Flame Spread Along PE-Insulated Wire in Sub-Atmospheric Pressure Enclosure

Flame spread over polymer-insulated wire in reduced (sub-atmospheric) pressure has been studied experimentally in order to evaluate the fire safety of electric circuit in the aircraft as well as the space habitats. Polyethylene (PE) insulated NiCr wire is used as the burning sample. Ambient gas is the mixture of nitrogen and oxygen, and the composition is fixed as air (79 vol.% of N₂ and 21 vol.% of O₂) throughout the study. Total pressure is reduced from atmospheric (101 kPa) to sub-atmospheric (20 kPa) in order to investigate the role of the reduced pressure on the flame spread along the wire. Spread event followed by the forced ignition is recorded by digital video camera to obtain any time-dependent flame behavior. Experimental results show that the flame shape is changed from typical “teardrop” to “round” (and even oval) with the decrease in total pressure. Flame spread rate increases in the reduced pressure although the partial pressure of oxygen is “reduced” with the total pressure. Such “pronounced” spread behavior is continuously observed until just before the extinction condition (∼25 kPa in the present study). The change in flame shape could enhance thermal input to the unburned PE through gas-phase conduction as well as conduction along the wire, and these should be responsible for the faster flame spread in sub-atmospheric pressure. Heat balance is roughly estimated with measured temperature and relative contribution of above two thermal input pathways is understood almost comparable. Importance of the presence of conductive material, such as metal wire, on flame spread is addressed in the current spread behavior.