Virginie Tihay

Smouldering natural fires: comparison of burning dynamics in boreal peat and Mediterranean humus

Smouldering of the forest subsurface can be responsible for a large fraction of the total fuel consumed during wildfires. Subsurface fires can take place in organic material stored in shallow forest layers such as duff or humus, and in deeper layers such as peat, landfills and coal seams. These fires play a major role in the global emission to the atmosphere, the destruction of carbon storage in the soil and the damage to the natural environment. Burning dynamics in two different ecosystems affected by smouldering wildfires are studied here; boreal peat and Mediterranean humus. A series of small-scale smouldering experiments have been conducted under laboratory conditions to study the ignition and the severity to the soil. The experimental set-up allowed the temperature and velocity of the fire front to be measured for different fuel moisture contents. The two fuels, peat and humus, were tested and the results are compared.

Ignition study of acetone/air mixtures by using laser-induced spark

The breakdown and the laser-induced spark ignition of acetone-air mixtures were experimentally studied using a nanosecond pulse at 1064 nm from a Q-switched Nd:YAG laser. The breakdown was first characterized for different mixtures with acetone and air. This part of the work highlighted the wide variation in the energy absorbed by the plasma during a breakdown. We also demonstrated that the presence of acetone in air tends to reduce the energy required to obtain a breakdown. Next, the ignition of acetone-air mixtures in the equivalence ratio range 0.9-2.4 was investigated. The probabilities of ignition were calculated in function to the laser energy. However, according to the variability of energy absorption by the plasma, we preferred to present the result according to the energy absorbed by the plasma. The minimum ignition energies were also provided. The minimum ignition energy was obtained for an equivalence ratio of 1.6 and an absorbed energy of 1.15 mJ. Finally the characteristics of the plasma (absorption coefficient and kernel temperature) were calculated for the experiments corresponding to minimum ignition energies.

Comparison of several kinetic approaches to evaluate the pyrolysis of three Mediterranean forest fuels

The thermal degradation of three Mediterranean forest fuels was studied by using thermogravimetric analysis. The different behaviours were compared and the degradation characteristics were determined for each fuel. Then a kinetic study was performed. Simple modelling of the thermal degradation was carried out with a global scheme. These results were compared with current kinetic models used in forest fire modelling. These methods predict the mass rate evolution globally. However, they do not describe the fluctuations well owing to the different pyrolysis steps. Next, the kinetic parameters were established in function of the degree of conversion with isoconversion methods for each fuel. The results calculated by the Friedman method were compared with the experimental results, showing good agreement. These data, which take into account the degree of conversion and the fuel, can be useful to model the burning rate of forest fuels.

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.

Study of the influence of fuel load and slope on a fire spreading across a bed of pine needles by using oxygen consumption calorimetry

A set of experiments using a Large Scale Heat Release Rate Calorimeter was conducted to test the effects of slope and fuel load on the fire dynamics. Different parameters such as the geometry of the flame front, the rate of spread, the mass loss rate and the heat release rate were investigated. Increasing the fuel load or the slope modifies the fire behaviour. As expected, the flame length and the rate of spread increase when fuel load or slope increases. The heat release rate does not reach a quasi-steady state when the propagation takes place with a slope of 20° and a high fuel load. This is due to an increase of the length of the fire front leading to an increase of fuel consumed. These considerations have shown that the heat release can be estimated with the mass loss rate by considering the effective heat of combustion. This approach can be a good alternative to estimate accurately the fireline intensity when the measure of oxygen consumption is not possible.

Testing of different skeletal and global mechanisms for modeling combustion of degradation gases involved in wildland fire

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 (CO2, CO, CH4 and H2O) 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.

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.