The Effect of Normoxic Atmospheres on Upward Flame Spread Over a Thin Fabric

The influence of environmental conditions on the flammability of combustible solids is of importance to spacecraft fire safety. In a manned spacecraft, the environment is maintained at a normoxic condition, which is the combination of ambient pressure and oxygen concentration that results in a partial pressure of oxygen equal to that of normal atmosphere at sea level. Future spacecraft will have atmospheres with reduced pressures and increased oxygen concentrations at normoxic conditions (Space Exploration Atmospheres–SEA), designed to reduce preparation time for extravehicular activities. This paper presents experimental results on upward spread of flames over a flat thin cotton fabric under normoxic conditions. Experiments are conducted with ambient pressures ranging between 100 and 60 kPa and oxygen concentrations between 21% and 35% by volume. Additional experiments are carried out with a fixed oxygen concentration of 21% (ambient air) and a pressure ranging between 100 and 30 kPa and varying oxygen concentration between 17% and 30% at a constant pressure (100 kPa) to isolate the roles of pressure and oxygen variations. Results show that moving to normoxic environments with reduced pressure and increased oxygen concentration increases the flammability of the fabric and the flame spread rate. Normoxic conditions also showed an increase in distance between the outer flame and the fuel bed at lower pressures. During the experiments, O₂, CO, and CO₂ were measured in the combustion products, and were used to calculate the heat release rate at different pressures and in normoxic conditions. The results show that as the cabin environment transitions to normoxic conditions with higher O₂ concentrations, the heat released by the flame also increases. The data presented here provide information about the flammability of spacecraft materials in the future SEA in support of NASA research, yielding insight for future designs when considering fire safety in spacecrafts.