Effect of flow direction on the extinction limit of spreading flame over wire insulation

Fire safety is one of the most important issues for manned space mission, because a fire in a spacecraft may results in irreversible alteration of the crew's environment. Electric wire is a major cause of fire accidents in spacecraft. So, it is very important to study the flammability of electric wire under microgravity conditions. In this paper experimental works to determine Limiting Oxygen Concentration (LOC) of spreading flame over electric wire insulation have been carried out and the difference between the LOC in opposed and concurrent flows has been evidenced under both normal gravity (1G) and microgravity (μG). Polyethylene (PE) insulated Nickel-Chrome (NiCr) wires with inner core diameter (dc) of 0.50 mm and insulation thickness (dp) of 0.30 mm are examined with external flow velocity ranging from 50mm/s to 200mm/s. Microgravity environment is provided along parabolic flights onboard the French A310 airplane. Results show that with increasing external flow velocity, the LOC in 1G first decreases and then increases (“U” trend) for both of opposed and concurrent cases. On the opposite the LOC in μG monotonically decreases with increasing external flow velocity. Furthermore the LOC in μG is lower than that at 1G because natural convection is vanished. Comparing opposed and concurrent flows in 1G condition, the LOC tendencies following the modification of the flow velocity are very similar and the minimum value is almost the same. On the other hand, in μG condition, the LOC value is almost the same within the low flow velocity region for both flow directions, but the LOC for concurrent flow is about 2% smaller in Oxygen concentration than that for opposed flow within the high flow velocity region. This research is supported by Japan Space Exploration Agency (JAXA) under the project of FLARE and the Centre National d'Etudes Spatiales (CNES) under contract #130615.