Graduate Thesis Or Dissertation


Computational Study of Smoldering Combustion in Cellulose and Hemicellulose Mixtures Public Deposited

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  • Due to global warming wildland fires are increasing in frequency and severity. In the case of wildland fires, the major modes of combustion occur include smoldering combustion and flaming combustion. Smoldering combustion occurs most commonly in porous fuels like peat, forest duff, and woody fuels, which are available in abundance in the forest. This makes the study of smoldering combustion in these types of fuels important. Woody fuels, in general, consist of cellulose, hemicellulose, and lignin in varying proportions, and densities can also differ. However, the effects of these properties on smoldering behavior are not well understood, which motivates this investigation. In this thesis, I developed a one-dimensional computational model that can simulate smoldering combustion in cellulose and hemicellulose mixtures. I first successfully validated the model using experimental results. After validating the model, I studied the effects of varying density and fuel composition on mean peak temperature and mean propagation speed. From this study, I found that the smoldering propagation speed increases with increases in hemicellulose content, since hemicellulose pyrolyzes earlier than cellulose, resulting in faster shrinkage of the fuel and thus quicker access to oxygen for oxidation reactions. On the other hand, propagation speed decreases with increases in density, because more mass of fuel needs to be converted to char and ash, slows the fuel shrinking and slowing access to oxygen. Mean peak temperature decreases with increasing density due to higher thermal conductivity of the condensed-phase species involved, and mean peak temperature increases with hemicellulose content due to formation of lower thermal-conductivity ash on the top, resulting in lower heat loss. I developed semi-empirical formulas for propagation speed and peak temperature capturing changes in density, mass fraction of cellulose, and mass fraction of oxygen. Next, I determined the effects of adding moisture content on peak temperatures and propagation speed for 100% cellulose, considering both expansion and lack of expansion with water addition. I found that propagation speed decreases with increasing moisture content when the fuel does not expand, due to an increase in the mass of wet fuel that needs to be dried. On the other hand, when the fuel expands on the addition of water, propagation speed decreases with additional moisture content because the fuel density decreases overall. Finally, I investigated whether the fuel composition affects the critical moisture content of ignition and extinction. I found that both critical moisture contents increase by 10% when hemicellulose content reaches 75% due to the increase in the peak temperature on the addition of hemicellulose.
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  • This work was funded by the Strategic Environmental Research and Development Program (SERDP) award RC-2651 under contract number W912HQ-16-C-0045.
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