Flame Spread over Polymer-Insulated Wire in Sub-Atmospheric Pressure: Similarity to Microgravity Phenomena

Abstract

Flame spread over polymer-insulated wire in reduced pressure environments have been studied experimentally. Horizontally-placed polyethylene (PE) insulated NiCr wire is used as the burning sample. Ambient gas is the mixture of nitrogen and oxygen, and composition is fixed at constant as air. Total pressure is reduced from atmospheric (101 kPa) to sub-atmospheric (~ 20 kPa) levels. Flame spread behavior over the sample wire followed by the forced ignition at the edge is examined. Experiments with backlight were also made to observe the deformation of molten PE during the spread event. Experimental results show that typical “teardrop” flame is gradually modified to round and even oval (wider in horizontal direction) as the total pressure decreases. Additionally, flame luminosity decreases, indicating the combustion becomes weaker, as the pressure decreases. Nonetheless, the spread rate does not drop in the “reduced” pressure condition, more precisely, low-pressured environment can bring faster spread rate depending under the condition considered in this study. This is due to the modification of mass and thermal transport processes depending on the pressure: relative importance of diffusive transport is pronounced while convective cooling becomes less important in the low-pressured environment. Similarity of the observed spread trend in sub-atmospherics pressure to that in microgravity with weak imposed flow via Grashof number correlation is discussed.