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 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.Item 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.Item 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.Item IGNITION QUALITY TESTER CHARACTERIZATION WITH PURE COMPONENT AND CONVENTIONAL NAVY FUELS(2016) Mendelson, Jacob Lee; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U.S. Navy is attempting to reduce dependence on conventional diesel fuels as a part of the environmental initiative commonly referred to as “The Great Green Fleet”. The purpose of this research was to characterize the measurements of ignition delay gathered by the Advanced Engine Technology Ignition Quality Tester (IQT) with conventional Navy diesel fuels, pure component biodiesel fuels, primary cetane standards, and toluene-hexadecane blends. The use of computational analysis with pressure traces gathered from the IQT allowed for the comparison of IQT ignition delay results with various methods of calculating start of combustion for various fuels. Physical and chemical ignition delays of each fuel were also calculated using different separation techniques and the chemical ignition delay results were compared with prior academic literature and with chemical ignition delays calculated with Lawrence Livermore kinetic theory.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 Investigation of Colorless Distributed Combustion (CDC) with Swirl for Gas Turbine Application(2013) Khalil Hasan, Ahmed EssamElDin; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Colorless Distributed Combustion (CDC) with swirl is investigated for gas turbine engine applications due to its benefits for ultra-low pollutants emission, improved pattern factor and thermal field uniformity, low noise emission, and stable combustion with the alleviation of combustion instabilities. Adequate and fast mixing between the injected air and internally recirculated hot reactive gases to form hot and diluted oxidant is critical for CDC, followed by rapid mixing with the fuel. This results in distributed reaction zone instead of a concentrated thin flame front as observed in conventional diffusion flames, leading to avoidance of hot spot regions and providing reduced NOx and CO emissions. The focus of this dissertation is to develop and demonstrate CDC in a cylindrical combustor for application to stationary gas turbine combustors. The dissertation examines the sequential development of ultra-low emission colorless distributed combustor operating at a nominal thermal intensity of 36MW/m3-atm. Initially, the role of swirl is evaluated through comparing the performance of swirling and non-swirling configurations with focus on pollutants emission, stability, and isothermal flowfield through particle image velocimetry. Different fuel injection locations have also been examined, and based on performance a swirling configuration have been down selected for further investigations demonstrating emissions as low as 1 PPM of NO with a 40% reduction compared to non-swirling configuration. Further investigations were performed to outline the impact of inlet air temperature and combustor pressure on reaction distribution and combustor performance. Next, Fuel flexibility has been examined with view to develop CDC combustors that can handle different gaseous and liquid fuels, both traditional and renewable. These fuels included diluted methane, hydrogen enriched methane, propane, ethanol, kerosene, JP-8, Hydrogenated Renewable Jet fuel, and novel biofuel. Swirling CDC combustor demonstrated emissions below 7.5 PPM of NO regardless of the fuel used, with emissions below 40PPM of CO for liquid fuels and 10 PPM for gaseous fuels. Further enhancement of swirling CDC combustor was sought next. Various fuel injection techniques have been examined, outlining the importance of fuel injection location with respect to air and hot reactive gases recirculation. The impact of air injection velocity on combustor performance have been examined in terms of increased recirculation (via isothermal flow field characterization using PIV) and enhanced performance with lower pollutants emission leading to 45% reduction in NO emissions with no impact on CO emissions. The impact of fuel dilution on mixing and performance has been also examined as a method to enhance mixing due to the increased fuel jet momentum. Dual air and fuel injection have been explored to outline the impact of multi injection on combustor performance for scaling up of the combustor. Planar Laser Induced Fluorescence technique was used to evaluate the reaction behavior and its distribution in the combustor through detection of activated OH radicals at different activation lines in different configurations. The different investigations performed (experimentally and numerically) have been compiled and analyzed with view to develop a "Distribution Index" that evaluated the reaction distribution in a given combustor based on certain parameters. These parameters include, but no limited to, hot reactive gases recirculation (entrainment) rate, air injection velocity, mixing between air and fuel, and operational equivalence ratio and inlet air temperature. The developed distribution index, DI, will be a valuable tool for future combustor design.Item CHARACTERISTICS AND CHEMICAL KINETICS OF HYDROGEN SULFIDE COMBUSTION IN THERMAL CLAUS REACTOR(2012) Selim, Hatem Mohamed Mohiy Elden; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrogen sulfide is a hazardous gas from both environmental safety and human health perspectives. Hydrogen sulfide presence in any combustion application results in the formation of acidic gases that affects ozone layer and causes acidic precipitation. Exposure to H2S levels at 100 ppm or higher can endanger human life. Hydrogen sulfide is commonly found to exist in crude natural gas and oil wells. With the decrease in fossil fuels reserves around the world, we will have to rely on extracting energy from wells that contain higher amounts of H2S. In addition, environmental regulations strictly regulate the H2S discharge into the atmosphere. Subsequently, efficient hydrogen sulfide treatment becomes of increasing importance with time. Hydrogen sulfide treatment is typically a chemical reaction process (Claus process) in which hydrogen sulfide is combusted to end-products of sulfur and water. Hydrogen sulfide combustion in thermal Claus reactor has been investigated in this research. A reduced reaction mechanism for H2S oxidation has been developed using a novel error-propagation-based approach for reduction of detailed reaction mechanisms. The reduced mechanism has been used for detailed investigation of chemical kinetics mechanistic pathways in Claus process. Experimental examination of H2S combustion in different flames, methane/air and hydrogen/air, is provided. Chemical kinetics pathways and reaction conditions responsible for sulfurous compounds formation (SO2, CS2, and COS) are addressed. Hydrogen sulfide flame emissions have been investigated for intermediate species identification using chemiluminescence flame spectroscopy. Effect of acid gas composition (H2S, CO2 and N2) on hydrogen sulfide combustion and Claus process efficiency is also provided. Finally, examination of the quality of captured sulfur with respect to reactor conditions is presented.Item IGNITION, COMBUSTION AND TUNING OF NANOCOMPOSITE THERMITES(2010) Sullivan, Kyle Thomas; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanocomposite thermites, or Metastable Intermolecular Composites (MICs), are energetic systems involving the reaction between nanoparticles of a metal fuel and another metal or metal oxide. When nanoparticles are used, the interfacial contact area and homogeneity of mixing are greatly improved, dramatically decreasing the characteristic mass diffusion length between the fuel and the oxidizer. Nano-sized aluminum is commonly used as a fuel, due to a combination of its abundance, good reactivity, and its ability to produce environmentally benign reaction products. A variety of oxidizers have been studied depending on the particular application. Nanocomposite thermites are currently being investigated for uses in propellants, pyrotechnics, and explosives, as well as some more exotic applications such as micro-propulsion and joining applications. Despite the research efforts and potential applications, the reaction mechanism remains poorly understood. As the particle size transitions into the nanometer regime, properties such as the melting temperature, surface energy, drag force, along with the characteristic time scales of thermo-chemical processes can change. In an exothermically reacting system, all of these considerations must be taken into account simultaneously, a rather daunting task. However, if we design parametric experiments to look at relative trends, we can develop scaling laws and determine which parameters are perhaps the most important in the reaction mechanism. This work largely involves combusting thermite materials in a pressure cell, and also uses new techniques such as inducing a reaction inside an electron microscope with a specially designed heating holder. The results suggest that the pressurization and optical emission can arise from fundamentally different phenomena. A reactive sintering mechanism occurs which rapidly decomposes the oxidizer and pressurizes the system. This is followed by the remainder of the fuel burning in a gaseous, pressurized environment, where the burning rate is controlled by the fuel. Also in this work, we combust new fuels and oxidizers such as nano-sized boron, AgIO3, and Ag2O. Boron can be used as an additive to increase the energy density in thermites. The silver-based oxidizers are currently being investigated in nanocomposite thermites for their ability to generate a product which can effectively destroy harmful biological spores, such as Anthrax.Item Direct Numerical Simulation of Non-Premixed Flame Extinction Phenomena(2010) Narayanan, Praveen; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Non-premixed flame extinction phenomena are relevant in a variety of com- busting environments, including but hardly limited to diesel engines, pool fires, and fire suppression scenarios. These disparate phenomena are controlled by various parameters that contain information on flame stretch, heat losses, composition of the fuel and oxidizer supply streams, etc. Direct Numerical Simulation (DNS) is used in the present study to provide fundamental insight on diffusion flame extinction under non-adiabatic combustion conditions. The list of DNS configurations include: (C1) counterflow laminar flames with soot formation and thermal radiation transport; (C2) coflow turbulent flames with soot formation and thermal radiation transport; (C3) counterflow laminar and turbulent flames interacting with a mist-like water spray. Configurations C1 and C2 use single-step chemistry while configuration C3 uses detailed chemistry (all cases correspond to ethylene-air combustion). Configuration C1 is also treated using large Activation Energy Asymptotics (AEA). The AEA analysis is based on a classical formulation that is extended to include thermal radiation transport with both emission and absorption effects; the analysis also includes soot dynamics. The AEA analysis provides a flame extinction criterion in the form of a critical Damköhler number criterion. The DNS results are used to test the validity of this flame extinction criterion. In configuration C1, the flame extinction occurs as a result of flame stretch or radiative cooling; a variation of configuration C1 is considered in which the oxidizer stream contains a variable amount of soot mass. In configuration C1, flame weakening occurs as a result of radiative cooling; this effect is magnified by artificially increasing the mean Planck soot absorption coefficient. In configuration C3, flame extinction occurs as a result of flame stretch and evaporative cooling. In all studied cases, the critical Damkohler number criterion successfully predicts transition to extinction; this result supports the unifying concept of a flame Damköhler number Da and the idea that different extinction phenomena may be described by a single critical value of Da.