Thermal Decomposition and Auto-ignition of Finite Thick PMMA in Forced Convective Airflow

An experimental apparatus consisting of a heating unit and a wind duct capable of flexibly adjusting radiation power and forced airflow velocity was used in this work to examine the heat transfer and thermal decomposition in condensed phase, mass diffusion of pyrolyzate in boundary layer in gas and the consequent ignition behaviors of PMMA (polymethyl methacrylate) in forced airflow condition. Constant heat flux (HF) was employed and spontaneous ignition was studied. Finite thick, 3, 6 and 10 mm, 5 cm squared samples and six sets of airflow velocities 0 to 1.2 m/s were selected in the tests. Surface temperature and ignition time under the designed conditions were collected and compared with corresponding numerical simulation results, performed by ANSYS fluid dynamics simulator, which consider thermal decomposition in solid and thermal insulation layer. The results shown that the ignition temperature of PMMA is positively correlated with increasing airflow velocity, indicating the critical temperature is not a reliable ignition criterion in these scenarios. The airflow velocity has little effect on surface temperature. For airflow velocities larger than 0.4 m/s, the ignition time increases significantly with the increase of airflow velocity and sample thickness. While for 0.4 m/s airflow velocity, the ignition temperature is lowered and the ignition time is shortened.