Experimental observations on the steady-state burning rate of a vertically oriented PMMA slab

Abstract

A series of experiments is performed to determine the local burning rate of vertically oriented burning slabs of PMMA (poly(methyl methacrylate)) under steady-state conditions. The burning rate is essential for the determination of flame temperatures, flame lengths, and burnout, all critical parameters to establish fire growth. The experimental procedure consists of a postprocessing of digitized images obtained with three CCD cameras placed on the sides of the PMMA slabs. The cameras are used to establish the displacement of the regressing surface as a function of time. The PMMA/gas interface is characterized by a bubbling layer that scatters light, making the bubbles visible. The video images are processed by establishing a gray-level intensity threshold that defines the depth of the bubbling layer. The local burning rate is then computed from the time derivative of the surface regression. Local burning rates for PMMA slabs, 30 cm high, 3 cm thick, and 2.5, 5, 10, 15, and 20 cm in width are obtained. Three-dimensional effects appear for 2.5-cm-wide slabs. For wider slabs, the local burning rate along the centre line is found not to be width-dependent and to be in good agreement with literature data. Results show that the pyrolyzing surface divides vertically into four regions. In the lowest region ( 0 < x < 4 cm ), the local burning rate increases with the distance from the leading edge, x. For 4 < x < 10 cm , the local burning rate follows a power-law decay. For slabs wider than 2.5 cm, the exponent approximates −0.35. This is higher than that predicted by the two-dimensional laminar boundary layer theory, i.e., −0.25. For 2.5-cm-wide slabs, the local burning rate decreases faster, with a power law dependency of −0.8. From x = 10 to 18 cm, the local burning rate is approximately constant, which is believed to correspond to the laminar/turbulent transition region. In the upper region, the flow becomes fully turbulent and the burning rate increases weakly with x.