R. Tobazeon

Charges and forces on conductive particles in roll-type corona-electrostatic separators

Mathematical modelling is a powerful tool in the optimization of electroseparation technologies and equipment. This paper proposes the use of a numerical method of field analysis to evaluate more accurately the charges and forces on millimeter-size conductive particles of virtually any shape in contact with an electrode. A program based on the boundary-element method was tested for this purpose, and provided the input data to an algorithm for the computation of particle trajectories. The results were in good agreement with the observations made on a laboratory electroseparator. The effect of space charge on the behavior of conductive particles was studied with a three-electrode system (a grounded grid, between a corona electrode and a metallic plate, energized by two high-voltage supplies of opposite polarities). The accuracy of numerical simulations of electroseparation processes is expected to increase when a mathematical model of particle motion in DC corona fields will be incorporated in the existing computer programs. >

Behaviour of conductive particles in corona-dominated electric fields

The authors present a theoretical analysis and an experimental study of the behavior of spherical and cylindrical conductive particles in contact with a metallic electrode, and affected by a corona-dominated electric field in atmospheric air. An accurate mathematical model of the conductive particle movement is introduced, taking into account the variation of the electric charge of the small object in a mono-ionized electric field, and the action of air convection produced by unipolar injection. Several experiments have been performed, using point-plane and wire-cylinder electrodes to quantify the contributions of the electric field, space charge density, and shape, size and mass of the particles. The results are compared to those for electric fields without space charge.<<ETX>>

Streamers in Liquids

Faults in devices subjected to an electric field are mostly due to electric breakdown of the insulations. Such a breakdown is the ultimate stage of irreversible processes which make any material switch from the insulating state to a conducting one (via an arc between two conductors). The situation is more or less catastrophic according to the nature of the insulator: solids, afterwards, are generally unable to sustain a further application of the voltage, whereas fluids (gases, liquids) can often do so, since they are easily renewed in the gap (by natural or forced convection).

Charging of insulating spheres on the surface of an electrode affected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pautheniers formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pautheniers formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed.

Factors which affect the corona charging of insulating spheres on plate and roll electrodes

Abstract Corona charging of particulates is employed in many electrostatic applications. Pautheniers equation for the ionic charging of spheres in uniform electric fields cannot be used for particles in the vicinity or in contact with an electrode. Therefore, this paper studies the charging of insulating spheres on plate and roll electrodes affected by monopolar space charge. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (radius: 1.5 mm), when subjected to the corona discharge generated between a high voltage electrode and either a plate or a roll electrode connected to the ground. The spheres were found to be less charged when the grounded electrode was covered by an insulating layer. When the insulating spheres were in contact with the rotating roll, their charge - at a given electric field intensity - was found to reduce with the increase of roll velocity. A similar correlation was found with the initial speed of the particle at the moment of deposing it on the surface of the rotating roll electrode. However, the saturation charge was the same and the increase of the electric field beyond a certain limit determined a diminution of the charge carried by the particles (auto-discharge effect). From a practical point of view, these observations have already served to optimise the operating parameters of roll-type corona electrostatic separators for mixed granular solids.

Cylindrical conductive particles in the proximity of an electrode affected by a high-intensity electric field

Abstract The electrostatics of cylindrical conductive particles between parallel plane electrodes is a research topic related to the development of various technologies which make use of electric fields for processing of granular materials. A program based on the boundary-element method was employed in order to compute the charge acquired by conductive cylinders and the force acting on them, while in contact with or in the proximity of an electrode, affected by a uniform field. The calculations shown that the field intensification generated by these particles is a function of their charge, of their dimensions and of their position with respect to an electrode of similar or opposite polarity. The basic experimental set-up consisted of two horizontally disposed parallel-plate electrodes, in air at atmospheric pressure. In some experiments, the upper electrode was substituted by a matrix of corona-emitting points, in order to evidence the peculiarities of particle behaviour in an ionized field. Conductive cylinders (diameters: 0.5–1.5 mm, lengths: 2–20 mm) were placed on the bottom electrode. The dielectric strength of air gaps containing single conductive particles was found to decrease in the presence of space charge. Mobile cylindrical particles were proved to be more harmful than the fixed ones.

DIELECTRIC BEHAVIOUR OF PARTICLE-CONTAMINATED AIR-GAPS IN THE PRESENCE OF CORONA

Abstract The operating conditions of corona-electrostatic separators, of various electroprecipitators, and of other similar high-voltage devices are influenced by the size and shape of the particles which pass through the inter-electrode air-gap. The presence of corona discharge affects the movements and the charge of the particles, and hence modifies the breakdown threshold. The numerical analysis of the electric field, using a charge simulation program, enabled the evaluation of the effects. Two other computer programs were written in order to analyse the conditions in which micro-discharges can occur between charged particles and electrodes of opposite polarities, and to estimate the consequences of such an event on the global dielectric strength of an ionized air-gap. Nevertheless, a systematical experimental study was necessary to fully-understand the phenomena. The basic electrode arrangement consisted in a matrix of corona-emitting points situated above a plate electrode. In some experiments, a grounded metallic grid was interposed between the two electrodes connected to high-voltage supplies of opposite polarities, so that the ionic current density and the intensity of the electric field be independently controlled. Both the computational results and the experimental data demonstrated that the presence of particles in air-gaps affected by corona discharges has a more important influence on the dielectric strength than in the case of a charge-free electric field.

Charge carrier elimination and production by electrodialytic polymers in contact with dielectric liquids

Abstract The mechanisms of injection and collection of ions in dielectric liquids using either commercial ion-exchange membranes or electrodialytic varnishes synthesized in order to adhere to metallic electrodes, have been studied. Depending on the nature of the liquid, of the polymer (structure or exchange groups), of the metal of the electrode, several mechanisms of carrier generation have been shown to occur. The polymer may simply act as a filter or play an essential role in electrochemical reactions; the regeneration of injected charge carriers can be mainly due to ionization of residual water bythe exchange groups. The peeling of adhesive films can be induced by gas release or by the electroosmotic effects of solvent dragging. Controlled injection and collection processes are needed in the field of fundamental research on liquid dielectrics (conduction, breakdown, electrohydrodynamics), electrooptics, liquid crystals, as well as in the field of electronic or electrical engineering.

Behavior of a Deionized Liquid Crystal Subjected to Unipolar Injection in Nematic and Isotropic Phase

Electrohydrodynamic phenomena in insulating MBBA subjected to unipolar injection of ionic carriers are described. Near the transition temperature, the transient currents following a voltage step were similar in the nematic and isotropic phases, over a wide range of voltages and electrode separations. High‐speed cinematography, Schlieren techniques, and the Kerr effect were resorted to in this investigation. Liquid motion was observed having a time lag and exhibited patterns closely similar to those previously observed in isotropic polar liquids. The velocities of hydrodynamic disturbances and of space‐charge fronts varied over a broad range, depending on experimental conditions.

Charging of insulating spheres in contact with an electrode affected by a mono-ionized field

The charge acquired by an insulating sphere in an uniform mono-ionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are in contact with an electrode. Under these circumstances, Pautheniers formula is no longer valid, because the field is nonuniform. The present paper addresses this problem from both a computational and an experimental point of view. A numerical method of electrostatic field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, in contact with plate electrode. The experiments were carried out on a laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (diameter: 3 mm) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (diameter: 150 mm) connected to the ground. The experimental data were in good agreement with the theoretical predictions. The charge of the particles was found to be maximum for a model of electrode, which was then proposed as the standard for the industrial corona-electrostatic separators.