Composite auto-ignition criterion for PMMA (Poly methyl methacrylate) exposed to linearly declining thermal radiation

This contribution experimentally studies auto-ignition of thermally thick PMMA (Poly methyl methacrylate) subjected to linearly declining heat flux (HF) radiated from a cone heater. Thermogravimetric tests (TGA) were conducted to derive the kinetics of PMMA by model fitting method. A numerical model dealing with heat transfer and pyrolysis is utilized to simulate the experimental measurements. Results show that auto- and no-ignition regions are separated by a critical decreasing rate of HF which is quantitively determined by numerical results. Measured surface temperatures are compared with the analytically and numerically estimated ones, and it is found both ignition time and critical decreasing rate are sensitive to critical temperature. These senilities are also confirmed by numerical model through parametric study. In no-ignition region near the critical condition, the maximum mass flux is higher than the measured critical value, 4.26 g/m²s, indicating critical mass flux is not a credible ignition criterion. Measured critical temperature increases with higher initial HF and yields an average value of 641 K. A composite auto-ignition criterion combining both critical mass flux and critical temperature is found valid in estimating ignition time. Meanwhile, the different affecting mechanisms of decreasing HF on auto-ignition between this work and literature is reasonably interpreted.