Kelly S. Robinson

Analysis of two-dimensional microdischarge distribution in dielectric-barrier discharges

The two-dimensional spatial distribution of microdischarges in atmospheric pressure dielectric-barrier discharges (DBDs) in air was studied. Experimental images of DBDs (Lichtenberg figures) were obtained using photostimulable phosphors. The storage phosphor imaging method takes advantage of the linear response of the phosphor for characterization of microdischarge intensity and position. A microdischarge interaction model in DBDs is proposed and a Monte Carlo simulation of microdischarge interactions in the discharge is presented. Comparison of modelled and experimental images indicates interactions and short-range structuring of microdischarge channels.

Multipolar interactions of dielectric spheres

Abstract The interactions of uncharged dielectric particles in an electric field can be described mathematically using multipolar expansions. Interparticle force calculations require that account be taken of the interactions of all these moments. Practical models for particle chains have been developed using a “dipole” approximation, where all moments except the dipole are neglected. To determine the accuracy of this dipole approximation, the effect of higher-order terms (quadrupole, octupole, etc.) upon the effective moment of particle chains is determined as a function of the number of particles in a chain, the relative dielectric constant ϵ p /ϵ m , and the spacing between particles d . It is found that the accuracy of the dipole model is severely compromised for closely spaced particles when ϵ p / ϵ m >4.0, due to slow convergence of conventional multipolar expansions. Similar limits on the accuracy of the dipole model are found for short and long chains. For chains of touching spheres with ϵ p / ϵ m ⪢1.0, impracticably large numbers of terms are required to achieve convergence.

Charge relaxation due to surface conduction on an insulating sheet near a grounded conducting plane

Electrostatic charges are responsible for a variety of problems in industrial processes and customer equipment that use webs or sheets. Problems include particle contamination from attracting dust, sheets that stick to each other, and electrical discharges resulting in logic resets or damage to electrical components. These problems can be mitigated by increasing the surface conductivity of the insulating sheets by coating the surface with a conductive layer or by increasing the relative humidity. To mitigate problems, electrostatic charge must dissipate quickly compared with the mechanical transport time of the process. Reported here are the results of a model calculation of the charge relaxation time showing explicitly that the charge relaxation time depends on both surface conductivity and geometry. The charge relaxation time is found to increase as the distance to a nearby, grounded conducting plane decreases. Charge relaxation is slowed because the tangential electric field needed to drive surface current becomes smaller as the distance to the grounded plane decreases. Inferred from this analysis is the dependence of charging on the electric Reynolds number (ratio of the electrical charge relaxation time to the mechanical transport time). Web charging can be divided into three regimes: dissipation (R/sub e/<0.1), transition (0.1

Particle-Wall Adhesion in Electropacked Beds

Electric fields applied across loosely packed beds of glass beads or sand induce strohg interparticle electrical forces. Reported here is an investigation of the closely related phenomenon of bed particles sticking electrically to wails. The yield locus of a metallic sled sliding over the surface of an electropacked bed is measured as a function of the applied electric field. The yield locus shifts with applied field because greater shear force is required to initiate movement, but the static friction coefficient remains unchanged. The effective electrical stress acting on the sled is estimated from the yield-locus shift, and the results are compared with data from other experiments, including fluidized-bed measurements and the angle-of- repose technique.

Slope Stability of Electropacked Beds

The mechanics of packed or fluidized beds of semi- insulating particles are substantially altered by strong applied electric fields. The angle of repose of a loosely packed bed of glass beads increases with the applied electric field; a sufficiently intense field will freeze the bed. An extensive series of angle of repose measurements are reported which suggest three distinct regimes of critical slope equilibria: infinite slope equilibrium for low electric field intensities; finite slope equilibrium for somewhat stronger fields, and the frozen bed limit for strong electric fields. The failure of an infinite slope equilibrium is characterized by individual particle motions confined to a thin layer near the surface. This regime is amenable to analysis, and the electrically induced cohesion can be inferred from the experimental data. The results are compared to Dietzs published expression for the cohesive electrical force acting between two contacting spherical particles. SEM photographs of the particles used experimentally provide evidence that surface asperities and very fine particles may be more important than previously suspected.

Maximize static dissipator neutralization efficiency

Plastic packaging materials are produced on roll-to-roll coating, printing and converting machines that convey insulating, polymer webs at speeds often exceeding 3 m/s. Static charges on these web cause a number of problems including sparks that ignite fires, that shock people, and that cause logic errors in the production machine control systems. Static charges attract airborne contaminates and, in sheeting and labeling operations, cause sheets and labels to stick and block. While many static dissipators are commercially available for controlling static on webs and sheets, their performance is highly variable. Here, the performance of static dissipators is analyzed to find that three key factors determine neutralization efficiency; the ion number density generated by the static dissipator (dissipator design), the length of the web exposed to ions (installation), and the web speed (process). A key result is that the static dissipator neutralization efficiency varies with the electric Reynolds number, the ratio of the dissipator time constant determined by the number of ions generated by the dissipator to the time that the web is exposed to ions from the dissipator. A second result is that the spacing between the static dissipator and the charged web is not a key factor. Rather, the web length exposed to ions from the dissipator is the key factor. Finally, a method is presented to measure the number of ions generated by a static dissipator to determine the dissipator time constant. Results from a commercially available static dissipator are given.