Material flammability in space exploration atmospheres.

摘要

In order to reduce the risk of decompression sickness associated with extravehicular activity, NASA is designing the next generation of exploration vehicles with a different cabin pressure and oxygen concentration than used previously. This work explores how the flammability of solid materials changes in this new environment. One method to evaluate material flammability is by its ease of ignition. To this end, piloted ignition delay tests were conducted in a small-scale wind tunnel subject to this new space exploration atmosphere (SEA -- 58.6 kPa and 32% oxygen) and compared to similar tests in standard atmospheric conditions. In these tests, polymethylmethacylate (PMMA) was exposed to a range of oxidizer flow velocities and externally applied heat fluxes. It was found that the ignition time was reduced by 27% in the intended space exploration atmosphere. It was also noted that the critical heat flux for ignition decreases in exploration atmospheres. These results show that materials are more susceptible to ignition than in current spacecraft atmospheres.;To further explore the effect of pressure and oxygen concentration, tests were performed for a wide range of pressures and oxygen concentrations. In all oxygen concentrations tested, the ignition delay time was seen to decrease with pressure, reach a minimum, and then increase with further reduction in pressure creating a classic u-shaped curve. No ignition was seen at sufficiently low pressures. The no ignition pressure depended on the oxygen concentration. Increasing the oxygen concentration uniformly decreases the ignition time; however, no significant differences were seen in oxygen concentrations above 24%. These results indicate there are several competing mechanisms controlling the ignition time. By reducing the pressure, the heat transfer coefficient and the mass flow rate of fuel to reach the lean flammability limit are reduced. Conversely, a reduction in pressure increases the gas-phase chemical induction time. The competition between these three mechanisms is responsible for the u-shaped dependence of ignition time on total pressure. In addition to gaining insight into the effect of pressure on piloted ignition, these results have practical applications including high altitude structures and airplane cabins.