Abstract
This chapter provides information on the physical and mathematical models of forest fuel layer ignition by the metal or non-metal particle heated up to high temperatures. Mathematically, the forest fuel layer ignition is described by a system of equations of heat conduction and diffusion with the corresponding initial and boundary conditions. Scenario modeling of forest fuel layer ignition was carried out in the temperature range characteristic to the occurrence of forest fires. A scenario of a catastrophic forest fire danger is considered when there is no moisture in the forest fuel layer.
TopPhysical Model
The following physical model is considered. On the underlying surface is a forest fuel layer, on which a particle falls, heated to high temperatures. The forest fuel layer is heated and thermally decomposed with the release of gaseous pyrolysis products (for example, CO), which diffuse into the air containing the oxidizing agent (oxygen) and inert components. In the gas phase, at certain temperatures and concentrations of the reacting gases, the oxidation reaction of gaseous combustible pyrolysis products proceeds and gas-phase ignition of the oxidizer-fuel mixture occurs. In a one-dimensional formulation, it is assumed that the gaseous pyrolysis products instantly appear above the particle, where they mix and ignite at a certain temperature and concentration of reagents. When considering the problem in two-dimensional and three-dimensional formulations, the pyrolysis products diffuse into the region of the gas phase on the sides of the particle, where they are mixed with an oxidizing agent. Ignition criteria: 1) the heat input from the chemical reaction exceeds the heat input from the heated particle; 2) the temperature in the gas mixture reaches a certain critical value.
The main assumptions adopted in the formulation of the problem: 1) A particle heated to high temperatures is modeled by a plate of finite thickness (one-dimensional formulation), a square (two-dimensional formulation), and a cube (three-dimensional formulation). 2) Carbon particles are considered (this option corresponds to the scenario of the occurrence of massive forest fires as a result of the transfer and release of charred hot twigs and forest fuel residues onto the forest fuel layer untouched by the fire) and steel particles are considered (this variant corresponds to the scenario of an anthropogenic fire source, for example, electric welding). 3) The gas mixture is taken as a three-component, containing an oxidizing agent (oxygen), combustible components (gaseous products of pyrolysis - carbon monoxide) and inert components (nitrogen, carbon dioxide). 4) It is believed that there is no moisture in the forest fuel (the assumption corresponds to a scenario of catastrophic fire danger quite typical for many territories).
Key Terms in this Chapter
Monitoring: Monitoring refers to the periodic calculation of the parameters of forest fire danger with a portion of information available in real time.
Lightning Activity: The production of a computer model of forest fire conditions and prerequisites, especially for the purpose of study.
Forest Fuel: It can be considered like dead and live forest fuel. Main types of forest fuel which can be involved in combustion during forest fire: ground forest fuel (needles, leaves and dry grass, small branches) and crown forest fuel (needles, small branches).
Forest Fire: Uncontrolled aerothermochemical phenomenon characterized by step-by-step mechanism which includes following stages: inert heating, moisture evaporation, high temperature terpens evaporation, dry organic matter pyrolysis, flammable combustion and smoldering.
Anthropogenic Load: Different human activities on forested territories lead to forest fire occurrence and characterized by presence of fire sources.
Ignition Delay: Time before flame flash after forest fuel heating.
Prediction: Under the prediction of forest fires is the calculation of the parameters of forest fire danger with a certain projection in advance in order to have enough time to anticipate an emergency. The calculation in this case is carried out in a mode ahead of the real time of the development of the catastrophe - the occurrence of a forest fire.
Ignition: Inflammation of forest fuel caused by definite source of high temperature or energy.