Mechanical Engineering
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Item NEAR-LIMIT SPHERICAL DIFFUSION FLAMES AND COOL DIFFUSION FLAMES(2023) Waddell, Kendyl; Sunderland, Peter B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To combat the rising threats of climate change, current combustion technologies must evolve to become cleaner and more efficient. This requires a better understanding of the fundamental properties of combustion. One way to gain this is through microgravity experiments, where the lack of buoyancy reduces flames to their most basic components, simplifying modeling efforts. The low-temperature combustion of warm and cool flames, which has applications in advanced engine technologies and implications in terrestrial and spacecraft fire safety, is favored in microgravity. In this work, microgravity spherical diffusion flames are generated aboard the International Space Station using a spherical porous burner. A transient numerical model with detailed chemistry, transport, and radiation is used to simulate the flames. This incorporates the UCSD mechanism with 57 species and 270 reactions. Hot, warm, and cool diffusion flames are all studied. Experimental flame temperature was measured using thin-filament pyrometry, which was calibrated using a blackbody furnace. The measured temperatures agreed reasonably well with numerical simulations for a wide range of conditions, and were in the range of 950-1600 K, with an estimated uncertainty of ± 100 K. The temperatures of the porous spherical burner were measured by a thermocouple embedded in its surface. These measured temperatures, combined with numerical simulations of the gas phase, yield insight into the complex heat transfer processes that occur in and near the porous sphere. Previous work has found that ethylene microgravity spherical diffusion flames extinguish near 1130 K at atmospheric pressure, regardless of the level of reactant dilution. The chemical kinetics associated with this consistent extinction temperature are explored using the transient numerical model. Species concentrations, reaction rates, and heat release rates are examined. Upon ignition, the peak temperature is above 2000 K, but this decreases until extinction due to radiative losses. This allows the kinetics to be studied over a wide range of temperatures for the same fuel and oxidizer. At high temperature, the dominant kinetics are similar to those reported for typical normal-gravity hydrocarbon diffusion flames. There are well defined zones of pyrolysis and oxidation, and negligible reactant leakage through the reaction zone. As the flame cools, there is increased reactant leakage leading to higher O, OH, and HO2 concentrations in the fuel-rich regions. The pyrolysis and oxidation zones overlap, and most reactions occur in a narrow region near the peak temperature. Reactions involving HO2 become more significant and warm flame chemistry appears. At atmospheric pressure, this low-temperature chemistry delays extinction, but does not produce enough heat to prevent it. As ambient pressure is increased, low-temperature chemistry is enhanced, allowing the flame to extend into the warm flame and cool flame regimes. Experimental results show that increasing the pressure from 1 atm to 3 atm decreased the ethylene extinction temperature by almost 60 K. Numerical simulations showed similar behavior, as well as the emergence of cool flame behavior when the pressure was increased to 50 atm. This allows the kinetics of spherical warm and cool diffusion flames and the role of increased HO2 participation to be examined. There are few options for studying cool diffusion flames experimentally that do not require expensive facilities that are unavailable to the average researcher. A method is presented for observing cool diffusion flames inexpensively using a pool of liquid n-heptane and parallel plates heated to produce a stably stratified stagnation flow. The flames were imaged with a color camera and an intensified camera. Measurements included gas phase temperatures, fuel evaporation rates, and formaldehyde yields. These are the first observations of cool flames burning near the surfaces of fuel pools. The measured peak temperatures were between 705 – 760 K and were 70 K above the temperature of the surrounding air. Autoignition first occurred at 550 K.Item Analysis of Mass Transfer in Electrochemical Pumping Devices(2022) Baker, Joseph P; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Considering the environmental challenges posed by traditional energy systems, we must strive to seek out innovative strategies to sustainably meet today’s demands for energy and quality of life. Energy systems using electrochemical (EC) energy conversion methods may help us to transition to a more sustainable energy future by providing intermittent renewable energy storage and improving building energy efficiency. EC pumping devices are a novel technology that use chemical reactions to pump, compress, or separate a given working fluid. These devices operate without any moving parts. Unlike mechanical pumps and compressors, they operate silently, producing no vibrations and requiring no lubrication. In this dissertation, I investigate EC pumping devices for use in two applications: ammonia EC compression for intermittent renewable energy storage and EC dehumidification for separate sensible and latent cooling. Hydrogen fuel cells are a promising technology for on-demand renewable power generation. While storage of pure hydrogen fuel remains a problem, ammonia is an excellent hydrogen carrier with far less demanding storage requirements. EC ammonia compression opens the door to several possibilities for separating, compressing, and storing ammonia for intermittent power generation. Using the same proton exchange membranes commonly used in fuel cells, I demonstrated successful ammonia compression under a variety of operating conditions. I examined the performance of a small-scale ammonia EC compressor, measuring the compression and separation performance. I also conducted experiments to investigate the steady-state performance of a multi-cell ammonia EC compressor stack, observing a maximum isothermal efficiency of 40% while compressing from 175 kPa to 1,000 kPa. However, back diffusion of ammonia reduced the amount of effluent ammonia by as much as 67%. Dehumidification represents a significant portion of air conditioning energy requirements. Separate sensible and latent cooling using EC separation of water may provide an energy efficient thermal comfort solution for the hot and humid parts of the world. I conducted experiments of several EC dehumidifier, considering both proton exchange and anion exchange processes. Diffusion of the working fluid was significant in this application as well. I observed a maximum Faradaic efficiency for dehumidification of 40% for a 50 cm2 cell using an anion exchange membrane under the most favorable case. I developed a novel open-air EC dehumidifier prototype. To alleviate the back diffusion issue, I investigated a method for mass transfer enhancement using high-voltage fields. I also developed a numerical model to simulate the performance of the EC dehumidifier devices, predicting the experimentally measured performance to within 25%.Item INVESTIGATION INTO PYROLYSIS AND GASIFICATION OF SOLID WASTE COMPONENTS AND THEIR MIXTURES(2021) Burra, Kiran Raj Goud; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unsustainable dependence on fossil fuel reserves for energy and material demands is leading to growing amounts of CO2 concentration in the atmosphere and irreversible climate changes. Carbon neutral sources such as abundant biomass reserves and landfill-destined high energy density wastes such as plastics, and tire-wastes can be utilized together for energy and material production for a sustainable future. Pyrolysis and gasification can convert these variable feedstocks into valuable and uniform synthetic gas (syngas) with versatile downstream applicability to energy, liquid fuels, and other value-added chemicals production. But seasonal availability, high moisture and ash content, and relatively low energy density of biomass can result in significant energy and economic losses during gasification. Furthermore, gasification of plastic wastes separately was found to result in feeding issues due to melt-phase, coking, and agglomerative behavior leading to operational issues. To resolve these issues, co-processing of biomass with these plastics and rubber wastes was found to be promising in addition to providing synergistic interaction leading to enhanced syngas yield and inhibitive behavior in some cases and thus motivating this work. This dissertation provides a deconvoluted understanding and quantification of the source and impact of these interactions for better process performance and alleviation of inhibitive interaction needed to develop reliable co-gasification of feedstock mixtures. To achieve this, plastic and tire wastes were investigated separately and mixed with different biomass species using a series of feedstock arrangements to understand synergistic influence on the syngas yield and kinetics in comparison to mono-conversion. Influence of operating conditions such as feedstock composition, temperature and gasifying agent was also examined for desirable conditions of energy recovery and high-quality syngas yield. Lab-scale semi-batch reactor studies equipped with online product gas analysis, along with thermogravimetric studies were utilized to obtain insight into the products yield, kinetics, and energy conversion. These results provided a better understanding of the influence of feedstocks and their interaction on the syngas and process behavior. They address the knowledge gap in versatile feedstock-flexible gasifier development for efficient and reliable syngas production from varying solid waste and biomass component mixtures with minimal changes to the operating conditions.Item A GENERALIZED METHODOLOGY TO DEVELOP PYROLYSIS MODELS FOR POLYMERIC MATERIALS CONTAINING REACTIVE FLAME RETARDANTS: RELATIONSHIP BETWEEN MATERIAL COMPOSITION AND FLAMMABILITY BEHAVIOR(2018) Ding, Yan; Stoliarov, Stanislav I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of effective flame retardant polymeric materials is of great interest to the fire protection community. To enable intelligent design of flame retardant polymeric materials, it is important to understand the relation between the material composition and the chemical and physical properties that control the fire growth process. This work details a generalized methodology to characterize flame retardant materials for the development of pyrolysis models that relate the fire behavior to material composition. The methodology employs thermogravimetric analysis, differential scanning calorimetry, and microscale combustion calorimetry, to measure the sample mass loss, heat required to decompose the sample, and the heat released from the complete combustion of the gaseous products evolved during the sample decomposition, respectively. Through inverse analysis of the milligram-scale experimental measurements using a numerical pyrolysis framework, ThermaKin2Ds, the decomposition kinetics and thermodynamics, and heats of combustion of gaseous pyrolyzate are determined. The chemical interactions between the polymer matrix and flame retardants are characterized by second-order (two-component) reactions. The resulting reaction model reproduces all aforementioned experiments with a high degree of detail as a function of heating rate and captures changes in the decomposition behavior with changes in the flame retardant contents. The methodology also utilizes a new bench-scale controlled atmosphere gasification apparatus to measure mass loss rate (MLR), back surface temperature, and sample shape profile evolution of 7-cm-diameter disk-shaped samples exposed to well-defined radiant heating. Inverse analysis of the bench-scale gasification experimental measurements using ThermaKin2Ds and the developed reaction model yields properties that define heat and mass transport in the pyrolyzing samples. This approach is demonstrated using two sets of materials: glass-fiber-reinforced polyamide 66 blended with red phosphorus and glass-fiber-reinforced polybutylene terephthalate blended with aluminum diethyl phosphinate and melamine polyphosphate. The resulting pyrolysis model is capable of predicting MLR data as a function of material composition and external heating condition. Idealized cone calorimetry simulations are conducted to demonstrate that, when the gas-phase combustion inhibition effect is excluded, aluminum diethyl phosphinate has a relatively minor impact on heat release rate, while the impacts of melamine polyphosphate and red phosphorus are significant.Item COMPACT ABSORBER FOR ADVANCED ABSORPTION HEAT PUMPS(2018) Bangerth, Stefan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.Item The Effects of Liquid Alkane Fuel Structure on Catalyst-Enhanced Combustion(2018) Dube, Grant; Oran, Elaine S; Lee, Ivan C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U.S. Army is developing micro-combustors for use in soldier-portable power generation systems. Many of the challenges associated with micro-combustion can be potentially overcome using a catalyst, but the effects of the catalyst on ignition under the low temperature, atmospheric conditions seen in the field are not well understood. To better understand catalytic ignition phenomena under these conditions, a Catalytic Ignition and Emissions Tester (CIET) was developed and used to investigate the effects of liquid alkane fuel structure during catalyst enhanced ignition. Various mixtures of n-octane and iso-octane, as well as single component n-dodecane and n-hexadecane, were chosen as simple, surrogate test fuels to represent gasoline, jet fuel, and diesel, respectively. Fuel reactivity was shown to decrease with increased branching for all metrics tested while the effects of chain length were less definitive. The global apparent activation energies of all fuels tested were found to be in the range of 41-61 kJ/mol with 95% confidence, significantly lower than those previously reported for non-catalytic hydrocarbon combustion (>100 kJ/mol).Item ENHANCED HYDROGEN PRODUCTION FROM ACID GAS(2017) El-Melih, Ahmed Mahmoud; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrogen sulfide is a colorless, corrosive, toxic and flammable gas that is notorious for its nuisance rotten egg odor. Hydrogen sulfide endangers environment, human health and industrial equipment. Despite its high heating value, utilization of hydrogen sulfide as fuel is strictly prohibited using conventional combustion technologies. This malignant gas naturally exists in crude oil and natural gas wells. The separated-out hydrogen sulfide from crude oil and gas has other impurities that include: carbon dioxide, nitrogen, ammonia, carbonyl sulfide, carbon disulfide as well as benzene, toluene and xylene (commonly referred to as BTX). With the increase in energy demand, there will be reliance on utilization of sourer feedstock. Consequently, hydrogen sulfide stream needs to be efficiently treated. On the other hand, hydrogen sulfide is considered a hydrogen rich feedstock. In addition, the hydrogen-constituted impurities, such as: methane, ammonia and BTX in the separated-out hydrogen sulfide stream from crude oil and gas furtherly enrich the hydrogen feedstock of this stream. Hydrogen can be produced from the decomposition of hydrogen sulfide into its two valuable constituents: hydrogen and sulfur. Thermal decomposition of hydrogen sulfide was studied in this work. Experimental examination of wide range of several key parameters that affect the amounts of hydrogen produced and destructed hydrogen sulfide was conducted. Effect of inlet acid gas composition as well as role of different contaminant gases naturally accompanying H2S on the chemistry, production of hydrogen and destruction of hydrogen sulfide were studied. A chemical reaction mechanism that characterizes hydrogen sulfide thermal decomposition as well as decomposition of a mixture of hydrogen sulfide with methane over wide range of conditions was developed. The developed mechanism addresses the chemical kinetics and possible pathways. The difference in dominant reaction pathways between the two cases of presence and absence of impurities facilitated the identification of the role played by the contaminants.Item Inhibition of Laminar Premixed Flames by Halon 1301 Alternatives(2015) Pagliaro, John Leonard; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Halon 1301 (CF3Br) has been banned (by the Montreal Protocol) because of its ozone depleting potential. Even though a critical-use exemption of CF3Br has been granted for commercial aircraft cargo bay applications, the European Union is requiring replacement in new aircraft by 2018 and in existing aircraft by 2040. As a result of the expected phase-out, the FAA tested three alternatives (C2HF5, C3H2F3Br, and C6F12O) in a cargo bay simulator, and under certain conditions, apparent combustion enhancement was observed (even though the agents showed promise in standard tests). To understand the enhancement, experiments and numerical analysis are performed to: 1) test the concepts developed via previous numerical simulations and analysis of the FAA tests, 2) reproduce the phenomena observed in the complex full-scale FAA experiments in laboratory-scale experiments which might serve as a screening tool, 3) provide preliminary validation of recently developed kinetic mechanisms (which are used to understand the phenomena), and 4) examine the performance of potential replacements that were not tested by the FAA. Two spherically expanding flame experiments were built to measure laminar burning velocity, peak pressure rise, and flame response to stretch. For each experiment, developments included designing the chamber, creating the operating procedure, setting up the necessary data acquisition and operation controls, and developing data reduction and post-processing routines. Numerical modeling with detailed kinetics was performed to interpret experimental results and to validate kinetic mechanisms. The most significant findings of this study include the enhancement of lean CH4-air flames by the proposed alternative agents, the potential of HCFC-123 as a halon replacement, and excellent agreement between burning velocity predictions (with detailed chemical mechanisms) and measurements for hydrocarbon-air flames inhibited by CF3Br, C2HF5, C3H2F3Br, C6F12O, and C2HF3Cl2.Item ROLE OF BENZENE, TOLUENE AND XYLENE TO ACID GAS DESTRUCTION IN THERMAL STAGE OF CLAUS REACTORS(2015) Ibrahim, Salisu; Gupta, Ashwani K.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Crude oil and natural gas often contain acid gases (H2S and CO2) and trace amounts of benzene, toluene and xylene (BTX) and these are all harmful to human health, environment and industrial equipment. Acid gases in chemical industry and vehicles cause corrosion to parts of engines, refinery equipment and catalysts deactivation in catalytic processes. Human exposure to H2S, even in low concentrations, causes burning of eyes, headache, dizziness, dyspnea, and skin irritations. Inhalation of high doses of BTX may cause skin and respiratory tract irritation. Increased energy demand and exploitation of sourer feedstock have made regulatory agencies worldwide to promulgate stricter regulations on sulfur emissions. US EPA requires a reduction of sulfur in gasoline from 30ppm to 10ppm by 2017. Crude oil and gas must be subjected to more efficient desulfurization processes. The separated acid gases and BTX is further processed in Claus process for chemical and energy recovery. Currently, BTX poses several technical and operational problems that result in higher operational costs and increased toxic gas emissions in Claus plants. BTX destruction in the thermal stage of Claus process was identified as the solution. Acid gas and BTX combustion in thermal stage of Claus reactor, which provides simultaneous recovery of both sulfur and thermal energy, is the subject of this research. Effect of BTX in H2S fueled flames on sulfur chemistry in thermal stage of a Claus reactor was characterized. Reactor conditions that promote BTX destruction are presented. Oxygen enriched combustion air for the destruction of BTX and acid gas is examined. Chemical kinetic pathways of BTX destruction under high temperature conditions of the Claus reactor are evaluated. Intermediate radicals and stable species formed during the combustion process are characterized using flame emission spectroscopy and gas chromatography (GC). Role of multiple contaminants, CO2 and benzene, toluene or xylene in H2S combustion is also investigated. Chemical kinetic pathways and reactor conditions that promote/hinder the formation of mercaptans (such as COS and CS2) is addressed. The results presented here assist in the design guidelines of advanced Claus reactors for enhanced sulfur capture in the thermal stage and to mitigate environmental issues.Item Enhanced Gas-Liquid Absorption Utilizing Micro-Structured Surfaces and Fluid Delivery Systems(2014) Ganapathy, Harish; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite intensive research and development efforts in renewable energy in recent years, more than 80% of the energy supply in the year 2040 is expected to come from fossil fuel-based sources. Increasing anthropogenic greenhouse gas emissions led the United States to legislatively limit domestic CO2 emissions to between 1000-1100 lb/MWh for new fossil fuel-fired power plants, thus creating an urgent need for efficient gas separation (capture) processes. Meanwhile, the gradual replacement of coal with cleaner burning natural gas will introduce additional challenges of its own since nearly 40% of the world's gas reserves are sour due to high concentrations of corrosive and toxic H2S and CO2 gases, both of which are to be separated. Next-generation micro-structured reactors for industrial mass and heat transfer processes are a disruptive technology that could yield substantial process intensification, size reduction, increased process control and safety. This dissertation proposes a transformative gas separation solution utilizing advanced micro-structured surfaces and gas delivery manifolds that serves to enhance gas separation processes. Experimental and numerical approaches have been used to achieve aggressive enhancements for a solvent-based CO2 absorption process. A laboratory-scale microreactor was investigated to fundamentally understand the physics of multiphase fluid flow with chemical reactions at the length scales under consideration. Reactor design parameters that promote rapid gas separation were studied. Computational fluid dynamics was used to develop inexpensive stationary (fixed) interface models for incorporation with optimization engines, as well as high fidelity unsteady (deforming) interface models featuring universal flow regime predictive capabilities. Scalability was investigated by developing a multiport microreactor and a stacked multiport microreactor that represented one and two orders magnitude increase in throughput, respectively. The present reactors achieved mass transfer coefficients as high as 400 1/s, which is between 2-4 orders of magnitude higher than conventional gas separation technologies and can be attributed to the impressive interfacial contact areas as high as 15,000 m2/m3 realized in this study through innovative design of the system. The substantial enhancement in performance achieved is indicative of the high level of process intensification that can be attained using the proposed micro-structured reactors for gas separation processes for diverse energy engineering applications. This dissertation is the first comprehensive work on the application of micro-structured surfaces and fluid delivery systems for gas separation and gas sweetening applications. More than ten refereed technical publications have resulted from this work, part of which has already been widely received by the community.