PE and ETFE wire insulation flame morphologies and spread rates under subatmospheric pressures

Understanding the effects of pressure and oxygen concentration on the flame spread behaviors of electrical wires is crucial for establishing fire safety strategies for plateau cities, aircraft, and spacecraft. During the combustion, the polymeric coating first liquefies to form molten agglomerations, after which these droplets boil to release combustible gases that propagate the combustion process, which brings a potential fire threat. To better evaluate this fire risk, the flame spread of polyethylene (PE) and ethylene tetrafluoroethylene (ETFE)-insulated nichrome wires was investigated in a subatmospheric cabin. During the combustion of the insulation, the PE flame always presents a closed teardrop-like structure, while at the limiting oxygen concentration condition, the ETFE flame presents an asymmetric pentagram-like structure due to uneven heat losses. In this experiment, dripping was exhibited only by the PE insulation mainly because of the difference in surface tension. When the oxygen concentration was raised from 21% to 30%, the critical pressure for dripping shifts from 60 kPa to 38 kPa. Air pressure has a slight negative effect on flame spread rates, because, at lower pressure, the burning rate is lower, but the heat loss and convective and radiant heats of the flame decrease.