Computational investigation of the influence of heating modes and moisture content on pyrolysis and ignition of live fuels
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
David R. Weise
Pyrolysis., Wildfires--Environmental aspects., Wildfires--Mathematical models., Eddies--Mathematical models., Fire ecology., Fluid dynamics.
The burning of an isolated leaf-like element was computationally investigated in a series of studies, motivated by recent burning experiments performed on live leaves of manzanita (Arctostaphylos glandulosa). In this study, the relative impor- tance of heating modes, effect of fuel moisture content on pyrolysis and combustion of live fuels is explored in stages. A preliminary study was conducted on a simpli- fied one-dimensional configuration, using Gpyro. The heating sources were modeled through convection and/or radiation as boundary conditions. Results showed that the increase in radiative source temperature substantially affects ignition time; how- ever, it has marginal influence on mass loss rate and charring rate. The increase of convective heating source temperature in presence of radiation had a marginal impact on ignition time and no influence on mass loss or charring rate. Next studies were conducted in full three dimensional configurations via Gpyro-3D/FDS. The solid fuel was under the radiative heating and a 5-step chemical kinetic mechanism was used for pyrolysis. Results indicated that temperature response and thermal degradation rate was higher for lower fuel moisture content (FMC) case and ignition occurred prior to the higher FMC case. In the gas phase, high volume fraction of water vapor observed in the region close to the combustion zone as well as away from this region illustrated that evaporation and ignition occur together. In the next task of the modeling activi- ties, an improved chemistry model was used, which included hemicellulose and lignin along with cellulose and moisture. A more advanced 12-step kinetic mechanism was used for the solid phase to simulate the multi-component decomposition process in detail. The solid fuel was oriented horizontally to mimic the burning experiments of individual leaves of manzanita by the Flat Flame Burner (FFB) apparatus and was exposed to convective heating. The simulations were consistent with the experimen- tal results in terms of ignition and burnout time prediction, fire initiation and spread pattern. Local evaporation of moisture and temperature rise at the periphery of the solid fuel was observed, also a significant amount of moisture remained at the center of sample at the time of ignition indicating that different points in the domain evap- orate and pyrolyze at different times. In the final study, the effect of both convection and radiation was investigated with the fuel element oriented vertically. Evaporation occurred at a higher rate near the leading, lateral and trailing edge of the solid fuel compared to the region located at the center. This pattern of heating was either due to the flame tilt observed during simulation or due to the effect of fluid dynamics. A boundary layer growth was observed above the surface of the solid fuel which reduces the heat transfer to the region located at the center. When radiation was used along with convective heating source, the peak value of mass loss rate was 20% higher than that in convection-only case.
Yashwanth, Bangalore Lingaraj, "Computational investigation of the influence of heating modes and moisture content on pyrolysis and ignition of live fuels" (2015). Dissertations. 82.