In this paper we present experimental and numerical studies of the electrohydrodynamic stretching of a sub-millimetre-sized salt water drop, immersed in oil with added non-ionic surfactant, and subjected to a suddenly applied electric field of magnitude approaching 1 kV/mm. By varying the drop size, electric field strength and surfactant concentration we cover the whole range of electric capillary numbers (Ca E ) from 0 up to the limit of drop disintegration. The results are compared with the analytical result by G.I. Taylor (Proc. R. Soc. A 280, 383 (1964)) which predicts the asymptotic deformation as a function of Ca E . We find that the addition of surfactant damps the transient oscillations and that the drops may be stretched slightly beyond the stability limit found by Taylor. We proceed to study the damping of the oscillations, and show that increasing the surfactant concentration has a dual effect of first increasing the damping at low concentrations, and then increasing the asymptotic deformation at higher concentrations. We explain this by comparing the Marangoni forces and the interfacial tension as the drops deform. Finally, we have observed in the experiments a significant hysteresis effect when drops in oil with large concentration of surfactant are subjected to repeated deformations with increasing electric field strengths. This effect is not attributable to the flow nor the interfacial surfactant transport.
ABSTRACT:The main aging mechanism of electrical cables with polymeric electrical insulation is the growth of water trees. Water trees are initiated if the relative humidity (RH) in the electrical insulation is above a critical level. Delaying the water ingress into the electrical insulation system delays the water tree initiation and reduces water tree growth, thus extending the service life of the cable. For a cable without any metallic water barrier, the water ingress can be significantly delayed by the use of an outer sheath material with low water permeability. An even greater delay in the water ingress into the electrical insulation can be achieved using a layered sheath system. To explore the possibilities of a layered sheath system, calculations of water ingress into a typical cable cross section has been performed using a finite element method. The water diffusion and sorption data used in the calculation has been measured for typical cable materials. Calculations have been performed for uniform temperature conditions and for a temperature gradient due to resistive current heating. The time to reach critical humidity levels and stationary humidity levels in the insulation system has been determined for several different arrangements of the sheath system. A sheath system with an outer layer of a material with low water permeability and an inner layer of a material with a high water absorption capacity is shown to give a significant delay of the water ingress into the electrical insulation. For the sheath materials used in this study, there is an optimum distribution of thickness of each layer. The calculations also show that a temperature gradient across the insulation system of a cable in operation gives an advantageous RH profile. With a temperature gradient the equilibrium RH level in parts of the electrical insulation can be lower than the critical value for water tree initiation.
Abstract-Field windings made of MgB2 wires or tapes are considered for their potential to reduce volume, weight and cost of large offshore wind turbine generators. To gain experience of how to use this relatively new material in full-scale generators, tests of different winding methodologies and techniques are needed. In this paper, we describe in detail the steps used to wind a racetrack coil with a length of 1 m and a width of 0.5 m out of 4.5 km of MgB2 superconducting tape. The width corresponds to a full-scale pole of a 10 MW generator, whereas the length of the straight section is shorter than the corresponding full-scale pole. The coil was built up of 10 double pancake coils. Each double pancake coil was wet-wound using a semi-automatic winding process, where Stycast 2850 was applied directly to the MgB2 tape without any other dedicated electrical insulation. The strengths and weaknesses of the winding process are discussed and compared to the dry-winding method.
Abstract-The behaviour of a single sub-millimetre-size water drop falling through a viscous oil while subjected to an electric field is of fundamental importance to industrial applications such as crude oil electrocoalescers. Detailed studies, both experimental and computational, have been performed previously, but an often challenging issue has been the characterization of the fluids. As numerous authors have noted, it is very difficult to have a perfectly clean water-oil system even for very pure model oils, and the presence of trace chemicals may significantly alter the interface behaviour. In this work, we consider a wellcharacterized water-oil system where controlled amounts of a surface active agent (Span 80) have been added to the oil. This addition dominates any trace contaminants in the oil, such that the interface behaviour can also be well-characterized. We present the results of experiments and corresponding two-phaseflow simulations of a falling water drop covered in surfactant and subjected to a monopolar square voltage pulse. The results are compared and good agreement is found for surfactant concentrations below the critical micelle concentration.
through a very thin filament interconnecting the drops during a short time and presumably generated by interface instability. Under strong enough applied field, there was also formation of a very fine mist around the zone of drop quasi contact; this mist is ascribed to the break-up of the transitory very thin ligament bridging the drops. Some explanations and considerations are proposed for the various observations; in particular, it is proposed that the contrast between the transitions to partial coalescence (at 60 °C) and to non-coalescence (at 40 °C) arises from geometrical differences in the temporary small bridge interconnecting the drops.
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