EFFECTS OF SOLID-PHASE PROPERTIES ON FLAMES SPREADING OVER COMPOSITE-MATERIALS

The problem of downward flame spread over composite, thermally thick m aterials is theoretically investigated by means of a thermal, analytic al model and a numerical model based on the reactive fully elliptic Na vier-Stokes equations. The solid, made of a thermoplastic polymer and inert additives, undergoes in-depth endothermic pyrolysis, for the act ive part, with volatile monomer formation. The effective thermal condu ctivity of the composite material depends on the content of inert and the variable content of the active part, whose thermal conductivity va ries between that of the polymer and that of the melted phase monomer. Even though surface regression is not taken into account, polymer con sumption is modeled through a mass balance. The changes in the flame s pread mechanisms with solid phase properties are investigated. The spr ead process is strongly affected by the solid perpendicular (to the sp read direction) thermal conductivity which, when it decreases, causes a continuous increase in the spread rate. On the other hand, both nume rical and analytical solutions give no dependence of the spread rate o n the solid parallel (to the spread direction) thermal conductivity fo r a wide range of variation. In agreement with previous experimental r esults, at very large values of the latter, the finite-rate reaction m odel predicts a decrease in the spread rate. However, in contrast to c ertain 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.