Ke CHEN (陈可), Lianghao WAN (万良淏), Bingyan CHEN (陈秉岩),Tao CHU (楚涛), Renyue GENG (耿任悦), Deyu SONG (宋德禹),Xiang HE(何湘),Wei SU(苏巍),Cheng YIN(殷澄),Minglei SHAN(单鸣雷)and Yongfeng JIANG (蒋永锋)

1 College of Science, Hohai University, Nanjing 210098, People’s Republic of China

2 Nanjing Suman Plasma Science and Technology Co., Ltd, Nanjing 211162, People’s Republic of China

3 Jiangsu Key Laboratory of Power Transmission and Distribution Equipment Technology, Changzhou 213022, People’s Republic of China

4 College of Mechanical and Electrical Engineering,Hohai University,Nanjing 210098,People’s Republic of China

Abstract The physical and chemical properties have significant differences for the positive and negative charged particles generated by discharge.In this work,a positive and negative corona discharge system was established, and two discharge reactors for charged particles restraining and acting were designed by a needle electrode covered with a quartz tube and a plate electrode filled with water.The corona discharges happened within the needle-plate electrodes were excited by a positive and negative high voltage source,and the characteristics of both water volatilization and oxides generation were examined within influence of the distances of both quartz tube inside and outside.The results show that the characteristics of both the water volatilization and oxides generation can be affected by the distances of both quartz tube inside and outside.When the distances of tube inside were increased from 5.00 to 13.00 mm, the water volatilizations decreased under negative corona, and increased firstly and declined immediately under positive corona.The maximum value of the water volatilization appeared in the distances of tube inside with 6.00-8.00 mm.In addition, the concentrations of the HNOx and H2O2 in treated water decreased with increasing the distances of tube inside.Moreover,with increasing the distances of tube outside from 4.00 to 14.00 mm, the change trends of both the water volatilizations and oxides presented the same as the distances of tube inside, and the maximum value of the water volatilization and oxides appeared in the distance of tube outside with about 9.00 mm.Overall,the positive corona can generate more water volatilizations and oxides in water than negative corona, and non H2O2 can be produced by negative corona.The results reflect the difference between positive and negative corona interaction with water, which can provide reference for plasma application.

Keywords: positive corona, negative corona, water volatilization, oxides generation

1.Introduction

High voltage discharge is accompanied by electron impact,light irradiance,shock wave and other processes[1,2].There are many reactive species in the discharge area, such as ultraviolet (UV), energetic electrons, hydroxyl radical (OH),oxygen radical (O), ozone (O3), and nitrogen oxides (NOx),and so on.Therefore, discharge plasmas have high potential application value in the environmental remediation [3, 4],advanced materials [5-7], life and health [8, 9], low-carbon energy [10, 11], advanced manufacturing [12], agricultural production [13, 14], and other fields [15-17].

Plasmas in and interaction with water have become the research hotspot for their complex process and wide application prospects[18-20].The key factors in effect of plasmaliquid system are the generation,transport and transformation of the reactive species from discharge within the gas-liquid two-phase [21-23].Some long-lived particles in these reactive species can react with water and produce short-lived particles.Among those reactive species, the hydroxyl radical(OH)has been great attracted because of high oxidation potential, which has been widely researched in its generation[22, 24, 25], formation [26], distribution [27], and detection[28, 29].Additionally, ozone (O3)[18, 30, 31] and nitrogen oxides (HNOx)[21, 32, 33] from discharge also have great application and research value.

Corona is a kind of self-sustaining discharge in an extremely non-uniform electric field [17, 34].According to the power supply polarity of the applied voltage loaded on the electrode, it can be divided into two types as the positive corona and negative corona.The polarity and energy of charged particles are different between the positive and negative corona.That can lead to different effects of the charged particles interaction with matter.For instance, the positive corona discharge can better modify the surface of white pine than the negative corona[35].Similarly,when the positive and negative discharges interact with liquid,particles with different polarities and energies have different effects.However, there are few studies on the difference in the interaction of positive and negative corona and water, which restricts the application of corona discharge.

In order to investigate the differential properties of the interactions between charged particles of different polarities and energies with water, we have designed corona discharge systems that can generate positive and negative particles and use them to treat water bodies.In this work, the needle-plate structure is used to generate corona.The needle electrode is connected with positive and negative high voltage.The plate electrode works as the ground and is immersed in deionized water.Then the corona is generated between a needle and the water surface.Additionally, a quartz tube is designed to constraint the movement of charged particle in the gap of the two electrodes, which could cause great influence on the effect of discharge [36, 37].The difference of interaction between positive and negative particles and water are researched by the water loss characteristics and oxides generation.The differences were analyzed including physical and chemical effects, and the work would provide a reference for the application of plasma interaction with water.

2.Experimental setup and method

2.1.Apparatus

The positive and negative particles will be driven to the opposite directions in different polarity electric fields as shown in figure 1.The electrons and negative ions under the negative discharge move to the space far away from the tip electrode and arrive at the water surface.At the same time,positive ions gather near the electrode surface.On the contrary, the positive ions migrate to the liquid surface while the electrons move to the needle electrode in the positive corona.During the process of particles motion, they obtain energy from the electrode field or react with each other because of continuous collisions.The particles with different polarities and energies reacting with water cause distinct phenomenon [35].

In this work, different polarity corona discharges are produced by the positive high voltage DC power supply and the negative one respectively.Figure 2 shows the experimental platform, including a discharge reactor, high voltage DC power supply (Teslaman, TCM3000), mixed domain digital storage oscilloscope (Tektronix, MDO3054), high voltage probe (North Star, PVM-5), low voltage probe(Tektronix, P2220), current sampling resistance, and microammeter (UNI-T, UT89XD).For the high voltage power supply, they are two sets of combined equipment with positive 0-30 kV and negative 0-30 kV, the maximum output current is about 1.0 mA.The positive and negative corona discharges are excited by the high voltage power supplies.The high voltage probe with oscilloscope monitors the applied voltage of the discharge reactor.The values of the applied current loaded on the discharge reactor are obtained by the micro-ammeter.Additionally,the characteristics of the discharge current are sampled by a resistance with 18.8 Ω(with 4 pieces of 4.7 Ω resistances in series).

Figure 3(a)shows the structure of the discharge reactor,which includes a quartz tube,a needle electrode connected to high voltage output,and a container (it is a plate electrode in contact with water and connected to ground).The quartz tube on the needle electrode makes the discharge distance divided into two parts, that is the distance of the tube inside and the distance of the tube outside.Figure 3(b)shows the discharge reactor structure to examine the characteristics of the water loss.The water loss in the experiment is at the milligram level.Hence, we designed a square reactor with the size of 10.00 mm × 10.00 mm × 4.00 mm,and the amount of water in the experiment is 400.0 ± 3.0 mg.

Figure 1.Diagram of particle motion.(a)Positive corona discharge, (b)negative corona discharge.

Figure 2.Experimental platform.

Figure 3(c)shows the discharge reactor structure to inspect the oxides generation in the water.The deionized water is pumped into the discharge reactor by a peristaltic pump (Shenchen LabV6, equipped a pump head YZ1515x).The concentrations of oxides(HNOxand H2O2)in the treated water are tested by a nitrite nitrogen analyzer (GDYS-101SX3)and hydrogen peroxide detector (DPM-H2O2),respectively.Figure 3(d)shows the discharge phenomenon in different distances of the quartz tube inside and outside.Additionally, all the experiments in this work are carried out under constant temperature.The experimental temperature is 18 °C and the humidity is 40%, and the water temperature is not changed by using discharge.

Figure 3.Diagram of discharge reactors.(a)Chart of interaction distance, (b)reactor for water loss, (c)reactor for oxides, (d)discharge phenomenon with different distances.

2.2.Methods

In order to evaluate the effects of positive and negative discharges interacting with water, the energy conversion efficiency of water loss and oxides generation also has been calculated.

The energy efficiency ratios (EERs)of both the water evaporation and oxides generation are defined as the numerical ratio of mass change (Δm, in the unit of mg)versus electric energy consumption (in the unit of J).In this paper,the mass changes (Δm)caused by positive and negative particles are the mass loss of water and the mass of oxides in the water.Therefore, the calculation formula of the energy efficiency ratio is as follows:

where Eeris the energy efficiency ratio, U is the applied voltage,I is the applied current,t is the treated time,which is fixed as 5.0 min in the experiment.

In the experiment of water loss rate, Δm is the mass of water loss, which is the volatile mass of deionized water in the reactor before and after treatment.

where m1is the weight of deionized water before treatment,and m2is the weight after treatment.

The water loss rate α is used to evaluate the water loss characteristics caused by positive and negative corona.The calculation formula is as follows:

In the oxides generation experiment, Δm is the amount of oxidation product, which is calculated as follows:

where, c is the concentration of oxides (mg l−1)including HNOxand H2O2, which are detected by the equipment of a nitrite nitrogen analyzer (GDYS-101SX3)and hydrogen peroxide detector(DPM-H2O2),respectively.V is the volume of treated water in each experiment (l).Unless otherwise specified, the volume of water in each experiment is fixed as about 50.0 ml in this work.

3.Results and discussion

3.1.Basic characteristics of discharge action

In order to examine the different characteristics of the interaction between discharges and water, we tested the discharge waveforms under positive/negative corona,and the treatment effects under different airflow rates.The experimental conditions are as follows, the applied voltage is 15.0 kV, the distances of quartz tube inside and outside are 5.0 mm and 10.0 mm respectively.

Figure 4 shows the characteristics of both applied voltage and current discharge for the positive and negative corona.Figures 4(a)and(c)show the waveforms with larger sampling time,that the applied voltage is stable and the applied current is occasional pulses.The details of a single current pulse are shown in figures 4(b)and (d).It can be seen that the applied current jumps rapidly and then recovers slowly at the moment of air breakdown.There are a lot of small peaks during the slow restoration of the applied current.

Figure 4.Waveforms of the applied voltage and current.(a)Macro waveforms for positive corona;(b)details for positive corona;(c)macro waveforms for negative corona; (d)details for negative corona.

Figure 5.Variation of water loss rate with gas flow.

In order to compare and contrast the effect of discharge polarity on water volatilization, we inspected the characteristics of water volatilization with the interaction including the positive corona, negative corona and flowing air with liquid water surface.In each round of experiment,the conditions are fixed as follows, the volumes of treated water are 400 μl, the treatment time is 5.0 min, the positive and negative applied voltages are 18 kV,and the distances of quartz tube inside and outside are 7.00 mm and 8.00 mm, respectively.

Figure 5 shows the volatilization characteristics of water.With no discharge, the water loss rate increases with increasing of gas flow.Compared with the positive and negative corona, the water volatilization has the same trend with no discharge.However, the corona discharges can enhance the total volatilization of water, and the positive corona can contribute more volatilization.Due to the flow gas can influence the water volatilization, unless otherwise specified, all experiments in our following research work were carried out in stagnant air.

3.2.Influence of the distances of tube inside

In the process of the interaction between discharge plasma and water,the charged particles with energy would cause the water volatilization, react with water or enter water to form oxides (such as H2O2, HNOx, and O3)[38].In the experiments, we focused on the water loss rates and oxides (H2O2and HNOx)in water.

It can be seen in figure 3(d),the discharge region reduced with increasing the distances of quartz tube inside.In order to study the influence of the distance of tube inside,we fixed the total distance at 15.00 mm, and adjusted the distance of tube inside to 5.00 mm, 7.00 mm, 9.00 mm, 11.00 mm and 13.00 mm, respectively.Figure 6 shows the water loss rates under the positive and negative discharges with 18.0 kV.When the distances of tube inside increased from 5.00 mm to 13.00 mm, the water loss rates increased first and then decreased with positive discharge.However, the water loss rates decreased with the negative discharge.Overall,the water loss rates caused by positive discharge were more than those caused by negative discharge.

Figure 6.Influence of the distance of tube inside on water loss rate.

Figure 7.Influence of the distance of tube inside on HNOx and H2O2.

The results can be explained as follows.The moving charged particles maintain an approximate free path in the discharge region due to collisions.Therefore, the average motion speeds of positive and negative particles are maintained in a certain range [34].The positive particles are mainly charged particles that interact with water, and the positive charged particles have large mass (such as the oxygen radical whose mass is 2.657 × 10−26kg)and energy.On the contrary, the negative discharge produced electron has less mass (9.10 × 10−31kg)and low kinetic energy.The positive particles are easier to have a series of physical and chemical reactions for its greater kinetic energy than the electrons, which leads to more water loss [35].

There forms an electric field because of the charged particles attached on the inner wall of quartz tube.Although the quartz tube restricted the movement of charged particles,it also inhibited the development of discharge to the surrounding space[36].This electric field intensity was related to the distances from the needle electrode to the quartz tube outlet, and it had the contrary direction to the electric field formed by the two high voltage electrodes, which led to the difference of the effect of the interaction between charged particles and water.If the distances of tube inside were small(less than 7.00 mm),the positive particles were more likely to collide with neutral particles and lose energy after sprayed out of the tube outlet.Therefore,when the distance of tube inside increases gradually, it was conducive to reduce the energy loss of collisions, and increase the water loss rate.

In an appropriate distance in the tube (about 8.00 mm),the collision probabilities of the neutral and the charged particles were reduced by the quartz tube.With the same electric field, the positive particles (mainly free radicals)and negative particles (mainly electrons)will reach the same free path.Because the positive particles have larger mass and greater kinetic energy than the electrons.Therefore,the effect of electric field in the tube on positive particles is less than that on negative particles.When the charged particles were passed through the quartz tube with large distances of tube inside(more than 8.00 mm),more charged particles would be attached on the inner wall of quartz tube, forming a larger reverse electric field.Hence, the kinetic energy and action effect of the charged particles would be weakened.

Figure 7 shows the results of oxides in treated water with different distances of quartz tube inside.The concentrations of oxides decreased with increasing of the distances of tube inside,and the generation of oxides in water had the same trend as the water loss rate.More oxides in water can be produced by positive discharges than negative discharges, and hydrogen peroxide(H2O2)in water cannot be detected by the negative corona.

Figure 8.Influence of the distance of tube outside on water loss rate.

Figure 9.Influence of the distance of tube outside on HNOx and H2O2.

This can be explained by the fact that the positive particles produced by the positive discharge have stronger oxidation ability and larger collision energy than the electrons with reduction ability and low collision energy produced by the negative discharge [31, 39, 40].

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In the discharge area, the electrons with kinetic energy collided the oxygen and water molecules and produced the positive particles(mainly the oxygen atom radical O)and the hydroxyl radicals (OH)[41-43].The main reactions are as follows:

With the positive corona discharge, the electric field force pushed the positively charged oxygen atom radicals (O)forward to the water surface, collided with water molecules and produced many hydroxyl radicals (OH).Further, a lot of hydroxyl radicals combined to form H2O2[41-43].The main reactions are as follows:

On the contrary, the kinetic energy of electrons is lower positively charged oxygen atom radicals (O), and has more difficult to generate OH [40].Therefore, the positive corona can produce more H2O2than the negative corona.

On the other hand, the positively charged O plays an important role in the formation of ozone (O3)and nitrogen oxide (HNOx).The oxygen atom radical O can react with O2to form O3with the help of heat carrier (M, N2or H2O)[44,45].Then the nitrogen molecules(N2)are oxidized by O to form nitric oxide(NO)and nitrogen atom(N)[42,46].The nitrogen atom reacts with OH to produce NO.At the same time,OH and NO react to form nitrite ion.Part of nitric oxide is oxidized by ozone to form nitrogen dioxide(NO2)[47-49].The nitric oxide and nitrogen dioxide formed by these reactions react with water to produce HNOx[47-49].Therefore,the positive corona can produce more HNOx.The main reactions are as follows:

With the negative corona discharge, the electric field force pushed the electrons forward to the water surface, collided with water molecules and produced OH,and.At the same time, the electrons acted on water surface and produced some hydr ated electrons ().This process caused the formation of HNOx.The main reactions are as follows[50]:

In general, the charged particles have quite different polarities and energies for positive corona and negative corona, and positively charged particles from positive corona have more kinetic energy than electrons from negative corona.Hence, the positive corona is more likely to produce oxides in water.

3.3.Influence of the distances of tube outside

The movement space of the charge particles would be expanded with increasing the distances of quartz tube outside,and the mean free path of charged particles would been influenced [34].In this section, we fixed the distance of tube inside as 7.00 mm, and then adjusted the distances of quartz tube outside from 4.00 to 14.00 mm.

Table 1.Applied power versus action length of tube outside.

Figure 10.The influence of applied current on water loss rate.(a)The water loss rate, (b)the energy efficiency ratio of water loss rate.

Figure 11.Oxides generation versus applied current.(a)Variation of HNOx and H2O2, (b)energy efficiency ratio.

In the figure 8, at a short distance of tube outside (from 3.00 to 7.00 mm), the water loss rate caused by positive discharge increased with the increase of the distance of tube outside.This can be explained as,when we increased the distances of tube outside,the free path of positive particles increased,and their kinetic energy increased.Additionally,in a shorter distance(less than 4.00 mm)of tube outside, the water loss is higher in the negative discharge than that of positive discharge.This phenomenon indicates that in a short distance, the electrons could cause more water loss than the positive ions.

Further, the generations of oxides were verified under different distances of tube outside,and the results were shown in figure 9.When we increased the distance of tube outside from 6.00 mm to 14.00 mm,the concentrations of both HNOxand H2O2increased first and then decreased by using the positive discharge, and the concentrations of HNOxdeclined gradually by using the negative discharge.The tendency of oxide generation was well in accord with the results of water volatilization.

In order to explore the internal reason of the change of oxide generation adjusted by the distances of tube outside,the applied power loaded on the discharge reactor was measured and shown in table 1.We can find that the applied power was regulated by the distance of the tube outside by using of both positive and negative discharges.Further, it can be seen that the tendency of the oxides generation was similar to that of the applied power, when we changed the distances of tube outside.

Overall,the energy of charged particles was regulated by the distances of both quartz tube inside and outside in both of positive and negative discharges.The effect of negative discharge decreased with increasing the distances both of tube inside and outside.For the effect of positive discharge, there was the best scale range of the distances both of tube inside and outside (the distances were about 6.00 mm to 8.00 mm and 8.00 mm for the tube inside and outside, respectively).Meanwhile, the applied power loaded discharge reactor played an important role, and the positive discharge showed more advantages for the oxides generation and water volatilization.

3.4.Influence of the applied current

In order to investigate the effect of the number of charged particles on the liquid phase,we change the applied current of the discharge reactor, so as to change the electric field intensity and the number density of charged particles in the discharge area.It should be noted that the regulation of the applied current is realized by regulating power supply voltage,and the specific value is the average current measured by microammeter.The water loss rate and reactive species in water after interaction are shown in figures 10 and 11,respectively.

Figure 10(a)shows that the water loss rates increased with the increase of applied currents of both negative and positive discharges.This can be explained as, the number densities of charged particles increased with increasing of the applied current, and more charged particles collided with water and caused the water volatilization.On the other hand,the water loss rates of negative discharge were slightly higher than that of positive discharge under same applied current.This would be related to the migration rates and electric field strengths in the discharge region.

In order to examine the electric field strengths under different applied currents,we observed the applied voltages of both negative and positive discharges under same discharge distance (with action distances as 7.00 mm and 8.00 mm of tube inside and outside, respectively)and treated time (with discharge time as 5.00 min), and found that the negative discharges require higher applied voltage than positive discharge did to maintain the same current (see figure 10(a)).This phenomenon can be explained as that the migration rate of negative ions in the air is greater than that of positive ions,and the current of negative discharge with the same applied voltage is larger than that of positive discharge.In other words, if we keep the same migration rates of charged particles(reflected as discharge current in this work),the applied voltages of negative discharges are lower than that of positive discharges [51].However, when the supply voltages loaded on the electrodes are large, the positive coronas are transformed into streamer discharges, which causes the migration rate of positive ions to be greater than that of negative ions.Therefore, for the same applied current, the applied voltages of negative discharges are higher than that of positive discharges [52, 53].

Figure 11(a)showed that the generations of HNOxin treated water increased with increasing of applied currents under both of negative and positive discharges,and H2O2had same trend with positive discharge.On the contrary, the positive discharges can generate more HNOxand H2O2in treated water than negative discharges.

In order to compare the energy conversion efficiency of water volatilization and oxides generation, we calculated the experimental results in figures 10(a)and 11(a)by using formula (1), and plotted the energy efficiency ratios in figures 10(b)and 11(b).It can be seen that the Eerof both water volatilization and oxides generation decreased gradually with the increase of the applied current.High applied current means more positive and negative particles acting on the liquid surface, which leads to more water loss and oxides generation.Moreover, the positive discharges are more efficient than the negative discharge under the same applied current.

4.Conclusion

In this work, the differences of positive and negative corona were investigated from the water volatilization and oxides generation using a needle-plate discharge reactor by means of a needle electrode covered with quartz tube and a plate electrode filled with water.The water volatilization and oxides generation by using positive discharge were found to be higher than those using negative discharge.The action effects of both positive and negative discharge were regulated by the distances of both quartz tube inside and outside.Meanwhile,there was a best scale range of the distances in both of tube inside and outside.It is anticipated that this method will be used as a reference for both the design and optimization of the corona discharge reactors in plasma applications.

Acknowledgments

This work was partially supported by Natural Science Foundation of the Jiangsu Province (No.BK20191162),Fundamental Research Funds for the Central Universities(No.B210203006), National Natural Science Foundation of China (No.11874140), and Changzhou Science and Technology Program (No.CJ20190046).

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