Frédéric Morandini

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.

Performance of a Protected Wireless Sensor Network in a Fire. Analysis of Fire Spread and Data Transmission

The paper deals with a Wireless Sensor Network (WSN) as a reliable solution for capturing the kinematics of a fire front spreading over a fuel bed. To provide reliable information in fire studies and support fire fighting strategies, a Wireless Sensor Network must be able to perform three sequential actions: 1) sensing thermal data in the open as the gas temperature; 2) detecting a fire i.e., the spatial position of a flame; 3) tracking the fire spread during its spatial and temporal evolution. One of the great challenges in performing fire front tracking with a WSN is to avoid the destruction of motes by the fire. This paper therefore shows the performance of Wireless Sensor Network when the motes are protected with a thermal insulation dedicated to track a fire spreading across vegetative fuels on a field scale. The resulting experimental WSN is then used in series of wildfire experiments performed in the open in vegetation areas ranging in size from 50 to 1,000 m2.

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.

Discrete Event Front-tracking Simulation of a Physical Fire-spread Model

Simulation of moving interfaces such as a fire front usually requires resolution of a large-scale and detailed domain. Such computing involves the use of supercomputers to process the large amount of data and calculations. This limitation is mainly due to the fact that a large scale of space and time is usually split into nodes, cells, or matrices and the solving methods often require small time steps. In this paper we present a novel method that enables the simulation of large-scale/highresolution systems by focusing on the interface and its application to fire-spread simulation. Unlike the conventional explicit and implicit integration schemes, it is based on the discrete-event approach, which describes time advance in terms of increments of physical quantities rather than discrete time stepping. In addition, space is not split into discrete nodes or cells, but we use polygons with real coordinates. The system is described by the behavior of its interface and evolves by computing collision events of this interface in the simulation. As this simulation technique is suitable for a class of models that can explicitly provide the rate of spread, we developed a radiation-based propagation model of wild land fire. Simulations of a real large-scale fire performed by implementation of our method provide very interesting results in less than 30 s with a 3-m resolution with current personal computers.

A two-dimensional model of fire spread across a fuel bed including wind combined with slope conditions

In a previous work (Santoni et al., Int. J. Wildland Fire, 2000, 9(4), 285–292), we proposed a twodimensional fire spread model including slope effects as another step towards our aim to elaborate a fire management tool. In the present study, we improve the model to include both wind conditions and wind combined with slope conditions. For this purpose the effect of wind and slope are considered similar, in the sense that they both force the flames to lean forward. However, this analogy remains acceptable only when flame tilt is below a threshold value. Simulation results are compared to experimental data under wind and no-slope conditions. The proposed model is able to describe the fire behaviour. Predictions of the model for wind and slope conditions are then considered and comparisons with observations are also provided.

Fire spread in chaparral – a comparison of laboratory data and model predictions in burning live fuels

Fire behaviour data from 240 laboratory fires in high-density live chaparral fuel beds were compared with model predictions. Logistic regression was used to develop a model to predict fire spread success in the fuel beds and linear regression was used to predict rate of spread. Predictions from the Rothermel equation and three proposed changes as well as two physically based models were compared with observed spread rates of spread. Flame length–fireline intensity relationships were compared with flame length data. Wind was the most important variable related to spread success. Air temperature, live fuel moisture content, slope angle and fuel bed bulk density were significantly related to spread rate. A flame length–fireline intensity model for Galician shrub fuels was similar to the chaparral data. The Rothermel model failed to predict fire spread in nearly all of the fires that spread using default values. Increasing the moisture of extinction marginally improved its performance. Modifications proposed by Cohen, Wilson and Catchpole also improved predictions. The models successfully predicted fire spread 49 to 69% of the time. Only the physical model predictions fell within a factor of two of actual rates. Mean bias of most models was close to zero. Physically based models generally performed better than empirical models and are recommended for further study.

Fire Front Width Effects on Fire Spread Across a Laboratory Scale Sloping Fuel Bed

In a previous study, a two-dimensional non-stationary model of fire spread across a fuel bed, which included slope effects, was proposed. Improvements of the flame radiant contribution are made in the present paper by taking into consideration long-range preheating mechanisms. Fire front is assumed to be a radiant panel and the amount of energy impinging on the unburned fuel ahead of the fire front is estimated by means of a view factor. However, computation time of this view factor is lengthy and a simplified computation is thus proposed. The predictions generated by both models are compared to data recorded during laboratory-scale experimental fires conducted across pine needle beds for slopes ranging from 0 to 30°. First, the slope effects on predicted rates of spread, temperature distribution and fire front shapes are provided. The influence of fire front width on model predictions is also presented. This effect appears to be a significant variable affecting the rate of fire spread and is qualitatively represented by the model.

Physical modelling of forest fire spreading through heterogeneous fuel beds

Vegetation cover is a heterogeneous medium composed of different kinds of fuels and non-combustible parts. Some properties of real fires arise from this heterogeneity. Creating heterogeneous fuel areas may be useful both in land management and in firefighting by reducing fire intensity and fire rate of spread. The spreading of a fire through a heterogeneous medium was studied with a two-dimensional reaction–diffusion physical model of fire spread. Randomly distributed combustible and non-combustible square elements constituted the heterogeneous fuel. Two main characteristics of the fire were directly computed by the model: the size of the zone influenced by the heat transferred from the fire front and the ignition condition of vegetation. The model was able to provide rate of fire spread, temperature distribution and energy transfers. The influence on the fire properties of the ratio between the amount of combustible elements and the total amount of elements was studied. The results provided the same critical fire behaviour as described in both percolation theory and laboratory experiments but the results were quantitatively different because the neighbourhood computed by the model varied in time and space with the geometry of the fire front. The simulations also qualitatively reproduced fire behaviour for heterogeneous fuel layers as observed in field experiments. This study shows that physical models can be used to study fire spreading through heterogeneous fuels, and some potential applications are proposed about the use of heterogeneity as a complementary tool for fuel management and firefighting.

Steady and Unsteady Fireline Intensity of Spreading Fires at Laboratory Scale

Fireline intensity is the rate of heat release per unit time and per unit length of a fire front. With rate of spread, it is one of the most relevant quantities used in forest fire science. It allows to evaluate the effects of fuel treatment on fire behavior, to establish limits for prescribed burning or to support fire suppression activities. Although it is widely used, conversely its measurement is often coarse and has received very little attention. Furthermore, literature only refers to steady state when dealing with this quantity. In the present paper, we measure directly the fireline intensity at laboratory scale by using the oxygen consumption calorimetry principle. This methodology allows us to provide this quantity not only for steady fires but also for unsteady spreading fires for the first time. We show that the current approach used to as- sess fireline intensity can lead to overestimation from about 20%. As the experiments were conducted under well- ventilated conditions, the heat release rate calculated by calorimetry was compared to mass loss rate and heat of combus- tion taking into account the combustion efficiency.

Validation study of a two-dimensional model of fire spread across a fuel bed

Abstract In a previous study, we proposed a two-dimensional equation to model fire spread. This study was in keeping with our long-term goal to create a forest fire simulator. The model was tested for laboratory fire experiments conducted on pine needle litter and allowed us to exhibit the main features of these fires. The study was carried out for a single fuel load however. The aim of the present paper is to further validate this model. First the influence of fuel load on the model predictions for horizontal fire spread is examined. These results which are in agreement with the experimental data, were then compared to predictions from other physical models. In addition, comparisons are made between numerical and experimental results under slope conditions for line-ignition fires. In this last case, the influence of the fuel load is also considered.