On the Piloted Ignition of Solid Fuels in Spacecraft Environments

The effect of environmental variables on the ignition of solid combustible materials is explored through a combination of experimental, analytical and numerical analyses. This research stems from NASA's design requirement to reduce the cabin internal pressure and increase the oxygen concentration in human space vehicles and in future lunar habitats of the Constellation Program. These new environmental conditions may result in an increased fire risk of combustible solid materials due to higher flame temperatures (attributed to enhanced oxygen) and reduced convective heat losses from heated surfaces (attributed to reduced pressure). In particular, the influence of low pressure on ignition is emphasized here because little is known concerning this topic. A series of experiments conducted in a laboratory-scale combustion wind tunnel with externally irradiated samples of PMMA (polymethyl-methacrylate) showed that both the ignition delay time and the fuel mass flux at ignition decrease when the ambient pressure is lowered. An analytical model is used to identify the governing processes that lead to these results and then a numerical model is applied to quantify the influence of ambient variables (particularly pressure) on the piloted ignition of PMMA. The numerical model verifies the phenomenological explanations inferred from the experimental findings and the qualitative analytical results, and correctly simulates the thermo-physical mechanisms leading to ignition. It is concluded that reduced pressure environments result in: 1) smaller convective heat losses from the heated material to the surroundings due to a thickening of the thermal boundary layer next to the solid fuel surface, allowing for the material to heat more rapidly and pyrolyze faster; and, 2) a lower mass flux of volatiles required to reach the lean flammability limit of the gases at the pilot, leading to earlier ignition, due mainly to an enlarged boundary layer and a thicker fuel species profile under reduced pressures. The findings from this research indicate that the flammability of combustible materials is enhanced at low ambient pressures and elevated oxygen concentrations, and may have significant consequences in the assessment of their fire risk in spacecraft and other environments where these conditions are encountered such as aircrafts and high altitude cities, among others