Abstract
The mechanisms by which flames spread over flat surfaces of polymethylmethacrylate (PMMA) have been studied. Spread rates of vertically downward and hrizontally spreading flames were measured, and the gas velocity and temperature profiles near the spreading flames were examined using particle tracer techniques, fine wire thermocouples, and a Mach-Zender interferometer.
For PMMA sheets thinner than 0.2 cm, the spread rate was inversely proportional to the thickness, and for those thicker than 2.0 cm, it was independent of the thickness. The velocity profiles in front of the preheat zone resembled those across the boundary layer near a uniform temperature wall, while those in the preheat zone and pyrolysis zone resembled those across the boundary layer with a diffusion flame.
The temperature profiles indicated that the heat flux to the surface increased with the decrease of the distance from the pyrolysis front.
A brief theoretical analysis based on the experimental results indicated that when a buring sheet was thin, the heat flux through the gas phase into the solid phase in the preheat zone was much larger than that into the solid phase in the pyrolysis zone, while when a buring sheet was thick, this relation between the heat fluxes became reverse. If the former and latter cases can be classified as thermally thin and thick sheet burings, respectively, a PMMA sheet can be considered to be thermally thin or thick when its thickness is smaller than 0.2 cm or larger than 2.0 cm, respectively.
