J.D. Cross

Breakdown Across a Dielectric Spacer in Insulating Oil and the Role of Electrohydrodynamics in Liquid Breakdown

Electrohydrodynamics is an important contributing factor in liquid breakdown, and may contribute to the breakdown across a dielectric spacer in oil. The two topics are discussed separately.

The effect of a metallic particle near a spacer on flashover phenomena in SF/sub 6/

A study of the flashover mechanism initiated by metallic particles in SF/sub 6/ gas is presented. Using a high-speed Imacon photographic camera, it was established that a metallic particle near the insulator surface plays an important role in flashover development. The influence of the particle on flashover development depends on the pressure of the SF/sub 6/ gas. At atmospheric pressure, the flashover was always initiated at a point on the particle farthest from the nearest electrode. At high pressures, it was found that breakdown develops from the electrode of relative positive polarity in the form of a leader. The velocity of the discharge crossing the gap increases exponentially when the distance between the particle and the insulator surface decreases. >

Removal of volatile organic compounds in water using low-energy electron beam

This paper presents the experimental results concerning removal of organic contaminants in water using low energy electron beam radiation. A laboratory scale apparatus for electron beam irradiation of water is described. The absorbed dose of radiation was controlled by water circulation time (1 to 10 min), accelerating voltage (100 to 170 kV), and electron beam current (0.5 to 1.2 mA) for a constant flow rate of 2 kg/min. The volume of the treated water was 1 dm/sup 3/. The electron beam was generated in vacuum (p<10/sup -5/ Pa), and electrons were injected into the water through the electron transparent window made of titanium foil with a thickness of 25 /spl mu/m TCE (trichloroethylene) and chloroform dissolved in deionized water were used in the experiment. The dependency of the relative concentration c=C/C/sub 0/; where C is the weight content of compound after electron irradiation and C/sub 0/ the initial contaminant concentration, on radiation energy density and the absorbed dose are presented. Although the initial contents of the compounds were higher than those occurring in real water sources, it has been found that it is possible to decompose both of the chemicals with high efficiency (total decomposition of TCE, and up to 90% reduction of chloroform) using a relatively low accelerating voltage (<200 kV). The results have indicated that the removal of TCE and chloroform mainly depended on the absorbed dose of electron radiation.

Photographic Observations of Breakdown Mechanism in Vacuum

The results of a high-speed photographic study of the initial stages of discharge development in a point-to-plane gap in vacuum are presented. An image converter camera operating, in a streak mode, up to a speed of 1 mm/ns was used in conjunction with an image intensifier. The first detectable light always appeared at the cathode. The light originating at the anode appears to be from a plasma expanding at thermal velocities. Collapse of the voltage across the gap coincides with the bridging of the gap by anode plasma.

Removal of nitrobenzene and volatile organic compounds using electron radiation

Experimental results concerning removal of nitrobenzene and volatile organic compounds (VOCs), such as toluene, trichloroethylene and benzene with the use of low energy electron beam are presented. Lab scale apparatus for electron beam irradiation of water is briefly described. Total dose of /spl beta/ radiation was controlled by the time of water circulation (1-50 min.), accelerating voltage (V/sub a/=100-125 kV) and electron beam current (0.1-1 mA) for flow rate of 1 kg/min. The electron beam was generated in vacuum (p<10/sup -7/ Torr) produced by a system of diffusion-sorption-rotary pumps. Electrons were injected into the treated water through an electron beam permeable membrane made of boron nitride poly-crystalline ceramics with a thickness of 10 /spl mu/m. A low mass density (/spl sim/2 g/cm/sup 3/) of the ceramic material enables to obtain a high transmission of the electron beam even for a low accelerating voltage. Dependencies of the removed content of contaminants on the total dose of /spl beta/ radiation are presented. Despite using low energy electron beam, the results have indicated a significant removal of nitrobenzene and VOCs treated.

Interfacial instability: a new approach to transient EHD motion with unipolar injection

The electrohydrodynamic (EHD) instability of an insulating liquid subjected to a transient unipolar injection is investigated. A novel model based on the EHD instability of the interface between the space-charge layer in front of an injecting electrode and the bulk of an insulating liquid is proposed. One of the important features for transient unipolar injection, the time delay, is studied using this model. There is good agreement between times predicted by this model and published data. It appears that the EHD instability is governed by the dynamics of the interface rather than by the development of a quasi-static critical voltage across the space-charge layer. >

Pulsed corona discharge for advanced oxidation in water purification technology

The possibilities of developing oxidation processes that remove and ultimately destroy hazardous organic volatile chemicals prior to entry into the environment are being considered. Pulsed corona has received significant attention for decomposition of toxic chemicals and disinfecting of microorganisms. Short duration pulses produced by highly non-uniform fields can produce intense corona discharge in water, without actually leading to breakdown of water itself. The purpose of this work is to use such pulsed corona discharge for advanced oxidation in the presence of air bubbles for the removal of organic substances like trichloroethylene (TCE), benzene, and toluene which are common organics arising from industrial wastes.

Effect of area on surface flashover voltage in vacuum

It is known that the breakdown voltage in a vacuum insulation system drops with an increase in linear dimension of electrodes and with an increase in the number of insulating spacers bridging the gap. Normally this is explained to be the result of the increase in the number of so-called weak links in the system. The influence of an external capacitance on the flashover voltage of an electrode gap bridged by a spacer was investigated. When a capacitor was connected inside the vacuum chamber near the discharge site in parallel with the gap, it was found that the additional capacitance lowered the breakdown voltage in a manner similar to that observed with an increase in electrode area. This supports the energy explanation of the area effect. A larger external capacitor, far from the discharge site, had no influence on the breakdown voltage. >

Inactivation of Yersinia enterocolitica Gram-negative bacteria using high-voltage pulse technique

High voltage pulses of peak voltages U=5-75 kV and rise times t/sub p/=500-1300 ns were applied with repetition frequency f=1 Hz in order to cause the irreversible electroporation of Gram negative bacteria Yersinia enterocolitica (Y. enterocolitica). The bacteria were suspended in NaCl solution of pH=7.2 and conductivity /spl gamma/=0.8-1.3 S/m. The suspension was placed in glass tube immersed in the cylindrical electrode system gap filled with distilled water. Such an electrode system will protect the bacteria suspension from the chemical processes at the electrode-liquid interface due to conduction and prebreakdown phenomena. The current chopping electrode system was connected in parallel to the sample in order to avoid heat generation from direct discharge of the pulse through the suspension. The dependence of the survival ratio s=N/N/sub 0/ (the number of bacteria per cm/sup 3/ after pulse treatment, N, divided by the number of bacteria per cm/sup 3/ before treatment, N/sub 0/) of Y. enterocolitica on peak voltage of the pulse, number of pulses applied and on various rise times of pulses have been measured. The reduction by 6 orders of magnitude of Y. enterocolitica living cells per cm/sup 3/ was achieved. The results show that considerable inactivation of microbes can be achieved by the application of short (t/sub p/<1000 ns) high voltage pulses for bacteria suspension without directly exposing the bacteria suspension to the electrodes. It is therefore possible to use the electrode system proposed as a means for sterilization of liquid foods.

Electrokinetic study in non-polar liquids by laser Doppler anemometry

Abstract The influence of EHD motion on charge convection in stressed contaminated non-polar liquids has been investigated. The test liquids, Norpar® 12 and Voltesso® 35 were artificially contaminated by adding a certain quantity of fine carbon black particles or Stadis® 450, an ionic impurity. The role played by the charge convection in the passage of current was analyzed by studying the transient current resulting from the application of a step voltage to parallel-plane electrodes separated by the test liquid. Non-intrusive optical techniques, Schlieren photography and laser Doppler anemometry were used for flow visualization and measurement of velocities both during the transient and steady state condition. The variation in velocity with time followed the same trend as the current. Both velocity and current measurements indicate that the peak value of velocity/current during transient state depended on the initial charge density at the liquid-electrode interface. Any increment in current was closely associated with increased liquid velocity indicating that charge transport was dominated by convection. The large difference between maximum velocities during transient state and mean velocities during steady state indicated that injection in non-polar liquids is weak and is a function of the impurity concentration.