Mechanical Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2263
Browse
2 results
Search Results
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 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.