A Global Kinetic Model for the Combustion of the Evolved Gases in Wildland Fires (original) (raw)
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To simulate forest fires, there is a need of simple models for gas oxidation. The aim of this work is to provide such a model. Using numerical methods, the transient equations for the conservation of mass, momentum, energy and chemical species were solved as well as the radiative transfer equation for a laminar flame. Skeletal and global mechanisms of combustion including the main degradation gases released by forest fuels (CO 2 , CO, CH 4 and H 2 O) were tested. Their evaluation was carried out following two criteria: their computational time and their accuracy. The skeletal mechanisms provide results close to the experiments. However, they require too long computational times whatever the number of reactions. Then, two global mechanisms considering different gases were investigated as they necessitate less computational time. The comparison between the simulated and predicted temperatures points out that the mechanism containing only carbon monoxide as fuel underestimates significantly the temperature in the fire plume. On the contrary, the results obtained with global mechanisms including both methane and carbon monoxide are in good agreement with the experiments. These conclusions lead to the proposal of a simple and reliable combustion model for forest fire simulations, which considers only two reactions steps including methane.
Reduced mechanism for the combustion of evolved gases in forest fires
Combustion and Flame, 2008
In wildland fires, gaseous fuel released from the thermal degradation of vegetation is burnt in the flame surrounding the solid. The gaseous fuel is a complex and variable mixture including mainly CO, CH 4 , CO 2 , and other light hydrocarbons (C 2 H 2 , C 2 H 4 , C 2 H 6 , C 3 H 6 ). For the first time, a detailed study of the gas-phase oxidation of a CO/CH 4 /CO 2 mixture is reported for wildland fire modeling purposes. The experiments were performed in a perfectly stirred reactor (PSR) at atmospheric pressure over the temperature range 773-1273 K at fuel/air equivalence ratios of 0.6, 1, and 1.4. Mole fraction profiles as a function of temperature were obtained for molecular species via sonic probe sampling and off-line chromatography analyses. These measurements were compared to the numerical predictions obtained with the PSR code from the CHEMKIN II package with a full mechanism (GRI-Mech 3.0). A skeletal mechanism was then developed; it was derived from the full reaction mechanism through sensitivity analysis and rate-of-production analysis of PSR calculations covering the range of interest. The skeletal mechanism consists of 49 elemental reactions and 20 reactive species. By using a steady state assumption for 13 reactive species, we developed a reduced four-step global mechanism with CH 4 , CO, H 2 , O 2 , H, CO 2 , and H 2 O as reactants and products. Both skeletal and reduced mechanisms provide a good description of the oxidation process. The numerical results for the species profiles are in agreement with full mechanism predictions and experimental data.
Wildfire and the atmosphere: Modelling the chemical and dynamic interactions at the regional scale
Forest fires release significant amounts of trace gases and aerosols into the atmosphere. Depending on meteorological conditions, fire emissions can efficiently reduce air quality and visibility, even far away from emission sources. In 2005, an arson forest fire burned nearly 700 ha near Lançon-de-Provence, southeast France. This paper explores the impact of this Mediterranean fire on the atmospheric dynamics and chemistry downwind of the burning region. The fire smoke plume was observed by the MODIS-AQUA instrument several kilometres downwind of the burning area out of the Mediterranean coast. Signatures of the fire plume on air pollutants were measured at surface stations in southeastern France by the air quality network AtmoPACA. Ground-based measurements revealed unusually high concentrations of aerosols and a well marked depletion of ozone concentrations on the day of the fire. The Lançon-de-Provence fire propagation was successfully simulated by the semi-physical fire spread model ForeFire. ForeFire provided the burnt area at high temporal and spatial resolutions. The burnt areas were scaled to compute the fire heat and water vapour fluxes in the three-dimensional meso-scale nonhydrostatic meteorological model MesoNH. The simulated fire plume kept confined in the boundary layer with high values of turbulent kinetic energy. The plume was advected several kilometres downwind of the ignition area by the Mistral winds in accordance with the MODIS and AtmoPACA observations. The vertical plume development was found to be more sensitive to the sensible heat flux than to the fire released moisture. The burnt area information is also used to compute emissions of a fire aerosollike tracer and gaseous pollutants, using emission factors for Mediterranean vegetation. The coupled model simulated high concentrations of the fire aerosol-like tracer downwind of the burning zone at the right timing compared to ground-based measurements. A chemical reaction mechanism was coupled online to the MesoNH model to account for gaseous chemistry evolution in the fire plume. High levels of ozone precursors (NO x , CO) were simulated in the smoke plume which led to the depletion of ozone levels above and downwind of the burning zone. This depletion of ozone was indeed observed at groundbased stations but with a higher impact than simulated. The difference may be explained by the simplified design of the model with no anthropogenic sources and no interaction of the smoke aerosols with the photolysis rates. Ozone production was modelled tens of kilometres downwind of the ignition zone out of the coast.
Before using a fluid dynamics physically based wildfire model to study wildfire, validation is necessary and model results need to be systematically and objectively analyzed and compared to real fires, which requires suitable data sets. Observational data from the Meteotron experiment are used to evaluate the fire-plume properties simulated by two fluid dynamics numerical wildfire models, the Fire Dynamics Simulator (FDS) and the Clark coupled atmosphere-fire model. Comparisons based on classical plume theory between numerical model and experimental Meteotron results show that plume theory, because of its simplifying assumptions, is a fair but restricted rendition of important plumeaveraged properties. The study indicates that the FDS, an explicit and computationally demanding model, produces good agreement with the Meteotron results even at a relatively coarse horizontal grid size of 4 m for the FDS, while the coupled atmosphere-fire model, a less explicit and less computationally demanding model, can produce good agreement, but that the agreement is sensitive to surface vertical-grid sizes and the method by which the energy released from the fire is put into the atmosphere.
A model of gas mixing into single-entrance tree cavities during wildland fires
2011
The level of protection to fauna provided by tree cavities during wildland fires is not well understood. Here we present a model for estimating the transport of combustion gases into cylindrical, single-entrance cavities during exposures caused by different wildland fire scenarios. In these shelters, the entrance occurs near the top of the cavity. This empirical model was developed from a suite of numerical experiments using the National Institute of Standards and Technology's Fire Dynamics Simulator, which spanned a range of entrance diameters, wind speeds, gas temperatures, and vertical angles of incidence. To evaluate the model's predictions, it was used to replicate, with great accuracy, a time series of carbon monoxide (CO) concentrations in a controlled experiment where a fabricated cylindrical cavity was exposed to combustion products. The time constant for cavity filling is proportional to the ratio of cavity volume to entrance area. Hot gases lead to significant stratification within the cavity during exposures. To demonstrate the model's potential use in predicting faunal exposures in the context of land management, we show that the model can be used to estimate dosage within red-cockaded woodpecker (Picoides borealis Vieillot, 1809) cavities without requiring temporally detailed, local measurements of wind speed and combustion product concentrations.
A physics-based approach to modelling grassland fires
International Journal of Wildland …, 2007
Physics-based coupled fire-atmosphere models are based on approximations to the governing equations of fluid dynamics, combustion, and the thermal degradation of solid fuel. They require significantly more computational resources than the most commonly used fire spread models, which are semi-empirical or empirical. However, there are a number of fire behaviour problems, of increasing relevance, that are outside the scope of empirical and semi-empirical models. Examples are wildland-urban interface fires, assessing how well fuel treatments work to reduce the intensity of wildland fires, and investigating the mechanisms and conditions underlying blow-up fires and fire spread through heterogeneous fuels. These problems are not amenable to repeatable full-scale field studies. Suitably validated coupled atmosphere-fire models are one way to address these problems. This paper describes the development of a three-dimensional, fully transient, physics-based computer simulation approach for modelling fire spread through surface fuels. Grassland fires were simulated and compared to findings from Australian experiments. Predictions of the head fire spread rate for a range of ambient wind speeds and ignition line-fire lengths compared favourably to experiments. In addition, two specific experimental cases were simulated in order to evaluate how well the model predicts the development of the entire fire perimeter.