Temperature measurements in PMMA during downward flame spread in air using holographic interferometry

Abstract

Temperature distributions within a thermally thick cast slab of poly(methylmethacrylate) (PMMA) during downward flame spread in air were determined by measurements of changes in refractive index of PMMA using a holographic interferometry technique. The surface temperature history was measured with a fine thermocouple as a reference for the interferogram over the glass transition region. Samples with widths of 0.32, 0.47 and 1.0 cm and thicknesses of 1.5 and 2.5 cm were used. The experimental configuration was arranged to make the phenomena two-dimensional in the direction of the laser beam. Detailed temperature and heat flux profiles, especially in the area of the immediate vicinity of the flame foot, were obtained. The results indicate that the surface temperature increases gradually from about 1 cm ahead of the vaporization front and reaches the glass transition temperature about 0.2 cm ahead of the vaporization front. Then, it sharply increases to the vaporization temperature, about 380°C. The visible flame front extends about 0.1 cm ahead of the vaporization front. The measured net heat flux from the gas phase to the sample surface increases sharply from about 1 W/cm 2 to 7 W/cm 2 within a distance of about 0.1 cm from the vaporization front. The net heat transfer rate from the gas phase to the sample is about 57% of the total heat transfer rate to the sample. It is concluded that this net heat transfer from the gas phase plays the dominant heat transfer path to heat the thick PMMA slab instead of conductive heat transfer through the condensed phase, as was previously believed with some question.