Date of Award
Doctor of Philosophy (PhD)
Fossil fuels supply over 80% of the world’s primary energy, and if current policy and technology trends continue, global energy and energy-related carbon dioxide (CO2) emissions are predicted to increase for at least several decades owing to population and economic growth, leading to serious concerns regarding global warming. Natural gas releases less CO2 than other fossil fuels (e.g., coal and oil) and can help meet future CO2 emission targets. Natural gas combustion, however, has poor radiation heat transfer when compared to other fossil fuels owing to its low propensity for soot formation, making it difficult to use as a drop-in replacement in applications requiring rapid heat transfer. Methane is the primary constituent of natural gas, and in the first part of this dissertation, a method of increasing and controlling soot formation, and subsequently thermal radiation, from methane flames is presented. Oxygen-enriched combustion in a unique tri-coflow flame configuration, coupled with fuel additives, is used to increase soot formation and radiation heat transfer in laminar methane coflow diffusion flames, while ultimately oxidizing the soot particles to ensure complete combustion and no soot emissions. Thermal radiation from the methane flame is increased by 110%, to levels comparable to, or greater than, fuels with a much higher propensity for soot formation, such as ethylene. In addition to increasing radiation heat transfer, the oxygen-enriched combustion approach employed has the additional benefit of facilitating carbon capture and storage (CCS), an established emissions reduction technology. In order to efficiently use oxygen-enriched combustion to facilitate CCS, a more thorough fundamental understanding of how oxygen-enrichment affects flame structure, soot formation, and flame extinction is necessary. To this end, in the second part of this dissertation, burner-supported gaseous ethylene microgravity spherical diffusion flames are investigated aboard the International Space Station. Microgravity affords a unique environment where the effects of buoyancy are eliminated, and thus fundamental information about flame structure and its effect on soot formation, flame extinction, and flame stability can be probed. It is found that radiative extinction of these flames at atmospheric pressure occurs at a critical temperature and reactant-based mass flux, regardless of initial conditions. Flame stability in the presence of radiation heat loss is studied, and the limiting conditions for steady-state spherical diffusion flames are identified. Furthermore, the effects of ultra-low strain on the kinetic structure and oxidation pathway of ethylene diffusion flames are elucidated. The results presented have important implications for spacecraft fire safety and terrestrial wildfires. In the last part of this dissertation, the subject of particle formation in flames was transitioned to identifying a method of producing materials in flames in a way that could lead to a novel approach to additive manufacturing. Specifically, the synthesis and deposition of high-temperature materials without contamination of the synthesis reaction byproduct was investigated. Material properties at high temperature are a common limiting factor for system performance and efficiency in energy technologies, such as plasma-facing materials, engine components and turbine blades. Flame synthesis with gas-phase precursors provides a unique opportunity to synthesize high-temperature materials (e.g., refractory metals, refractory multi-principal element alloys, and ultra-high temperature ceramics) with no limit on the melting point of the material. In this part of this dissertation, a novel method of manufacturing components through flame synthesis is presented, and preliminary measurements are conducted. A thin layer of titanium is synthesized and deposited on a substrate with minimal contamination of the byproduct of the synthesis reaction This method holds promise for yielding improved high-temperature materials that could lead to significant advancements in additive manufacturing and the potential technologies that could benefit from it.
Richard L. Axelbaum
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