Limiting Oxygen Concentrations of Burning PMMA Cylinders under External Radiant Heating and Subatmospheric Pressure

This work considers the effect of ambient pressure and an external radiant flux on the limiting oxygen concentration (LOC) for flames spreading downward over the surface of cylindrical samples of black polymethyl methacrylate (PMMA). The objective is to determine the effect of an external radiant flux on the LOC of combustible solid materials in environments expected in future spacecraft cabins. The experimental apparatus and testing methodology is a combination of the LOI and LIFT test apparatuses. The radiant heating ranges from 0 to 3.3 kW/m^2 and the ambient pressures tested from 100 kPa to 40 kPa. An upward forced flow of a mixture of oxygen and nitrogen with a velocity of 10 cm/s is used to determine the LOC of the PMMA as a function of ambient pressure and external heat flux. Results show that increasing the ambient pressure or external radiant flux increases the flame spread rate and decreases the LOC of the PMMA. Correlating the LOC data in terms of the partial pressure of oxygen and the ambient pressure shows that the LOC occurs at an approximately constant oxygen mole fraction that depends weakly on the ambient pressure and radiant flux. The combustion mechanisms leading to this result are discussed based on simple equations and phenomenological arguments. The data from this work will be compared with data from experiments to be conducted in the International Space Station (ISS) under the SoFIE-MIST project, to provide further understanding of the effect spacecraft environments on the LOC of materials. The results will give further insight into the flammability of materials, particularly at subatmospheric ambient pressures, such as in spacecraft, aircraft, and high-altitude locations.