Development of a Lagrangian-Eulerian Modeling Framework to Describe Thermal Degradation of Porous Fuel Particles in Simulations of Wildland Fire Behavior at Flame Scale

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The dynamics of wildland fires involve multi-physics phenomena occurring at multiple scales ranging from sub-millimeter scale representative of small vegetation particles to several kilometers representative of meteorological scales. The objective of this research is to develop an advanced physics-based computational tool for detailed modeling of the coupling between the solid-phase and the gas-phase processes that control the dynamics of flame spread in wildland fire problems. This work focuses on a modeling approach that resolves processes occurring at flame and vegetation scales, i.e., the formation of flammable vapors from the porous biomass vegetation due to pyrolysis, the subsequent combustion of these fuel vapors with ambient air, the establishment of a turbulent flow because of heat release and buoyant acceleration, and the thermal feedback to the solid biomass through radiative and convective heat transfer. A modeling capability called PBRFoam is developed in this dissertation based on the general-purpose Computational Fluid Dynamics (CFD) library OpenFOAM and an in-house Lagrangian Particle Burning Rate (PBR) model that treats drying, thermal pyrolysis, oxidative pyrolysis, and char oxidation using a one-dimensional porous medium formulation. This modeling capability allows the description of fire spread in vegetation fuel beds comprised of mono- or poly-dispersed porous particles including thermal degradation processes occurring during both flaming and smoldering combustion.The modeling capability is calibrated for cardboard and pine wood using available micro- and bench-scale experimental data obtained. Then it is applied to simulate the fire spread across the idealized fuel beds made of laser-cut cardboard sticks that have been studied experimentally at the Missoula Fire Sciences Laboratory. The simulations are conducted with prescribed particle and environmental properties (i.e., fuel bed height, fuel bed packing, particle size, moisture content, and wind velocity) that match the experimental conditions. The model is first validated against experimental measurements and observations such as the rate of spread of the fire and the flame residence time. The modeling capability is then used to provide insights into local as well as global behavior at the individual particle level and the fuel bed level with variations of the fuel packing. The modeling capability is also applied to simulations of fire spread across idealized vegetation beds corresponding to mixed-size cylindrical-shaped sticks of pine wood under prescribed wind conditions. Depending on the particle size distribution, the simulations feature complete fuel consumption with a successful transition from flaming to smoldering combustion or partial fuel consumption with no or limited smoldering. These simulations show the existence of either a mixed mode of heat transfer through convection and radiation for small particles or a radiation-dominant heat transfer mode for larger particles. The results are interpreted using a novel diagnostic called the Pseudo Incident Heat Flux (PIHF) and 2-D maps that characterize single particle response as a function of the PIHF and the flame residence time.