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Current–voltage characteristics of alternating electric field charger
Journal of Electrostatics 58 (2003) 77–89
Current–voltage characteristics of alternating electric field charger M. Lackowski*, A. Jaworek, A. Krupa Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, P.O. Box 621, 80-952 Gdansk, * Poland Received 21 May 2002; received in revised form 21 October 2002; accepted 27 October 2002
Abstract Charging of liquid droplets or solid particles is widely used in many industrial electrostatic processes such as surface coating or painting, particle separation, scrubbing or two-stage electrostatic precipitation. Employing electrical forces allows improving efficiency of these processes or save energy. The throughput of these processes can be increased by maximizing the charge imparted to the particles and minimising their loss during the charging. This goal can be accomplished by charging the particles in an alternating electric field and ionic current. The rate of particle charging by this type of charger is proportional to the space charge density in the charging zone. The effect of electrode geometry, gas velocity and the frequency of alternating electric field on the space charge in the alternating electric field charger are presented in this paper. The discharge current and the space charge increase with the frequency increasing in the range from 50 to 400 Hz, and next they decrease. The gas velocity has only a slight effect on the discharge current, but a small maximum for gas velocity of about 0.2 m/s was observed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Droplet charging; AC charger; Aerosol particle; Corona charge
1. Introduction The alternating electric field charger [2] is a device, which can be used for charging aerosol particles to extremely high electric charge with reduced loss of the particles within the charger. This device can be applied in many technological processes such *Corresponding author. Tel.: +48-58-3460881ext 292; fax: +48-58-3416144. E-mail address: [email protected] (M. Lackowski). 0304-3886/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 2 ) 0 0 2 0 0 - 0
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as surface coating, particle separation, or flue gas cleaning, for improving their efficiency. Electrostatic charging of small particles in DC corona discharge is usually not effective because the charge, which can be acquired by the particles is much lower than the saturation charge predicted by the Paunthenier formula. The particles during the charging process are deflected in the electric field to the corona-counter electrode where they are precipitated. High flow velocity is usually used as remedy against particle loss, but the time during which the particle remains in the ionized gas is too short, and the charge cannot be very high. Charging in ionized gas and alternating electric field was therefore proposed to solve the problem with particle precipitation, and to maximize the charge imparted to the particle. Two types of chargers utilising this concept were proposed in literature. In the device developed by Masuda et al. [1], the ions were generated by high frequency (16 kHz) surface discharge, and alternating electric field of low frequency (500 Hz) was superimposed by additional pair of electrodes to cause oscillating motion of the particles. In the charger designed by Jaworek and Krupa [2], the ions were emitted by two corona discharge electrodes located at opposite side of a channel, and the alternating electric field was generated by two additional grids, placed between them. The particles are charged in the charging zone between the grids by ionic current flowing alternatingly from opposite directions. The advantage of the bidirectional charging is that the charged particles undergo oscillations due to the alternating electric field between the grids and are not precipitated on the electrodes. Particles of high resistivity are also easier charged by the ionic currents flowing from both directions because the charge can be more uniformly distributed over the particle surface. The charge imparted to the particles by this type of charger depends on the ionic current flowing through the charging zone, charging time, and the electric field inside it. The effect of ionic current, electric field, and frequency on particle trajectories was studied by Adamiak et al. [3], and Lackowski [4]. The magnitude of the ionic current and the space charge in the charging zone depend, however, on the charger geometry, i.e., the distance between the discharge electrodes and the grids, and also on the space between the grids. The discharge and charging processes proceed in flowing gas. The discharge in flowing gas, as met in alternating electric field charger, is quite different from that in stationary conditions. The issue of corona discharge in flowing gas has not been extensively studied in literature. Most of papers concern characteristics of corona discharge in point to plane geometry, and only a few of them discuss the problem of corona discharge in flowing gas [5–8]. Discharge in the alternating field charger is generated by a multipoint electrode, but only a few papers were devoted to this problem. Lama and Gallo [9], Boulloud et al. [10], and Abdel Salam et al. [11], and recently Noll [12] investigated the discharge in a two-point-plane geometry. More complex geometrical configurations of discharge electrode, which are of more practical value have only be investigated by Thanh [13], McKinney et al. [14] and Jaworek and Krupa [15–17]. Thanh determined the current–voltage characteristics of a multiple point electrode with needles arranged linearly or circularly. The distribution of the corona current at the
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grounded plate against a barbed (multipoint) discharge electrode was studied by McKinney et al. [14]. They discovered that the space charges generated by two adjacent points do not overlap each other. The characteristics of corona discharge between a multipoint and plate electrodes for stationary conditions and in flowing gas were determined by Jaworek and Krupa [15–17], but for geometry different from that used in the alternating electric field charger. The authors noticed that the discharge current changes significantly for low gas velocity, lower than 1 m/s, but for higher (up to 4 m/s) the current stabilizes. The experimental results of investigations of the effect of charger geometry and gas velocity on the discharge and charging current are presented in this paper. The ionic current distribution between the electrodes in the alternating electric field charger, its dependence on the AC voltage, its frequency and gas velocity are also given. The research is aimed at the optimization of the alternating electric field charger with respect to maximization of the charging current and energy efficiency.
2. Experimental set-up The scheme of the alternating electric field charger is shown in Fig. 1. The charger consists of two discharge electrodes and two grids spaced forming the charging zone. The grids C and D were made of 7 stainless-steel rods 2 mm in diameter, and 130 mm high. The discharge electrodes, A and B, were composed of array of 8 13 stainlesssteel needles, spaced 15 mm each other. The diameter of the needles was 0.8 mm, its length 6 mm and the angle of the cone about 151C. The electrodes of the charger were supplied from two high voltage transformers by the diode circuit, as shown in Fig. 1. The windings of the transformers were connected in anti-phase to obtain opposite polarity of the potential at each grid at the same half-cycle. In the first half-cycle of the AC voltage, the electrodes A and C are at positive potential, while the discharge electrode B is at negative potential, and the grid D is grounded. The negative ions emitted from the electrode B are accelerated by the electric field to the grid C. A fraction of the ions pass through the grid to the charging zone between the grids C and D. Particles flowing across the charging zone are bombarded by these ions and gain an electric charge. In the second half-cycle of the AC voltage the electrodes’ potential is reversed, i.e., the negative ions are emitted from the electrode A, the grid C is grounded, and ions flow throughout the charging zone to the grid D. In order to determine the ionic current flowing through the charging zone, the circuit was modified as shown in Fig. 2. By this modification the charger operates only in one half-cycle of the AC voltage, while in the other the electrodes are grounded. In this circuit, the discharge current of the electrode A, the ionic current flowing to the nearest grounded grid C, and the charging current flowing to the opposite grid D can be measured separately. The ionic current flowing to the electrodes in one half-cycle is not distorted by the current flowing oppositely in the second half-cycle. It was therefore possible to determine the ionic current flowing through the charging zone in one half-cycle only.
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Fig. 1. Schematic diagram of the alternating electric field charger.
The currents were measured by standard dc moving-coil ammeters, which are immune to pulse character of the discharge current, and can operate at high potentials without special precautions. It should be noticed that, in this circuit (Fig. 2), the charger operates similarly to a corona triode, and the results can be directly transferred to the corona-triode devices. The effects of gas velocity on the discharge current and the space charge in the charging zone were investigated in a channel of 160 160 mm cross section. The gas flow was forced by a suction pump and the gas velocity was measured by the hotwire anemometer model 8455 produced by TSI.
3. Results and discussion The effects of electrode geometry on the discharge current, ionic charging current, and the space charge in the charging zone were determined experimentally.
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Fig. 2. Circuit for measurement of ionic currents in the alternating electric field charger.
The charge q imparted to a particle in the charger is given by the following differential equation: qq qðtÞðer þ 2Þ 2 ¼ 3pnðtÞmEðtÞr2 1 ; ð1Þ qt 12pe0 EðtÞr2 er where r is the particle radius, er is the particle relative permittivity, m is the ion mobility, n is the space charge density in the charging zone, and E is the electric field. The saturation charge, acquired by the particle after an infinite time of charging is qs ¼ 12pe0 r2 Em
er ; er þ 2
ð2Þ
where Em is the amplitude of the alternating electric field. The trajectory of the particle depends on the electric field, gravitational force and air drag. The particle motion is given by the following Newton equation: d2 x% dx% m 2 ¼ mg% þ qE% þ 6pZrcx u% ; ð3Þ dt dt
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where g is the gravitational acceleration, q is the particle charge, m is the particle mass, Z is the gas viscosity and cx is the drag coefficient. In Fig. 3 are presented the particle trajectories obtained by numerical simulation. The particles flowing through the charger oscillate following the alternating electric field vector. The amplitude of oscillation decreases with increasing frequency, and for this reason higher frequencies are more appropriate for particle charging. Lower amplitude of oscillation results in lower probability of particle precipitation on the electrodes. A trajectory of a particle in a charger of similar geometry but supplied
5 50Hz 10kV-ac 4 Z Position [mm]
10kV-dc 3 2
100Hz 10kV-ac
1 0 -1
0
5 150Hz 10kV-ac
10
15
flow
20
Vx = 0,5m/s
-2 X Position [mm]
Fig. 3. Particle trajectories for DC voltage and AC voltage of different frequencies.
Fig. 4. A photo of particle trajectories in the alternating electric field charger for frequency of 150 Hz (top view).
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Discharge current [µA]
40 35
40-40-40 45-40-45
30
50-40-50 25
55-40-55
20
60-40-60
15 10 5 0 6
8
10
12
14
16
14
16
Voltage [kV]
(a)
Discharge current [µA]
40 35
40-55-40
30
45-55-45 50-55-50
25
55-55-55 60-55-60
20 15 10 5 0 6
(b)
8
10
12
Voltage [kV]
Fig. 5. Current voltage characteristics for the discharge electrode. Distance between the grids: (a) 40 mm, (b) 55 mm. The number in the legend refer to the following distance: 1st: between first discharge electrode and grid, 2nd between the grids, 3rd the distance between second grid and discharge electrode (passive).
with DC voltage is also drawn in Fig. 3. It is evident from the figure that after a short time, the particle trajectory is deflected towards an electrode, where it is lost, whereas the particles oscillating in the alternating electric field can be born by the flowing air out of the charger. A photo of particle trajectories in the alternating electric field charger is shown in Fig. 4. The particles flow along the charger with oscillations perpendicular to the grids without tendency to precipitation. The charge imparted to the particle in time interval dt is proportional to the space charge n in the charging zone (cf. Eq. (1)), which depends on the charging current. The charging current is the current, which flows to opposite grid through the charging zone (i.e. to the grid D when A is the discharge electrode). It depends on the discharge current and electrode geometry. The corona discharge current–voltage characteristics for the discharge electrode of the charger are shown in Fig. 5. The discharge current decreases with the increase in the distance between the discharge electrode and the nearby grid. The charging current as function of AC voltage is presented in Fig. 6. The charging current changes according to the discharge current,
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Charging current [µA]
16 14
40-40-40
12
45-40-45 50-40-50
10
55-40-55 60-40-60
8 6 4 2 0 6
8
10
12
14
16
14
16
Voltage [kV]
(a)
Charging current [µA]
16 14
40-55-40
12
45-55-45
10
50-55-50 55-55-55
8
60-55-60
6 4 2 0 6
(b)
8
10
12
Voltage [kV]
Fig. 6. Charging current vs. AC voltage in alternating electric field charger. Distance between the grids: (a) 40 mm, (b) 55 mm. (legend as in Fig. 5).
but it can be seen that the charging current slightly decreases with the increase of the distance between the grids. This is caused by the decrease of the electric field in the plane of the grid C when the distance increases. An intrinsic property of the charger is that only a fraction of the discharge current flows through the charging zone. This property lowers the total energy efficiency of the charger. The energy efficiency of the charger can be measured as the ratio of the charging current to the discharge current. It is assumed that the charging current is that flowing to the counter grid D. This current ratio vs. supply voltage, for various distances between the electrodes is presented in Fig. 7. The analysis of the measurements indicate that the ratio of the charging current to discharge current is roughly about 1/2 and only weakly depends on voltage, but slightly increases when the distance between the discharge electrode and the grid increases. The rate of particle charging is proportional to the space charge density n in the charging zone (cf. Eq. (1)), and high magnitude of space charge is required to make the charging process most efficient. The space charge density can be estimated
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Charging current/ Discharge current
1
0.8
0.6
0.4
0.2
40-40-40
45-40-45
50-40-50
55-40-55
60-40-60
0 6
8
10
12
14
16
14
16
Voltage [kV]
(a) 1
Charging current/ Discharge current
40-55-40 0.8
45-55-45 50-55-50 55-55-55
0.6
60-55-60 0.4 0.2 0 6
(b)
8
10
12
Voltage [kV]
Fig. 7. The ratio of the charging current to discharge current vs. voltage. AC frequency f ¼ 50 Hz. Distance between the grids: (a) 40 mm and (b) 55 mm (legend as in Fig. 5).
from the relation: n¼
Id ; mUA
ð4Þ
where I is the charging current, d is the distance between the grids, m is ion mobility, U is AC voltage, and A is the surface of a grid. The phase shift between the current and the voltage was neglected in Eq. (4). The effect of voltage and electrode distances on the space charge density is shown in Fig. 8. The space charge density between the grids (Fig. 8a) decreases with the distance between the discharge electrode and the grid increasing because the discharge current decreases. This is mainly an effect of the decrease in the discharge current (cf. Fig. 5). The effect of distance between the grids on the space charge density is presented in Fig. 8b. The space charge is practically independent of the distance between the grids, because of the decrease in the charging current (cf. Fig. 6) is balanced by increasing space between the grids (cf. (4)). Short distance between the grids causes an increase in the charging current, however, this remains in contradiction with possibly wide charging zone required, because too narrow zone can cause particle precipitation on the electrodes. The
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Space charge density [µC/m3]
12
13kV
10
12kV
8
11kV 6
10kV
4 2 0 40
45
50
55
60
Distance between discharge electrode and grid [mm]
(a)
Space charge density [µC/m3]
8 13kV
6
12kV 4 11kV 10kV 2
0 40
45
50
55
60
Distance between grids [mm]
(b)
Fig. 8. Space charge in the charging zone vs. (a) distance between the discharge electrode and grid (for distance between the grids 40 mm), (b) distance between the grids (for distance between the discharge electrode and grid 55 mm).
Charging current [µA]
12
13kV
10
12kV
8
11kV 6
10kV 4 2 0 0
100
200
300
400
500
Frequency [Hz] Fig. 9. The charging current as a function of frequency. Distance between the discharge electrodes and grids 55 mm, distance between the grids 40 mm.
distance between the grids should therefore be a result of the trade-off between the maximum current ratio (energy efficiency) and wide inter-grid space required (particle loss). For further analysis the space between grids was adjusted to 40 mm, and the distance between each discharge electrode and nearby grid to 55 mm.
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The effect of frequency of the AC voltage on the charging current, and charging to discharge current ratio is presented in Figs. 9 and 10. The results show that charging current increases with frequency increasing up to about 400 Hz and next starts to slightly decrease. That is probably caused by low mobility of ions, which do not follow changes in the electric field within each half-cycle of the AC voltage (Fig. 10). The current ratio is only weakly dependent on frequency, with flat minimum at the frequency of about 150 Hz (Fig. 11), that is difficult to interpret in terms of ion mobility. The discharge and charging currents as function of gas velocity are shown in Figs. 11 and 12. There is only a low effect of gas velocity on these currents, except for gas velocity of about 0.2 m/s, for which the discharge current slightly decreases. The ratio of the charging current to discharge current increases for low velocities, as shown in Fig. 13 and next stabilizes.
Charging current/ Discharge current
1.00 0.80 0.60
13kV 12kV
0.40
11kV 0.20
10kV
0.00 0
100
200
300
400
500
Frequency [Hz]
Fig. 10. The ratio of the charging current to discharge current vs. frequency. Distance between the discharge electrodes and grids 55 mm, distance between the grids 40 mm.
Discharge current [µA]
16 13kV 14 12kV
12 10
11kV
8 10kV
6 4 2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 11. The discharge current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
M. Lackowski et al. / Journal of Electrostatics 58 (2003) 77–89
88
Charging current [µA]
10
13kV 8
12kV 6
11kV 4
10kV 2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 12. The charging current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
Charging current/ Discharge current
1 0.8 0.6
10kV 11kV 12kV 13kV
0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 13. The ratio of the charging current to the discharge current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
4. Conclusions The measurement of ionic currents in an alternating electric field charger were carried out and presented in the paper. Of particular interest was the current flowing through the charging zone of the charger, which is of fundamental importance for charging of aerosol particles by this device. The results show that the most optimal distance between the discharge electrode and grid is 55 mm because the energy efficiency is maximum. The maximum of charging to discharge current ratio was obtained for the space between the grids equal to 40 mm. It was noticed that the space charge density in the charging zone is practically independent of the distance between the grids. A choice of appropriate frequency of supply voltage is particularly important in this device because the amplitude of particle oscillation is lower for higher frequencies, and precipitation of particles on the charger electrodes becomes also low. However, for too high frequency, above 400 Hz, the charging current
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decreases due to low ion mobility. The investigations of the effect of gas velocity on the discharge current show that for velocity up to about 1 m/s, the changes in the discharge current are rather small, only for velocity of about 0.2 m/s a small minimum in the discharge current was observed.
References [1] S. Masuda, M. Washizu, A. Mizuno, K. Akutsu, Boxer charger—a novel charging device for high resistivity powders, Proceedings of the IEEE/IAS Annual Meeting, Toronto, Canada, 1978, pp. 16–22. [2] A. Jaworek, A. Krupa, Airborne particle charging by unipolar ions in ac electric field, J. Electrostat. 23 (1989) 361–370. [3] K. Adamiak, A. Krupa, A. Jaworek, Unipolar particle charging in an alternating electric field, Inst. Phys. Conf. Ser. 143 (1995) 275–278. [4] M. Lackowski, Unipolar charging of aerosol particles in alternating electric field, J. Electrostat. 51–52 (2001) 225–231. [5] J.A. Chalmers, The effect of wind on point-discharge pulses, J. Atmos. Terr. Phys. 27 (1965) 1037–1038. [6] K.J. Nygaard, Frequency of corona discharge Trichel pulses in air flows, J. Appl. Phys. 37 (7) (1966) 2850–2852. [7] T. Yamamoto, P.A. Lowless, L.E. Sparks, Narrow gap point-to-plane corona with high velocity flow, IEEE Trans. Ind. Appl. 24 (5) (1988) 934–939. [8] K. Okumura, S. Sumihiro, T. Nozue, T. Anai, T. Kawamura, Measurements of fluctuations and pulse waveform of DC negative corona pulse with needle to plane gap in air flow, 11th International Conference on Gas Discharges and their Applications, Chuo University, Tokyo, 11–15 September 1995. [9] W.L. Lama, C.F. Gallo, Interaction of the ‘Trichel’ current pulses of a pair of negative coronas, J. Phys. D 6 (16) (1973) 1963–1972. [10] A. Boulloud, J. Charrier, W.B. Stark, R.T. Waters, Interaction between adjacent positive glow corona, Seventh International Conference on Gas Discharges, London, 31 August–3 September 1982. [11] M. Abdel Salam, A. Abdel Fattah, M.M. Saied, M. Tharwat-El-Mohandes, Positive corona pulse characteristics from two interacting needles in Air, IEEE Trans. Ind. Appl. 21 (4) (1985) 912–918. [12] C.G. Noll, Charge-carrier extraction from air and nitrogen gas streams that entrain charge from dc corona ionizers, J. Electrostat. 54 (2002) 271–282. [13] L.C. Thanh, Negative corona in a multiple interacting point to plane gap in air, IEEE Trans. Ind. Appl. 21 (2) (1985) 518–522. [14] P.J. McKinney, J.H. Davidson, D.M. Leone, Current distribution for barbed plate-to-plane coronas, IEEE Trans. Ind. Appl. 28 (6) (1992) 1424–1431. [15] A. Jaworek, A. Krupa, Corona discharge in multipoint-to-plane geometry, Inst. Phys. Conf. Ser. 143 (1995) 289–292. [16] A. Jaworek, A. Krupa, Electrical characteristics of a corona discharge reactor of multipoint-to-plane geometry, Czech. J. Phys. 45 (12) (1995) 1035–1047. [17] A. Jaworek, A. Krupa, Corona discharge from a multipoint electrode in flowing air, J. Electrostat. 38 (1996) 187–197.
Current–voltage characteristics of alternating electric field charger M. Lackowski*, A. Jaworek, A. Krupa Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, P.O. Box 621, 80-952 Gdansk, * Poland Received 21 May 2002; received in revised form 21 October 2002; accepted 27 October 2002
Abstract Charging of liquid droplets or solid particles is widely used in many industrial electrostatic processes such as surface coating or painting, particle separation, scrubbing or two-stage electrostatic precipitation. Employing electrical forces allows improving efficiency of these processes or save energy. The throughput of these processes can be increased by maximizing the charge imparted to the particles and minimising their loss during the charging. This goal can be accomplished by charging the particles in an alternating electric field and ionic current. The rate of particle charging by this type of charger is proportional to the space charge density in the charging zone. The effect of electrode geometry, gas velocity and the frequency of alternating electric field on the space charge in the alternating electric field charger are presented in this paper. The discharge current and the space charge increase with the frequency increasing in the range from 50 to 400 Hz, and next they decrease. The gas velocity has only a slight effect on the discharge current, but a small maximum for gas velocity of about 0.2 m/s was observed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Droplet charging; AC charger; Aerosol particle; Corona charge
1. Introduction The alternating electric field charger [2] is a device, which can be used for charging aerosol particles to extremely high electric charge with reduced loss of the particles within the charger. This device can be applied in many technological processes such *Corresponding author. Tel.: +48-58-3460881ext 292; fax: +48-58-3416144. E-mail address: [email protected] (M. Lackowski). 0304-3886/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 2 ) 0 0 2 0 0 - 0
78
M. Lackowski et al. / Journal of Electrostatics 58 (2003) 77–89
as surface coating, particle separation, or flue gas cleaning, for improving their efficiency. Electrostatic charging of small particles in DC corona discharge is usually not effective because the charge, which can be acquired by the particles is much lower than the saturation charge predicted by the Paunthenier formula. The particles during the charging process are deflected in the electric field to the corona-counter electrode where they are precipitated. High flow velocity is usually used as remedy against particle loss, but the time during which the particle remains in the ionized gas is too short, and the charge cannot be very high. Charging in ionized gas and alternating electric field was therefore proposed to solve the problem with particle precipitation, and to maximize the charge imparted to the particle. Two types of chargers utilising this concept were proposed in literature. In the device developed by Masuda et al. [1], the ions were generated by high frequency (16 kHz) surface discharge, and alternating electric field of low frequency (500 Hz) was superimposed by additional pair of electrodes to cause oscillating motion of the particles. In the charger designed by Jaworek and Krupa [2], the ions were emitted by two corona discharge electrodes located at opposite side of a channel, and the alternating electric field was generated by two additional grids, placed between them. The particles are charged in the charging zone between the grids by ionic current flowing alternatingly from opposite directions. The advantage of the bidirectional charging is that the charged particles undergo oscillations due to the alternating electric field between the grids and are not precipitated on the electrodes. Particles of high resistivity are also easier charged by the ionic currents flowing from both directions because the charge can be more uniformly distributed over the particle surface. The charge imparted to the particles by this type of charger depends on the ionic current flowing through the charging zone, charging time, and the electric field inside it. The effect of ionic current, electric field, and frequency on particle trajectories was studied by Adamiak et al. [3], and Lackowski [4]. The magnitude of the ionic current and the space charge in the charging zone depend, however, on the charger geometry, i.e., the distance between the discharge electrodes and the grids, and also on the space between the grids. The discharge and charging processes proceed in flowing gas. The discharge in flowing gas, as met in alternating electric field charger, is quite different from that in stationary conditions. The issue of corona discharge in flowing gas has not been extensively studied in literature. Most of papers concern characteristics of corona discharge in point to plane geometry, and only a few of them discuss the problem of corona discharge in flowing gas [5–8]. Discharge in the alternating field charger is generated by a multipoint electrode, but only a few papers were devoted to this problem. Lama and Gallo [9], Boulloud et al. [10], and Abdel Salam et al. [11], and recently Noll [12] investigated the discharge in a two-point-plane geometry. More complex geometrical configurations of discharge electrode, which are of more practical value have only be investigated by Thanh [13], McKinney et al. [14] and Jaworek and Krupa [15–17]. Thanh determined the current–voltage characteristics of a multiple point electrode with needles arranged linearly or circularly. The distribution of the corona current at the
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grounded plate against a barbed (multipoint) discharge electrode was studied by McKinney et al. [14]. They discovered that the space charges generated by two adjacent points do not overlap each other. The characteristics of corona discharge between a multipoint and plate electrodes for stationary conditions and in flowing gas were determined by Jaworek and Krupa [15–17], but for geometry different from that used in the alternating electric field charger. The authors noticed that the discharge current changes significantly for low gas velocity, lower than 1 m/s, but for higher (up to 4 m/s) the current stabilizes. The experimental results of investigations of the effect of charger geometry and gas velocity on the discharge and charging current are presented in this paper. The ionic current distribution between the electrodes in the alternating electric field charger, its dependence on the AC voltage, its frequency and gas velocity are also given. The research is aimed at the optimization of the alternating electric field charger with respect to maximization of the charging current and energy efficiency.
2. Experimental set-up The scheme of the alternating electric field charger is shown in Fig. 1. The charger consists of two discharge electrodes and two grids spaced forming the charging zone. The grids C and D were made of 7 stainless-steel rods 2 mm in diameter, and 130 mm high. The discharge electrodes, A and B, were composed of array of 8 13 stainlesssteel needles, spaced 15 mm each other. The diameter of the needles was 0.8 mm, its length 6 mm and the angle of the cone about 151C. The electrodes of the charger were supplied from two high voltage transformers by the diode circuit, as shown in Fig. 1. The windings of the transformers were connected in anti-phase to obtain opposite polarity of the potential at each grid at the same half-cycle. In the first half-cycle of the AC voltage, the electrodes A and C are at positive potential, while the discharge electrode B is at negative potential, and the grid D is grounded. The negative ions emitted from the electrode B are accelerated by the electric field to the grid C. A fraction of the ions pass through the grid to the charging zone between the grids C and D. Particles flowing across the charging zone are bombarded by these ions and gain an electric charge. In the second half-cycle of the AC voltage the electrodes’ potential is reversed, i.e., the negative ions are emitted from the electrode A, the grid C is grounded, and ions flow throughout the charging zone to the grid D. In order to determine the ionic current flowing through the charging zone, the circuit was modified as shown in Fig. 2. By this modification the charger operates only in one half-cycle of the AC voltage, while in the other the electrodes are grounded. In this circuit, the discharge current of the electrode A, the ionic current flowing to the nearest grounded grid C, and the charging current flowing to the opposite grid D can be measured separately. The ionic current flowing to the electrodes in one half-cycle is not distorted by the current flowing oppositely in the second half-cycle. It was therefore possible to determine the ionic current flowing through the charging zone in one half-cycle only.
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Fig. 1. Schematic diagram of the alternating electric field charger.
The currents were measured by standard dc moving-coil ammeters, which are immune to pulse character of the discharge current, and can operate at high potentials without special precautions. It should be noticed that, in this circuit (Fig. 2), the charger operates similarly to a corona triode, and the results can be directly transferred to the corona-triode devices. The effects of gas velocity on the discharge current and the space charge in the charging zone were investigated in a channel of 160 160 mm cross section. The gas flow was forced by a suction pump and the gas velocity was measured by the hotwire anemometer model 8455 produced by TSI.
3. Results and discussion The effects of electrode geometry on the discharge current, ionic charging current, and the space charge in the charging zone were determined experimentally.
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Fig. 2. Circuit for measurement of ionic currents in the alternating electric field charger.
The charge q imparted to a particle in the charger is given by the following differential equation: qq qðtÞðer þ 2Þ 2 ¼ 3pnðtÞmEðtÞr2 1 ; ð1Þ qt 12pe0 EðtÞr2 er where r is the particle radius, er is the particle relative permittivity, m is the ion mobility, n is the space charge density in the charging zone, and E is the electric field. The saturation charge, acquired by the particle after an infinite time of charging is qs ¼ 12pe0 r2 Em
er ; er þ 2
ð2Þ
where Em is the amplitude of the alternating electric field. The trajectory of the particle depends on the electric field, gravitational force and air drag. The particle motion is given by the following Newton equation: d2 x% dx% m 2 ¼ mg% þ qE% þ 6pZrcx u% ; ð3Þ dt dt
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where g is the gravitational acceleration, q is the particle charge, m is the particle mass, Z is the gas viscosity and cx is the drag coefficient. In Fig. 3 are presented the particle trajectories obtained by numerical simulation. The particles flowing through the charger oscillate following the alternating electric field vector. The amplitude of oscillation decreases with increasing frequency, and for this reason higher frequencies are more appropriate for particle charging. Lower amplitude of oscillation results in lower probability of particle precipitation on the electrodes. A trajectory of a particle in a charger of similar geometry but supplied
5 50Hz 10kV-ac 4 Z Position [mm]
10kV-dc 3 2
100Hz 10kV-ac
1 0 -1
0
5 150Hz 10kV-ac
10
15
flow
20
Vx = 0,5m/s
-2 X Position [mm]
Fig. 3. Particle trajectories for DC voltage and AC voltage of different frequencies.
Fig. 4. A photo of particle trajectories in the alternating electric field charger for frequency of 150 Hz (top view).
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Discharge current [µA]
40 35
40-40-40 45-40-45
30
50-40-50 25
55-40-55
20
60-40-60
15 10 5 0 6
8
10
12
14
16
14
16
Voltage [kV]
(a)
Discharge current [µA]
40 35
40-55-40
30
45-55-45 50-55-50
25
55-55-55 60-55-60
20 15 10 5 0 6
(b)
8
10
12
Voltage [kV]
Fig. 5. Current voltage characteristics for the discharge electrode. Distance between the grids: (a) 40 mm, (b) 55 mm. The number in the legend refer to the following distance: 1st: between first discharge electrode and grid, 2nd between the grids, 3rd the distance between second grid and discharge electrode (passive).
with DC voltage is also drawn in Fig. 3. It is evident from the figure that after a short time, the particle trajectory is deflected towards an electrode, where it is lost, whereas the particles oscillating in the alternating electric field can be born by the flowing air out of the charger. A photo of particle trajectories in the alternating electric field charger is shown in Fig. 4. The particles flow along the charger with oscillations perpendicular to the grids without tendency to precipitation. The charge imparted to the particle in time interval dt is proportional to the space charge n in the charging zone (cf. Eq. (1)), which depends on the charging current. The charging current is the current, which flows to opposite grid through the charging zone (i.e. to the grid D when A is the discharge electrode). It depends on the discharge current and electrode geometry. The corona discharge current–voltage characteristics for the discharge electrode of the charger are shown in Fig. 5. The discharge current decreases with the increase in the distance between the discharge electrode and the nearby grid. The charging current as function of AC voltage is presented in Fig. 6. The charging current changes according to the discharge current,
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Charging current [µA]
16 14
40-40-40
12
45-40-45 50-40-50
10
55-40-55 60-40-60
8 6 4 2 0 6
8
10
12
14
16
14
16
Voltage [kV]
(a)
Charging current [µA]
16 14
40-55-40
12
45-55-45
10
50-55-50 55-55-55
8
60-55-60
6 4 2 0 6
(b)
8
10
12
Voltage [kV]
Fig. 6. Charging current vs. AC voltage in alternating electric field charger. Distance between the grids: (a) 40 mm, (b) 55 mm. (legend as in Fig. 5).
but it can be seen that the charging current slightly decreases with the increase of the distance between the grids. This is caused by the decrease of the electric field in the plane of the grid C when the distance increases. An intrinsic property of the charger is that only a fraction of the discharge current flows through the charging zone. This property lowers the total energy efficiency of the charger. The energy efficiency of the charger can be measured as the ratio of the charging current to the discharge current. It is assumed that the charging current is that flowing to the counter grid D. This current ratio vs. supply voltage, for various distances between the electrodes is presented in Fig. 7. The analysis of the measurements indicate that the ratio of the charging current to discharge current is roughly about 1/2 and only weakly depends on voltage, but slightly increases when the distance between the discharge electrode and the grid increases. The rate of particle charging is proportional to the space charge density n in the charging zone (cf. Eq. (1)), and high magnitude of space charge is required to make the charging process most efficient. The space charge density can be estimated
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Charging current/ Discharge current
1
0.8
0.6
0.4
0.2
40-40-40
45-40-45
50-40-50
55-40-55
60-40-60
0 6
8
10
12
14
16
14
16
Voltage [kV]
(a) 1
Charging current/ Discharge current
40-55-40 0.8
45-55-45 50-55-50 55-55-55
0.6
60-55-60 0.4 0.2 0 6
(b)
8
10
12
Voltage [kV]
Fig. 7. The ratio of the charging current to discharge current vs. voltage. AC frequency f ¼ 50 Hz. Distance between the grids: (a) 40 mm and (b) 55 mm (legend as in Fig. 5).
from the relation: n¼
Id ; mUA
ð4Þ
where I is the charging current, d is the distance between the grids, m is ion mobility, U is AC voltage, and A is the surface of a grid. The phase shift between the current and the voltage was neglected in Eq. (4). The effect of voltage and electrode distances on the space charge density is shown in Fig. 8. The space charge density between the grids (Fig. 8a) decreases with the distance between the discharge electrode and the grid increasing because the discharge current decreases. This is mainly an effect of the decrease in the discharge current (cf. Fig. 5). The effect of distance between the grids on the space charge density is presented in Fig. 8b. The space charge is practically independent of the distance between the grids, because of the decrease in the charging current (cf. Fig. 6) is balanced by increasing space between the grids (cf. (4)). Short distance between the grids causes an increase in the charging current, however, this remains in contradiction with possibly wide charging zone required, because too narrow zone can cause particle precipitation on the electrodes. The
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Space charge density [µC/m3]
12
13kV
10
12kV
8
11kV 6
10kV
4 2 0 40
45
50
55
60
Distance between discharge electrode and grid [mm]
(a)
Space charge density [µC/m3]
8 13kV
6
12kV 4 11kV 10kV 2
0 40
45
50
55
60
Distance between grids [mm]
(b)
Fig. 8. Space charge in the charging zone vs. (a) distance between the discharge electrode and grid (for distance between the grids 40 mm), (b) distance between the grids (for distance between the discharge electrode and grid 55 mm).
Charging current [µA]
12
13kV
10
12kV
8
11kV 6
10kV 4 2 0 0
100
200
300
400
500
Frequency [Hz] Fig. 9. The charging current as a function of frequency. Distance between the discharge electrodes and grids 55 mm, distance between the grids 40 mm.
distance between the grids should therefore be a result of the trade-off between the maximum current ratio (energy efficiency) and wide inter-grid space required (particle loss). For further analysis the space between grids was adjusted to 40 mm, and the distance between each discharge electrode and nearby grid to 55 mm.
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The effect of frequency of the AC voltage on the charging current, and charging to discharge current ratio is presented in Figs. 9 and 10. The results show that charging current increases with frequency increasing up to about 400 Hz and next starts to slightly decrease. That is probably caused by low mobility of ions, which do not follow changes in the electric field within each half-cycle of the AC voltage (Fig. 10). The current ratio is only weakly dependent on frequency, with flat minimum at the frequency of about 150 Hz (Fig. 11), that is difficult to interpret in terms of ion mobility. The discharge and charging currents as function of gas velocity are shown in Figs. 11 and 12. There is only a low effect of gas velocity on these currents, except for gas velocity of about 0.2 m/s, for which the discharge current slightly decreases. The ratio of the charging current to discharge current increases for low velocities, as shown in Fig. 13 and next stabilizes.
Charging current/ Discharge current
1.00 0.80 0.60
13kV 12kV
0.40
11kV 0.20
10kV
0.00 0
100
200
300
400
500
Frequency [Hz]
Fig. 10. The ratio of the charging current to discharge current vs. frequency. Distance between the discharge electrodes and grids 55 mm, distance between the grids 40 mm.
Discharge current [µA]
16 13kV 14 12kV
12 10
11kV
8 10kV
6 4 2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 11. The discharge current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
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Charging current [µA]
10
13kV 8
12kV 6
11kV 4
10kV 2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 12. The charging current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
Charging current/ Discharge current
1 0.8 0.6
10kV 11kV 12kV 13kV
0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
Gas velocity [m/s] Fig. 13. The ratio of the charging current to the discharge current vs. gas velocity. Distance between the discharge electrode and grid 55 mm, distance between the grids 40 mm.
4. Conclusions The measurement of ionic currents in an alternating electric field charger were carried out and presented in the paper. Of particular interest was the current flowing through the charging zone of the charger, which is of fundamental importance for charging of aerosol particles by this device. The results show that the most optimal distance between the discharge electrode and grid is 55 mm because the energy efficiency is maximum. The maximum of charging to discharge current ratio was obtained for the space between the grids equal to 40 mm. It was noticed that the space charge density in the charging zone is practically independent of the distance between the grids. A choice of appropriate frequency of supply voltage is particularly important in this device because the amplitude of particle oscillation is lower for higher frequencies, and precipitation of particles on the charger electrodes becomes also low. However, for too high frequency, above 400 Hz, the charging current
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decreases due to low ion mobility. The investigations of the effect of gas velocity on the discharge current show that for velocity up to about 1 m/s, the changes in the discharge current are rather small, only for velocity of about 0.2 m/s a small minimum in the discharge current was observed.
References [1] S. Masuda, M. Washizu, A. Mizuno, K. Akutsu, Boxer charger—a novel charging device for high resistivity powders, Proceedings of the IEEE/IAS Annual Meeting, Toronto, Canada, 1978, pp. 16–22. [2] A. Jaworek, A. Krupa, Airborne particle charging by unipolar ions in ac electric field, J. Electrostat. 23 (1989) 361–370. [3] K. Adamiak, A. Krupa, A. Jaworek, Unipolar particle charging in an alternating electric field, Inst. Phys. Conf. Ser. 143 (1995) 275–278. [4] M. Lackowski, Unipolar charging of aerosol particles in alternating electric field, J. Electrostat. 51–52 (2001) 225–231. [5] J.A. Chalmers, The effect of wind on point-discharge pulses, J. Atmos. Terr. Phys. 27 (1965) 1037–1038. [6] K.J. Nygaard, Frequency of corona discharge Trichel pulses in air flows, J. Appl. Phys. 37 (7) (1966) 2850–2852. [7] T. Yamamoto, P.A. Lowless, L.E. Sparks, Narrow gap point-to-plane corona with high velocity flow, IEEE Trans. Ind. Appl. 24 (5) (1988) 934–939. [8] K. Okumura, S. Sumihiro, T. Nozue, T. Anai, T. Kawamura, Measurements of fluctuations and pulse waveform of DC negative corona pulse with needle to plane gap in air flow, 11th International Conference on Gas Discharges and their Applications, Chuo University, Tokyo, 11–15 September 1995. [9] W.L. Lama, C.F. Gallo, Interaction of the ‘Trichel’ current pulses of a pair of negative coronas, J. Phys. D 6 (16) (1973) 1963–1972. [10] A. Boulloud, J. Charrier, W.B. Stark, R.T. Waters, Interaction between adjacent positive glow corona, Seventh International Conference on Gas Discharges, London, 31 August–3 September 1982. [11] M. Abdel Salam, A. Abdel Fattah, M.M. Saied, M. Tharwat-El-Mohandes, Positive corona pulse characteristics from two interacting needles in Air, IEEE Trans. Ind. Appl. 21 (4) (1985) 912–918. [12] C.G. Noll, Charge-carrier extraction from air and nitrogen gas streams that entrain charge from dc corona ionizers, J. Electrostat. 54 (2002) 271–282. [13] L.C. Thanh, Negative corona in a multiple interacting point to plane gap in air, IEEE Trans. Ind. Appl. 21 (2) (1985) 518–522. [14] P.J. McKinney, J.H. Davidson, D.M. Leone, Current distribution for barbed plate-to-plane coronas, IEEE Trans. Ind. Appl. 28 (6) (1992) 1424–1431. [15] A. Jaworek, A. Krupa, Corona discharge in multipoint-to-plane geometry, Inst. Phys. Conf. Ser. 143 (1995) 289–292. [16] A. Jaworek, A. Krupa, Electrical characteristics of a corona discharge reactor of multipoint-to-plane geometry, Czech. J. Phys. 45 (12) (1995) 1035–1047. [17] A. Jaworek, A. Krupa, Corona discharge from a multipoint electrode in flowing air, J. Electrostat. 38 (1996) 187–197.