Effect of Ignition Condition on the Extinction Limit for Opposed Flame Spread Over Electrical Wires in Microgravity

Flame spread over wire insulation plays a crucial role in spacecraft fire safety. To quantify the effect of the initial ignition condition on the limiting oxygen concentration (LOC) of spreading flame over wire insulation, opposed flow flame spread experiments with wire insulation were conducted in microgravity (parabolic flights). Both ignition power (32.7 W to 71.8 W) and heating time (5 s to 15 s) were varied for an external flow of 100 mm/s. The sample wires were made of polyethylene-coated nickel-chrome (NiCr) and Copper (Cu), respectively, both with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm. A 0.50 mm diameter coiled Kanthal wire wrapped around the sample wire 6 times with 8 mm length was used as the igniter. The experimental results show that the LOC gradually decreases as the ignition power or heating time increases and eventually it reaches a constant value. Also, the effect of ignition condition on LOC was more pronounced for Cu wires than for NiCr wires. The variation range of LOC in the tested ignition condition in microgravity was larger than that of horizontal flame spread in normal gravity. This conclusion can have implication for future experiment in the International Space Station to avoid the wrong LOC value because of the insufficient initial ignition energy and will eventually lead to an improved fire safety in spacecrafts.