Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item Experimental Analysis and Numerical Modeling of Ignition of Lignocellulosic Building Materials Subjected to Glowing Firebrand Piles(2023) De Beer, Jacques Andre; Stoliarov, Stanislav I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The prevalence and severity of Wildland-Urban Interface (WUI) fires is on the rise globally. Wildfires that spread into urban areas are known as WUI fires, with firebrand exposure a leading cause of structure losses during these WUI fire events. However, the complex ignition process of WUI building materials by glowing firebrand piles has not been fully resolved. The objective of this research was to develop a numerical ignition model capable of predicting the ignition probability of any horizontally-mounted flammable substrate when exposed to a pile of glowing firebrands. This development process was based on: extensive experimental data quantifying the mechanisms controlling the ignition process; a novel empirical firebrand pile heat flux model; and comprehensive pyrolysis models of three commonly-used lignocellulosic building materials.Experiments were conducted in a bench-scale wind tunnel where a glowing firebrand pile of controlled geometry was deposited onto a horizontally-mounted substrate. Forced air flow velocities in the range of 0.9 – 2.7 m s-1 and two firebrand pile coverage densities (0.06 and 0.16 g cm-2) were used, significantly expanding the range of conditions used in earlier laboratory-scale studies. The firebrand pile thermal exposure and burning intensity were quantified using time-resolved back surface temperature and combustion heat release rate data. Flammable substrate flaming ignition and extinction statistics, as well as burning intensity data, were also collected. A custom inverse heat flux modeling technique, utilizing a solid-phase pyrolysis solver, ThermaKin, and infrared thermal imaging back surface temperature data, was employed to generate incident firebrand pile heat flux profiles directly underneath and in front of a glowing firebrand pile. The time-dependent firebrand pile heat flux behavior was captured using a three-step piecewise linear function. Further, a novel empirical firebrand pile heat flux model was developed, capable of generating time-dependent firebrand pile heat flux profiles over a range of forced air flows (0 – 4 m s-1) and for all firebrand pile coverage densities and geometries. A hierarchical modeling approach was used to develop a comprehensive pyrolysis model for each lignocellulosic substrate through inverse analysis of milligram- and gram-scale experimental data. All relevant kinetics, thermodynamics, and thermal transport properties of pyrolysis was parameterized. Finally, a novel numerical ignition model used to predict the ignition probability of any flammable target substrate when exposed to a glowing firebrand pile under wind was developed. A newly-defined dimensionless flame stability parameter was used as a material-independent criterion to characterize the ignition of a flammable substrate surface. The model captured the stochastic ignition behavior of flammable substrates by firebrand piles, as well as the reduced burning intensity of the piles deposited onto a flammable substrate surface. A logistic growth function was found to most accurately capture the ignition probability dependence on the dimensionless flame stability parameter and was, on average, capable of predicting the ignition probability within 14% of the experimental data. Further, using a critical dimensionless flame stability parameter, the absolute average difference between all experimental and predicted ignition timing and burning duration data was 11 and 26 s respectively.Item COMPARISON OF IGNITION AND COMBUSTION CHARACTERISTICS OF PRESSURE TREATED WOOD AND TREX EXPOSED TO THERMALLY CHARACTERIZED GLOWING FIREBRAND PILES(2023) Lauterbach, Alec; Stoliarov, Stanislov I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent decades, the intensity of wildfires worldwide has escalated, leading to a rise in the destruction of structures and loss of lives within the Wildland-Urban Interface (WUI). Firebrands are small fragments of ignited vegetation or structural material that are carried by the plume of a wildfire, traveling in advance of the main fire front. Firebrand exposure has been recognized as the primary mechanism for the propagation of wildfires as well as a source of ignition of structural elements. However, this complex ignition process of structural elements in the WUI has yet to be fully understood. The ignition and combustion characteristics of a thermoplastic-wood composite (Trex) and Pressure Treated Wood (PTW), two frequently used WUI decking materials, when exposed to glowing firebrand piles were studied using a bench scale wind tunnel. An inert insulation material, ii Kaowool PM, was also used as a deposition substrate to quantify the heat feedback and combustion characteristics of solely the firebrand pile. Firebrand pile densities of 0.16 g cm-2 and 0.06 g cm-2 were deposited on each substrate in rectangular 10 cm x 5 cm orientations and exposed to air flow velocities of 0.9 m s-1, 1.4 m s-1, 2.4 m s-1, and 2.7 m s-1. Infrared camera measurements were used to determine the back surface temperatures of Kaowool PM tests. Using DSLR cameras, surface ignitions of the decking material in front of the firebrand pile (preleading zone ignition events), ignitions on top of the firebrand pile (pile ignition events), and surface ignitions of the decking material behind the firebrand pile (downstream ignition events) were visually quantified via their probability of ignition, time to ignition, and burn duration at each testing condition. A gas analyzer was used to compare combustion characteristics of Trex, PTW, and Kaowool PM tests through heat release rate (HRR) and modified combustion efficiency (MCE). Peak back surface temperatures of the firebrand pile were found to increase with increased air flow up to 2.4 m s-1, and then plateau. The same trend was observed for the ignition probabilities of preleading zone and pile ignition events. The probability of downstream ignition events increased with increasing air flow velocity. Peak HRR increased with increasing air flow velocity. Trex exhibited significantly less smoldering combustion than PTW yet was prone to more intense flaming combustion. When the rectangular 5 cm x 10 cm firebrand pile (10 cm edge facing the airflow), of which the majority of tests were conducted on, was rotated 90 degrees so that the 5 cm edge faced the airflow, the result was a significant decrease in the probability of ignition for both Trex and PTW, along with notable reductions in their HRR and MCE profiles.Item Firebrand Pile Thermal Characterization and Ignition Study of Firebrand Exposed Western Red Cedar(2021) Alascio, Joseph Anthony; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildfires across the world has grown, resulting in increased number of structures in the Wildland–Urban Interface being destroyed, and lives lost. An ignition pathway that has been identified to contribute to most structures destroyed during a wildland fire is that of firebrand ignition. Firebrands are small burning pieces of vegetative material that are lofted ahead of the fire front. This study seeks to quantify thermal conditions experienced by building materials exposed to accumulated firebrands and to identify conditions that lead to ignition of these materials. A bench scale wind tunnel was used to house a decking material, western red cedar, on which the firebrands were deposited, which allowed for testing at different air flow velocities, while simultaneously analyzing the temperature of the solid substrate and gaseous exhaust flow constituents to identify trends in flaming and smoldering combustion. Higher peak temperatures and larger heating rates were found with the exposure of a higher air flow velocity. An increased air flow velocity also allowed for quicker, more frequent, and longer sustained flaming of the firebrand pile. A Modified Combustion Efficiency (MCE) value of 0.81 ± 0.02 for the firebrand pile across all testing conditions was quantified, which is indicative of a hybrid–smoldering/flaming combustion mode.Item Thermal Characterization of Firebrand Piles(2017) Hakes, Raquel Sara Pilar; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildland-urban interface (WUI) fires has increased drastically, resulting in thousands of structures lost globally each year. The cause of the majority of structure losses is ignition via firebrands, small pieces of burning material which are generated from burning vegetation and structures. In this thesis, a methodology for studying the heating to recipient fuels by firebrands is developed. Small-scale experiments designed to capture heating from firebrand piles and the process of ignition were conducted using laboratory-fabricated cylindrical wooden firebrands. The methodology compares two heat flux measurement methods. Experimental results compare the effects of varying firebrand diameter, pile mass, and wind speed. An ignition condition is described using temperature and heat flux.