Physical Modeling of Forest Fuel Ignition by the Molten Metal Particles

Introduction

The Russian Federation has significant territories covered by boreal forests (Baranovskiy and Kuznetsov, 2017)]. In such forests, a proportion of pine and birch is noticeable. It should be noted that the areas of pine and birch include almost the entire territory of the Russian Federation (Devisilov, 2010). This allows us to consider Siberian forests as the most typical territories, on the example of which issues of forest fire danger can be considered. Frequent fires of recent years in the forests of the Siberian region have led to a significant destruction of the forest fund. The occurrence of fires in the forest is due to a wide range of natural and man-made causes (Baranovskiy and Kuznetsov, 2017). It must be said that the anthropogenic load on a number of forested areas is growing in connection with their economic development. The probability of a forest fire is affected by the type of source of elevated temperature and the specific ignition mechanism that is realized when the forest fuel is ignited (Babrauskas, 2003).

The literature presents data on experimental and theoretical studies of the ignition of fuels by particles heated to high temperatures (Hadden et al., 2011). Such particles can be formed both as a result of destruction of wood of trees or morphological parts of plants (Tohidi et al., 2015; Manzello et al., 2006), and as a result of destruction of building wood-glued materials in urban areas (Suzuki and Manzello, 2016). Moreover, they can have different geometry and shape. It is known that burning particles (firebrands, coals) are the main mechanism for the spread of forest fires and fires passing to the village (Manzello et al., 2006). In the case of the natural mechanism of particle formation, they undergo the following path: a) the formation and separation of the heated particle from a tree or plant; b) the rise (vertical transfer) of a particle due to the action of a convective column over a fire; c) horizontal transport by wind; d) sedimentation of a particle on a layer of forest fuel and the occurrence of a fire. For particles of natural origin, a number of models have been developed that describe their trajectory in the surface layer of the atmosphere (Matvienko et al., 2016; Kortas et al., 2009; Wadhwani et al., 2017). In addition, a number of properties of such particles (Zakharevich and Zygin, 2016) and the occurrence of fires when they are exposed to a layer of forest fuel (Baranovskiy and Zakharevich, 2016) were studied. For an experimental study of such particles, a generator has been developed that allows physical modeling of these processes (Manzello et al., 2008).

In addition, molten or burning particles can form as a result of collision of wires of high-voltage power lines with large winds (Hadden et al., 2011; Fernandez-Pello, 2017; Pleasance and Hart, 1977). It should be noted that the processes of ignition of forest fuels by such heated particles were studied only in the course of single field studies (Stokes, 1990; Rowntree and Stokes, 1994; Manzello et al., 2006b).

One of the stages of ignition of fuels is their thermal decomposition (pyrolysis). It should be noted that the problem of gasification of solids is widely described in the literature, for example, in the review (Di Blasi, 1993). The simplest approach to this problem implies that a solid decomposes with the release of volatile compounds directly at a critical temperature (often denoted by Tp). Accordingly, the critical temperature is a parameter of the problem. This approach (often called the ablation model) is mathematically similar to the Stefan problem (Carlslaw and Jaeger, 1984) and its variation with respect to the melting process is described in the classical Landau article and later implemented in a number of works. The second approach includes a kinetic mechanism for the decomposition process, which is usually established as a result of thermogravimetric analysis (Liu and Wang, 2018).

Thus, the analysis of scientific periodicals shows that the formation, transport, and the effect of wood particles or building materials on forest fuels have been studied quite well. However, only a few works have been published on the ignition of forest fuel by crystallizing metal particles, which are formed as a result of the collision of the wires of power lines (Stokes, 1990; Rowntree and Stokes, 1994; Manzello et al., 2006b). The present work is devoted to the development of an experimental setup and the physical modeling of the ignition of forest fuel by crystallized metal particles that are formed as a result of electric welding in urbanized or forested areas.