Physical Modeling of Forest Fuel Ignition by the Heated Up to High Temperatures Particle

Introduction

It is possible to ignite forest fuels with a single particle heated to high temperatures. Real particles, as a rule, have the shape of irregular polyhedrons and are in a solid state during deposition. In order to predict the forest fire danger, it is impossible to take into account the real configuration of particles falling onto the surface of a fuel. For experiments, particles similar to parallelepipeds and cylinders were selected.

The results of experimental and theoretical studies of the ignition of natural fuels by particles heated to high temperatures have been published. It is known (Hadden et al., 2011) that burning particles (firebrands, coals) are the main mechanism for the propagation of forest fires and fires passing to the village (WUI Fires - Wildland Urban Interface Fires). Often, such sources of local heating are formed during the burning of forest fuels and are transferred by the wind or as a result of the plume (convective column) from the fire to the forest fuel layer untouched by the fire and lead to its ignition. For example, an analysis of the 1994 fire statistics in Sydney showed that 75% of houses caught fire as a result of heated particles and only 25% caught fire from particles and radiation from a flame (Ramsey and McArthur, 1995). In addition, molten or burning particles can be formed as a result of collision of power line wires at high wind speeds (Hadden et al., 2011). The processes of forest fuel ignition by heated particles have not been sufficiently studied, with the exception of some field studies (Stokes, 1990; Rowntree and Stokes, 1994; Manzello et al., 2006a,b; Pitts, 2007; Manzello et al., 2008; Caine et al., 2009). In (Hadden et al., 2011), the results of an experimental and theoretical study of the forest fuel ignition by heated particles are presented. Steel spheres that were inert to chemical interaction were considered as a source of ignition. At the same time, cellulose in the form of a powder was used as a fuel, since it formed homogeneous layers with known thermophysical properties. The following range of sizes of hot particles is known, which are formed as a result of burning vegetation or the interaction of power transmission line wires: fragments with characteristic sizes of 200 to 10 mm in diameter are formed from Douglas spruce wood (Manzello et al., 2008a,b; Yoshioka et al., 2004). The objectives of the research (Hadden et al., 2011) was to evaluate the ignition conditions; smoldering; decay, turning into a flame; lack of ignition. These conditions depend on the size and state of the particle (temperature, heat capacity, smoldering or burning in a flame mode), as well as on the properties of the fuel. It was established (Manzello et al., 2008a) that when using single hot fragments from Douglas spruce (5 mm and 10 mm in diameter, 51 mm and 76 mm in length, respectively) at a blown stream speed of 0.5 m / s or 1 m / s, paper scraps ignited in smoldering mode, but did not occur in pine needles and hardwood. For burning particles, ignition was recorded in all experiments, with the exception of “mulch” of deciduous trees. In (Caine et al., 2009), hot springs obtained as a result of electric heating of a spiral were used. No explicit relationship was found between the diameter of the heating source and the temperature necessary for ignition. As a result of theoretical studies, the Joule heat release from the spiral was evaluated as an ignition criterion by analogy with the minimum ignition energy for the concept of gas ignition (Stokes, 1990; Rowntree and Stokes, 1994). However, this approach was evaluated critically by the authors of (Babrauskas, 2003), since particles of different sizes with the same energy did not always initiate ignition. In (Babrauskas, 2003), it was concluded that the ratios of its size and initial temperature are important for ignition by a particle. The first works on ignition of condensed matter were published in the 60-70s of the last century (Goldschleger et al., 1973; Zinn, 1962; Boddington, 1963; Friedman, 1963; Thomas, 1965) and later applied to the study of the ignition of natural fuels (Jones, 1993; 1994; 1995). Later, a mathematical simulation of ignition was carried out when a fuel was heated by sources of constant temperature and large sizes (Zvyagilskaya and Subbotin, 1996; Grishin et al., 1998; Lautenberger and Fernandez-Pello, 2008).

The purpose of this study is a comparative analysis of the results of experimental studies of the ignition of forest fuels by a metal or non-metal particle heated to high temperatures.