An experimental study on flame spread over electrical wire with high conductivity copper core and controlling heat transfer mechanism under sub-atmospheric pressures

The present study revealed experimentally flame spread behavior over high-conductivity copper (Cu) core electrical wire in sub-atmospheric pressures, and compares those with relative low-conductivity nickel-chrome (NiCr) core electrical wire, to quantify the evolutions of "wire-driven" and "flame-driven" heat transfer mechanisms in supporting flame spread with pressure decreasing. Polyethylene (PE) insulated Cu electrical wires with inner core diameter (d(c)) of 0.30 mm, 0.50 mm 0.80 mm and insulation thickness (delta(p)) of 0.15 mm, 0.20mm and NiCr core electrical wires with inner core diameter (d(c)) of 0.30 mm, 0.50mm and insulation thickness (delta(p)) of 0.15 mm were studied with ambient pressure ranged from 100 kPa to 40 kPa. Results showed that the blue flame height (or the "flame standoff distance" from the bottom edge of the high temperature yellow flame region to wire surface), and the flame base width both increased as the pressure decreased. The flame spread rate (FSR) of NiCr-core electrical wire just increased; however, that of Cu-core electrical wire first decreased then increased as the ambient pressure decreased. The critical pressure (P*) for the turning of FSR variation of Cu-core wire with pressure was larger as the core diameter was smaller or the insulation was thicker. The "wire-driven" and "flame-driven" heat transfer mechanisms in controlling the flame spread behavior over electrical wires with metal core were well verified by the experimental results shown in the present study. The relative importance of "wire-driven" heat transfer mechanism decreased, while that of "flame-driven" increased, as the ambient pressure is reduced. The findings of the present study will be essential for simulative study of flame spread behavior over electrical wires concerning the core conductivity effect and the evolutions of controlling heat transfer mechanisms.