UMD Theses and Dissertations
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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 given thesis/dissertation in DRUM.
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Item A STUDY OF INTERMITTENT CONVECTIVE HEATING OF FINE LIVE WILDLAND FUELS(2020) Orcurto, Ashlynne Rose; Gollner, Michael; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent studies have investigated the role of convective heating advancing wildland fire spread through intermittent bursts or pulses of flames onto unburned fuels. This study seeks to expand on initial experiments investigating the role of this intermittent convective heating by exploring the ignition of live fuels with different moisture contents under these conditions. A modified Rubens' tube is used to generate periodic pulses of a small-scale diffusion flame over the surface of a fuel sample. Infrared imaging is used to track the surface temperature of the fuel leading up to ignition while the intermittent temperature is characterized using a fine-wire thermocouple. Ponderosa pine needles with a variety of moisture contents are tested at different heating frequencies to determine patterns in the process of ignition. The fuel moisture content is seen to have a significant effect on the ignition times of fuels, while the temperature at ignition is seen to vary with heating frequency. Ignition results are compared with past tests of dried dead fuels using the same apparatus. Model predictions of ignition times as a function of ignition temperature and moisture content are also compared to experimental values and discussed.Item Fluid dynamics of boundary layer combustion(2017) Miller, Colin; Gollner, Michael J; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reactive flows within a boundary layer, representing a marriage of thermal, fluid, and combustion sciences, have been studied for decades by the scientific community. However, the role of coherent structures within the three-dimensional flow field is largely untouched. In particular, little knowledge exists regarding streamwise streaks, which are consistently observed in wildland fires, at the base of pool fires, and in other heated flows within a boundary layer. The following study examines both the origin of these structures and their role in influencing some of the macroscopic properties of the flow. Streaks were reproduced and characterized via experiments on stationary heat sources in laminar boundary layer flows, providing a framework to develop theory based on both observed and measured physical phenomena. This first experiment, performed at the University of Maryland, examined a stationary gas burner located in a laminar boundary layer with stationary streaks which could be probed with point measurements. The gas temperature within streaks increased downstream; however, the gas temperature of the regions between streaks decreased. Additionally, the heat flux to the surface increased between the streaks while decreasing beneath the streaks. The troughs are located in a downwash region, where counter-rotating vortices force the flame sheet towards the surface, increasing the surface heat flux. This spanwise redistribution of surface heat flux confirmed that streaks can, at least instantaneously, modify important heat transfer properties of the flow. Additionally, the incoming boundary layer was established as the controlling mechanism in forming streaks, which are generated by pre-existing coherent structures. Finally, the amplification of streaks was determined to be compatible with quadratic growth of Rayleigh-Taylor Instabilities, providing credence to the idea that the downstream growth of streaks is strongly tied to buoyancy. The next phase of the project was performed at the Missoula Fire Sciences Laboratory, where a hot plate in a laminar boundary layer was examined. In addition to manipulating the wind speed, the local buoyant force was controlled via the surface temperature of the hot copper plate. Infrared thermography was employed to detect streaks by means of local surface temperature fluctuations, and a novel and consistent method for tracking streaks and quantifying important properties was developed. Streak spacing was seen to be lognormally distributed, and the initial spacing, which was consistently between 60-70 dimensionless wall units, was shown to be governed by the incoming boundary layer. Streak spacing increased downstream of the plate, with higher plate temperatures resulting in larger magnitudes of spanwise fluctuations in surface temperature. Finally, streak behavior became more chaotic downstream, as streaks would meander rapidly and persist for shorter durations. The final phase of the study, performed at the Missoula Fire Sciences Laboratory, examined a saturated fuel wick in the same experimental configuration as the hot plate. Streaks were detected in the flame via high speed video, and tracked using the previous developed algorithm. Streak spacing was lognormally distributed, with the initial spacing (60-75 wall units) again being controlled by the incoming boundary layer. Spacing between coherent structures increased downstream, likely due to buoyant amplification. The width of streaks grew to an apparent assymptote, indicating a settling of length scale controlled by the time and rate of growth. Further downstream, coherent structures no longer resembled well-ordered streaks but more complex structures resulting from streak aggregation. Overall, trends for streaks are consistent in both the hot plate and the flame, indicating that the behavior of streaks is governed by similar mechanisms in both scenarios. Although the initial instabilities are governed by the incoming wind, buoyant forces cause the growth and aggregation of these structures. These local instabilities are capable of affecting macroscopic properties of the flow, including heat transfer to the surface, indicating that a two-dimensional assumption may fail to adequately describe heat and mass transfer during flame spread and other reacting boundary layer flows.Item THE INTERACTION OF SPRINKLER SPRAYS AND FIRE PLUMES(2017) Link, Eric D.; Marshall, Andre W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The critical factor for successful suppression using fire sprinklers is the delivery of water to burning surfaces. Water delivery is dependent on initial spray characteristics and subsequent spray interactions with the opposing fire plume, which can deflect or reverse the sprinkler spray away from the targeted fire source. Measurements provide a comprehensive validation data set for computational fluid dynamics (CFD) spray models, as well as insight and engineering guidance to the spray-plume interaction important in fire sprinkler applications. An experimental facility consisting of an array of four sprinklers, similar to that of typical suppression system installations, is used to evaluate both quiescent spray dispersion and spray-plume interaction conditions. A 0.2 m x 0.2 m centrally located forced air jet, with velocities ranging up to 4 m/s is used (in place of a fire) to provide a well-characterized, repeatable kinematic challenge to the spray. Measurements include quiescent case spray dispersion and local volume flux delivery to the plume source to evaluate spray penetration through the plume. Additional measurements include air jet centerline drop velocity and drop size at variable source injection velocities to evaluate plume penetration behavior. These spray dispersion experiments capture the dominant transport physics and kinematic behavior of the spray plume interaction. Scaling analysis of the spray plume interaction is explored for two regimes of spray penetration; individual drop action and group spray action. In the individual drop action regime the droplets have a negligible effect on the plume and penetration scales with the ratio of drop terminal velocity to plume velocity. In the group action regime, a spray work criterion is proposed, accounting for drag interactions with the plume. The complete set of spray dispersion and plume penetration measurements comprise a data set of high resolution and well-characterized boundary conditions (including detailed initial spray measurements for each sprinkler in the array) useful for CFD validation.Item Development of the ASTM E681 Standard(2016) Lomax, Peter; Sunderland, Peter; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ASHRAE 34, based on ASTM E681, was improved by identifying and rectifying deficiencies in ASTM E681. An ASTM E681 apparatus and procedure was developed with gaseous refrigerant testing in mind. The plumbing was improved by ensuring that the pressure readings could be constantly monitored while decreasing leakage potential. An original electrical system was designed and constructed for the ignition system. Additionally, a control panel was constructed to isolate hazardous electrical elements, and facilitate the testing, while simultaneously organizing the critical plumbing and ignition components. 3D printing efficiently produced heat-resistant, nonreactive, and structurally stable lower electrode spacers, propellers, and propeller bars. The heating system was designed to ensure even temperature throughout the apparatus. The humidity system was designed to accurately condition the air. Recommendations to improve ASTM E681 are provided. The research can be built on to improve the accuracy and reproducibility of ASTM E681.Item Development of a Turbulent Wolfhard-Parker Burner with Suppressing Co-flow(2014) Link, Eric; Marshall, Andre; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A burner and surrounding co-flow air system were developed for experimental study of flame suppression in a diluted oxygen environment. Development of the burner was guided by design criteria requiring the flame to be a turbulent buoyant diffusion flame in a line configuration presenting a realistic challenge to suppression methods such as oxidizer-stream dilution. Design constraints included buoyancy, turbulence, and flame height criteria. Oxygen concentration and temperature measurements, along with a smoke wire flow visualization technique were utilized to evaluate the system and ensure the design criteria were successfully met. Additional analysis was performed on image recordings of the flame to provide a robust and repeatable flame height measurement to evaluate the flame height constraint. Ultimately, an experimental apparatus with the ability to suppress a flame in a controlled, well-characterized environment was developed to provide a good base for model validation measurements.Item Upward Flame Spread over Discrete Fuels(2014) Miller, Colin; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Upward flame spread over discrete fuels has been analyzed through experimental research on vertical arrays of alternating lengths of PMMA and insulation. By manipulating the lengths of the PMMA fuel and the insulation, several important trends related to flame spread were identified and assessed. The peak flame spread rate for arrays with 4 cm lengths of PMMA occurred at a fuel percentage of 67%; for arrays with 8 cm PMMA, a peak flame spread rate occurred at fuel percentages of 67%, 80%, and 89%. It has been hypothesized that increased air entrainment at these fuel percentages maximizes the flame spread rate. Based on observed trends, this study proposed a method for approximation of the fuel spread rate at various fuel percentages. Given reasonable estimates for the homogeneous flame spread rate and the lowest fuel percentage that sustains spread, an estimation for intermediate spread rate values is feasible.Item Using Inverse Fire Modeling With Multiple Input Signals to Obtain Heat Release Rates in Compartment Fire Scenarios(2014) Price, Michael David; Marshall, Andre; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A set of multi-room compartment fire experiments were conducted to obtain measurements of hot gas layer temperature and depth. These measurements were used as an input to an inverse fire model that coupled a genetic algorithm with a zone fire model to calculate a unique solution to the original fire size and door opening used in the experiments. The objective of this research was to calculate simultaneously the real-time fire size and fire door opening of the experiment using a combination of hot gas layer temperature and hot gas layer height measurements from a multi-room compartment in concert with an inverse fire model. This research focused on increasing the robustness of an inverse fire model (IFM) with respect to physical accuracy and multi-variable calculations. The IFM successfully identified a unique solution and calculated fire size within 10-40% of experimental values.Item Detailed Measurements of Fire-Induced Mixing Phenomena(2014) Layton, Thomas George; Marshall, Andre W; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study successfully validated the use of salt-water analog modeling as an effective diagnostic tool for predicting fire-induced flows. A technique was developed for taking measurements using combined Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), and results were analyzed with respect to smoke filling as well as transient ceiling layer dynamics, and turbulent mixing intensity. Data was shown to be in good agreement with theory, further validating the salt-water analogy as a tool for diagnostics, prediction, and scaling of fire phenomena.Item Impacts of a changing fire frequency on soil carbon stocks in interior Alaskan boreal forests(2014) Hoy, Elizabeth Embury; Kasischke, Eric S; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Increasing temperatures and drier conditions, related to climate change, have resulted in changes to the fire regime in interior Alaskan boreal forests, including increases in burned area and fire frequency. These fire regime changes alter carbon storage and emissions, especially in the thick organic soils of black spruce (Picea mariana) forests. While there are ongoing studies of the size and severity of fire using ground- and remote-based studies in mature black spruce forests, a better understanding of fire regime changes to immature black spruce forests is needed. The goal of this dissertation research was to assess impacts of changing fire frequency on soil organic layer (SOL) carbon consumption during wildland fires in recovering Alaskan black spruce forests using a combination of geospatial and remote sensing analyses, field-based research, and modeling. The research objectives were to 1) quantify burning in recovering vegetated areas; 2) analyze factors associated with variations in fire frequency; 3) quantify how fire frequency affects depth of burning, residual SOL depth, and carbon loss in the SOL of black spruce forests; and 4) analyze how fire frequency impacts carbon consumption in these forests. Results showed that considerable burning in the region occurs in stands not yet fully recovered from earlier fire events (~20% of burned areas are in immature stands). Additionally, burning in recovering black spruce forests (~40 yrs old) resulted in SOL depth of burn similar to that in mature forests which have burned. Incorporating these results into a modeling framework (through adding an immature black spruce fuel type and associated ground-layer carbon consumption values) resulted in higher ground-layer carbon consumption (and thus total carbon consumed) for areas that burned in 2004 and 2005 than that of a previous version of the model. This research indicated that the dominant controls on fire behavior in this system were fuel type and amount, not fuel condition, and that changes in vegetation associated with more frequent fire (shift to deciduous and shrub vegetation which does not traditionally burn as readily) may represent a long-term negative feedback on burned area. These new results provide insight into the fire-climate-vegetation dynamics within the region and could be used to both inform and validate modeling efforts to better estimate soil carbon pools and emissions as climate continues to change.Item They Don't Know Us Here(2012) DeCarlo, Carolyn Cecelia; Norman, Howard; English Language and Literature; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To be human is to be shaped by memory: what is remembered, what is forgotten, and what lies quietly dormant. But what of the unique mind, for whom this balance is upset? The novella They Don't Know Us Here imagines a place where David Whelan experiences past and present on a continuous plane. Confined to Ward 12 of St. Elizabeths Hospital in Washington, D.C., David's mind soars between life on the ward and memories from before his confinement. But when things change in the present, what is shaken loose in the past? Through looking both inward on David and out to the other men residing on the ward, They Don't Know Us Here explores what happens when unquiet minds are confined to bodies that rest.