Jacques-Henri Balbi

The contribution of radiant heat transfer to laboratory-scale fire spread under the influences of wind and slope

Abstract In a previous study, a two-dimensional non-stationary model of fire spread across a fuel bed including slope effects was proposed. Enhancement of the radiant heat transfer process ahead of the fire front under wind and slope conditions is provided in the current paper. Predictions are compared to data recorded during laboratory-scale experimental fires conducted across different pine needles bed under wind and slope conditions. Influence of these environmental factors on rate of spread, temperature–time profiles and fire front shapes is then presented. Predictions reveal that effects of flame radiation alone can account for observations up to limit value of slope angle and wind velocity for which convective mechanisms cannot be neglected. Below this limit observations are quantitatively well reproduced by the model.

A MODEL FOR THE SPREAD OF FIRE ACROSS A FUEL BED INCORPORATING THE EFFECTS OF WIND AND SLOPE

ABSTRACT A two-dimensional nonstationary model of a fire spreading across a bed of fuel is proposed, incorporating the effects of wind and slope. The contributions of both radiative and convective preheating ahead of the fire front are included. The radiation impinging on the top of the fuel bed is determined, assuming the flame is a radiant surface. Convective heat transfer in the fuel layer is considered using a simplified description of the flow through the fuel bed. Model predictions are compared to laboratory-scale experiments. Dedicated experiments were carried out for horizontal fire spread in still air across beds of pine needles to measure flame and fire front properties using infrared camera, thermocouples, and heat flux sensors. Experiments conducted under wind and slope conditions are also considered.

A 3D PHYSICAL REAL-TIME MODEL OF SURFACE FIRES ACROSS FUEL BEDS

Abstract This work presents a new modelling approach to the elaboration of a simple model of surface fire spread. This model runs faster than real-time and will be integrated in management tools. Until now, models used in such tools have been based on an empirical relationship. These tools may be efficient for conditions that are comparable to those of test-fires but the absence of a physical description makes them inapplicable to other situations. This paper proposes a physical 3D model of surface fire able to predict fire behaviour faster than real-time. This model is tested on experiments carried out across fuel beds under slope and wind conditions.

Coupled Atmosphere‐Wildland Fire Modelling

A tight interaction exists between the development of a wildfire and the local meteorology near the front. The convective effects induced by the fire heat release can modify the local wind circulation and consequently affect the fire propagation. In this study we use a meso-scale numerical model in a Large Eddy Simulation (LES) configuration coupled to a simplified physical front tracking wildfire model to investigate the differences induced by the atmospheric feedback in propagation speed and behaviour. Simulations of typical experimental configurations show a good response of the coupled fire-atmospheric model. Numerical results matches qualitatively observed values for fire induced winds and convection. Both numerical models already have operational usage and might ultimately be run to support decisions in wildfire management.

Modelling of two-dimensional flame spread across a sloping fuel bed

A significant contributing factor to wildland fire development is the slope effect which causes the fire spread rate to increase considerably as compared to horizontal spread. This leads to difficulties in determining the development of the fires hence in coordinating forest fighting efforts. In the present study, a two-dimensional non-stationary model for a fire spreading across a sloping fuel bed made up of Pinus pinaster litter is described. Based on a series of hypotheses, we first defined a medium equivalent to the pine needle litter for which we provided a thermal balance. By coupling this balance to a diffusion flame model we obtained the fire spread model numerically solved by means of the SIMPLEC procedure. The fire spread rates given by the simulations were then compared to experimental results generated by small-scale laboratory fires for a range of slope values. Predicted flow field structure, and temperature field are also discussed.

On the wind advection influence on the fire spread across a fuel bed: modelling by a semi-physical approach and testing with experiments

This paper is devoted to the study of the advection effect on the fire spread across a fuel bed by means of a semi-physical model. This work is a step forward in our general process which consists in elaborating a simple model of fire spread to be used in a simulator. To this end, a thermal balance including an advective term coupled with a wind velocity profile in the burning zone is presented. Following our general procedure that consists in using the multiphase approach to elaborate our semi-physical model, we used the momentum equation of the multiphase model to set this wind profile. The predictions of the model were then compared to experimental data obtained for fire spread conducted across pine needle litters. Different slope values and varying wind velocities were considered. The experimental tendency for the variation of the rate of spread was predicted, especially for the higher values of wind.

Wildland Fire Behaviour Case Studies and Fuel Models for Landscape-Scale Fire Modeling

This work presents the extension of a physical model for the spreading of surface fire at landscape scale. In previous work, the model was validated at laboratory scale for fire spreading across litters. The model was then modified to consider the structure of actual vegetation and was included in the wildland fire calculation system Forefire that allows converting the two-dimensional model of fire spread to three dimensions, taking into account spatial information. Two wildland fire behavior case studies were elaborated and used as a basis to test the simulator. Both fires were reconstructed, paying attention to the vegetation mapping, fire history, and meteorological data. The local calibration of the simulator required the development of appropriate fuel models for shrubland vegetation (maquis) for use with the model of fire spread. This study showed the capabilities of the simulator during the typical drought season characterizing the Mediterranean climate when most wildfires occur.

Simplified Flame Models and Prediction of the Thermal Radiation Emitted by a Flame Front in an Outdoor Fire

The authors proposed a comparison between 2 simplified flame models. The first flame model uses the radiant surface approach with a new analytical expression for the heat flux. The second one is derived from the Radiative Transfer Equation. The fire front has been considered as a line characterized by some geometric and physical parameters. Two assumptions are used to model the flame, either a radiant plane or a volumetric flame. The flame parameters have been identified from experiments using video records and applying an inverse method. These two models were tested against fires carried out in a fire tunnel and found to perform very well considering the complicate nature of the flame geometry and flame characteristics. The need to determine the heat flux from a large-scale fire has lead to make a number of assumptions. By means of the proposed modeling, the authors try to determine the extent to which the range of assumptions made disqualifies some simplified flame models from use.

Proposal for Theoretical Improvement of Semi-Physical Forest Fire Spread Models Thanks to a Multiphase Approach: Application to a Fire Spread Model Across a Fuel Bed

This paper is devoted to the improvement of semi-physical fire spread models. In order to improve them, a theoretical approach based on the multiphase concept was carried out. The multiphase approach which considers the finest physical phenomena involved in fire behaviour was reduced by making several assumptions. This work led us to a simplified set of equations. Among these, a single equation for the thermal balance was obtained by using the thermal equilibrium hypothesis. This approach has been applied to the improvement of our semi-physical model in order to take into account increasing wind influence. The predictions of the improved model were then compared to experimental data obtained for fire spread conducted across pine needle fuel beds. To this end, different slope values and varying wind velocities were considered. The experimental tendency for the variation of the rate of spread was predicted. Indeed, it increases with increasing wind velocity for a given slope as well as for a given wind with increasing slope.

Reduction of a multiphase formulation to include a simplified flow in a semi-physical model of fire spread across a fuel bed

The aim of our ongoing research is to propose a forest fire simulator. To this end, we have developed a semi-physical model of fire spread that has been validated experimentally thanks to laboratory-scale pine needle bed fires under both slope and low wind conditions. This model described the physical phenomena in a simple manner while providing the main characteristics of spread. However, it did not allow to describe accurately the experimental tendency of an increasing spread rate with increasing wind velocity, particularly because of the strong assumption of considering a constant wind over the entire spreading zone. In the present study, we propose a simplified description of the flow that is coupled to our model. To proceed, we carry out the reduction of a multiphase model of reference. This reduction of the complete equations that describe the flow allows us to develop a simplified flow by considering mainly the buoyancy effect induced by combustion in the flaming zone. The results are subsequently compared to laboratory experiments under varying wind and slope conditions. A substantial improvement of the predicted rates of spread is provided.