Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Advancing Understanding of Canonical Fire Phenomena through Novel Experimental Techniques and Data Analysis(2021) Ren, Xingyu; Gollner, Michael J.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Physical modeling of both stationary fires and wildland fire spread requires a thorough understanding of underlying heat transfer processes which result from the interaction of flames with their surrounding environment. However, the hostile fire environment makes it difficult to conduct detailed experiments that measure and describe common thermal phenomena in many of these configurations, limiting both our understanding and the opportunity for model validation. In this dissertation, new measurement techniques were developed to characterize the thermal and fluid structures of three canonical fires: a buoyant-driven flame, a wind-driven flame, and an inclined flame, providing both enhanced understanding and new data for model validation. The first experiment applied a dual-thermocouple technique to turbulent buoyant flame measurements with a newly developed method for uncertainty analysis. A 15 kW turbulent buoyant diffusion flame was established over a round gas burner with a 13.7 cm inner diameter at FM Global’s laboratory. A dual-thermocouple probe, consisting of two fine-wire thermocouples with 25 μm and 50 μm wire diameters, was used to determine a compensated turbulent gas temperature. Flame temperatures including the mean, root-mean-square (rms) and probability density function were obtained in a two-dimensional plane across the flame centerline. These temperature measurements, alongside existing data such as the radiant power distribution, local soot volume fraction and soot temperature, as well as future gas velocity measurements will provide a detailed dataset of this flame for validation and development of radiation models. The second experiment, performed at the University of Maryland, investigated convective heat transfer from a wind-driven flame under the effect of freestream turbulence. An image analysis technique was developed to extract the sub-scale flame structures: flame streaks and troughs. It was observed that freestream turbulence initiated an earlier onset of visible coherent flame streaks. Both spacing and fluctuation frequency of the flame streaks showed a nearly quadratic growth at high turbulence intensities. This quadratic growth promoted the transition of flames to a turbulent state, which ultimately modified the overall flame heating dynamics. The forward attachment length of the flame was found to be negatively correlated to the turbulence intensity. Two heating modes, a momentum-dominated and a plume mode, were observed and found to be segregated by a critical Richardson number. The downstream heat flux was found to increase from 30 to 40 kW/m2 in the momentum-dominated regime, when flow turbulence intensity changed from less than 1% to a level of 14.9 –16.8%. Finally, it was observed that placing a bar upstream of the burner tripped the flow to the point where the downstream flame structure closely resembled flames under the highest turbulence intensity investigated, suggesting a simplistic configuration for future study. The third experiment developed a temperature-correlation velocimetry technique (TCV) to examine the thermal structure and flow dynamics of inclined fires. The experimental data was provided by the USDA Forest Service Missoula Fire Sciences Laboratory. A 10 kW partially premixed propane flame was first produced over a small tilt table. Shadowgraph images were taken to illustrate the motion of the flow governing the resulting inclined fire plume. Large-scale fire tests with heat-release rates ranging from 81 kW to 2.25 MW were also conducted over a large tilt table. The angle of inclination, θ, was varied between 0° and 30°. A micro-thermocouple array along the centerline of the table was used to measure downstream gas temperatures. Flames were seen to start attaching to the inclined surface at θ = 18°, independent of the fire intensity. The centerline temperatures under attached flame conditions are consistent with McCaffrey’s buoyant flame temperature correlation, suggesting the buoyancy-driven nature of the inclined fires. The local gas velocity was measured using cross-correlation velocimetry through the streamwise temperature signals. Results show that the local flow was accelerated in the attached flame region driven by buoyancy before reaching a peak. Velocity of the flow slowed after the peak due to weaker buoyancy within the intermittent and plume regions. The mean surface velocity of the attached flame scales directly with the angle of inclination (sin^2(2θ)) and the fire intensity, providing a promising method to evaluate convective heat transfer using the geometry of an inclined fire.Item A Study Of Intermittent Convective Heating Effects On Fine Fuel Ignition(2019) Benny, Lana; Gollner, Michael J.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent studies have suggested the potential importance of intermittent convective heating on the ignition of fine fuels during wildland fire spread. In this study, a novel pulsed-gas line burner similar to a Rubens' tube, driven by acoustic oscillations, is used to re-create the pulsations observed in wildland fires in a controlled environment. After acoustically stimulating a long tube with perforations at the top, creating a pulsed linear flame, thin fuels with different densities and diameters are quickly placed in the center of the flame. The temperature of these fuels is measured using an infrared camera, distinguishing the temperature at which the fuel starts to pyrolyze. As expected, smaller-diameter fuels ignite faster when exposed to flames; however, they also are least affected by intermittent heating. Larger-diameter fuels are more dramatically affected by intermittent heating frequencies, in large part due to cooling effects between pulses and the larger thermal mass of the fuels. The results are discussed and compared with a simple numerical model incorporating measured velocities and temperatures present in the burner and their effect on a thermally-thin fuel element over time.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.