Graduate Thesis Or Dissertation
 

The Influence of Fuel Characteristics on Smoldering Behavior through Porous Fuels

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  • Smoldering can play a dangerous role in the ignition and spread of wildfires. Naturally occurring fuels consist of multiple layers of organic materials with varying particle sizes due to differences in decomposition stages with fuel depth, yet the influence of particle size on burning behavior is poorly understood. Previous research has investigated the smoldering behavior of organic fuels (e.g. peat, cotton, cellulose, hemi-cellulose, lignin) relative to fuel characteristics (e.g. moisture content, inorganic content, bulk density), however the smoldering behavior of many other naturally occurring organic fuels is poorly understood. Additionally, most laboratory smoldering experiments are performed in ambient conditions in the absence of wind, while wildfires are rarely subjected to windless conditions. This work is split into three studies. In the first study, the objectives were to identify the sensitivities of smoldering behavior to particles with characteristic diameters near 1 mm and elucidate the role of heat transfer and oxygen availability in causing observed sensitivities. In the second study, the objective was to identify the sensitivities of smoldering behavior to wind speeds. In the third study, the objective was to expand on previous smoldering research by investigating how fuel characteristics (i.e., moisture content, bulk density, fuel thickness) impact the smoldering behavior of Douglas-fir duff. In the first study, experiments were conducted where fuel beds of granulated Douglas-fir wood pellets of varying particle sizes were put in contact with an ignition source. The propensity to sustain horizontal and downward smoldering combustion and the resulting propagation rates were quantified for three different ignition temperatures. Tests of homogenous and mixed particle sizes were investigated. In general, particle sizes with d > 0.85 mm did not transition to self-sustained smoldering, while particle sizes with d < 0.85 mm transitioned to self-sustained smoldering, irrespective of the ignition temperature. Increasing the size of particles reduced the spread rate and surface temperatures, despite having lower overall densities. Larger particles can have sustained smoldering if sufficient concentrations of smaller particles are present in the pores. Fuel beds with increased porosity and permeability (i.e., larger particles) have reduced propensity to sustain smoldering. In the second study, smoldering experiments using 100% cellulose were conducted in a wind tunnel. The effects of wind speed on the wind aided smoldering spread rates and temperatures were quantified. With cellulose at 200 kg/m3 packing density, smoldering spread rates and peak temperatures increased with the wind speed. Spread rates from the lowest wind speed measured (1 m/s) were twice as fast as spread rates of the same fuel under windless conditions. In the third study, Moisture content, bulk density, organic bulk density and fuel thickness were measured and compared to spread rates and surface temperatures of Douglas-fir duff samples collected underneath mature trees from the McDonald research forest in Corvallis, Oregon. The data suggests that spread rate decreases with moisture content and bulk density and increases with fuel thickness. Surface temperatures decrease with moisture content and bulk density, while increasing with duff thickness. Models for horizontal spread rate and surface temperatures for measured fuel characteristics were created. The models suggest that spread rate decreases with moisture loading and surface temperatures decrease with fuel loading and moisture content-fuel thickness interaction.
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  • This research was made possible through funding from the Strategic Environmental Research and Development Program (SERDP) under contract number (W912HQ-16-C-0045).
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