Theses and Dissertations from UMD
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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 WOOD EMBERS UNDER SIMULATED MOVEMENT(2022) Baldwin, James H; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A major mechanism for wildland fire spread are spot fires, where small combusted organic particulate (firebrands) are lofted and transported to a remote location where they can then ignite new fires. The modeling of these spot fire ignitions is limited by the unknown surface temperature and emissivity of firebrands, which is challenging to measure due to the small size of firebrands (precluding the use of intrusive temperature methods such as thermocouples) as well as the dependency of conventional non-intrusive temperature measurements (e.g. Infrared Imagers) on emissivity. A solution to this is presented in Color Pyrometry, which uses color pixel intensities to determine an object's temperature based on a calibration against an object of known temperature/emissivity. The presented method is a Ratio Pyrometry approach between green and red pixel intensities normalized to camera settings, which demonstrates the benefit of being independent of object emissivity as validated by Planck's Law, and is based on a Blackbody Furnace calibration. To determine the method's applicability to realistic firebrand imaging conditions, which would provide the most comprehensive understanding of firebrand ignition, the individual impact of firebrand movement speed on the pyrometry's surface temperature predictions is considered. An apparatus is developed that decouples firebrand movement speed from the surface wind speed (which is known to impact firebrand surface temperature) as well as allows for modulation of the firebrand's simulated movement speed, and involves rotating the imaging device about a fixed axis relative to a stationary firebrand. Five trials at a set orientation were conducted to verify the apparatus' repeatability, and subsequent trials of varying rotation speed, distance, applied wind speed, and mounting orientation were conducted. Both qualitatively and through a statistical analysis consisting of ANOVA and non-parametric distribution testing, firebrand movement speed and orientation are shown to have no individual impact on surface temperature. Average ember surface temperatures were found to be 922.1 ± 20.4 °C with a 1 m/s applied wind speed and 955.0 ± 20.2 °C with a 2 m/s applied wind speed, which is in agreement with previous studies. It is proven that the presented Pyrometry method's results are independent of a major complicating factor associated with realistic firebrands, which thereby further supports future efforts into wildland fire spread modeling.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.Item Critical Ignition Conditions of Structural Materials by Cylindrical Firebrands(2019) Salehizadeh, Hamed; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Smoldering firebrands (embers) are a major cause of ignition and eventual structural damage during wildfires at the wildland-urban interface (WUI). These small pieces of wood can loft several kilometers ahead of the main flame front and ignitespot fires directly on structural elements such as decks. In this research, dense structural materials such as wood and engineered wood will be studied with a focus on capturing the critical thermal conditions necessary for ignition. Unique to this study will be a configuration where whole piles of firebrands are placed on the recipient material, emulating observations from WUI fires. In order to design appropriate fire safety standards at the WUI and, someday, to model the propagation of these fires, the conditions leading to ignition of common WUI materials by piles of lofted firebrands must be quantified. Firebrands were modeled using small cylindrical wooden dowels which were ignited and placed in a small-scale wind tunnel. Two tests were performed for each loading condition of firebrands, one studying ignition of wooden structural elements such as decking and marine-grade plywood and another measuring temperatures and heat fluxes over an inert piece of ceramic insulation. A single-point water-cooled heat flux gauge was used for time-resolved measurements of heat flux at the center of the inert setup surrounded by thin-skin calorimeters and K-type thermocouples which allowed for a spatial characterization of heating. The wind speed was the main quantity of interest changed during the test to determine the effects of wind speed on the heat flux released from the glowing dowels to recipient fuels. The results showed a drastic increase in heating from piles of firebrands vs. individual brands. The piles also produced higher heat fluxes under increasing winds. This is due, for the most part, to higher surface temperatures resulting from increased surface oxidation under higher wind speeds. Both smoldering and flaming ignition of wood was found to be similarly dependent on wind speed. Larger piles also produced higher peak heat fluxes at the center of the pile, highlighting the role of re-radiation within the pile influencing heat fluxes to recipient fuels. Critical heat flux and firebrand loading conditions required to achieve smoldering and flaming ignition of structural materialsused in the WUI are determined by comparing tests with inert and flammable fuels. These critical conditions can be used to model the propagation of WUI fires over structural elements to design appropriate fire safety standards at the WUI. A non-dimensional relationship incorporating fuel type, geometry, and ambient conditions is also proposed to describe the results.