The effects of insulator thickness on molten PE dripping and dielectrophoresis phenomena are also characterized. The initial condition for the vortex formation is identified as fAC,cr∼VAC−2Dout, which supports that such a phenomenon is induced by the magnetic field. Irregular rotation of secondary molten PE is a new observation. When alternating current electric fields were applied to electrical wires, several dynamic behaviors of molten PE were observed: the ejection of the vapor jet of molten insulation and an internal circulation induced by Marangoni convection [24], the dripping of molten PE [23-27], an electrospray phenomenon that ejects small insulating droplets from the surface [25–28], and a dielectrophoresis phenomenon that causes the flame splitting [23–28].
These dynamic behaviors of molten PE were found to have a significant influence on the flame spread rate (FSR). The most noteworthy difference was that the molten PE dripping phenomenon did not occur with the Cu-core but with the NiCr-core when AC electric fields were applied. Due to the complex dynamic behavior of Cu-core molten PE, FSR characterization has not previously been successfully performed [28].
The length of the test section was 170 mm after including the part connected to the wire holder. Close-up images were also taken with a DSLR video camera (Nikon, D7500, 60 fps) with backlight to visualize the dynamics behavior of molten PE.
Overall feature of flame spread
The close-up images of molten PE show continuous migration of part of molten PE towards the fired wire via dielectrophoresis [28] and formation of a thin long film of molten PE. At 200 Hz, the flame oscillates significantly due to electrospray, showing ejection of fine droplets from molten PE surface, which has been observed previously [26–28] . Note that a spherical molten PE is formed near the flame front in all cases, including the baseline cases of Dout= 1.1 and 1.5 mm as marked as red circles in Figure 2, except for the cases with flame fluctuation at both flame ends, which will be explained later .
At 100 and 200 Hz, in addition to the formation of a spherical molten PE, the molten PE rotates around the burned wire (dripping on the ground) when its size is appropriate (large). At 600 Hz, the flame oscillates somehow regularly in both end regions, while both are reasonably symmetric, which has previously been reported to be associated with the occurrence of magnetic field-induced vortex flame [26, 40], while the flame height remains almost constant and fine. droplets are ejected from the surface of the molten PE. At 1000 Hz, in addition to the similar phenomena at 600 Hz, a secondary molten PE separates from the primary molten PE due to dielectrophoresis, while fine droplets are still ejected from the surface of the main molten PE (electrospray).
This causes the magnetic field-induced vortices at both ends to become asymmetrical, so that the flame size on the burned wire side is larger than that on the unburned wire side due to migration of the secondary molten PE to the side of the burnt wire. Note that the frequency of molten PE dripping increases as the frequency increases from 100 to 600 Hz, that is, for 100 and 200 Hz, in addition to the formation of a spherical molten PE, a secondary molten PE moves from the main molten PE to the core migrates. burnt wire due to dielectrophoresis.
When the size of the primary molten PE becomes large in such a case, it drips onto the ground, while the secondary molten PE occasionally rotates around the burnt wire. For 600Hz, a secondary molten PE separates from the primary molten PE together with the formation of a spherical molten PE. The details of the dynamic behavior of molten PE, such as dripping of molten PE, dielectrophoresis, electrospray and rotation of a secondary molten PE, as well as the splitting phenomenon of a single-peak flame into a double-peak one and the formation of a spherical molten PE explained later.
The results show that the flame front positions over time are reasonably linear, even with dynamic behavior of molten PE such as dripping, dielectrophoresis, electrospray and.
Flame spread rate
The rate of change in flame front position is affected by the wire diameter and AC electric field. For 4kV and 30Hz in (a), the slope of flame front position becomes smaller over time as Dout increases, while that for the baseline cases is larger in all cases. With varying AC frequencies (b-d), the slope of flame front position over time forDout= 0.8 mm decreases slightly in the increase of fAC increases from 30 to 100 Hz and then increases to 200 Hz.
For Dout=1.1mm, the slope of the flame front position decreases over time to 200Hz and then increases to 1000Hz, while it monotonically decreases to 1000Hz for Dout=1.5mm. Note that at Dout= 1.1 and 1.5 mm the flames do not extinguish up to 1000z tested in this study. The flames also do not extinguish up to 1000 Hz and 5 kV for Dout= 1.1 and 1.5 mm tested in this study.
Based on the behavior of FSR with AC frequency for several outer wire diameters, we can define four regimes: a decreasing FSR in regime I, an increasing FSR in regime II, a decreasing FSR in regime III and an increasing FSR in regime IV, respectively.
Effects of AC electric field on spreading flame and molten PE behaviors
However, for Cu core wire with Dout= 0.8mm previously investigated [28], the flame size was significantly reduced (expect a reduction of FSR) when a spherical molten PE was formed near the spreading flame front. This di-electrophoresis phenomenon forces a part of molten PE from the main molten PE to move intermittently or continuously towards the burned wire side. So an electrophoretic force is downward, it causes a downward movement of the skin layer of molten PE via a shear stress.
As shown in Figure 6, the molten PE generally hangs down due to the force of gravity. In this situation, the electric field strength in the upper part of the molten PE may be greater than that in the lower part of the molten PE, causing the molten PE to move to the upper region. This means that the dielectrophoretic force together with the upward buoyancy force can drive the rotational motion of the molten PE.
A new phenomenon is observed such that the secondary PE melted from the main molten PE rotates around the wire in the region of an incarnadine color for Dout = 1.1 and 1.5 mm in Figure 7. In regime III, a part of the molten PE was transferred to continuous. due to the phenomenon of di-electrophoresis, covering the bare wire with melted PE for Dout = 0.8 mm. As forDout = 1.1 and 1.5 mm, a secondary molten PE forms intermittently, although the formation of a liquid film of molten PE is very likely.
Even when the main molten PE has an appropriate size by the dripping of molten PE, the main molten PE rotates around the wire. Note that the dripping of molten PE did not occur for Dout = 0.8mm, but for Dout = 1.1 and 1.5mm (even the baseline cases with no electric field). The region with a very regular droplet number of molten PE is marked as a blue dotted line in Figure 7.
Two types of flame quenching modes have been previously identified: excess mass loss of molten PE and reduction in flame size.
Characterization of Flame spread rate
At the flame front, spherical molten PE forms only in regime III for Dout= 0.8 mm, while it forms in all regimes, including the base cases for Dout= 1.1 and 1.5 mm. As the frequency increases, the size of the trailing edge of the flame increases due to the occurrence of electrophoresis during electrospray on the surface of molten PE. For regime III, where the FSR decreases again with frequency, a liquid film (or secondary droplet) is mostly (occasionally) formed due to the phenomenon of dielectrophoresis, while dripping of the main molten PE also occurs frequently.
In particular, at Dout= 1.5 mm, dripping of molten PE is more likely to be observed at high voltages and frequencies in regime III. Spherical molten PE is formed near the flame front only in regime III for Dout = 0.8 mm, while in all cases, including the baseline cases with no electric field for Dout = 1.1 and 1.5 mm. With an increase to 1.1 and 1.5 mm in Dout, the secondary molten PE occasionally separates from the main molten PE due to di-electrophoresis phenomena, while the formation of a liquid film is more likely.
When the main molten PE has an appropriate size by dripping of main molten PE for regime III for Dout = 1.5mm, such a rotational movement was observed. In regime IV, flame vortices are formed as a result of magnetic fields at the leading and trailing flame edges, such that an electrospray from molten PE surface occurs. As the AC frequency further increases, a portion of the molten PE separated from the molten PE continues to migrate to the burned wire side due to the dielectric phenomenon, forming a molten PE film.
In this case, the size of the flame vortex increases due to the magnetic field at the trailing flame edge because the vortex motion increases the evaporation rate of the molten PE film. Fujita, “Effect of low external flow on flame spread over polyethylene-insulated wire in microgravity,” Proc. Zhang, “Flame spreading over electric wire with high thermal conductivity metal core at different slopes,” Proc.
Kim, “Flame spreading across electric wire with AC electric fields: internal circulation, fuel vapor jet, spread rate acceleration, and molten insulator dripping,” Combust. Lim, “Effect of AC electric field on flame propagation in electric wire: variation in polyethylene insulation thickness and di-electrophoresis phenomenon,” Combust. Yoo, “Effect of metal core on flame spreading across electric wire with applied electric fields,” Proc.
