Valérie Leroy-Cancellieri

An analytical model based on radiative heating for the determination of safety distances for wildland fires

In a wildfire, radiative heat transfer is often the main thermal impact on people fighting the fire or on structures. Thus, the estimation of the radiation from the fire front and the heating of a target is of primary importance for forest and urban managers. An analytical formulation of this radiative heat transfer, based on a solid-flame assumption, is used. The realistic description of finite fire-front widths allows the proposal of a new criterion for the estimation of the radiative impact of the fire, which is based on the ratio of the fire-front width to the flame length, which is opposite to the classical approach of considering only the flame length. A numerical solution is necessary to calculate the safety distance for a fixed radiative threshold value, so an analytical approximation is proposed to obtain a simple and useful formulation of this Acceptable Safety Distance. A sensitivity analysis is conducted on the different physical and geometrical parameters used to define the flame front. This analysis shows that the flame temperature is the most sensitive parameter. The results of the analytical model are compared with the numerical solution of the flame model and previous approaches based only on flame length. The results show that the analytical model is a good approximation of the numerical approach and displays realistic estimations of the Acceptable Safety Distance for different fire-front characteristics.

A GLOBAL KINETIC MODEL FOR THE COMBUSTION OF THE EVOLVED GASES IN WILDLAND FIRES

The analysis of combustion kinetics in the gas-phase is decisive for wild land fire behavior modeling. However, the use of detailed reaction mechanisms, which involves a large number of species and reactions, is impractical due to large computational time requirements. The present work proposes a five-step chemical kinetic mechanism to simulate the gas phase combustion processes taking place in wildland fires. Both experimental data and data from simulations run using the PSR code from the CHEMKIN-II package with a detailed kinetic mechanism (GDF-kin 3.0) have been used to calibrate and evaluate the global model under typical wild land fire conditions in terms of the inlet mixture composition, equivalence ratio, and range of temperatures.

New experimental diagnostics in combustion of forest fuels: microscale appreciation for a macroscale approach

In modelling the wildfire behaviour, good knowledge of the mechanisms and the kinetic parameters controlling the thermal decomposition of forest fuel is of great importance. The kinetic modelling is based on the massloss rate, which defines the mass-source term of combustible gases that supply the flames and influences the propagation of wildland fires. In this work, we investigated the thermal degradation of three different fuels using a multi-scale approach. Lab-scale experimental diagnostics such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), use of the cone calorimeter (CC) or Fire Propagation Apparatus (FPA) led to valuable results for modelling the thermal degradation of vegetal fuels and allowed several upgrades of pyrolysis models. However, this work remains beyond large-scale conditions of a wildland or forest fire. In an effort to elaborate on the kinetic models under realistic natural fire conditions, a massloss device specifically designed for the field scale has been developed. The paper presents primary results gained using this new device, during large-scale experiments of controlled fires. The mass-loss records obtained on a field scale highlight the influence of the chemical composition and the structure of plants. Indeed, two species with similar chemical and morphological characteristics exhibit similar mass-loss rates, whereas the third presents different thermal behaviour. The experimental data collected at a field scale led to a new insight about thermal degradation processes of natural fuel when compared to the kinetic laws established in TGA. These new results provide a global description of the kinetics of degradation of Mediterranean forest fuels. The results led to a proposed thermal degradation mechanism that has also been validated on a larger scale.

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. 1. 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. 2. 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. 3. 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. To 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.