The contribution of space charge in a high non-homogenous electric field to the creation of negative differential conductivity

The paper describes anomaly origination on current or voltage characteristics in time of positive potential on small curvature radius electrode rod against plane electrodes. An anomaly phenomenon occurred in the narrow voltage area in a high non-homogenous electric field close to the electrode (E > 10⁷ V m^?1). A mathematical – physical analysis of the observed processes in close proximity to the electrode with the above mentioned form is made. The differential equations, which analytically and theoretically describe this phenomenon, are compiled and solved. The space charge created by element particles (electrons, ions) which causes negative differential conductivity origination in narrow voltage area by their behaviour in electric field, plays a substantial role in this phenomenon. Current–voltage characteristics at both polarities of corona electrode were continuously measured at the study of static and dynamic processes occurring during discharge in the surrounding of a small curvature radius electrode. It was proved that an anomaly, in the form of negative differential conductivity (dI/dU < 0), appeared on a highly curved electrode at positive polarity in a narrow voltage area. This phenomenon was subjected to detailed experimental research including investigation of the influence of the shape and material of the electrode (output voltage), electrode temperature, influence of photoionization on the profile of the anomaly, the contribution of exoelectrons to particle distribution, and study of electrochemical potential of metal electrode. Oscillation of low ionized plasma at positive and negative polarity of small curvature radius electrodes has been analyzed. At the same time, the influence of the external forced electric field on the change of current–voltage characteristic profile was investigated. Theoretical justification of the anomaly phenomenon resulted from a change of energy conditions in the investigated place (the distance limit from the electrode is 10^?4–10^?6 m).     

Characterization of brush type discharge electrodes and impact of enhanced corona discharge on operation of electrostatic precipitators

This project presents the results of investigation of current/voltage characteristics of brush type discharge electrodes (BTDE) in tube type electrostatic precipitators and the effect on operation. Experimental investigations were conducted with discharge electrodes of different wire diameter and different brush diameter. The effect of electrode geometry on current/voltage behavior was recorded. Corona current with brush type discharge electrodes was modeled and compared with experimental data. Brush type discharge electrodes produce an enhanced corona current compared with wire type discharge electrodes. Limited enhanced corona has improving effect on collection efficiency. An adjusted correlation was therefore deduced from experimentally obtained current/voltage data with BTDE.     

Negative corona discharge from a water droplet under the pulsating DC field

Corona discharge from a fine water droplet always involves deformation of the droplet shape or Taylor-cone formation, emission of fine water jets or disruption of droplet. Therefore, corona discharge from a water droplet always manifests complicated aspects. In addition, disruption of Taylor cone simultaneously affects not only discharge current but also motion of water droplet. To confirm corona discharge phenomena from a water droplet protruded from a tip of a metal capillary tube with a diameter of 1 mm, negative corona discharge was investigated by using a water droplet located at a tip of grounded rod electrode facing a ring electrode with positive dc voltage superimposed by ac one. Since the droplet has inherent resonant vibrating frequency defined by the size or volume, the volume of water droplet was adjusted at 20 nL where the corresponding resonant frequency was 500 Hz. The period of the event of successive corona discharge is exactly consistent with resonant frequency defined by the size of the water droplet. As a result, corona pulse trains with a definite duration appeared intermittently corresponding to its resonant vibration. When dc voltage superimposed by ac voltage with resonant frequency of 500 Hz was applied to the water droplet, corona pulse trains appeared at the period corresponding to the frequency. The maximum value of corona current reasonably increased with the applied voltage. Even when the frequency of ac field superimposed on dc field was varied from the resonant frequency, corona pulse trains occur corresponding to not only the superimposed field frequency but also resonant frequency.     

Experimental study of a modified dual-type high-voltage electrode for electrostatic separation applications

Several electrode arrangements have been proposed to enhance the efficiency of insulating materials charging by corona discharge. Recent studies pointed out that the presence of metal strips in the proximity of a dual-type high voltage electrode increases the total current measured at the surface of the collecting electrode, decreases the corona onset voltage value and enlarges the reparation of current density as well. The aim of this paper is to evaluate the benefits of using such an electrode arrangement for corona charging of non-conductive particulate materials in belt-type corona-electrostatic separators. The experimental study was carried out with samples of Aluminum and Polystyrene particles in the size class 125–250 μm. The presence of grounded strips reduces the electric wind, which is associated to corona discharge but not tolerated in such processes that involve micronized materials. At the same time, it improves the corona charging conditions of non-conductive materials and as consequence the overall efficiency of the corona-electrostatic separation process. The use of the new electrode configuration is characterized by both high recovery rates and better purities of the separated products.     

Effect of the experimental parameters on the number of free electrons in the drift region of a wire-cylinder electrostatic precipitator

The aim of this paper is to highlight the number evolution of free electrons in the drift region of a wire-cylinder electrostatic precipitator in negative voltage depends on the experimental parameters, more particularly of gas composition. A numerical model of the negative DC corona discharge developed by Chen et al. was used and modified to investigate the negative discharge corona for different gases. A parametric study was conducted to examine the effect on the electron distribution of operating conditions. The results showed the electron concentration increases with temperature, decreases when the pressure increases, and is closely related to gas composition.     

Effect of relative humidity on current–voltage characteristics of an electrostatic precipitator

This paper aims at characterizing the behavior of dc corona discharge in wire-to-plane electrostatic precipitators (ESPs) as influenced by the relative humidity (RH) of the inlet air. The current–voltage characteristics and time evolution of the current are analyzed. Experimental results show that discharge current is strongly affected by the RH level of the inlet air. For instance, the time-averaged current is lower at higher RH for a given voltage, except when RH = 99%. Time evolution of the discharge current is affected by the humidity especially in the case of negative corona.     

Modelling of corona discharge in cylinder-wire-plate electrode configuration

Numerical computation of the electric field strength and ionic space charge density in electrode systems consisting of ionizing wire and non-ionizing cylinder, connected to the same DC high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. In a previous study a simple numerical method has been proposed to calculate the spatial distributions of electric field and ionic space charge in a case of a continuum and uniform corona discharge originating at the surface of the wire. The aim of the present paper is to improve the physical model of the corona discharge in this particular electrode configuration, by assuming a more realistic law of charge injection on the wire circumference. The computations were carried out for an ionizing wire of radius r=0.1 mm, located at different distances h from a metallic tubular support of radius R=13.4 mm.The initial conditions of the corona discharge took into account the non-uniformity of the charge injection around the ionizing wire electrode. The computational results were compared with those obtained under the assumption of uniform corona discharge. The comparison pointed out that neither the non-uniformity of the electric field nor that of the charge injection can be neglected. They depend on the geometry of the electrode system and affect the distribution of the electric field and of the space charge density in the inter-electrode gap.     

Corona discharges in asymmetric electrode configurations

The dual electrode, which consists of an ionizing wire in parallel with a metallic cylindrical support, both connected at same high voltage supply, has been extensively studied in relation with various electrostatic applications. In practical situations, the dual electrode may be installed in the proximity of metallic objects that will affect the electric field repartition and, hence, the development of the discharge. The aim of the present work is to analyze the operating conditions of such electrodes in the presence of metallic rods or plates connected at fixed or floating potentials. The Superficial Charge Simulation Method was then employed for the numerical analysis of several electrode arrangements involving a dual corona electrode and a metallic rod parallel to it. The paper also reports the results of current–voltage characteristics and current density repartition measurements for the dual corona electrode alone or in the presence of other bodies at same or floating potential. The proximity of metallic objects leads to the increase of corona inception threshold and shifts the I–V characteristics to higher voltages. The objects at floating potential may reduce the discharge current to very low values, while those energized at the same voltage as the ionizing wire may simply anneal the discharge.     

Effect of needle cone angle and air flow rate on electrostatic discharge characteristics of a corona-needle ionizer

In this study, the corona-needle ionizer was designed, constructed, and characterized. Experimental characterizations of the electrostatic discharge in terms of current–voltage relationships of the corona ionizer, including the effects of discharge electrode cone angle and air flow rate were presented. It was found that the charging current and ion concentration in the charging zone increased monotonically with corona voltage. Conversely, discharge currents decreased with increasing angle of the needle cone. The negative corona was found to have higher current than the positive corona. At higher air flow rates, the ion current and concentration were found to be relatively high for the same corona voltage. The effect of air flow rate was more pronounced than the corona voltage. It was also shown that the ion penetration through the ionizer decreased with increasing corona voltage, and increased with increasing air flow rate. The highest ion penetration through the ionizer of the 10° needle cone angle was found to be about 93.7 and 7.7% for positive and negative coronas, respectively. The highest ion penetration for the needle cone angle of 20° was found to be 96.6 and 6.1% for positive and negative coronas, respectively.     

Corona-discharge-based neutralization of charged granular insulating materials in contact with an electrode of opposite polarity

Charge neutralization is a key operation in many electrostatic processes. A wide-range of charge neutralizers have already been developed for various applications: eliminate shock and ignition hazards, avoid electrostatic discharges that might affect the operation of electronic equipment, reduce electrostatic adhesion forces that might stick granular materials in pneumatic conveyors, and so on. The aim of the present paper is to evaluate several methods of charge neutralization for charged granular insulating materials spread on an electrode energized from a voltage source of opposite polarity, a situation encountered – for instance – in two-metallic-belt tribo-aero-electrostatic separators, and which is not covered by commercially-available solutions. Three corona-discharge-based charge neutralization systems are studied: (i) DC-biased AC voltage applied to a corona dual-type electrode; (ii) DC-biased AC voltage applied to a triode-type electrode system; (iii) DC voltage applied to a corona dual-type electrode. The charged samples of polycarbonate granules are obtained at the outlet of a tribo-aero-electrostatic separator. The electric charge per mass ratio of each sample is measured before and after neutralization. The third of the above-mentioned charge neutralization solutions seems to be the most effective solution, but it requires an appropriate adjustment of the DC voltage applied to the corona dual-type electrode.     

Corona and back discharges in flue-gas simulating mixture

Results of spectroscopic investigations and current–voltage characteristics of electrical discharges between a needle and plate electrodes in a gas mixture simulating flue gases from coal fired power plants at atmospheric pressure are presented in the paper. In these investigations, back discharge was generated at the plate electrode covered with fly ash layer in order to simulate the conditions similar to those in electrostatic precipitators. To characterize the physical processes in back discharges, the emission spectra were measured and compared with those obtained for normal corona discharge generated in the same electrode configuration but with fly ash removed from the electrode. The emission spectra provide information on elemental and molecular composition of the layer. It was also shown that discharge characteristics in flue gas are quite different from those occurring in ambient air.     

Optical emission spectroscopy of point-plane corona and back-corona discharges in air

Results of spectroscopic investigations and current-voltage characteristics of corona discharge and back discharge on fly-ash layer, generated in point-plane electrode geometry in air at atmospheric pressure are presented in the paper. The characteristics of both discharges are similar but differ in the current and voltage ranges of all the discharge forms distinguished during the experiments. Three forms of back discharge, for positive and negative polarity, were investigated: glow, streamer and low-current back-arc. In order to characterize ionisation and excitation processes in back discharge, the emission spectra were measured and compared with those obtained for normal corona discharge generated in the same electrode configuration but with fly ash layer removed. The emission spectra were measured in two discharge zones: near the tip of needle electrode and near the plate. Visual forms of the discharge were recorded with digital camera and referred to current-voltage characteristics and emission spectra. The measurements have shown that spectral lines emitted by back discharge depend on the form of discharge and the discharge current. From the comparison of the spectral lines of back and normal discharges an effect of fly ash layer on the discharge morphology can be determined. The recorded emission spectra formed by ionised gas and plasma near the needle electrode and fly ash layer are different. It should be noted that in back arc emission, spectral lines of fly ash layer components can be distinguished. On the other hand, in needle zone, the emission of high intensity N₂ second positive system and NO _γ lines can be noticed. Regardless of these gaseous lines, also atomic lines of dust layer were present in the spectrum. The differences in spectra of back discharge for positive and negative polarities of the needle electrode have been explained by considering the kind of ions generated in the crater in fly ash layer. The aim of these studies is to better understand the discharge processes encountered in electrostatic precipitators.     

Study of magnetically enhanced corona pre-charger

In this paper, experimental investigations of the discharge characteristics of magnetically enhanced corona discharges, for the purpose of capturing fine aerosol particles, are presented. The discharge mechanism during such a process is analyzed as well. The effects of magnetic enhancement under different magnet flux densities, and in positive- or negative-corona discharges, were experimentally compared. The magnetically enhanced effects in different inter-electrode regions were studied. Experimental results demonstrated that the magnetic field could efficiently increase the concentrations of both the negative ions and the free electrons during negative-corona discharge. The dominant mechanism of magnetic enhancement in a corona discharge involves the Larmor precessions of free electrons which enhance ionization of the gas molecules near the discharge electrode. A convenient configuration for enhancing corona discharge was formed by placing permanent magnets with a local strong magnetic field near the discharge electrode. A magnetically enhanced negative-corona (MNC) pre-charger was assembled in front of an electrostatic enhancement filter. The influence of the MNC pre-charger on the efficiencies of an electrostatic enhancement filter was measured and compared with that of a conventional corona pre-charger. The free-electron-charging mechanism of the MNC pre-charger was preliminarily analyzed. Our results show that the new pre-charging technique is promising for capturing fine aerosol particles in electrostatic enhancement filters or electrostatic precipitators.     

Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10⁸ Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.     

Electrohydrodynamic force produced by a corona discharge between a wire active electrode and several cylinder electrodes – Application to electric propulsion

Low-speed electric propulsion systems for long-duration near-space travels by using solar energy could be based on the electrohydrodynamic force produced inside a corona discharge. This paper is a contribution to a better understanding of these types of thrusters, in order to enhance the produced thrust and their electromechanical effectiveness. Three different simple designs are experimentally studied and compared. The first one is composed of a wire active electrode and a single cylinder grounded one. For the second three-electrode design, the single grounded cylinder is replaced by two cylinders. Finally, the last design is composed of an active wire supplied by a positive voltage, two grounded electrodes and two others cylinders at a negative voltage. On one hand, results show that the use of two grounded electrode instead of a single one results in an increase of the discharge current. Moreover, whatever the electrode gap d, the current-to-thrust conversion is more effective with the three-electrode design. It changes from 31 to 58 N/A (+87%), from 74 to 85 N/A (+15%), and from 104 to 120 N/A (+15%), for electrode gaps d = 10, 20 and 30 mm, respectively. The thruster effectiveness θ is improved by 2 mN/W. On the other hand, the use of two collecting electrodes supplied by a negative high voltage does not result in an effectiveness enhancement because the power consumption is significantly increased.     

Corona discharge in a cylindrical triode charger for unipolar diffusion aerosol charging

A cylindrical triode charger for unipolar diffusion charging of aerosol particles was designed, constructed, and evaluated. The corona discharge characteristics were studied in this cylindrical triode charger. For the process the current–voltage characteristics were determined, as were the ion number concentration, the n_it product, and the mean charge per particle as a function of particle diameter. The discharge and charging currents, and ion number concentration in the charging zone of the charger increased monotonically with corona voltage. The negative corona had a higher current than the positive corona. At the same corona voltage, the ion number concentration in the discharge zone was larger than the charging current for positive and negative coronas, with values of about 197 and 32 times and 645 and 99 times for the ion-driving voltages of 0 and 310 V, respectively. The average ion penetration for positive and negative coronas was 0.64 and 0.19% and 3.62 and 1.93% for the ion-driving voltages of 0 V and 310 V, respectively. The higher flow rate, shorter residence time, gave a lower N_it product. By calculation 14% of charged particles of 10 nm in diameter were lost to the outer cylinder because of the electrostatic field effect. The charger does not use a sheath of air flow along the walls or the perforated screen opening, it has low diffusion and space charge losses due to the short column charging zone, and is a low complexity and inexpensive system. It worked as well as more sophisticated and expensive commercially available chargers.     

The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator

An analysis of the electrostatic gas cleaning fundamental phenomenon shows an essential influence of discharge electrode construction on the gas cleaning process efficiency.In the physical model tests there were used rigid discharge electrodes with corona emitting elements of various geometries. Different constructions of discharge electrode were tested in the aspect of discharge current uniform distribution on collecting electrode surfaces. Measurements of discharge current distribution has been carried out for discharge electrodes with different spike shapes and in different electric field geometry. The research aim was to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Collection efficiency measurements of selected fly ash samples (from coal fired boilers) were carried out on a laboratory testing bench in a horizontal electrostatic precipitator model.     

The study of the ionic wind blower with multi-needles/ring type electrodes disposed on inner wall of the cylindrical blower

Corona discharge is one of many methods that convert electrical power into mechanical force. It has been studied for various industrial fields because of its many advantages over conventional motor, such as its no moving parts, simpler structure, minimizing size and so on. In this paper, a discharge system with multiple corona electrodes disposed in a ring format, is studied by focusing on the electrical and mechanical characteristics. Effective ionic wind generation is due to the corona discharge which depends on electric field. Therefore, the electric field is affected by the voltage, discharge spacing, and distance between each corona electrodes.     

Sliding discharge in air at atmospheric pressure: electrical properties

Several studies have shown that a surface non-thermal plasma may be used as an electrofluidodynamic actuator for airflow control. For few years, we has been working on this subject, especially in the case of DC corona discharges and AC barrier discharges established at the wall of profiles. The present paper deals with a new type of surface plasma using a sliding discharge. This discharge, excited here by a negative AC voltage with a positive DC component, is created in a three-electrode geometry: one DC positive electrode and two negative AC electrodes at the same voltage. Then a barrier discharge is established between the positive electrode and the first negative one when a surface corona discharge or sliding discharge is generated between the positive electrode and the second negative one. In this preliminary study, the goal is to obtain a stable sliding discharge. Then the electrical properties of this discharge are observed and briefly discussed.     

Analysis of geometric scaling of miniature,multi-electrode assisted corona discharges for ionic wind generation

The assisted corona discharge is a unique discharge configuration that utilizes multiple collecting electrodes to minimize the voltage required to initiate a corona discharge and to generate an ionic wind. In this work, the geometric parameters that govern the formation of the assisted corona discharge and subsequent ionic wind are evaluated. Flow velocity measurements suggest that the geometry of the electrode spacings is optimized for ionic wind generation when the current flowing to the collector electrode is maximized, and that as the electrode gap is decreased to microscale dimensions, ionic wind production is inhibited.