Effect of the thickness of polyethylene insulation on flame spread over electrical wire with Cu-core under AC electric fields

Abstract

Effect of the thickness of polyethylene (PE) insulator on flame spread over electrical wire with high thermal conductivity of Cu-core is experimentally investigated by varying the applied AC voltage (VAC) and frequency (fAC) in the ranges of 0 – 5 kV and 0 – 1000 Hz, respectively. Three outer wire diameters (Dout= 0.8, 1.1, and 1.5 mm) with the same core diameter (Dc= 0.5 mm) are used. For the baseline cases without applying electric field, the flame spread rate (FSR) decreases with an increase in the outer wire diameter. When AC electric fields are applied to the wires, flame shapes and FSRs are appreciably influenced. The FSR behaviors with AC electric fields are classified into four sub-regimes as the frequency increases: a decreasing FSR (Regime I) having a leaning flame toward the burnt wire side, an increasing FSR (Regime II) showing a splitting phenomenon from a single-peak flame to a double-peak one due to a dielectrophoresis phenomenon, a decreasing FSR again (Regime III) generating a globular molten PE near the flame front and forming a liquid film of molten PE via continuously migrating a part of molten PE toward the burnt wire side, and an increasing FSR again (Regime IV) showing vortex flames at the front and rear flame edges along with an electrospray of molten PE. These emphasize diverse effects of the dynamic behaviors of molten PE. The onset condition for the formation of the vortex structure is identified as fAC,cr∼VAC−2Dout, supporting that such phenomenon is generated by induced magnetic field. An intermittent rotation of secondary (main) molten PE around the wire is a new observation. The mechanism is suggested by a dielectrophoretic force. Excluding the cases experiencing an intermittent leaning flame toward the burnt wire side and an irregularly splitting flame, FSR under the influence of such complex dynamic-behaviors of molten PE could reasonably be characterized with the gradient of electric field intensity taken at the outer surface of PE insulator, the area ratio between insulator and core, and AC frequency in all the regimes.