Study on unsteady molten insulation volume change during flame spreading over wire insulation in microgravity

Abstract

Tests with flames spreading over wire insulation in microgravity were performed at varying external opposed flow conditions to examine the influence of flow velocity in the time dependent volume change of molten insulation. In the experiments, low density polyethylene insulated Nickel–chrome wire specimens were used and the oxygen concentration was fixed at 30% (N2 balanced). The results show that the time dependent changes in molten insulation volume are related to the opposed flow velocity. Further, as opposed flow velocity increases, the volume change rate decreases monotonically. By subtracting the volume change rate from the volume supply rate from the solid part to the molten part, which is calculated by multiplying the rate of spreading of molten insulation at the leading edge by the cross sectional area of the insulation, a pyrolysis volume rate for the polyethylene was established. The pyrolysis volume rate is defined as the amount of consumed molten insulation volume per unit time. After these calculations, it was found that the pyrolysis volume rate increases monotonically with increases in the opposed flow velocity. Further, numerical calculations of time dependent volume change in the molten insulation at different flow velocities were made. The numerical results show good agreement with the experimental results of the molten insulation volume change during the 0–4.5s of microgravity measured here. By using the numerical calculations for this initial short period, the time dependent volume change in molten insulation during longer-term microgravity is predicted. The calculated results show that the volume finally reaches a steady state value in flow velocities of 10–250mm/s investigated here. These results provide insight into the mechanism of flame spreading over wire insulation, especially the unsteadiness of the flame in flame spreading events.