Heat transfer components at the surface of burning thick PMMA slabs
Résumé
Mass pyrolysis rate is the key parameter to predict fire behavior. It is generally deduced from the energy balance at the surface of the solid material. However, due to lack of knowledge, existing pyrolysis models use simplifying assumptions neglecting all or part of in-depth losses into the solid material or the net radiation at its surface. In order to improve the accuracy of pyrolysis models, experiments are conducted to quantitatively evaluate the heat transfer components at the surface of burning thick clear poly-methyl-methacrylate (PMMA) slabs at steady state. The contributions of each transfer mode including radiation and convection from the flame, surface re-radiation, and in-depth losses, to total heat flux are determined from two series of experiments. Pure pyrolysis (non-flaming) cone calorimeter experiments are first carried out to evaluate in-depth losses in horizontally-oriented slabs exposed to an incident heat flux below that of ignition. A specific procedure based on video processing is used to track the position of the PM MA regressing surface with time. The second series of experiments consist in burning vertically-oriented slabs from 2.5 cm to 20 cm in height, 10 cm in width and 3 cm in thickness. It is found that only a small part of flame radiation is transmitted through the virgin solid, most in-depth radiation being absorbed by the bubble surface, which in turn strongly emits radiation inward. An excellent agreement is obtained between the local mass loss rate deduced from the energy balance and literature data.