Effect of core metal on flame spread and extinction for horizontal electrical wire with applied AC electric fields

The effect of metal core on flame spread behavior over polyethylene (PE)-insulated electrical wire was experimentally investigated by varying applied AC voltage (VAC) and frequency (fAC). The present experimental results with Cu-core were compared with those conducted previously with NiCr-core. Flame spread rate (FSR) with Cu-core for the baseline case with no electric field was larger than that with NiCr-core. Both FSR and flame size were appreciably influenced by applied AC electric fields and behaved differently for Cu- and NiCr-cores. For Cu-core, FSR behavior could be classified into four regimes as fAC increases: increasing FSR (regime I), decreasing FSR (II), and again increasing FSR (III) and decreasing FSR (IV). While for NiCr-core, it has been categorized into two regimes I and II. FSR behavior was qualitatively similar to that of flame width in regimes I, II, and IV. While such a relationship was not satisfied in regime III, which can be attributed to the formation of molten PE film in the burnt wire side as well as the formation of globular molten PE in front of spreading flame edge, emphasizing the important role of complex molten PE behavior. Molten PE dripping was observed for NiCr-core, while for Cu-core, such dripping was not observed. Electrospray and di-electrophoresis phenomena occurred with Cu-core along with a formation of molten PE film on the burnt wire side via continuous di-electrophoresis phenomenon from molten PE droplet. When the frequency was excessive, flame extinction occurred via two routes: appreciable reduction of flame size (both for Cu- and NiCr-cores) and detachment of molten PE (for Cu-core). These extinction frequencies were correlated well with the voltage.