The limiting oxygen volume fraction for opposed flame spread extinction

Abstract

The limiting oxygen volume fraction (LOC) for flame spread is a parameter used to determine flammability and fire hazard. The LOC depends on the solid material, geometry, and environmental conditions. It is important to understand the relationship between these conditions and the LOC, particularly for environments different than Earth atmospheres. Future spacecraft will have sub-atmospheric cabin pressures designed to reduce preparation time for extravehicular activities. This work attempts to provide further information on this aspect of solid fuel flammability by conducting experiments to determine the effect of ambient pressure and external radiant flux for flames spreading downward over cylindrical samples of black polymethyl methacrylate (PMMA). The experimental methodology is based on test methods ASTM 2863 (LOI) and ASTM E-1321 (LIFT), that are used to determine some aspects of material flammability, specifically extinction and flame spread under external radiant heating. Experiments are conducted in a pressure chamber at pressures from 40 to 100 kPa with radiant heating from 0 to 3.3 kW/m2. Results show that decreasing either the ambient pressure or the external radiant flux increases the LOC of the PMMA. The results are correlated in terms of the ratio of the partial pressure of oxygen, radiant flux, and ambient pressure with an equation that includes the LOC at atmospheric pressure. It is found that the heat transfer and chemical kinetic mechanisms that control the flame spread are primarily dependent on the oxygen molar fraction, explaining the experimental observations. The data from this work will be compared with experiments to be conducted in the International Space Station (ISS) under the SoFIE-MIST project to provide further understanding of the effect spacecraft environments have on the LOC of materials. The results will give further insight into the flammability of materials, particularly at sub-atmospheric ambient pressures found in spacecraft, aircraft, and high-altitude locations.