Effects of solid-phase properties on flames spreading over composite materials

The problem of downward flame spread over composite, thermally thick materials is theoretically investigated by means of a thermal, analytical model and a numerical model based on the reactive fully elliptic Navier-Stokes equations. The solid, made of a thermoplastic polymer and inert additives, undergoes in-depth endothermic pyrolysis, for the active part, with volatile monomer formation. The effective thermal conductivity of the composite material depends on the content of inert and the variable content of the active part, whose thermal conductivity varies between that of the polymer and that of the melted phase monomer. Even though surface regression is not taken into account, polymer consumption is modeled through a mass balance. The changes in the flame spread mechanisms with solid phase properties are investigated. The spread process is strongly affected by the solid perpendicular (to the spread direction) thermal conductivity which, when it decreases, causes a continuous increase in the spread rate. On the other hand, both numerical and analytical solutions give no dependence of the spread rate on the solid parallel (to the spread direction) thermal conductivity for a wide range of variation. In agreement with previous experimental results, at very large values of the latter, the finite-rate reaction model predicts a decrease in the spread rate. However, in contrast to certain experiments performed for non homogeneous composite samples, an increase of the spread rate with the effective solid parallel thermal conductivity or thermal capacity has not been found.