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Effect of Environmental Variables on the Flammability of Fire Resistant Materials


This work investigates the effects of external radiation, ambient pressure and microgravity on the flammability limits of fire-resistant (FR) materials. Future space missions may require spacecraft cabin environments different than those used in the International Space Station, 21%O2, 101.3kPa. Environmental variables include flow velocity, oxygen concentration, ambient pressure, micro or partial-gravity, orientation, presence of an external radiant flux, etc. Fire-resistant materials are used in astronauts, firefighter, and racecar driver suits, cable harnesses, airplane components, etc. However, their fire resistant characteristics, including flammability limits may depend on the environmental conditions and require further study.

The addition of an external radiant flux is able to extend the flammability limits of materials. Based on the results, there is a Limiting Oxygen Concentration (LOC) above which flame spread occurs without the aid of an external radiant flux. Below this critical oxygen concentration flame spread may occur if there is an external radiant flux large enough to allow the fabric to continue the pyrolysis process. Experiments with four different kinds of fabrics showed that FR material content affects the value of the minimum external radiant flux for flame spread. As FR material content increases, so does the value of the minimum external radiant flux.

Regarding the effect of ambient pressure, pressures above 70 kPa result in small changes in the LOC. However, as ambient pressure drops below 70 kPa reducing ambient pressure results in an increase in the LOC. This flammability behavior can be phenomenologically explained in terms of factors such as heat transfer between the flame and unburned solid, buoyancy induced flows, and chemical kinetics. A reduction in ambient pressure decreases the heat transfer from the flame to the unburned fuel while at the same time reducing buoyancy induced flows and the associated heat losses. Examining the Flame/No-Flame spread boundaries in terms of the ambient pressure and oxygen partial pressure revealed a nearly linear relationship between p and pO2. The reduction in pO2 as a function of p suggests that is possible for materials to exhibit significantly different flammability behavior when following a constant oxygen partial pressure curve, like for example normoxic atmospheres similar to those suggested for future spacecraft.

The combination of microgravity and/or addition of an external radiant flux are able to extend the flammability limits of ETFE insulated wires. Under no external radiant flux microgravity conditions resulted in a 6% decrease in the Limiting Oxygen Concentration (LOC) for opposed flame spread. When microgravity conditions were combined with an external radiant flux of 25 kW/m2 the decrease in the LOC was 12%. Microgravity limits heat losses, making possible to reduce the oxygen concentration and flame temperature while maintaining a critical solid decomposition rate. External radiation is able to further compensate for reductions in flame temperature, resulting in a noticeable reduction between the normal gravity and microgravity LOC. The normal gravity and microgravity LOC values of ETFE insulated wires were also compared to those of polyethylene (PE) insulated wires. This comparison showed that microgravity results in a bigger change in the LOC of ETFE than PE. This observation is explained in terms of thermal parameters, critical mass flux and chemical kinetic effects.

Adoption of testing methodologies similar to those described in the present work can produce flammability maps that provide more information than Pass/Fail methods while presenting a clearer picture of the flammability of materials. The results of this work are important given that the flammability of materials is routinely tested without considering the effect of environmental variables, which according to the results presented in this dissertation, may not be indicative of the absolute flammability limits of materials.

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