Concurrent flame spread over externally heated Nomex under mixed convection flow

Fire-resistant materials are used in multiple applications where protection from fire is needed. Their fire-resistant capacity is often tested under specific conditions that might not represent the situation in an actual fire. Particularly relevant for this work is the application for astronaut spacesuit, since a spacecraft environment may be different than earth atmospheres. There, a material is exposed to low velocity flows, microgravity, reduced pressure, and enriched oxygen concentration. Under these conditions, material flammability can be altered. In addition, flammability tests are based primarily on the exposure of the material to an external radiant flux to simulate an adjacent fire, but not a real flame. In this work, an experimental study was performed to investigate the effect of ambient pressure and oxygen concentration on the concurrent/upward flame spread over a fire-resistant fabric (Nomex HT90-40) exposed to two different external heat sources. One is the radiation from infrared lamps, and the other is the flame from a burning polymethyl methacrylate (PMMA) sheet placed below the fabric. The experimental results show that an external heat flux extends the limiting oxygen concentration (24% LOC) of Nomex. This effect is more pronounced when the PMMA flame provides the heat flux (17% LOC). For oxygen concentrations larger than the Nomex LOC, the flame spread rate decreases as the ambient pressure is decreased, indicating that reducing buoyancy reduces the flame spread rate. A simple analysis of concurrent flame spread that incorporates mixed flow heat transfer correlates well with the experimental data. This suggest that flame spread in microgravity can be predicted in terms of a mixed flow velocity that includes the Reynolds and Grashof numbers. The results of this work provide further information about the effect of the type of external heating on material flammability. They may also guide future fire safety design in space exploration.