Flame spread over electric wire with high thermal conductivity metal core at different inclinations

This paper reveals experimentally the flame spread rate (FSR) [both upward (concurrently) and downward (opposed)] over electric wire with high thermal conductivity metal core at different inclination angles, which is new in view of that previous works about such inclination effect are mainly focusing on the material (wood, PMMA……) where the conductivity through media itself is not so important. Polyethylene (PE) insulated copper (Cu) wires with inner core diameter (dc) of 0.30 mm, 0.50 mm 0.80 mm and insulation thickness (δp) of 0.15 mm, 0.30 mm are studied with inclination angles ranged from −90° to +90°. Their behaviors are examined in both naturally normal (Hefei city with altitude of 50 m; 100 kPa) and a reduced (Lhasa city with altitude of 3650 m; 64 kPa) ambient pressure atmosphere. Results show that with increase in inclination angles from −90° to 90°, the FSR first decreases and then increases (“U” trend) with its value being lowest at nearly horizontal condition (0°) in both pressures, which is quite different from what we normally know for other materials with low thermal conductivity. Two characteristic lengths, the flame base width (Wf) and the pyrolysis zone length (Lp), are found to account for this special variation behavior with their variation trend with inclination angle being consistent with that of FSR. A simplified heat balance analysis concerning core thermal conduction effect is performed to calculate the FSR in relation to these two characteristic lengths, thermal conductivity of the metal core as well as the effective convection heating of the wire by the flame base. The calculated FSR are shown to be in fairly good agreement with the measured values at different inclination angles for different inner core (wire) diameters in both ambient pressures.