Influence of moisture content and heat and mass exchange with the surrounding medium on the critical conditions of initiation of surface fire

Ignition of a layer of combustible forest materials (CFM) by a cylindrical heat source is considered. Critical values of the radius of a brand at which the brand can cause a surface fire are determined on the basis of numerical calculations. Dependences of the critical radius on moisture content and on conditions of heat and mass exchange between the CFM layer and the surrounding medium are determined. An assessment is made of how neglect of both the internal structure of the brand and the processes occurring in it influence the critical size. It is found that initially heterogeneous ignition of the brand occurs, and then it is followed by ignition of litter.