Effect of applied AC electric field on flame spread over electrical wire with cross-linked polyethylene insulation

Abstract

The effect of applied AC electric field on flame spread over electrical wires with NiCr-core insulated by crosslinked polyethylene (XLPE) is experimentally investigated by varying the AC voltage and frequency. Results are compared with those for low-density polyethylene (LDPE) insulation, commonly studied in fire safety research. For the baseline case without applying electric field, XLPE-insulated case exhibits distinct behaviors such as flame splitting and a unique molten dripping via merging of newly-formed globular molten XLPE, which were not observed in LDPE-insulated one. Under applied electric fields, the flame spread rate (FSR) and molten insulation dynamics differ markedly between XLPE and LDPE. Two regimes of FSR behavior are identified for XLPE and three for LDPE, depending on voltage and frequency. At high voltage and frequency, induced magnetic fields promote flame vortex formation, increasing flame width and FSR, while excessive conditions lead to flame extinction through mass loss via electrospray and dielectrophoresis. Scaling analyses are applied to elucidate the underlying mechanisms. The flame spread rates are phenomenologically characterized depending on these various phenomena in terms of the frequency and voltage, especially emphasizing the electric field intensity on the unburned wire surface. The extinction conditions are correlated with AC voltage and frequency.