Effect of AC electric field on flame spread in electrical wire: Variation in polyethylene insulation thickness and di-electrophoresis phenomenon

Abstract

The effect of AC electric field on flame spread over polyethylene (PE)-insulated electrical wire was experimentally investigated by varying the applied AC voltage (VAC) and frequency (fAC) with special attention to the effect of the thickness of the PE insulation material (Tins). The diameter of the Ni–Cr wire was 0.5 mm and Tins was 0.15, 0.3, and 0.5 mm, yielding outer diameters (Dout) of 0.8, 1.1, and 1.5 mm, respectively. For the baseline cases with no electric field, the flame spread rate (FSR) (flame size) decreased (increased) with insulation thickness. Both FSR and flame size were appreciably influenced by applied AC electric fields. The FSR behaviors under applied electric fields could be classified into three sub-regimes as AC frequency increased: regime I exhibited a decreasing FSR as AC frequency increased, regime II exhibited an increasing FSR, and regime III again exhibited a decreasing FSR. Molten PE dripped from the wire (resulting in mass loss); exhibited di-electrophoresis (some molten PE moved from the main molten zone toward the burnt wire, forming globules in the process); and developed electrosprays (ejection of small droplets from the molten PE surface). For Dout = 0.8 mm, the FSR behavior was similar to that of the flame width, such that the behavior could be explained by the thermal balance mechanism. When a low voltage and high frequency were applied to wires with Dout = 1.1 and 1.5 mm, molten PE droplets detached and moved to the burnt wires continuously (although sometimes intermittently) from the main body of molten PE; the FSR behavior thus deviated from that of regime I. Droplet detachment was attributable to a di-electrophoresis. The distance moved correlated well with the difference in electric field intensities of burnt and unburned wires. Appreciable dripping of molten PE occurred at high voltages and moderate frequencies in regime II. When the frequency was excessive, flame extinction occurred via two routes: appreciable reduction of flame size when Dout = 0.8 mm in regime I and appreciable fuel mass loss via dripping of molten PE during flame spread when Dout = 0.8 and 1.1 mm in regime III. These extinction frequencies correlated well with VAC/Dout. When high voltage and frequency were applied in the Dout = 1.5 mm case, droplets detached and moved to the burnt wire via di-electrophoresis; subsequently a series of fine droplets ejected from the surface via electrospraying, while the molten PE region grew and subsequently dripped. In such cases, flame extinction did not occur because di-electrophoresis increased the flame width and thereby the FSR over the experimental ranges of VAC and fAC.