Numerical analysis of composition effects on the ignition delay of polymeric composites

Abstract

A numerical approach based on the enthalpy method is applied to predict endothermic pyrolysis and piloted ignition delay of thermoplastics and polymeric composites exposed to an external radiant heat flux. The model considers the coupled thermo-chemical processes that take place in the condensed phase, and therefore describes the piloted ignition of solid combustible materials only under conditions of fast chemical kinetics in gas phase. The condensed phase processes considered in the model include oxidative and thermal pyrolysis, phase change, heat and mass transfer in a multi-phase and multi-composition medium, and in-depth radiation absorption. Ignition is considered to occur when a critical pyrolysate mass flow is reached at the composite surface. The implication is that the ignition of the material is subjected to the attainment of a minimum concentration of fuel in the gaseous mixture adjacent to the pilot (lean flammability limit). The ignition delay is predicted over a wide range of external heat fluxes for low-density polyethylene, low-density polyethylene/fiberglass and polypropylenelfiberglass composites with varied composition. The results agree well with the available experimental data, and show that the ignition delay and critical external heat flux for ignition depend on the composition of the material, increasing as the fiber content is increased. From the results it is possible to identify the fiberglass content required to prevent ignition at a given radiant flux, a result that could provide guidelines toward the development of fire safe composites.