Effect of radiation absorption modes on ignition time of translucent polymers subjected to time-dependent heat flux

Majority of previous solid ignition models, including numerical and analytical ones, considered only surface absorption of incident heat flux for simplification. However, the influence of in-depth absorption on pyrolysis and subsequent ignition cannot be ignored for infrared translucent polymers. This work addresses this problem and focuses on time-dependent heat flux to establish an analytical model for ignition behaviors prediction by means of theoretical analysis. Ignition temperature was utilized as the ignition criterion, and both surface and in-depth absorption scenarios were considered. Thermally thick polymethyl methacrylate and polyamide 6 were selected as reference materials to verify the reliability and applicability of the proposed model by comparing the analysis results with experimental data as well as numerical simulations. A method for determining the approximation parameters of the theoretical analysis was presented to derive the relationship between ignition time and the coefficients in heat flux expressions. The results show that the higher surface temperature owing to surface absorption accelerates the pyrolysis rate and results in a shorter ignition time, while in-depth absorption affects the ignition time inversely. The effect of surface heat loss was also evaluated quantitatively through both analytical and numerical models. The uncertainty of the proposed model is mainly caused by the selection of the approximation parameters. Nevertheless, it provides an alternative approach to estimate the ignition time of translucent polymers besides numerical simulation.