Spatial forms of a combustion wave in energetic heterogeneous systems

The effect of the content of combustible particles on the spatial forms of the combustion wave in a gasless thermite-inert material model system is studied. It is shown that the lower flammability concentration limit for this mixture is related to the formation of a three-dimensional binding percolation cluster or to a percolation phase transition. Near the lower concentration limit, the combustion of model mixtures enters a smoldering regime in which separate combustion sources move randomly in the volume of the unburned material. If the initial composition contains more than 47% thermite particles, a motionless heated surface forms, along which separate incandescent combustion sources move. The motion of this surface in the unburned material occurs only locally as a result of motion of the combustion sources on it. The formation of this unified heated surface corresponds to the occurrence of a second percolation phase transition. When the volumetric content of thermite particles in the initial mixture is higher than 70%, separate combustion sources merge into a unified surface of layer-by-layer combustion due to the occurrence of a third percolation phase transition.