Computational analysis of pyrolysis and ignition of polymeric materials

A numerical analysis has been developed to predict the pyrolysis and piloted ignition of polymeric materials exposed to an external radiant heat flux and a forced airflow parallel to their surface. This study provides a theoretical basis for determining the fire hazard characteristics of combustible materials that could be used in space-based facilities. The numerical analysis considers the coupled thermochemical processes that take place in both the condensed phase and the gas phase, and has been applied here to Polymethyl methacrylate (PMMA) and a Polypropylene/Fiberglass (PP/GL) composite. A temperature-enthalpy hybrid method developed in this study has been used to explore the interactions between the melting, oxygen diffusion and oxidative pyrolysis. Using a boundary layer analysis in the gas phase, the controlling mechanisms of ignition and the critical conditions at ignition have been examined. The effects of the airflow velocity, the sample's composition and thickness on ignition have been investigated.; Numerical results have shown that a critical fuel mass fraction at the pilot location is needed for ignition to occur. This critical fuel mass fraction appears to be independent of the external heat flux and the airflow velocity. Results have also shown that the pyrolysate mass flux on the sample surface at the instant of ignition is independent of the external heat flux for a given airflow velocity. The sample surface temperature at ignition appears to increase with increasing external heat flux and with increasing airflow velocity. Thus at a given airflow velocity, the critical pyrolysate mass flux can be adopted as an ignition criterion to reduce the computing effort by eliminating the necessity of solving the gas phase equations. Using this criterion, the ignition delay times and the critical heat fluxes in microgravity, where airflows can be reduced significantly below the natural-convection-limited values, have been predicted to be close to half of the values in normal gravity. This indicates that materials may ignite more easily under the conditions expected in space-based facilities. Also, the ignition delay and the critical heat flux for ignition have been shown to increase with increasing fiberglass content in the PP/GL composite materials. For a given external heat flux, there is a critical volume fraction of fiberglass beyond which the composite material does not ignite. These observations have potential fire safety implications for composite materials and could provide guidelines for the development of fire-safe composites.