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Two-dimensional numerical simulations are carried out to evaluate the enhancement of natural convection heat transfer in a dielectric liquid resulting from injection induced annular electro-convection. The liquid is confined between two concentric horizontal cylinders and subjected to the simultaneous actions of a direct current electric field and a thermal gradient. A unipolar injection from the inner cylinder introduces free space charges into the liquid. Numerical results are presented for the configuration of inner to outer diameter ratio of 0.5 and silicon oil as the working medium. It is found that the charge injection may induce a strong radial fluid motion, which consequently augments the heat transfer rate. Due to the linear stability nature of electro-convection, there exists a threshold value of the electric driving parameter, above which the electrical enhancement becomes manifest. In addition, when the electric driving parameter is sufficiently high, the flow is fully dominated by the Coulomb force, and thus heat transfer becomes independent of the thermal driving parameter. [ABSTRACT FROM AUTHOR]

We numerically investigated the annular electroconvection that takes place in a dielectric liquid lying between two concentric cylinder electrodes. A uniform injection of arbitrary strengths either from the inner or outer cylinder introduces free charge carriers into the system, and the resulting Coulomb force induces electroconvection. The problem is characterized by a linear instability that corresponds to the onset of flow motion. The linear stability criteria were determined from direct numerical results and by linear stability analysis, and the results obtained with the two approaches show an excellent agreement. We focused on the fully developed flow pattern in the finite amplitude regime. We observed very different flow motions that were highly dependent on the injection strength. [ABSTRACT FROM AUTHOR]

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (~ MV/cm), and of propagation (requiring low electric field, ~ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data. [ABSTRACT FROM AUTHOR]

What has been the contribution of surface potential studies during the last four decades to our understanding of insulation physics? Which are the promising techniques emerging in this field? The present review covers the industrial context accounting for the development of surface potential measurements on insulating materials, the way they have been implemented, as well as the great variety of models produced to explain the charging and potential decay mechanisms. Several polarization or transport processes can be responsible for the decay of the potential. Though most of the models initially stemmed from electrostatics and semiconductor physics, around the notion of mobility, experiments on polymers required "thermodynamic" models, describing progressive charge detrapping. We also underline here that the three main different physical processes likely to be involved in the potential decay (dipolar relaxation, dispersive transport, slow detrapping) can lead in disordered materials to the same time response, the challenge being here is to design inventive new procedures to distinguish them. This comment also applies to most of electrical measurements on disordered materials. Kelvin probe microscopy, return voltage and thermostimulated potential measurements also illustrate the multiform development of surface potential measurement techniques, as their future prospects. [ABSTRACT FROM AUTHOR]