Although the modelling of the spreading of a forest fire has made considerable progress recently, there remains a lack of reliable field measurements of thermodynamic quantities. We propose in this paper a method and a set of measuring structures built in order to improve the knowledge on the fundamental physical mechanisms that control the propagation of wildland fires. These experimental apparatus are designed to determine: the fire front shape, its rate of spread, the amount of energy impinging ahead of it, the vertical distribution of the temperature within the fire plume as well as the wind velocity and direction. The methodology proposed was applied to a fire spreading across the Corsican scrub on a test site.The recorded data allowed us to reconstruct the fire behaviour and provide its main properties. Wind and vegetation effects on fire behaviour were particularly addressed.
One of the objectives of the present study is to gain a deeper understanding of the heat transfer mechanisms that control the spread of wildfires. Five experimental fires were conducted in the field across plots of living vegetation. This study focussed on characterising heat transfer ahead of the flame front. The temperature and heat flux were measured at the top of the vegetation as the fire spread. The results showed the existence of two different fire spread regimes that were either dominated by radiation or governed by mixed radiant–convective heat transfer. For plume‐dominated fires, the flow strongly responds to the great buoyancy forces generated by the fire; this guides the fire plume upward. For wind‐driven fires, the flow is governed by inertial forces due to the wind, and the fire plume is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of the flame front) and wind velocity fluctuations change according to the fire regime. The longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. The data showed that neither the convective Froude number nor the Nelson convection number – used in the literature to predict fire spread regimes – reflect the observed behaviour of wind‐driven fires.
A two-dimensional non-stationary 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 bed of fuel. Dedicated laboratory-scale experiments have been carried out across beds of pine needles for horizontal fire spread in still air, in order to validate the model. Experiments conducted under wind and slope conditions are also considered. Model predictions are then compared to these experimental measurements.
Simulation of moving interfaces, like a fire front usually requires the resolution of a large scale and detailed domain. Such computing involves the use of supercomputers to process the large amount of data and calculations. This limitation is mainly due to the fact that large scale of space and time is usually split into nodes, cells or matrices, and the solving methods often require small time steps. This paper presents a novel method that enables the simulation of large scale/high resolution systems by focusing on the interface. Unlike the conventional explicit and implicit integration schemes, it is based on the discrete-event approach, which describes time advance in terms of increments of physical quantities rather than discrete time stepping. Space as well is not split into discrete nodes or cells, but we use polygons with real coordinates. The system is described by the behaviour of its interface, and evolves by computing collision events of this interface in the simulation. As this simulation technique is suited for a class of models that can explicitly provide rate of spread for a given configuration, we developed a specific radiation based propagation model of physical wildland fire. Simulations of a real large scale fire performed with an implementation of our method provide very interesting results in less than 30 seconds with a 3 metres resolution with current personal computers.
Fire spread across forest fuel is usually characterized by the rate of spread or the fireline intensity.The determination of the fireline intensity represents an essential aspect for understanding the behaviour of the fire and the involved combustion processes. The heat released during fire spread cannot be a-priori estimated from the fundamental properties of the fuel material and experiments need to be carried out to determine it. This paper presents a global characterization of horizontal fire spread in still air across fuel beds in terms of heat release, rate of spread, flame geometry and radiant and convective fractions. The influence of the fuel load on these main fire properties is investigated.A series of experiments was conducted using a Large Scale Heat Release apparatus. The fire tests were carried out on a combustion table located on a load cell. The fuel consisted in a 2 m long and 1 m wide bed of pine needles. The fireline intensity was accurately estimated by means of oxygen consumption calorimetry and some other methods to assess this quantity were also tested.Combustion efficiency and effective heat of combustion were discussed. The heat fluxes emitted during the fire spread were also investigated. In the studied configuration, radiation was the dominant heat transfer mechanism in the preheating zone; whereas some transfers combining
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