Large eddy simulations of the structure of spreading line fires at flame scale

Abstract

Our general objective in the present study is to develop tools to better describe the coupling between solid phase and gas phase processes that control the dynamics of flame spread in wildland fire problems. We focus on a modelling approach that resolves processes occurring at flame scales, i.e., the formation of flammable vapors from the 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. The modelling capability is based on a general-purpose Computational Fluid Dynamics (CFD) library called OpenFOAM and an in-house Lagrangian particle model that treats drying, thermal pyrolysis, oxidative pyrolysis and char oxidation using a one-dimensional porous medium formulation that allows descriptions of thermal degradation processes occurring during both flaming and smoldering combustion. The modelling capability is calibrated for pine wood and is first applied to simulations of fire spread across a surrogate vegetation bed corresponding to thin, monodisperse, cylindrical-shaped sticks of pine wood with prescribed particle and environmental properties (i.e., bed height, surface-to-volume ratio, packing ratio, moisture content, and wind velocity). While the model can be used in sloped terrain, the present simulations are limited to a flat ground surface. The current emphasis is on determining threshold conditions for successful spread, differences between the plume-dominated and wind-driven flame regimes, possible transitions to a steady or time-dependent flame structure, and differences in the relative weights of the flaming and smoldering regions.

Numerical simulations of flame spread in pine needle beds using simple thermal decomposition models

“…beyond spread rate) as the quality of the models depend on their ability to predict underlying physical phenomena which dictate global fire behavior. Such models have been compared to experimental data related to the burning of individual plants [7,8], ignition and flame spread in simple fuels beds (pine needles, excelsior, wood sticks) at laboratory scale [9][10][11][12][13][14], and more complex fuel beds at field scale [15]. Nevertheless, this approach for modeling flame spread in natural fuels requires a wide range of detailed input parameters, and the models are still in need of a considerable degree of testing and validation.…”

“…Ahmed et al [14] also used a three-component pyroylsis model to simulate flame spread through pine wood sticks. In many cases, however, very detailed species-specific characterization of the decomposition mechanisms are not available and it also may not be practical to calibrate complex decomposition models to every possible fuel type.…”