The relative position of pyrolysis onset and flame front location for downward flame spread

Abstract

The interaction of solid and gas phase phenomena in downward flame spread is studied under buoyant conditions. Phosphor thermometry (PT) was used to determine the spatiotemporally resolved surface temperatures both ahead and beneath the flame while CH* chemiluminescence allowed for the quantification of the flame location relative to the fuel surface. The combination of these measurements allowed for the superposition of the flame front location and the surface temperature distribution. The standoff distance (∼1.1 mm) and preheated distance along the surface (∼3.5 mm) were quantified. The application of PT allowed for the continued measurement of surface temperatures in the pyrolyzing region beneath the flame, where the surface temperature was found to remain relatively constant between 300 and 350 °C. The combination of measurements allowed for the calculation of the gas-phase heat flux distribution ahead of the flame front using a point-source approximation, which compared well with previous studies. PT and CH* measurements were used to expand previous work to quantify the thermal gradients through the solid both ahead of and behind the flame front. Surface temperature measurements were coupled with thermogravimetric analysis (TGA) and a simple pyrolysis model to determine that the onset of pyrolysis occurs approximately 1 mm ahead of the flame front. This approach was also used to illustrate that pyrolysis gases were generated in sufficient volume to result in a flammable mixture at the leading edge of the flame. This study highlights the benefit of optical diagnostic techniques in exploring the controlling mechanisms of flame spread, and provides insight into the interaction of solid and gas-phase phenomena in downward flame spread.