Limiting condition for auto-ignition of finite thick PMMA in forced convective airflow

Abstract

This contribution experimentally addresses the auto-ignition of finite thick PMMA (polymethyl methacrylate) subjected to parallel forced convective airflow. 1–15 mm thick samples, 30–60 kW/m2 heat fluxes (HF) and 0–1.2 m/s airflow velocities (u) were utilized to examine the auto-ignition mechanism and the limiting condition of non-ignition cases. Critical temperature, surface temperature evolution and ignition time were recorded. A modified numerical model, considering both PMMA and thermal insulation layers, and theoretical correlations developed for thermally intermediate solid were employed to analyze the experimental data. Results show that three regimes corresponding to different controlling mechanisms are identified: (1) solid-dominant regime, u ≤ 0.4 m/s, where auto-ignition is primarily determined by the heat transfer and pyrolysis in solid; (2) transitional regime, 0.4 m/s < u < 1.0 m/s, where both solid and gas phases have nonnegligible contributions; (3) gas-dominant regime, u ≥ 1.0 m/s, where gas phase chemical kinetics is responsible for the observed non-ignition phenomenon. Critical temperature is reliable only in the solid-dominant regime. In all the ignition scenarios, critical temperature increases with increasing sample thickness and u, but it barely changes with varying HFs. In the gas-dominant regime, surface temperature measurement suffers greatly from the generated large bubbles, worsening the agreement with numerical predictions. Based on the simulated in-depth temperature, estimated Biot number and thermal penetration depth, 1 mm PMMA is not thermally thin, whereas 15 mm sample is approximately thermally thick. Furthermore, thermal diffusivity of PMMA and the critical HF are estimated by fitting the measured ignition times.