Albert Simeoni

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.

Investigation on the Emission of Volatile Organic Compounds from Heated Vegetation and Their Potential to Cause an Accelerating Forest Fire

An experimental study is conducted on the emission of volatile organic compounds (VOCs) emitted by Rosmarinus officinalis plants when exposed to an external radiant flux. The thermal radiation heats the plant and causes the emission of VOCs. The thermal radiation simulates the radiant flux received by vegetation in a forest fire. The results of the experiments are used in a simplified analysis to determine if the emissions of VOCs in an actual forest fire situation could produce a flammable gas mixture and potentially lead to the onset of an accelerating forest fire. The experiments consist of placing a plant in a hermetic enclosure and heating it with a radiant panel. The VOCs produced are collected and analyzed with an automatic thermal desorber coupled with a gas chromatograph/mass spectrometer (ATD-GC/MS). The effects of the fire intensity (radiant panel heat flux) and the fire retardant on the VOCs emission are then investigated. Two thresholds of the VOCs emission are observed. The first is for plant temperatures of around 120°C and appears to be caused by the evaporation of the water in the plant, which carries with it a certain amount of VOCs. The second one is around 175°C, which is due to the vaporization of the major parts of VOCs. The application of a fire retardant increases the emission of VOCs due to the presence of the water (80%) in the fire retardant. However, the use of the retardant results in a lower production of VOCs than using water alone. The measurements are used to estimate the concentration of VOCs potentially produced during the propagation of a specific fire and compared to the flammability limits of α-pinene. It is concluded that the quantities of VOCs emitted by Rosmarinus officinalis shrubs under certain fire conditions are capable of creating an accelerating forest fire.

Fire spread across pine needle fuel beds: characterization of temperature and velocity distributions within the fire plume

The aim of this article is twofold. First, it concerns the improvement of knowledge on the fundamental physical mechanisms that control the propagation of forest fires. To proceed, an experimental apparatus was designed to study, in laboratory conditions, the flame of a fire spreading across a pine needle fuel bed. Characterization of temperature was managed by using a reconstruction method based on a double thermocouple probe technique developed recently. The vertical gas velocity distribution was derived from the previous reconstructed signals by measuring the transit time of a thermal fluctuation between two points of the flow. Second, the experimental data were used for the testing of a physical two-phase model of forest fire behavior in which the decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented.

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.

Volatile and semi-volatile organic compounds in smoke exposure of firefighters during prescribed burning in the Mediterranean region

Prescribed fires can be used as a forest management tool to reduce the severity of wildfires. Thus, over prolonged and repeated periods, firefighters are exposed to toxic air contaminants. This work consisted in collecting and analysing smoke released by typical Mediterranean vegetation during prescribed burning. Sampling was performed at five active zones on the island of Corsica. Seventy‐nine compounds were identified: volatile organic compounds and semi‐volatile organic compounds, including polycyclic aromatic hydrocarbons. Depending on exposure levels, the toxins present in smoke may cause short‐term or long‐term damage to firefighters’ health. The dangerous compounds emitted, benzene, toluene, ethylbenzene and xylenes, were quantified. Their concentrations varied as a function of the study site. These variations were due to the intrinsic and extrinsic characteristics of the fire site (e.g. plant species, fire intensity and wind). Our results show that benzene concentration is high during prescribed burning, close to the exposure limit value or short‐term exposure limit. Benzene can be considered as a toxicity tracer for prescribed burning because its concentration was above the exposure limit value at all the study sites. The authors suggest that respirators should be used to protect staff during prescribed burning operations.

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.

On the Interest of Studying Degradation Gases for Forest Fuel Combustion Modeling

The aim of this work is to determine the influence of the degradation gases on the combustion of forest fuels and whether they have to be taken into account in numerical modeling. A laboratory experimental apparatus was designed to generate laminar, axisymmetric, time-varying and non-premixed flames from crushed forest fuels. The experiments highlight that the mass burning rate of the fuel controls the flame dynamics whereas the combustion kinetics depends on the degradation gases. From the analysis of the degradation gases released by one fuel, different combustion mechanisms and gas mixtures were tested numerically. The reaction rates computed with these mechanisms were examined and the temperature distributions were compared with the experimental data. The predictions obtained with the combustion mechanism including both methane and carbon monoxide did agree with the experiments.

Chapter 5 – Modeling Peat-Fire Hazards: From Drying to Smoldering

Peat is an organic and flammable material used for energy generation and involved in wildfires. Smoldering fires do not have the visual impact of flaming fronts but are an important aspect of wildfires because of the associated large carbon emissions and damage to valuable ecosystems. Moreover, in the case of extreme dry conditions or strong winds, smoldering fires develop easily into scrub or forest flaming fires. An advanced knowledge on the mechanisms and kinetic parameters controlling the drying process and the thermal decomposition of peat is of importance for understanding smoldering peat fires and quantifying the associated risks. In this context, the present chapter proposes a contribution to determine kinetic parameters to model peat-fire hazard. Three main topics were developed: thermal behavior, kinetic analysis, and fire hazard modeling. n n1. Investigations were conducted on boreal peat samples from two regions (Scotland and Siberia) and for three depths: two high-moor peat types collected in Edinburgh and in Tomsk, and one transition peat from Tomsk. For each sample, the botanical composition, degree of decomposition, ultimate analysis, and moisture content were determined and correlated to the thermal behavior obtained using two thermal apparatuses: a Thermogravimetric Analyzer and a Differential Scanning Calorimeter, along with a humidity analyzer. The complementary use of these thermal techniques provides details about the partial processes and reaction kinetics. The peat samples were submitted to thermal stress from the ambient temperature to 773 K with heating rates of 10, 20, and 30 K/min. A significantly different degradation behavior was observed for the different peat types. However, three main degradation steps can be isolated. The first one consists of a drying step between ambient temperature and 473 K, the second one is a devolatilization step between 473 and 650 K, and last one is a combustion step from 650 to 773 K. A large part of this chapter is dedicated to the description of these phenomena and their correlations with the various physicochemical, botanical, and physiological parameters of the peat previously determined. n n2. Based on the experimental results, another part of the chapter is devoted to the determination of kinetic constants. The kinetic triplet of each reaction involved during thermal degradation was estimated. Water evaporation is known to be an important mechanism that dictates the ignition and spread of peat fires. In this context, the thermokinetic constants for drying were determined using two different methods: the Inverse Kinetic Problems and Kissinger Akahira Sunose methods. Moreover, it is well known that devolatilization conditions have a major bearing on the yield of char and its reactivity in the process. This evidence, together with the need to develop models that predict peat behavior when it is subjected to a heating process, confers a great interest on the kinetic study of smoldering and combustion. For these two complex exothermic phenomena, the kinetic triplets of each reaction were estimated using a more detailed Hybrid Kinetic Method. This method has the advantage of eliminating errors caused by the ambiguity of the phenomenon. n n3. Despite the negative impact of peat fires, a science-based system has not yet been developed for the prediction of their occurrence. Such a method would allow one to assess the probability of occurrence of peat fire, taking into account the natural and anthropogenic pressure, weather conditions, terrain characteristics, and dynamics of the moisture content of the peat layer. Therefore, the description of peat drying is a major parameter in the prediction of fire danger. The last part of the chapter devoted to the development of a mathematical model of the drying of a peat layer is based on a single temperature. The iteration–interpolation method was used to numerically solve the mathematical model. The thermokinetic constants previously obtained were used to implement the simulations. The model allows one to obtain the time variation of the volume fraction of water, gas phase, and temperature of the peat layer. n nTo conclude, an accurate set of mathematical models that can be used for predicting peat-fire hazard is presented. These models take into account all known reasons for ignition of natural fires. The specific meteorological conditions, anthropogenic pressure, and simulation of the dynamics of the moisture content of the peat layer are included in the analysis.

Computational and experimental study of laminar flames from forest fuels

Experiments and simulations have been conducted to study the burning of three different vegetative fuels involved in forest fires. An experimental apparatus was designed to generate, in laboratory conditions, laminar, axisymmetric, time- varying and non-premixed flames of these fuels. Characterization of temperature in such flame was managed. The experimental data were used for the testing of a very simple formulation for fuel oxidation. To proceed, the gases released from the pyrolysis of one of the above vegetative fuels were analysed by means of a tube furnace apparatus connected to a gas chromatograph. Using numerical methods the transient equations for the conservation of mass, momentum, energy and chemical species were solved for the flame as well as the radiative transfer equation. The calculated distribution of temperature is presented for this fuel. It does agree with the experimental data recorded as a function of time, at different heights in the flame.