A 3D physical model to study the behavior of vegetation fires at laboratory scale

Abstract

A 3D multi-physical model referred to as "FireStar3D" has been developed in order to predict the behavior of wildfires at a local scale (< 500m). In the continuity of a previous work limited to 2D configurations, this model consists of solving the conservation equations of the coupled system composed of the vegetation and the surrounding gaseous medium. In particular, the model is able to account explicitly for all the mechanisms of degradation of the vegetation (by drying, pyrolysis, and heterogeneous combustion) and the various interactions between the gas mixture (ambient air + pyrolysis and combustion products) and the vegetation cover such as drag force, heat transfer by convection and radiation, and mass transfer. Compared to previous work, some new features were introduced in the modelling of the surface combustion of charcoal, the calculation of the heat transfer coefficient between the solid fuel particles and the surrounding atmosphere, and many improvements were brought to the numerical method to enable affordable 3D simulations. The partial validation of the model was based on some comparisons with experimental data collected at small scale fires carried out in the Missoula Fire Sciences Lab's wind tunnel, through various solid-fuel layers and in well controlled conditions. A relative good agreement was obtained for most of the simulations that were conducted. A parametric study of the dependence of the rate of spread on the wind speed and on the fuelbed characteristics is presented.

Mathematical Modeling of Heat Transfer in an Element of Combustible Plant Material When Exposed to Radiation from a Forest Fire

“…As a result, there is no fresh portion of gaseous combustible pyrolysis products for sustained combustion [69]. As a result, heat transfer has an impact on the probability of a forest fire and the scenarios for its further spread over the grass cover [70].…”

Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy

“…FIRESTAR3D is a fully physics-based wildfire model [21] based on a multiphase formulation approach [22]. The details of the model have been widely presented in previous publications [12,20,21,23,24], and the most detailed description of FIRESTAR3D model can be found in [21]. The model consists of two parts that are solved on two distinct grids.…”

Front’s dynamics of quasi-infinite grassland fires

“…Note that Hchar is not constant, it depends on the proportion of CO to CO2 produced during charcoal combustion (Morvan et al, 2018), it varies between 9 MJ/kg (for an incomplete combustion) and 30 MJ/kg (for a complete combustion). The fireline intensity is then obtained from I = HRRss/w, where HRRss is the average value of the heat release rate obtained at steady state and w = 100 m is the fireline width.…”

“…The fully physical approach has also a great potential in the management of fire hazard in wildland-urban interfaces, such as the dimensioning of a fuel break, the evaluation of heat flux on a target located inside the WUI, the interaction between two fire fronts (Morvan, 2015, Mell et al, 2010 The 3D model used in this study (FireStar3D) is based on a multiphase formulation and solves the conservation equations of the coupled system formed by the vegetation and the surrounding gaseous medium (Grishin andAlbini, 1997, Morvan et al, 2018). The predictive potential of FireStar3D model was tested at a small scale in the case of litter fires (Morvan et al, 2018), as well as at a larger scale in the case of grassland fires (Frangieh et al, 2018) that have been extensively studied experimentally (Cheney et al, 1998). In this study, the fire-front dynamics and spread through a homogeneous grassland is analyzed, for different wind speeds, in terms of rate of fire spread, fire intensity, and characteristic-wavelength of the fire-front structures.…”

“…The interaction between the gaseous phase and the solid one obtained through coupling terms that appear in both parts of the model. The coupling in the momentum and turbulence equations is obtained by adding aerodynamic drag terms (Morvan et al, 2018) that include a drag coefficient (evaluated empirically, equal to 0.15 in the present study) multiplied by a reference surface, defined as the Leaf Area Density (LAD). Heat transfer between the gas mixture and the solid fuel is based on empirical correlations for convective transfer (Incropera and DeWitt, 1996), and on the resolution of the radiative transfer equation (Modest, 2003) that accounts for the presence of soot in the flaming zone and for the presence of hot particles in the vegetation layer (embers) (Grishin and Albini, 1997).…”