A global model for the combustion of gas mixtures released from forest fuels

International audienc

Skeletal and global mechanisms for the combustion of gases released by crushed forest fuels

International audienc

A global model for the NO released in vegetation fires

Vegetation fires are a major emission source of CO, CO2, NOx, volatile organic compounds and particulates to the atmosphere, which in turn can form secondary pollutants with implications at local, regional or global scale. To describe the impact of these fires on the environment, especially on the atmosphere, and to model their pollutant emissions, it is necessary to understand the processes involved in the combustion of vegetation. Likewise, the analysis of combustion kinetics in the gas-phase is decisive for modeling vegetation fires behavior given that the rate and amount of energy released from the fuel are derived from the fundamental chemistry of the fuel and its combustion. However, the use of detailed reaction mechanisms, which involves a large number of species and reactions, is impractical owing to large computational time requirements. In this work, a numerical approach has been used to study the mechanisms of NO formation in vegetation fires at the source level, given that NO plays an important role on the formation of ground-level ozone. The major reaction mechanisms involved in NO chemistry have been identified using reactions path analysis and sensitivity analysis with a detailed kinetic mechanism (GDF-kin® 3.0). Moreover, a two-step global kinetic mechanism has been proposed herein to account for the conversion of volatile fuel-bound nitrogen to NO in the gas phase, considering that the volatile fraction of fuel-bound nitrogen is released as NH3. Data from simulations using the PSR code from CHEMKIN-II package with a detailed kinetic mechanism (GDF-kin® 3.0) have been used to formulate reaction rate expressions of the global model under typical wildfire conditions in terms of the inlet mixture composition, equivalence ratio and range of temperatures

Heat transfers and energy released in the combustion of fine vegetation fuel beds

The study of the heat transfers and the energy released during the fire spread through beds of pine needles (Pinus pinaster) was conducted under slope and no slope conditions by using a Large Scale Heat Release (LSHR) calorimeter. The results revealed two regimes of propagation. For experiments along a flat surface, the heat release rate (HRR) reached a quasi-steady state whereas for experiments taking place under a slope, the HRR increased during the fire spread. This difference was due to a distortion of the fire front. Under no slope condition, the fire front was indeed linear and propagated with a nearly constant rate of spread. On the contrary, under slope conditions, the fire front had a V-shape leading to an increase of the derivative of the burnt surface per unit of time and therefore to a rise of the heat release rate during the fire spread. The study was then devoted to the characterization of the heat transfers. Firstly, the radiant fractions of the flame and the embers were calculated from the measurements of radiant heat fluxes, the HRR and the geometrical properties of the fire front. Then, the convective fraction was determined by using the temperatures recorded in the exhaust duct of the LSHR. The contribution of each mechanism of heat transfer to the fire spread was therefore quantified. The results showed an increase of the radiation heat transfer, when the fire spread under slope condition, which was mainly due to an increase of the radiant fraction of the flames. All these data give global information on the fire spread and are thus they are very useful to improve and validate physical models of wildland fires

Experimental study of laminar flames obtained by the homogenization of three forest fuels

International audienc

Analyzing the recent dynamics of wildland fires in Quercus suber L. woodlands in Sardinia (Italy), Corsica (France) and Catalonia (Spain)

Wildland fires represent a major threat to Quercus suber L. ecosystems, which provide relevant socioeconomic and ecological services in the Mediterranean Basin. In this work, we analyzed recent wildland fire dynamics in cork oak woodlands along the fire-prone areas of Sardinia (Italy), Corsica (France) and Catalonia (Spain). We first characterized geographic extent and main characteristics of cork oak woodlands in these regions and analyzed how environmental (climate and elevation) and socioeconomic factors (population and land uses) vary in the areas covered by Quercus suber L. We then evaluated how wildfires affected cork oak stands and, by logistic regression analysis, to what extent wildfires in cork oak areas were related to the above set of environmental and anthropic explanatory variables. Results revealed specific variations across study areas in cork oak characteristics as well as in environmental and social factors. We highlighted the spatial and temporal patterns of wildfires on cork oak woodlands, in terms of extent, seasonality, frequency and main driving factors. In the period 2003–2015, the percentage of cork oak woodlands burned ranged from 3.42% in Corsica to 11.30% in Sardinia. A few large and severe wildfires accounted for most of the area burned in cork oak woodlands. The most significant predictive variable that explained the spatial variation in wildland fire ignitions inside or nearby cork oaks was summer precipitation, while the weight of other factors varied depending on the region. This study provides evidence on recent fire dynamics in cork oak woodlands and gives valuable information and insights for the implementation of forest management and planning strategies in the Mediterranean area.This work was supported by the Sardinia Region under the project “Multifunzionalità delle foreste a quercia da sughero in Sardegna” (Multifunctionality of cork oak forests in Sardinia) funded within L.R. n. 7/2007 “Promozione della ricerca scientifica e della innovazione tecnologica in Sardegna,/Tender no. 2″, and by the “Proterina-2” Project funded by the Italia Francia-Marittimo Programme

Recent dynamics of forest fires in <i>Quercus suber</i> stands in Sardinia, Corsica and Catalonia

In this study, we analyzed the recent dynamics of forest fires in Quercus suber stands in Sardinia (Italy), Corsica (France), and Catalonia (Spain) for the period 2003-2015

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements