Numerical analysis of piloted ignition of polymeric materials

A numerical model is developed to analyze piloted ignition of polymeric materials exposed to an external radiant heat flux in a convective oxidizer flow. The model considers the coupled thermo-chemical processes that take place both in the condensed and gas phases. Condensed phase processes include oxidative and thermal pyrolysis, phase change, and in-depth radiation absorption. Heat and mass transport in the gas phase are described by the reactive boundary layer conservation equations. Ignition is determined by the onset of thermal runaway in the gas phase. PMMA is used as the testing material, and ignition delay are predicted and compared with experimental data for different external heat fluxes and flow velocities. The numerical model is used to explore the ignition controlling mechanism and the critical conditions at ignition under various external heat fluxes and different airflow velocities. It is also used to provide an ignition criterion that could simplify the theoretical description of the ignition process. It is found that for a given flow velocity the pyrolysate mass flux at ignition on the sample surface is independent of the external heat flux, whereas the sample surface temperature at ignition varies with the external heat flux. Therefore, if the oxidizer flow velocity is a fixed parameter, it is possible to adopt a critical pyrolysate mass flux as an accurate ignition criterion. The use of this criterion allows the use of a simplified model to predict piloted ignition that solely considers the condensed phase. (C) 2002 by The Combustion Institute.