UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

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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.
Burning Emulations of Condensed Phase Fuels Aboard The International Space Station
(2022) Dehghani, Parham; Sunderland, Peter B; Quintiere, James G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Little is known about the fire hazards of solids and liquids in microgravity. Ground-based tests are too short to overcome ignition transients and testing dozens of condensed fuels in orbit is prohibitively expensive. Burning rate emulation is one way to address this gap. It involves emulating condensed fuels with gases using a porous burner with embedded heat flux gages. This is a study of microgravity burning rate emulation aboard the International Space Station. The burner had porous round surfaces with a diameter of 25 mm. The fuel mixture was gaseous ethylene, and it was diluted with various amounts of nitrogen. The resulting heats of combustion were 15 – 47.2 kJ/g. The flow rate, oxygen concentration in the ambient, and pressure were varied. Heat flux to the burner was measured with two embedded heat flux gages and a slug calorimeter. The effective heat of gasification was determined from the heat flux divided by the fuel flow rate. Radiometers provided the radiative loss fractions. A dimensional analysis based on radiation theory yielded a relationship for radiative loss fraction. RADCAL, a narrow-band radiation model, yielded flame emissivities from the product concentrations, temperature, flame length, and pressure. Previously published analytical solutions to these flames allowed prediction of flame heights and radius, and when combined with the radiation empirical relationship led to corrections of total heat release rate from the flames due to radiative loss. Average convective and radiative heat flux were obtained from the analytical solution and a model based on the geometrical view factor of the burner surface with respect to the flame sheet, that was used to calculate the heat of gasification. All flames burning in 21% by volume oxygen self-extinguished within 40 s. However, steady flames were observed at 26.5, 34, and 40% oxygen. The analytical solution was used to quantify flame steadiness just before extinction. The steadiest flames reached more than 94% of their steady-state heat fluxes and heights. A flammability map as a plot of the heat of gasification versus heat of combustion was developed based on the measurement and theory for nominal ambient oxygen mole fractions of 0.265, 0.34, and 0.4.
FUNDAMENTAL STUDY OF INJECTION, MIXING AND STABILITY IN MODEL ROTATING DETONATION ENGINES
(2022) Redhal , Shikha Chaudhry; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In light of the growing demand for more efficient aviation engines, detonation-based engines are being investigated as possible replacements to traditional rockets and jet engines. Rotating Detonation Engine (RDE) is one of the novel engine concepts, gaining much interest from the aero propulsion community, including both industry and academia. RDE is a continuous-detonation engine, which consists of an annular chamber where the reactants are injected axially while the detonation wave propagates along the chamber in an orthogonal direction to the flow axis. Potential advantages of RDEs include greater thermal efficiency, improved fuel economy, simpler design, reduced weight, better scalability, and possible exempt from combustion instability concerns. One of the purposes of this research is to better understand the nature of RDE propulsion concepts and pertinent interaction between various physical processes. The main focus is on the effect of injection and mixing on the detonation wave propagation and heat release inside RDE combustors. Complex interaction between detonation waves and injector flow-fields is investigated for various injector geometries and flow compositions. The effects of injection and mixing are investigated for two different types of injector geometry, including unlike impinging doublet injectors and recessed partially-premixed jet injectors. Counter propagating waves are observed in the detonation tunnel as well as in other RDE tests. Based on the present results, the onset of the counter-propagating waves can be attributed to the reignition of the unburned reactants trapped in the wake of the wave. By visualizing the injector internal flowfield, it was also shown that detonation wave propagates into the interior of the recessed injectors. This finding is important for properly predicting the refresh injection timing. Also, the RDE stability and mode selection phenomenon are investigated using Rayleigh criterion to provide physical explanations based on acoustic energy for sustenance of periodic motion. The experimental data for this analysis is acquired using simultaneous sampling of transient chemiluminescence and local pressure measurements in the extended linear model detonation engine. The results show that, in the case of decoupled detonation wave, there is a distinct time delay between the pressure peak and the peak in the chemiluminescence signal. The ensuing Rayleigh index analysis can be used to explain and predict RDE mode selection processes. The results provide both qualitative and quantitative insights on the effect of RDE geometry as well as on the detonation wave stability. They provide better understanding of the complex interaction between RDE combustor processes.
SYNTHESIS AND CHARACTERIZATION OF ENERGETIC NANOMATERIALS WITH TUNABLE REACTIVITY FOR PROPULSION APPLICATIONS
(2020) Kline, Dylan Jacob; Zachariah, Michael R.; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Combustion is the world’s leading energy conversion method in which a fuel and oxidizer react and release energy, typically in the form of heat. Energetic materials (propellants, pyrotechnics, and explosives) have combustion reactions that are so fast that they are generally limited by how quickly the fuel and oxidizer can reach each other. Recent research has employed nanomaterials to reduce the distance between reactants to increase energy release rates. This dissertation attempts to uncover and quantify structure-function relationships in energetic nanomaterials by modifying chemical and physical properties of the materials and characterizing the observed changes using new diagnostic tools. This dissertation begins with the development of diagnostic tools that can capture the dynamics of energetic material combustion using a high-speed color camera to measure temperature. This tool has also been modified into a high-speed microscope that allows for spatial and temperature measurements at microscale length (µm) and time (µs) scales. Changes to chemical formula have been explored for energetic nanomaterial systems, though visualization of the reaction dynamics limited detailed results on reaction mechanisms. The first study performed here probed the role of gas generation vs. thermal effects in energy release rate where it was found that combustion inefficiencies from reactive sintering could be mitigated by introducing a gas-generating oxidizer. To explore combustion improvements in the fuel, a metal fuel nanoparticle manufacturing method was explored, though the combustion performance was again limited by reactive sintering. Another effort to reduce reactive sintering with a gas generator proved successful, but also unveiled the importance of different heat transfer mechanisms for propagation. The role of physical architecture on propellant combustion was also investigated to improve efficiency and versatility in solid propellants. It was found that addition of a poor thermal conductor to a propellant mixture increased the propagation rate of the material and this was attributed to the result increase in burning surface area resulting from inhomogeneous heat transfer. Lastly, this dissertation explores a method to remotely ignite materials using microwaves and titanium nanoparticles. This work sets the stage for a remotely staged solid propellant architecture that would provide control over solid propellant combustion in-operando.
Effects of Fire Whirl Generator Dimensions on Flame Length and Burning Rate
(2020) Dowling, Joseph Lee; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In-situ burning remains an efficient method of oil spill cleanup, but the implementation of fire whirls over the spilled fuel has the potential increase the speed and efficacy of the process by increasing burning rate and temperature. Logistical requirements would then be placed on the size of the fire whirl generator. A range of wall heights between 0 and55 cm were tested for a fixed-frame fire whirl generator with a liquid fuel source 10.5cm in diameter to analyze the effect on the burning rate and flame length of resulting fire whirls. For very short walls, with heights approximately equal to the fuel pool diameter,an increase of almost double was shown in the mass loss rate. The flame length for the fire whirl increased drastically for wall heights above a critical value of 35 cm, forming stable on-source fire whirls. This indicates that the inflow boundary layer of the fire whirl is a crucial feature causing an increase in the burning rate, while a critical wall height is necessary for aerodynamic effects to form stable on-source fire whirls with extended flame engths.
NUMERICAL SIMULATION OF THE BLUE WHIRL: A REACTING VORTEX BREAKDOWN PHENOMENON
(2019) Chung, Joseph Dong il; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a new potential way for low-emission combustion. This thesis uses numerical simulations to present, for the first time, the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames - a diffusion flame and a premixed rich and lean flame - all of which meet in a fourth structure, a triple flame which appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. This thesis also presents the development and testing of the numerical algorithms used in the simulation of the blue whirl. This work is a critical step forward in understanding how to use this new form of clean combustion.
SHAPE AND DOUBLE BLUE ZONES IN LAMINAR CO-FLOW DIFFUSION FLAMES
(2019) Wang, Zhengyang; Sunderland, Peter B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Many studies have examined the stoichiometric lengths of laminar gas jet diffusion flames. However, these have emphasized normal flames of undiluted fuel burning in air. Many questions remain about the effects of fuel dilution, oxygen-enhanced combustion, and inverse flames. In addition, past experimental and computational work indicates that double blue zones are possible in hydrocarbon diffusion flames. However, much remains unknown about double blue zones in diffusion flames. Thus, in this dissertation, the shape and double blue zones of the laminar co-flow jet diffusion flames are studied for more than 300 normal and inverse diffusion flames. Flame conditions including fuel type, reactant mole fraction, reactant flow rate, dilution agents, burner port material, burner port diameter, and flame Tad and Zst are varied. Chemiluminescence associated with excited species (C2*, CO2* CH*, and OH*) are measured through image deconvolution and broadband CO2* emission correction. Temperatures are measured with B-type thermocouples and TFP. Nitrogen addition to the fuel and/or oxidizer is found to increase the stoichiometric lengths of both normal and inverse diffusion flames, but this effect is small at high reactant mole fraction. This counters previous assertions that inert addition to the fuel stream has a negligible effect on the lengths of normal diffusion flames. The analytical model of Roper is extended to these conditions by specifying the characteristic diffusivity to be the mean diffusivity of the fuel and oxidizer into stoichiometric products and a characteristic temperature that scales with the adiabatic flame temperature and the ambient temperature. The extended model correlates the measured lengths of normal and inverse flames with coefficients of determination of 0.87 for methane and 0.97 for propane. Double blue zones, separated by up to 1.6 mm (and 0.9 mm) at the flame tip for IDFs (and NDFs), are observed in all the flames we measured. For both flame types, the blue zone toward the fuel side is rich and blue-green, while that toward the oxidizer side is stoichiometric, blue, and thicker. The rich zone results from emissions from CH* and C2*. The stoichiometric zone results from CO2* emissions and is coincident with the peak in OH*. All the deconvolved spectral emissive power peaks are higher in the IDF than in the NDF owing to higher scalar dissipation rates. The temperature profile of an NDF (and an IDF) was measured by B type thermocouple (and TFP). The result support the finding that the temperature peaks at the stoichiometric location for both NDFs and IDFs.
USING A BURNING RATE EMULATOR (BRE) TO EMULATE CONDENSED FUELS AND STUDY POOL FIRE BEHAVIOR IN 1G
(2019) Auth, Eric; Sunderland, Peter B; Quintiere, James G; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The Burning Rate Emulator (BRE) is a device constructed to emulate condensed fuels using gaseous fuel mixtures by matching heat of combustion, heat of gasification, smoke point, and surface temperature. The burner’s heat flux gauges are calibrated for local heat flux measurements and the copper top-plate calorimeter is calibrated for measuring net heat flux to the surface, which allows for determination of an effective heat of gasification to compare to condensed fuels. Seven condensed fuels with known properties are burned and emulated using methane, ethylene, and propylene gas diluted with nitrogen. Propane gas is used to study the general pool fire characteristics displayed by gaseous flames on the BRE. Flame anchoring, flammability regions, flame height, and convective heat transfer are analyzed. Based on a radial heat flux distribution, the readings from the heat flux sensors agree with the calorimeter when applied to a flame. Example flame images are shown.
EXPLORING THE RELATIONSHIPS BETWEEN FUEL AND OXIDIZER REACTION OF BIOCIDAL ENERGETIC MATERIALS
(2019) Wu, Tao; Zachariah, Michael R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Energetic materials are defined as a class of material with extremely high amount of stored chemical energy that can be released when ignited, along with intensive light emission and shock generation. Developing new energetic materials with high efficiency neutralization of biological warfare agents has gained increased attention due to the increased threat of bioterrorism. The objective of this dissertation is to develop new energetic materials with biocidal capabilities and apply them in various nanothermite systems to explore the relationships between fuel and oxidizer reactions. Aerosol techniques offer a convenient route and potentially direct route for preparation of small particles with high purity, and is a method proven to be amenable and economical to scale-up. Here I demonstrate the synthesis of various iodine oxides/iodic acids microparticles by a direct one-step aerosol method from iodic acid. A previously misidentified phase of I4O9 hydrate is in fact a new polymorph of HIO3 which crystalizes in the orthorhombic space group P212121. Various iodine oxides/iodic acids, including I2O5, HI3O8 and HIO3, were employed as oxidizers in thermite systems. Their decomposition behaviors were studied using a home-made time resolved temperature-jump/time-of-flight mass spectrometer (T-Jump/TOFMS). In addition, nano-aluminum (nAl), nano-tantalum and carbon black were adopted as the fuel or additive in order to fully understand how iodine containing oxidizers react with the fuel during ignition. The ignition and reaction process of those thermites were characterized with T-Jump/TOFMS. Carbon black was found to be able to lower both initiation and iodine release temperatures compared to those of Al/iodine oxides and Ta/iodine oxides thermites. Their combustion properties were evaluated in a constant-volume combustion cell and results show that nAl/a-HI3O8 has the highest pressurization rate and peak pressure and shortest burn time. However, an ignition delay was always present in their pressure profiles while combusting. To shorten or eliminate this ignition delay, a secondary oxidizer CuO is incorporated into Al/I2O5 system and four different Al/I2O5/CuO thermites by varying the mass ratio between two oxidizers are prepared and studied in a constant volume combustion cell. Significant enhancement is observed for all four thermites and their peak pressures and pressurization rates are much higher than that of Al/I2O5 or Al/CuO. Two other oxidizers also demonstrate similar effects as to CuO on promoting the combustion performance of Al/I2O5. A novel oxidizer AgFeO2 particles was prepared via a wet-chemistry method and evaluated as an oxidizer in aluminum-based thermite system. Its structure, morphologies and thermal behavior were investigated using X-ray diffraction, scanning electron microscopy, TGA/DSC, and T-Jump/TOFMS. The results indicate the decomposition pathways of AgFeO2 vary with heating rates from a two-step at low heating rate to a single step at high heating rate. Ignition of Al/AgFeO2 at a temperature just above the oxygen release temperature and is very similar to Al/CuO. However, with a pressurization rate three times of Al/CuO, Al/AgFeO2 yields a comparable result to Al/hollow-CuO or Al/KClO4/CuO, with a simpler preparation method. T-Jump/TOFMS was used to study the ignition and decomposition of boron-based thermites. The ignition behaviors of bare boron nanopowders and boron-based nanothermites at various gaseous oxygen pressure were investigated using the T-Jump method. High-heating rate transmission electron microscopy studies were performed on both B/CuO and B/Bi2O3 nanothermites to evaluate the ignition process. I propose a co-sintering effect between B2O3 and the oxidizer play an important role in the ignition process of boron-based nanothermites.
Nanocomposite and Soluble Energetic Additives for Burning Enhancement of Hydrocarbon Fuels
(2017) Guerieri, Philip Michael; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Metallizing liquid fuels and propellants to improve performance of energy conversion and propulsion systems has been of interest for decades but past attempts to do so using micron-sized metal powders demonstrated inefficient combustion and low burning rates of modified hydrocarbons. “Nanofuels” composed of energetic nanoparticles like nanoaluminum suspended in liquid fuels have slowly emerged in scientific research over the last two decades with promising results. Increased burning rates, lower ignition delays, and high suspension stabilities compared to slurry fuels of micron-sized particles have been demonstrated; however, the effects of various energetic nanoparticles on the combustion of hydrocarbons remain poorly understood while particle agglomeration remains a performance-limiting problem. The research in this dissertation identifies strategies for inclusion of aluminum into hydrocarbons which promote combustion performance in a free-droplet burning experiment developed herein. Considering the low burning rates which plagued micron particle-based slurry fuels, specific attention is paid to characterizing and understanding effects on droplet burning rate constants. Classical characterization of this metric based on the D-squared-law for isolated droplet combustion is found to be unsuitable with heterogeneous energetic additives and thusly an original scheme for experimental approximation of burning rate constant is set forth. Several beneficial strategies for aluminum inclusion and burning rate enhancement are studied including co-addition of nanoaluminum with the gas generator nitrocellulose (NC), dissolution of Al-containing molecules including organometallic clusters into hydrocarbons, and burning rate enhancements realized with oxygen-carrying nanoparticle co-additives. Arguably the most impactful strategy identified however is the preassembly of active nanoparticles into NC-bound clusters or controlled agglomerates, termed “mesoparticles” (MPs), by electrospray which drastically improves droplet burning rate increases and nanofuel suspension stabilities observed compared to nanofuels of unassembled nanoparticles. Mechanisms of the various additives studied are probed with a variety of diagnostic techniques and burning rate enhancements are linked to physical effects of droplet disruptions on the diffusion-limited burning droplet system. The MP architecture causes a feedback loop between physical disruptions by gas liberation from droplets, transport of active additives into the flame where they react, and promotion of further gas evolution repeating and accelerating this process.

UMD Theses and Dissertations

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