Underwater current distribution induced by spark discharge on a water surface

Journal of Electrostatics 71 (2013) 823e828

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Underwater current distribution induced by spark discharge on a water surface Nur Shahida Midi*, Ryu-ichiro Ohyama, Shigeru Yamaguchi Graduate School of Science and Technology, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2012 Received in revised form 21 February 2013 Accepted 3 March 2013 Available online 16 March 2013

Discharge current distributions generated underwater by spark discharges from the atmosphere to free water surfaces with conductivities in the range 0.07e10.0 S/m were investigated using a laboratory-scale electrode system consists of a discharge electrode and nine underwater grounding electrodes. Discharge emission on the water surface, which shows significant change with slight increase in conductivity, affects the current distribution in the water. The electric potential of the water surface also changes significantly with slight increase in conductivity. Results of numerical calculations of the underwater discharge current based on the water surface potential agree with the experimental results. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Spark discharge Conductivity Water surface Underwater Discharge current

1. Introduction Electric discharge phenomena in dielectric two-phase gas-liquid systems have been extensively investigated from a physical viewpoint for many years. From an electrohydrodynamics viewpoint, electrostatic effects in insulating liquids have been well recognized for the role of interfacial sheer stress [1], flow pattern transitions [2], instability in fluid motion [3], and non-dimensional numbers for electrified interfaces [4] through dielectric and conductive forces. On the other hand, from a chemical perspective, exposure of conductive liquids interfaces with gases (which act as liquid electrodes) to corona or glow discharge is being widely investigated for various applications. Examples include plasma generation for aqueous phenol decomposition [5] and NOx treatment methods [6], and maintenance of wet polluted insulators on high-voltage transmission lines [7]. Most investigations of discharge on water surface have focused on understanding its mechanism and nature by employing various types of discharges and electrode configurations. Examples of such studies include investigations of discharge transitions [8], plasma characteristics in an air gap [9,10], and leader extension of a spark discharge on a water surface [11].

* Corresponding author. Tel.: þ81 463 58 1211 ext. 4021. E-mail addresses: [email protected] (N.S. Midi), [email protected] (R.-i. Ohyama), [email protected] (S. Yamaguchi). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.03.002

When considering atmospheric discharge processes above liquid interfaces, it is important to determine the behavior of the surface potential and the current in the liquid. Recently, electrolyte-cathode discharge in the atmosphere with water as the medium or as an electrode is one of the promising plasmas for water treatment [12e16]. Besides that, in concerns of lightning stroke to wind turbine generator system on the ocean [17], prediction of the discharge current within a seawater region circumference will be desired for the consideration of safe defense. Water, which contains ions and other impurities (except for pure water), makes a good conductor as the impurities increased [18], and the discharge characteristics in water environment also change under the condition of varied conductivity [19]. Surprisingly, the behavior of underwater current generated by spark discharges from the atmosphere to a water surface is still largely unknown, unlike the extensive knowledge about underwater discharges [20e22]. It is thus important to understand the effect of water properties on electrical conduction phenomena. This work experimentally investigates the effect of water conductivity on the underwater current distribution generated by spark discharge from the atmosphere to a water surface. In this work, an electrode system with a free airewater interface was constructed to investigate the current characteristics dependence on the water conductivity during spark discharge in air. Spark discharges over a water surface (SDWS) were generated in this system by a applying a standard lightning impulse voltage. The

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Current probe Impulse high voltage generator

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Fig. 1. Experimental setup; all dimensions are in mm.

temporal and spatial underwater current distributions were measured at nine grounding electrodes installed in a laboratoryscale water tub (depth: 130 mm; diameter: 750 mm). The potential of the water surface during spark discharge was measured and numerical analysis was performed based on the measured boundary conditions. 2. Experimental setup Fig. 1 shows an axially symmetric electrode system whose grounding electrode consists of nine underwater electrodes, which are numbered from 1 to 9 radially. Electrodes 1e6 were aligned parallel to the water surface at a depth of 130 mm, while electrodes 7e9 were perpendicular to the water surface. Electrode 1 was a circular copper plate (thickness: 1 mm; diameter: 40 mm), while electrodes 2e6 were ring-shaped copper plates of the same thickness and their outer radius was 40 mm greater than their inner radius. Electrodes 7e9 were copper tape (thickness: 0.1 mm; width: 40 mm) attached to the inner surface of a 750-mm-diameter water tub made of vinyl chloride. There was a 15-mm-gap between adjacent electrodes. Electrode 9 was placed at the water surface with 20 mm of its width submerged beneath underwater to observe the discharge current at the water surface. Although there were only two current probes showed in Fig. 1, discharge current

observations were done at all electrodes 1e9. The placement and number of electrode were designed to cover all parts of water (i.e. water surface and underwater). The discharge electrode (diameter: 12 mm; tip angle: 45 ) was placed 5 mm above the water surface so that there was an air gap between it and the water surface. A 400 kV impulse voltage generator (Tokyo Transformer, 5 kJ) was used as the voltage source. SDWS was generated by applying a 25 kV peak voltage with a 1.2/50 ms standard lightning impulse voltage. The applied voltage waveform was measured by a resistance divider (voltage dividing ratio 39,882:1) and the current waveforms of all the electrodes were measured by two current probes (Pearson, 6585); displayed on a digital oscilloscope (Tektronix, TDS5054B). Discharge emission images of the water surface were captured using a digital camera (Ricoh, CX3) with an exposure time of 1/7 s. It was orientated at 20 relative to the water surface and faced the discharge electrode. Measurements were conducted by varying the conductivity of water s by varying the concentration of sodium chloride dissolved in tap water. The conductivity between 0.07 and 10.0 S/m at 25  C was measured using a conductivity meter (Horiba, ES-51). In this experimental electrode system, the physical parameter of the electric potential on the water surface is important. To measure the potential, a probe was placed perpendicular to the water surface, where the tip was in contact with the surface as shown in

Fig 2. Typical discharge emission profiles on water surface for three values of s. (a) s ¼ 0.07 S/m, (b) s ¼ 0.2 S/m, (c) s ¼ 5.0 S/m.

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the maximum extend length of filamentary discharge on the water surface as a function of s; each plot was obtained from the average maximum extend length of 10 observations. The result suggests an adequate inverse power law correlation between the average maximum extend length of filamentary discharge and s. The SDWS characteristics are thus expected to be influenced by even a slight change in s.

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3.2. Voltage and current waveforms during SDWS

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Conductivity, σ [S/m] Fig. 3. Average maximum extend length of filamentary discharge on water surface as a function of s.

Fig. 1. The probe was made of a 0.8-mm-diameter tungsten rod and was fully insulated except for the 1 mm tip. It was connected to the digital oscilloscope through a 1000:1 high-voltage divider circuit with an input impedance of 1 MU. Measurements were conducted from the center (r ¼ 0 mm) to the edge (r ¼ 320 mm) in intervals of 20 mm, where r is the distance from the discharge electrode axis. 3. Results and discussion 3.1. Discharge emission profile of SDWS Fig. 2 shows typical discharge emission profiles on the water surface for s ¼ 0.07, 0.2, and 5.0 S/m. SDWS was observed as a filamentary discharge path sprouting on the water surface. The maximum extend length of the filamentary discharge decreased with increasing of s, and this phenomenon was also observed in pulsed corona discharge between a planar high-voltage made from reticulated vitreous carbon (RVC) and water surface with an immersed ground electrode [23]. For tap water (s ¼ 0.07 S/m; Fig. 2(a)), the spark discharge progressed as a filamentary discharge to the water surface. This is also known as incomplete discharge on a water surface due to resistive barrier discharge [24,25], where the distributed resistance of the water prevents discharge localization. For water with salinity equivalent to that of seawater (s ¼ 5.0 S/m; Fig. 2(c)), only spark discharge localized at the discharge point almost without filamentary discharge was observed. Fig. 3 shows

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Fig. 4 shows typical voltage and current waveforms at the discharge electrode (Vd and Id, respectively); and the current waveform at the grounding electrode 3 (Ig3) during SDWS for s of 0.07 and 5.0 S/m. Current waveforms at the other grounding electrodes Igi (i ¼ 1, 2, 4e9) were similar to that of Ig3 in this figure, although their magnitude differed. The flashover voltage between the discharge electrode and the water surface decreased with increasing s; which were 20.3 and 15.8 kV for 0.07 and 5.0 S/m, respectively. For 0.07 S/m, Vd did not decrease rapidly due to resistive barrier discharge at the water surface, as shown in Fig. 2(a). This waveform is similar to the underwater discharge when the discharge electrode is immersed in tap water [22,26]. As s increases, the flashover voltage tended to decrease, and the peak in the discharge current appeared in a shorter time. Although not shown here, the voltage and current waveforms were similar to those in Fig. 4(b) for s ¼ 0.2 S/m and above. For s ¼ 5.0 S/m, a drastic decrease of Vd to only a few kV was observed during the flashover at 0.5 ms. Most of the discharge currents increased during the flashover duration. These observations clearly indicate that SDWS induces different discharge current waveforms when s is varied in the range of 0.07e10.0 S/m. In correspondence with that, under applied voltage of 25 kV, the maximum extend length of filamentary discharge propagating as partial discharge on water surface decreased as s increase. During low s, waveform with a low-voltage drop was observed. Id was limited due to the water resistance and this is observed as the filamentary discharge propagating on the water surface. On the other hand, during high s, a high-voltage drop was obtained and Id was highly induced. The relatively high s of water produced a small propagation of filamentary discharge on the water surface. In summary, the difference in voltage and current waveforms due to s is associated with the filamentary discharge on water surface. In addition, it can be concluded that the equivalent resistance for 0.07 S/m is higher than other conductivity used in this work, with notable difference between 0.07 S/m; and 0.2 S/m and above, relevant to the changes of voltage and current waveforms. The appearance of a second current peak during SDWS in high conductivity is considered to be due to the effect of distortion of the water surface [27].

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Fig. 4. Typical voltage and current waveforms for s of (a) s ¼ 0.07 S/m, (b) s ¼ 5.0 S/m. Vd: voltage at discharge electrode, Id: current at discharge electrode, Ig3: current at grounding electrode no. 3.

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0

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3.4. Electric potential distribution on water surface Fig. 6 shows typical waveforms of water surface potential for

s ¼ 0.07 and 5.0 S/m, where each waveform is the average of 10 observations. For s ¼ 0.07 S/m (Fig. 6(a)), the time at which the

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Fig. 5. Normalized discharge current density ii.nor for grounding electrodes in water.

3.3. Distribution of discharge current at grounding electrodes Fig. 5 shows the distribution of the normalized current density ii.nor [¼ (Igi.peak/Id.peak)/Ai], where Igi.peak is the current peak at grounding electrode i, Id.peak is the current peak at the discharge electrode, and Ai is the area of electrode i. The current peak was observed after the same elapsed time at every grounding electrode, where the elapsed time depended on s. Highest ii.nor was at electrode 1 for all s since electrode 1 was located nearest to the discharge electrode. ii.nor initially decreased with increasing distance of the grounding electrode from the discharge electrode but then increased as the grounding electrode approached the water surface. ii.nor changes significantly at electrodes 2e5 when s increases slightly from 0.07 to 0.2 S/m; this is caused by filamentary discharge at the water surface (see Fig. 2). At these electrodes, a higher ii.nor was observed for 0.07 S/m for which the longest filamentary discharge was observed and ii.nor decreased with increasing s because the effect of filamentary discharge decreases as s increases. The average maximum extend length of filamentary discharge for 0.07 S/m was 100 mm, which is approximately the same r as electrode 3 (110 mm from the discharge electrode axis). A significant difference in ii.nor was also observed at electrode 9, suggesting that the water surface current differs between s ¼ 0.07 S/m and higher s. This also affects ii.nor of nearby electrodes (6e8). In these electrodes, ii.nor increased with increasing s, confirming the influence of the water surface current observed at electrode 9. These observations confirmed the correspondence of

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maximum potential occurred increased as r increased to about 100 mm. The propagation velocity of the surface discharge in this range of r estimated from the delay time was of the order of 104 m/s, which is similar to the order of the propagation velocity of local discharge on the electrolyte surface [28,29]. For s ¼ 5.0 S/m (Fig. 6(b)), the potential waveforms were similar to the discharge current waveforms, as shown in Fig. 4(b). The propagation velocity of the discharge is high due to the high s since there was no significant delay in the maximum potential as r increases. Fig. 7 shows the water surface potential V as a function of r, where V is the maximum of potential immediately after the commencement of discharge. The influence of probe connection toward this electric potential distribution on water surface is concluded as negligible. This is because from the observation results of light emission profiles (Fig. 2), maximum extend length of filamentary discharge (Fig. 3), voltage and current waveforms (Fig. 4), and current density of electrodes 1e9 (Fig. 5), the difference in measurement due to the probe was not observed. Although the flashover voltage of SDWS changed with difference in s (as mentioned in Section 3.2), it is confirmed that the magnitude of V tends to decrease with increasing s. The V distribution differs significantly between tap water (0.07 S/m) and water with higher s. This result qualitatively agrees with the changes of the filamentary discharge on the water surface, which differed drastically between 0.07 S/m and other s, as shown in Fig. 3. From these electric potential data, the electric field distribution on water surface in r-direction, Er was calculated where Er ¼ gradF. From the calculation result, the electric field is mostly distributed at the vicinity of filamentary discharge as shown in Fig. 2 and Fig. 3. However, only small values of electric field were exhibited, indicating weak electric field strength on water surface with the filamentary discharge propagation. From this, it can be assume that electric field strength is high at the vicinity (or at the tip) of the discharge electrode considering that streamer is initiated in the region with high field strength and propagates to enter a region with weak electric field [30], which is the water surface in this case.

(b) 1.5 Water surface potential [kV]

Normalized current density, ii.nor

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filamentary discharge on water surface with the underwater discharge current distribution. As the filamentary discharge is affecting the current distribution to the electrodes, the electrodes’ distance from water surface is also an important factor for the current distribution. This can be seen from the electrodes’ placement where electrodes 1e6 were installed at the bottom of the water tub while electrodes 7e9 were at the side of the water tub.

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Fig. 6. Typical water surface potential waveforms for s of (a) s ¼ 0.07 S/m, (b) s ¼ 5.0 S/m.

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3.5. Numerical calculation of discharge current distribution Electric field calculations with the potential on water surface as the only boundary condition were performed to roughly evaluate the discharge current observed in the experiment. As this work was done using a pulsed power supply, a time dependence calculation is ought to be employed. For this, the charge density, diffusion and recombination of charged particles are needed to be known beforehand. Furthermore, in this work, the nature of filamentary discharge on water surface and its effect toward the quantitative parameter such as charge density and ion mobility also need to be considered. However, at this current moment, these data are still not fully accumulated. As an alternative, a DC calculation of current distribution with measured water surface potential as the boundary condition was attempted. In this numerical calculation, an axially symmetry two-dimensional model of the water-phase region shown in Fig. 8 was used, where the electric potential V (r, z) on the water surface for the boundary condition was the measured value shown in Fig. 7. The calculation employs ion flow field calculation which includes Poisson’s equation, div (grad V) ¼ r/ε and current continuity equation, i ¼ mrE with several general assumptions. By assuming that water behaves as a low resistance, constant liquid permittivity, ε and conductivity, s (¼ rm; where r: space charge density, m: ion mobility) were employed in this calculation. Simple calculations of the electric field, E (r, z) ¼ egrad V and the current density, i (r, z) ¼ sE (r, z), were performed by the finite element method with 1000 triangular meshes. Fig. 9

Φ = measured value

water surface

3 4 5 6 7 No. of grounding electrode

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Fig. 9. Analysis result of discharge current density at grounding electrodes.

compares the calculated current density ii at the grounding electrodes with the experimental values of the peak current. Although this result was obtained by DC analysis of the numerical data, the calculation and experimental data agree with each other, confirming the correspondence between the potential gradient on the water surface and the distribution of ii. 4. Concluding remarks Electrical conduction during impulse flashover on a water surface interfaced with the atmosphere was investigated. An axially symmetric electrode system was constructed with nine underwater grounding electrodes. The distribution of the discharge current at the electrodes, potential distribution on the water surface, and spreading of the filamentary discharge path on the water surface were evaluated at different water conductivities. In the case of tap water (conductivity of 0.07 S/m), the spark discharge profile exhibited behavior indicating resistive barrier discharge at the water surface. This behavior did not appear when the conductivity was increased to 0.2 S/m. A small difference in the conductivity was found to significantly affect the spark discharge process, which can be seen from the discharge current waveforms and filamentary discharge at the water surface. We conclude that the water conductivity has significantly influence to the electric potential distribution on the water surface, and determines the discharge current distribution at the underwater grounding electrodes. From the findings of this work, development of a discharge model of spark discharge on water surface that will consider all the parameters regarding this type of discharge is expected. The development of this model will allow the visualization of the discharge; in consequently can be utilized in various applications of SDWS. Acknowledgment

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The authors would like to thank M. K. A. Mohamad for the electric potential data. The first author is also grateful to the International Islamic University Malaysia (IIUM) and Ministry of Higher Education (MoHE) Malaysia for a graduate fellowship to support her PhD research. References

r Fig. 8. Computational model for discharge current analysis.

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