Combustion Behaviour Over ETFE Insulated Wire in Slow External Flow under Microgravity

Abstract

Opposed flame spreading over ETFE (ethylene-tetrafluoroethylene co-polymer) insulated wire, which is used for actual wire in space, in low air flow velocity has been investigated under microgravity environment. The experiments were performed with Japan Microgravity Center (JAMIC) 10 s dropshaft and NASA's KC 135 parabolic flight. Experiments were performed with changing O₂ concentration, 35 and 40%, external air flow velocity, 0 (quiescent) -22 cm/s and dilution gas, N₂, CO₂, He. The sample of 0.32 mm inner core diameter with 0.15 mm insulation thickness was used. The results showed that flame spread rate over wire insulation was strongly affected by air flow velocity and the dilution gases. Flame spread with N₂ dilution had maximum value in a low flow velocity region. However, CO₂ dilution resulted in monotonic increase in flame spread rate with decrease in air flow velocity and the maximum flame spread at V_e=0 cm/s (quiescent). The different dependency of the spread rate on flow velocity with different dilution gas was explained by the reabsorption effect of CO₂ gas ahead of flame front. Reabsorption of radiation heat with CO₂ gas recovers the radiation heat from the flame and prevent the flame temperature decrease even in very low flow velocity region, which is known as radiation control region for optically thin gas.