A numerical and experimental study of the ignition of insulated electric wire with long-term excess current supply under microgravity

The ignition of electric wire after long-term excess current supply under microgravity was studied using both experimental and numerical methods. The experiments were conducted in parabolic flights that provided about 20 s of microgravity, while the numerical simulation was carried out in one-dimensional cylindrical co-ordinate system. The experimental results showed wire insulation was ignited by much lower electric currents under microgravity than under normal gravity and that the ignition delay time increased as the applied current decreased. It was further found that the total electric energy required for ignition increased with a decrease in the current value. The numerical analysis suggested that the increase in the required energy for ignition was caused by an increase in heat losses by conduction and radiation from the wire insulation surface to the ambient air, which increased the ignition delay time. As the ignition delay time became longer, heat loss by radiation became more dominant in the total heat loss. This was because a slow exothermic oxidation reaction occurred in the gas phase near the sample, and the temperature gradient next to the wire surface became smaller. We conclude that the ultimate ignition limit over a very long exposure to microgravity is mainly determined by radiation heat loss. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.