Albert Simeoni

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.

Calculation methods for the heat release rate of materials of unknown composition

The Heat Release Rate (HRR) is a critical parameter to characterise a fire. Different methods have been developed to estimate it. The most widespread techniques are based on mass balance. If the heat of combustion of the fuel is known, the measure of the mass loss allows its evaluation. If the burning material can not be identified, calorimetric principles can be used. They rely on oxygen consumption (OC) or carbon dioxide and carbon monoxide generation (CDG) measurements. Their asset comes from the observation that the amount of energy release per unit mass of O2 consumed or per unit mass of CO2 produced is relatively constant for a large number of materials. Thus, an accurate HRR can be obtained without knowing the composition of the burning fuel. The aim of this work is to assess this last statement and define how essential the knowledge of the chemistry to calculate HRR for complex materials such as polymers including fire retardants and/or nanocomposites, energetic materials or pine needles is. This assessment ends in an OC and CDG calorimetry comparison of several materials in order to investigate the propensity to determine whether converging or diverging HRR results when average energy constants are used. Copyright

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.

Heat and mass transfer in fires: scaling laws, ignition of solid fuels and application to forest fires

Fire is a phenomenon that covers a multiplicity of scales depending on the different processes involved. Length scales range from the nanometres when addressing material flammability to the kilometres when dealing with forest fires, while time scales cover a broad spectrum too. Heating of structural elements can be measured in hours while characteristic chemical times for reactions do not exceed the millisecond. Despite these wide ranges, a series of simple scaling laws seem to describe well a multiplicity of processes associated with fire. In this paper, flaming ignition of a solid fuel will be presented within the context of general scaling laws and forest fires. Therefore, the case of highly porous vegetable fuels will be investigated to extend the theory to the forest fires application.

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.

Bulk and particle properties of pine needle fuel beds – influence on combustion

This paper presents a study to assess the influence of pine needle layer characteristics on combustion for three pine species of the Mediterranean region of France. It identifies the key parameters that explain the combustion of this fuel bed component. A relationship between permeability of the litter layer, fuel bed porosity and needle geometrical properties is presented. Although permeability was found to influence the rate of heat release from the combustion of litter independent of litter species, this was not the case for litter layers with similar mass and porosity. This study also stresses the important role of particle properties on their time to piloted ignition. The surface-to-volume ratio (SVR) of the species is the essential parameter driving the time to ignition as it defines the thermal thickness of single needles. This parameter also influences the combustion dynamics of litters under forced convection. In that case, the heat release rate of pine needle litters with the same permeability increases with the SVR of the species.

On the role of bulk properties and fuel species on the burning dynamics of pine forest litters

This work aims to characterize pine needles as a fuel for a better understanding of the burning dynamics of forest floor fuels in wildland fires. Three Mediterranean species of pine have been studied: Pinus halepensis, Pinus pinaster and Pinus laricio. These species have been chosen because they present close but slightly different physical and chemical properties. The study focuses on the influence of the bulk and particle properties on the burning dynamics of pine needles litters. The permeability of the porous fuel beds as well as the physical and chemical characteristics of each fuel have been determined experimentally. The combustion experiments were performed using the FM Global Fire Propagation Apparatus. The heat release rate estimation was done by using oxygen consumption calorimetry with a correction of the energy constant since the composition of the fuels was known. Two kinds of sample holders were used, one with holes to allow different air flow rates to pass through the fuel sample and one blocked to stop the flow at the bottom and the sides of the fuel sample. The different air flow rates were natural convection and different rates of forced flows. A mean value of the energy released during the flaming stage per unit mass loss can be determined for all species under the different flow conditions. This energy is related to the fire-line intensity, which is an important quantity for foresters, and firefighters that allows them to evaluate the fire impact and the means which are necessary to fight a fire. Permeability appears to be an important parameter when analyzing the combustion dynamics of highly porous forest fuel beds. The energy released during flaming depends greatly on permeability but for the same permeability, the fuel species have an influence on the burning dynamics.

Clarifying the meaning of mantras in wildland fire behaviour modelling: reply to Cruz et al. (2017)

In a recent communication, Cruz et al. (2017) called attention to several recurring statements (mantras) in the wildland fire literature regarding empirical and physical fire behaviour models. Motivated by concern that these mantras have not been fully vetted and are repeated blindly, Cruz et al. (2017) sought to verify five mantras they identify. This is a worthy goal and here we seek to extend the discussion and provide clarification to several confusing aspects of the Cruz et al. (2017) communication. In particular, their treatment of what they call physical models is inconsistent, neglects to reference current research activity focussed on combined experimentation and model development, and misses an opportunity to discuss the potential use of physical models to fire behaviour outside the scope of empirical approaches.

A preliminary study of wildland fire pattern indicator reliability following an experimental fire

An experimental fire was conducted in 2016, in the Pinelands National Reserve of New Jersey, to assess the reliability of the fire pattern indicators used in wildland fire investigation. Objects were planted in the burn area to support the creation of the indicators. Fuel properties and environmental data were recorded. Video and infrared cameras were used to document the general fire behavior. This work represents the first step in the analysis by developing an experimental protocol suitable for field studies and describing how different fire indicators appeared in relation to fire behavior. Most of the micro- and macroscale indicators were assessed. The results show that some indicators are highly dependent on local fire conditions and may contradict the general fire spread. Overall, this study demonstrates that fire pattern indicators are a useful tool for fire investigators but that they must be interpreted through a general analysis of the fire behavior with a good understanding of fire dynamics.

A dimensional analysis of forest fuel layer ignition model: Application to the ignition of pine needle litters

This article deals with the physical modelling of forest fuel layer ignition. A model based on momentum-, fluid- and solid-phase energy equations is written for a fuel layer and a dimensional analysis is performed. This analysis allows to enlighten on two relevant dimensionless groups regarding the dimensionless time to ignition of a fuel layer and also provides a suited scaling for the fluid velocity inside the fuel layer during ignition. A correlation for the time to ignition is then fitted on experimental data obtained using an FM-Global fire propagation apparatus for different pine species with a closed basket. A good agreement is found, emphasizing the relevance of the dimensionless groups and the thermally thick behaviour of the solid particles during the ignition process under incident radiant heat flux as low as 8 − 12 kW m − 2 .