Auto-ignition of thermally thick PMMA exposed to linearly decreasing thermal radiation

This contribution experimentally investigates the heat transfer and the consequent auto-ignition of thermally thick PMMA (polymethyl methacrylate) exposed to linearly declining heat flux (HF) coupling with a theoretically derived model. Thermal decomposition in solid was ignored and critical temperature was utilized. Both scenarios neglecting and considering surface heat loss were studied in the proposed model. An asymptotic solution, extending the derivation method from constant to time-declining heat flux, was obtained to explicitly estimate the surface temperature and ignition time. The credibility of the established model was validated through comparison with experimental data and a previously developed numerical model. The results show that in both considered scenarios ignition is confined in a region where the decreasing rate of heat flux is lower than a critical value. No ignition occurs if the decreasing rate increases beyond this region. The newly developed model is compatible with the correlations of constant heat flux, and it provides identical accuracy in predicting ignition time when compared with other existing models in the literature. A constant surface heat loss coefficient was used in the model, and it was found little error is introduced by invoking this approximation. Also, a parametric study was implemented, indicating that the critical temperature affects the ignition time prediction significantly. Meanwhile, the effects of surface heat loss on surface temperature and ignition time were quantitatively evaluated.