Radiative properties of plastics fires

Abstract

A variant of the Schmidt Method, in which a narrow-view-angle radiometer looks at a flame against a blackbody background of known temperature, has been used for measuring radiative properties of plastics pool fires or fires spreading over horizontal fuel slabs. Measurements performed at several background temperatures, including cold background (radiance measurement), are interpreted by two alternative models. A gray-emitter homogeneous model yields an effective radiation temperature and emissivity. Alternatively, the assumption of a spectral absorption coefficient inversely proportional to wavelength, ascribing the absorption exclusively to soot, yields radiation temperature and the product of equivalent soot volume fraction and optical path length. The respective gray absorption coefficients and equivalent soot volume fractions were determined from path lengths measured as averages from flame photographs. Data for pool fires of polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polyoxymethylene (POM) and fires spreading over polyurethane (PU) foam are presented. At a height of 51 mm above the fuel surface, radiation temperatures ranged from 1190 K (PS) to 1408 K (PU foam), emissivities of 0.3×0.3 m fires from 0.05 (POM) to 0.81 (PS), absorption coefficients (not determined for POM) ranged from 1.3 m −1 (PMMA, PU foam) to 5.3 m −1 (PS) and equivalent soot volume fractions from 0.46×10 −6 (PU foam) to 2.8×10 −6 (PS). Finally, the correlation between emissivities and burning rates, which varied from 6.5 g/m 2 s (POM) to 14.5g/m 2 s (PS), strongly suggested that for sufficiently large fires burning rate is controlled primarily by radiative transfer. Data for a 0.73 m dia PMMA pool fire are also presented.