particles. The enhancement of figure-of-merit ZT or efficiency of thermoelectric materials is dependent on reducing the thermal conductivity. This dissertation synthesized and characterized the advanced two-component Si-Ge nanocomposites with a focus on lowering the thermal conductivity. The ball-milled two-component Si-Ge material demonstrated 50% reduction in thermal conductivity than the single component material used in the radioisotope thermoelectric generators and 10% reduction than the p-type SiGe alloy.Item A Potential Flow Model of a Fire Sprinkler Head(2014) Myers, Taylor Macks; Marshall, Andre W; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding fire sprinkler sprays fills a critical gap in the modeling of fire suppression systems. Previous research has shown that a modeling framework consisting of an instability model coupled with a stochastic transport model can paint most of the sprinkler spray picture, but requires input in the form of the thickness and velocity of unstable fluid sheets. The model outlined forgoes traditional CFD to solve for water jet-deflector interactions, and instead describes the sheet formation as a potential flow boundary value problem, utilizing a free surface formulation and the superposition of the Green's function. The resulting model allows for the determination of the complete flow field over a fire sprinkler head of arbitrary geometry and input conditions. A hypothetical axisymmetric sprinkler is explored to provide insight into the impact of sprinkler head geometry on local fluid as well as complete spray behavior. The resulting flow splits, sheet thicknesses, and sheet velocities are presented for various sprinkler head geometries.Item Assessing the Cost of Risk for New Technology and Process Insertion(2013) Lillie, Edwin Thomas; Sandborn, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Adoption and insertion of new technologies and processes into systems is inherently risky. A cost model that forecasts the cost of risk associated with inserting new technology into a system has been developed. The model projects the cost of inserting new processes, projects the impact of the processes on the cost of risk for the system, and performs a cost-benefit analysis on the adoption of proposed new processes. The projected cost of failure consequences (PCFC) is defined as the cost of all failure events (of varying severity) that are expected to occur over the service life of the system. The PCFC is uncertain, and the potential positive impact of adopting new technologies into the system is to reduce the cost of risk and/or reduce its uncertainty. A case study that assesses the adoption of a lead-free solder control plan into systems that previously used tin-lead solder has been performed.Item A DECISION SUPPORT SYSTEM FOR THE SPATIAL CONTROL OF INVASIVE BIOAGENTS(2010) Hebou, Luc; Montas, Hubert J; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Decision Support System (DSS) is developed and applied to the spatial control of invasive bioagents, exemplified in this study by the resident Canada goose species (Branta Canadensis) in the Anacostia River system of the District of Columbia. The DSS incorporates a model of goose movement that responds to resource distribution; a twocompartment Expert System (ES) that identifies the causes of goose congregation in hotspots (Diagnosis ES) and prescribes strategies for goose population control (Prescription ES); and a Geographic Information System (GIS) that stores, analyzes, and displays geographic data. The DSS runs on an HP xw8600 64-bit Workstation running Window XP Operating System. The mathematical model developed in this study simulates goose-resource dynamics using partial differential equations - solved numerically using the Finite Element Method (FEM). MATLAB software (v. 7.1) performed all simulations. ArcGIS software (v. 9.3) produced by Environmental Systems Research Institute (ESRI) was used to store and manipulate georeferenced data for mapping, image processing, data management, and hotspot analysis. The rule-based Expert Systems (ES) were implemented within the GIS via ModelBuilder, a modular and intuitive Graphical User Interface (GUI) of ArcGIS software. The Diagnosis ES was developed in three steps. The first step was to acquire knowledge about goose biology through a literature search and discussions with human experts. The second step was to formalize the knowledge acquired in step 1 in the form of logical sentences (IF-THEN statements) representing the goose invasion diagnosis rules. Finally, in the third step, the rules were translated into decision trees. The Prescription ES was developed by following the same steps as in the development of the Diagnosis ES, the major difference being that, in this case, knowledge was acquired relative to goose control strategies rather than overpopulation causes; and additionally, knowledge was formalized based on the Diagnosis and on other local factors. Results of the DSS application indicate that high accessibility to food and water resources is the most likely cause of the congregation of geese in the critical areas identified by the model. Other causes include high accessibility to breeding and nesting habitats, and supplementary, artificial food provided by people in urban areas. The DSS prescribed the application of chemical repellents at feeding sites as a goose control strategy (GCS) to reduce the quality of the food resources consumed by resident Canada geese, and therefore the densities of geese in the infested locations. Two other prescribed GCSs are egg destruction and harvest of breeding adult geese, both of which have direct impacts on the goose populations by reducing their densities at hotspots or slowing down their increase. Enclosing small wetlands with fencing and banning the feeding of geese in urban areas are other GCSs recommended by the ES. Model simulations predicted that these strategies would reduce goose densities at hotspots by over 90%. It is suggested that further research is needed to investigate the use of similar systems for the management of other invasive bioagents in ecologically similar environments.Item Generic Dynamic Model for a Range of Thermal System Components(2010) Xuan, Shenglan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The simulation of a thermal system consists of a simulation of its components and their interactions. The advantages of thermal system simulations have been widely recognized. They can be used to explore the performance of a newly designed system, to identify whether the design meets the design criteria, to develop and test controls, and to optimize the system by minimizing the cost or power consumption, and maximizing the energy efficiency and/or capacity. Thermal system simulations can also be applied to existing systems to explore prospective modifications and improvements. Much research has been conducted on aspects of thermal system and component simulation, especially for steady-state simulation. Recently, transient simulations for systems and components have gained attention, since dynamic modeling assists the understanding of the operation of thermal systems and their controls. This research presents the development of a generic component model that allows users to easily create and customize any thermal component with a choice of working fluids and levels of complexity for either transient or steady-state simulation. The underlying challenge here is to design the code such that a single set of governing equations can be used to accurately describe the behavior of any component of interest. The inherent benefits to this approach are that maintenance of the code is greatly facilitated as compared to competing approaches, and that the software is internally consistent. This generic model features a user-friendly description of component geometry and operating conditions, interactive data input and output, and a robust component solver. The open literature pertaining to thermal component models, especially the components of vapor compression systems, is reviewed and commented on in this research. A theoretical evaluation of the problem formulation and solution methodology is conducted and discussed. A generic structure is proposed and developed to simulate thermal components by enabling and disabling a portion of the set of governing equations. In addition, a system solver is developed to solve a system composed of these components. The component/system model is validated with experimental data, and future work is outlined.Item The Impact of Thermal Imaging Camera Display Quality on Fire Fighter Task Performance(2008) Rowe, Justin Lawrence; Mowrer, Frederick W.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermal imaging cameras (TIC) have become a vital fire fighting tool for the first responder community but there are currently no standardized quality control regulations. The purpose of the study was to understand the impact of TIC display image quality on a fire fighter's ability to perform a hazard recognition task. Test subjects were asked to identify a fire hazard by observing infrared images. The image matrix considered the interactions of several image characteristics including contrast, brightness, spatial resolution, and noise. The results were used to create a model function to predict the effect of image quality on user performance. This model was recommended to be incorporated in image quality test methods in development at the National Institute of Standards and Technology. These recommendations will also be provided to the National Fire Protection Association for use in an upcoming standard on fire fighting TIC.Item Discharge Characteristics of Canonical Sprinkler Sprays(2007-04-05) Blum, Andrew; Marshall, Andre; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Detailed characterization of spray behavior and its relationship to nozzle geometry, fluid properties, and injection characteristics is needed to advance water-based suppression technology and fire related computational fluid dynamics (CFD) tools. In this study, a series of experiments have been conducted to measure discharge characteristics of sprays produced by basic injector configurations modeled after conventional pendant sprinklers. Liquid jets of various sizes were injected downwards onto flat deflectors, tined deflectors, and boss-modified tined deflectors to establish the three canonical configurations explored in this study. Spray measurements including the initial angle of the sheet at the deflector exit, the sheet breakup radius, the drop size distribution 1 m below the deflector surface, and the volume density distribution were performed for these configurations. These systematic experiments provide discharge characteristics of practical interest while providing valuable data for CFD based atomization model development.Item Development of Magnetorheological Fluid Elastomeric Dampers for Helicopter Stability Augmentation(2005-12-06) hu, wei; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Conventional lag dampers use passive materials, such as elastomers, to dissipate energy and provide stiffness, but their damping and stiffness levels diminish markedly as amplitude of damper motion increases. Magnetorheological (MR) fluids based dampers have controllable damping with little or no stiffness. In order to combine the advantages of both elastomeric materials and MR fluids, semi-active magnetorheological fluid elastomeric (MRFE) lag dampers are developed in this thesis. In such a damper configuration, magnetic valves are incorporated into the chamber enclosed by elastomeric layers. Preliminary MRFE damper design analysis was conducted using quasi-steady Bingham-plastic MR flow mode analysis, and MRFE damper performance was evaluated analytically. To investigate the feasibility of using a combination of magnetorheological (MR) fluids and elastomeric materials for augmentation of lag mode damping in helicopters, a semi-active linear stroke MRFE lag damper was developed as a retrofit to an existing elastomeric helicopter lag damper. Consistent with sinusoidal loading conditions for a helicopter lag damper, single frequency (lag/rev) and dual frequency (lag/rev and 1/rev) sinusoidal loadings were applied to the MRFE damper. Complex modulus and equivalent damping were used to compare the characteristics of the MRFE damper with the passive elastomeric damper. The experimental damping characteristics of the MRFE damper were consistent with the analytical results obtained from the Bingham plastic analysis of the MR valve. Based on measurements, the Field-OFF MRFE characteristics are similar to the passive elastomeric damping, and controllable damping as a function of different flight conditions is also feasible as the applied current is varied in the MR valve. A second key objective of the present research is to develop an analytical model to describe the nonlinear behavior demonstrated by an MRFE damper. Since the damping behavior of both elastomers and MR fluids is dominated by friction mechanisms, a rate-dependent elasto-slide element is developed to describe the friction characteristics. An MR model developed from a single elasto-slide element successfully emulated the yield behavior of the MR damper, and this model captured nonlinear amplitude and frequency dependent behavior of MR dampers using constant model parameters. Meanwhile, using a distributed elasto-slide structure, an elastomeric model was developed to describe the stiffness and damping behavior of the elastomer as the amplitude of excitation increases. The fidelity of this five parameters time domain model is demonstrated by good correlation between modeling and experimental results for both the complex modulus and steady-state hysteresis cycles. Since an MRFE damper was shown to be a linear combination of the elastomeric and MR component, a time domain MRFE damper model was constructed based on the linear combination of the MR and elastomer models to describe the nonlinear behavior of the MRFE damper. Good correlation between the model and experimental data demonstrates the feasibility of the MRFE model for future MRFE damper applications.