(ISS) Material effect of flame spread over electric wire

1. Background

In spacecraft, many electric devices and especially electrical wires represent a potential source of fires. Fires in spacecraft are dangerous because of its special characteristics, ”Closed space” and ”Microgravity (μg)”.

All spacecrafts are consisted on closed spaces therefore it is very hard to deal with fire hazard. In addition, the research conducted by Takahashi et al.[1] revealed that the flame spreading over polyethylene (PE) insulated electrical wire in μg is more flammable than that in normal gravity (1g). In this work, the authors indicated that the main cause of flammability increase is the vanishment of natural convection. Electrical wires contain metal core wire (typically it is copper) which has high thermal conductivity and it acts important role for changing flammability. The flame in 1g produces buoyancy flow and it cools down the burned bare core wire. Consequently the molten insulation (fuel) is also cooled down because of the high thermal conductivity of the core. By contrast the flame in μg wraps around the core wire and supply more heat to the insulation through the core wire.

Figure 1 shows comparison of 1g flame and μg flame. As a result the difference of gravity causes the flammability increase. Nevertheless, still we haven’t obtained theoretical and qualitative understanding, "how much flammability varies in each conditions?".

Comparison of 1g and ug

Figure1: Comparison of ig and μg flame



 

2. Our Research

To this date there are some researches focused on the flame spreading wire, but most of them use PE insulated wire which is not the practical one. Therefore it is still unclear how we apply these result to practical, fire-resistant material. The main topic of our research is ”Material Effects on Flammability”. In 2013, we conducted the experiment using Ethylenetetrafluoroethylene (ETFE) insulated wire. Figure 2 and Figure 3 shows the subscribe of inside the chamber and experiment rack used in experiment. The result shows that the difference of limiting oxygen concentration for flame propagation (LOC) between 1g and μg in ETFE wire sample is greater than that in PE. Now we are continuing the research to reveal which factor is the key point and how much it affects.

実験装置

Figure2: Experiment rack

chamber

Figure3: Subscribe for combustion chamber



 

References

[1] S. Takahashi, H. Ito, Y. Nakamura, O. Fujita, Combustion and Flame 160 (2013) pp.1900-1902.

[2] A. F. Osorio, K. Mizutani, C. Fernandez-Pello, O. Fujita, Proceedings of the Combustion Institute 35 (2015) pp.2683-2689.

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