Effect of low external flow on flame spread over polyethylene-insulated wire in microgravity

Abstract

An experimental study of flame spread phenomena over polyethylene-insulated wires has been performed in opposed flow under microgravity. The experiments were performed at the Japan Microgravity Center (JAMIC) 10 s drop shaft. Two samples with different insulation thicknesses, 0.075 and 0.15 mm, and with the same inner core diameter, 0.5 mm, were used. Experiments were performed with O 2 concentrations of 21%–50% and external flow velocities 0 (quiescent) to 30 cm/s. The results show that the rate of flame spread is affected by the flow velocity and that the effect is much stronger at high oxygen concentrations. According to the results, flame spread phenomena of wire insulation can be classified into four different regimes based on the flow velocity: (1) an oxygen transport control regimes, (2) a geometrical effect regime, (3) a thermal regime, and (4) a chemical kinetic control regime. A special feature of the flame spread over wire insulation is the existence of the geometrical effect regime and a maximum spread rate between the oxygen transport control and geometrical effect regimes. The mechanism that gives rise to the unique features is discussed based on changes in preheat length, standoff distance, and flame temperature. The importance of the three effects and their relation to sample geometry, enhancement of diffusive oxygen supply, reduction of standoff distance, and logarithmic effect for the heat transfer are discussed.