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This review paper introduces fundamental knowledge about and presents recent developments in modeling and simulation of ion transport and electrohydrodynamic flow in dielectric liquids. Starting with the formulation of the governing equations, we introduce background ideas as well as the related modeling of ion dissociation, recombination, injection, and discharge, which play key roles in the charge creation and transport problem. Numerical methods and solutions are then presented for the onedimensional problem over the domain between a pair of parallel electrodes. In particular, we show how the electrical properties of a nonpolar liquid, m-xylene, can be derived from the matching of the numerical solutions with Novotny's experimental data. Two asymptotic solution methods, one for the overlap-layer and the other for the thin-layer regimes, are then presented; in particular, we show a remarkably high accuracy for the solutions. The current-voltage relation with ion dissociation and recombination is shown to be completely different from that without them. A historical review of the modeling and simulation of ion transport and electrohydrodynamic flow is finally touched upon. [ABSTRACT FROM PUBLISHER]

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the pre-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity. [ABSTRACT FROM PUBLISHER]

Since gas evolvement deteriorates the dielectrical properties of insulating fluid, its ability to resist decomposition under electrical discharge is of paramount importance for the safety of power transformers under operating conditions. In this paper, a series of experiments have been performed under electrical discharge according to ASTM D6180. Various insulating fluids were considered. The samples include minerals oils, synthetic and natural esters, and silicone fluid. The results indicate that the gassing tendency of natural esters is much lower than their counterparts. No significant differences between synthetic esters and silicone fluid were observed. Severely hydrotreated and hydrocracked oils indicate a higher gassing tendency. It is also shown that service-aged oil gassing tendency decreases with reclamation passes. The reclamation of service aged oil by three commercially available Fuller?s Earth (FE) showed a declined gassing tendency when FE helped reducing the DDF and turbidity. In addition, the theoretical premises that, oil born decay products contribute to the oil gassing, is experimentally confirmed under laboratory conditions. Extended stability test (3 times longer than the standard specification) also provide convincing evidences that each brand of mineral insulating oil might contain a limited amount of unstable hydrocarbons and volatile compounds. This finding indicates that when in service conditions these are gradually decomposed, the gassing tendency should go down [ABSTRACT FROM AUTHOR]

The main objective of the present work is the measurement and comparison of the dielectric properties of mixtures of two insulating liquids: synthetic ester liquid Midel 7131, used in the power industry, and THESO high permittivity oil developed by Tetra Corp. for pulsed power applications. It is shown that these two liquids can form stable mixtures with tailored dielectric properties. The dielectric properties of Midel 7131/THESO liquid mixtures such as their low frequency dielectric permittivities, dc conductivities and breakdown volt-time characteristics have been measured and will be discussed. [ABSTRACT FROM AUTHOR]

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/μs) to 1 MV/cm (dV/dt 350 kV/μs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials ? average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 μs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry. [ABSTRACT FROM AUTHOR]

A charge injection atomizer functions by introducing electric charge discharged from a high voltage electrode into a dielectric liquid, which subsequently atomizes the ejected liquid jet. Atomizer evolution thus far has proceeded through trial and error analysis of the experimentally measured electrical characteristics of the atomizer and of the quality of atomization. Within the atomizer, a coupled space charge and electric field exist, which can alter the internal flow pattern, thus creating electrohydrodynamic (EHD) instabilities that affect atomizer operation. Such a system has not been simulated in the past under forced flow conditions. In this work we simulate the internal flow of such a charge injection device in two dimensions; using experimental based boundary conditions. Initial results indicate that in the linear injection regime defined by the experimental data, the flow is only slightly unstable but in the transitional and highly non-linear regimes, the coupled space charge and electric field produce more instability in the liquid that must be investigated further. [ABSTRACT FROM AUTHOR]

Effect of electric conduction phenomenon on the mixing mechanism is studied numerically to thermally homogenize a dielectric liquid with an initial nonuniform temperature distribution. The fluid is stored in a spherical reservoir, and the electrodes are embedded on the reservoir surface such that the resultant local electric body forces mix the fluid. The electric field and electric body force distributions along with the resultant velocity field at the final steady-state condition are presented. The mixing mechanism is illustrated by the time evolution of temperature distribution inside the reservoir. The effects of primary dimensionless numbers on the mixing time are studied. [ABSTRACT FROM AUTHOR]

Abstract: The boiling process is a very fast and effective heat exchange process that requires high demands on the heat transfer surface and its roughness. In former publications we described a new kind of heat transfer tubes with microstructures on the exterior wall surfaces. The microstructures, generated by the ion track and replica technique, enhance distinctly the vapour bubble generation process and therefore the boiling rate too. In this report we will present new results obtained by heating of massive copper rods whose surfaces were improved by replica microstructures. The copper rod, provided by a heat power cartridge, was completely dipped in a dielectric liquid that starts boiling on the rod surface and causes a high vapour bubble density. Depending on the heat flux, the overheat temperature of the heating copper rod was measured and compared with that for non-structured copper surfaces. Compared with plane surfaces, the heat transfer rate was increased by 200–300%. [Copyright &y& Elsevier]