Dynamic behaviors of flame and molten insulation in electrical wire fire under applied electric field

Abstract

This study reviews recent findings on the dynamic behaviors of flame and molten insulation material observed in spreading flames over electrical wires under applied electric fields, which is a relevant scenario in electrical wire safety. The important roles of various unique dynamic behaviors in flame spreads are discussed, including fuel-vapor jet ejection from molten polyethylene (PE) surface, internal circulation of molten PE driven by Marangoni convection, dripping of molten PE, electrospray ejecting multiple small droplets from molten PE surface, and lateral dielectrophoresis by migrating a part of main molten PE toward the burnt wire side by forming a secondary molten PE or a liquid film of molten PE and sometimes leading to a formation of splitting flame. Additional behaviors such as vibration/rotation of molten PE due to a vertical dielectrophoresis, flame-leaning toward the burnt wire side caused by ionic wind, and magnetic field induced flame vortices near flame edges are also reviewed. The physical mechanisms of these dynamic behaviors are explained. Various regimes are identified depending on the occurrence of abovementioned phenomena. The dependence of flame spread rate on relevant physical parameters is reviewed, revealing a non-monotonic response to applied AC voltage and frequency, due to the intricate interactions among various dynamic phenomena. Phenomenological correlations are established for the FSR using key physical parameters including wire diameter, wire core diameter, applied voltage and frequency, and radial electric field gradient. To better understand the dynamic behaviors of molten insulation, the combustion of a droplet suspended on a wire was investigated, isolating the effects of solid-to-liquid phase change and the asymmetric distribution of molten PE between the burnt and unburned sides of the wire. The dynamic behaviors of such burning droplets under applied electric fields, along with their underlying mechanisms, are also reviewed and discussed.