The Influence of Wind and Forest Combustible Material Properties on Forest Fires Spread

This paper gives a new mathematical setting and method of numerical solution of a problem of crown forest fires initiation and spread. Mathematical model of forest fire was based on an analysis of experimental data and using concept and methods of reactive media mechanics Natural investigations of these problems are merely impossible and methods of mathematical modeling are urgent. Great interest of the problem concerned is explained by the influence of meteorological conditions (temperature, wind velocity, moisture and etc.) and properties of forest combustible materials on the propagation of crown forest fires fronts. The forest and combustion products are considered as a homogeneous two temperatures, reacting, non - deformed medium (gas – dispersed and solid phase). The first medium includes a dry organic substance, water in the liquid-drop state, condensed products of pyrolysis and combustion products (coke, ash) and mineral part of forest fuels. The gaseous phase consists of the components necessary to describe reactions of combustion (oxygen, gas products of pyrolysis and the inert components). It is considered that 1) the flow has a developed turbulent nature, molecular transfer being neglected, 2) gaseous phase density doesn't depend on the pressure because of the low velocities of the flow in comparison with the velocity of the sound, 3) forest canopy is supposed to be non-deformed porous medium. The research is based on numerical solution of Reynolds equations and concentration equations for of chemical components and equations of energy conservation for gaseous and condensed phases. The transfer of energy by radiation is described using diffusion approximation. To obtain discrete analogies a method of finite volume is used. The boundary-value problem is solved numerically using the method of splitting according to physical processes. As a result of mathematical modeling the fields of temperatures, mass concentrations of components of gaseous phase, volume fractions of components of solid phase, as well as vectorial fields of velocity at different instants of time. It allows to investigate dynamics of forest fire initiation and spread under influence of various external conditions: a) meteorology conditions (air temperature, wind velocity etc.), b) type (various kinds of forest combustible materials) and their state(load, moisture etc.). In this paper attention is given to questions how the shapes of contours and the rate of forest fires propagation are dependent on these different conditions.