Theses and Dissertations from UMD
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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 DEVELOPMENT AND USE OF PIXEL-BY-PIXEL PYROMETRY METHODS ON SMOLDERING WOOD EMBERS AND PILES(2022) Tlemsani, Mahdi; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wildfire, especially along the Wildland Urban Interface (WUI), presents a large threatto life and property globally. Firebrands can increase the size and spread of wildfires rapidly. More than half of spot fires and WUI fires are caused by firebrands. Firebrand generation, transport, and morphology has been studied in recent literature, but few papers have reported firebrand temperatures, even fewer on groups of firebrands across varying configurations. Those that have reported temperatures have typically relied on expensive IR and thermocouple data that may not be as accurate at determining temperature or emissivity. Color camera pyrometry presents a high resolution alternative to previous pyrometry methods and can be done using a cheap color camera, and with certain techniques can derive temperature independent of emissivity. This research builds on previous color camera pyrometry, automating the process to allow for large datasets to be analyzed as opposed to single images. Two-color, grayscale, and hybrid pyrometry [20] were used to recreate pyrometry results of previous literature. Similar average single firebrand temperatures in the range of 900-950C were reported. A novel Pixel-by-Pixel hybrid pyrometry was developed to incorporate more data into established hybrid pyrometry methods. This method introduced large amounts of noise into the temperature results, making them unreliable. Additionally, a method was developed for determining the temperature of 8-gram ember piles at various wind speeds of 1.4, 2.4, and 2.7m/s through a borosilicate glass window. Modified grayscale temperatures assuming constant emissivity were used for these experiments and were fit to firebrand temperature data from Kim and Sunderland [20]. A total of 720 ember pile images were analyzed in the final dataset at an effective emissivity of 0.76. Peak ember pile average temperatures ranged from 700-900C. Normalized temperature (T /Tmean) PDFs were produced. Data was approximated as a normal distribution with mean of 1 and standard deviation ranging from 0.048 - 0.057.Item IMAGING PYROMETRY OF SMOLDERING WOOD EMBERS AT VARIOUS DISTANCES AND ILLUMINATIONS(2020) decker, kyle; Sunderland, Peter B.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wildland fires in the WUI present a constant threat to life and property in the United States and across the globe. Many wildland fires are caused by ember spotting, a process in which firebrands are lofted significant distances away from the fire front by combinations of winds and gas flows. These firebrands have the potential to collect and cause new spot fires independent of the original wildland fire. While firebrand mechanisms such as ember generation and transport have been thoroughly studied and quantified, the capacity in which firebrands cause these fires is not as well known. Recent studies have made progress towards determining the surface temperature of these firebrands; however, none have provided repeatable temperature data from a variety of test conditions. This paper presents firebrand surface temperature using color imaging ember pyrometry techniques for various imaging distances and illuminations. A digital color camera was calibrated to a blackbody furnace with a temperature range of 600 – 1200 °C. Calibration to the blackbody allows the normalized pixel values of each image to be converted to temperature using G/R ratio, grayscale, and hybrid pyrometry. Signal to noise ratios of around 850 and 46 for grayscale and ratio pyrometry were obtained. Two simultaneous images of a single ember from distances of 0.5 and 1 m, as well as additional images from 4 m were observed and quantified. The firebrand surface temperature was determined to be independent of imaging distance. The mean surface temperature across all imaging distances was calculated to be 931 ± 6.2 °C. Ratio pyrometry was observed to be the preferred method of imaging pyrometry due to its independence from surface emissivity and transmissivity as well as it’s applicability to real fire scenarios for future research. Firebrands were also imaged in sequences containing various illumination and background color. Illumination was observed to disrupt G/R ratio pyrometry due to an overwhelming increase in green pixel values.