Flame spread over horizontal surfaces of polymethylmethacrylate

Abstract

The U.S. Bureau of Mines conducted a large-scale study to determine the horizontal flame spread rates over the top surface of polymethylmethacrylate (PMMA) sheets. This study is part of the Bureau's program to gain a better understanding of the fire propagation of combustible materials in ventilated mine entries. The experiments were performed in a fire gallery at airflows of 0.80, 1.4, and 3.8 m s −1 with samples 9.75 m long by 1.07 m wide by 6.35 mm thick. The PMMA sheets were placed on top of 12.5-mm-thick calcium silicate boards and the upstream edge ignited with a 1 liter heptane tray fire. Pyrolysis front spread rates were calculated from temperature measurements obtained from thermocouples mounted with their beads about 1 mm above the PMMA surface and spaced along the centerline. Gas temperatures and average downstream concentrations of CO, CO 2 , and O 2 were also measured and peak heat release rates estimated. The experimentally measured pyrolysis front rates of 6±2 cm s −1 were essentially independent of airflow, and are an order of magnitude higher than those of large-scale experiments reported by other investigators whose sample lengths were too limited. These largescale spread rates are also about three orders of magnitude greater than those measured in small-scale studies where laminar flow and diffusion control the pyrolysis front rate. The rapid pyrolysis front rates reported here give good agreement with theoretical predictions based on a radiation dominated heat transfer mechanism from an optically thick flame plume, a realistic ignition temperature and physical properties for the PMMA solid.