Numerical Modeling of Endothermic Pyrolysis and Ignition Delay of Composite Materials Exposed to an External Radiant Flux

Monday, October 11, 1999 - 9:30am - 10:30am
Keller 3-180
Carlos Fernandez-Pello (University of California, Berkeley)
Joint work with YingYing Zhou.

A numerical model is proposed to predict endothermic pyrolysis and piloted ignition delay of composite materials (formed by blending a polymer with loose fiberglass) that are exposed to an external radiant heat flux. Under fast chemical kinetics, the ignition of composite materials is determined primarily by the coupled physical and chemical processes in the condensed phase. In order to investigate these factors� effects on the ignition delay, pyrolysis, phase change (for thermoplastics) as well as heat and mass transfer in a two-phase media are considered in this analysis. A first- order of Arrhenius law is incorporated to model the polymer pyrolysis process and an enthalpy method is adopted to model phase change. Physical properties are taken as the summation of each constituent weighted by its transient volume or mass fraction. To model the effects of radiation penetration, radiation absorption in depth by the composite is incorporated. The coupled conservation equations of mass and energy for the two composite components are solved numerically to calculate temperature distribution, porosity distribution, pyrolysate mass flow rate and melting front location if phase change occurs. By using a pre-determined pyrolysate mass flow rate as a criterion for ignition, the ignition delay is computed under a wide range of external heat fluxes and compared to experimental data for PP/GL (Polypropylene/Fiberglass composite). The effects of the factors mentioned above on ignition delay for composite materials are also addressed theoretically.