Understanding the interaction behaviors between sessile droplets under imposed high voltages is very important in many practical situations, e.g., microfluidic devices and the degradation/aging problems of outdoor high-power applications. In the present work, the droplet coalescence, the discharge activity and the surface thermal distribution response between sessile multidroplets and chloride salt droplets under high tangential electric fields have been investigated with infrared thermography, high-speed photography and pulse current measurement. Obvious polarity effects on the discharge path direction and the temperature change in the droplets in the initial stage after discharge initiation were observed due to the anodic dissolution of metal ions from the electrode. In the case of sessile aligned multidroplets, the discharge path direction could affect the location of initial droplet coalescence. The smaller unmerged droplet would be drained into the merged large droplet as a result from the pressure difference inside the droplets rather than the asymmetric temperature change due to discharge. The discharge inception voltages and the temperature variations for two salt droplets closely correlated with the ionization degree of the salt, as well as the interfacial electrochemical reactions near the electrodes. Mechanisms of these observed phenomena were discussed.

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Sessile multidroplets and salt droplets under high tangential electric fields

Water droplets on polymeric insulating surfaces provide an intensified E-field stress at the common interface between water droplet, insulating surface and surrounding air, which may lead to partial discharges, surface deterioration or even flashover. While intensive research has been performed for the AC case and also for the DC case, a new kind of stress may arise by the combination of both, which is present when a DC and an AC system are installed on the same tower. Water droplet deformations at AC, DC and combined AC-DC voltages as well as their PD characteristics are experimentally investigated and discussed in this contribution. The water droplet at AC voltage oscillates at different oscillation modes, while it elongates towards electrodes at DC voltage. The deformation under combined ACDC voltage stress consists of an oscillation superimposed to an elongation towards the electrodes. The water droplet elongates towards the AC electrode when the polarity of DC source is negative, while it elongates towards the DC electrode in case of positive DC voltage. Oscillation modes of the water droplet at AC voltage have an influence on its PD inception E-field strength. Under DC stress the PD signals consist of two or more different PD levels which are due to the differently sharped edges of the deformed water droplet. Increasing the DC component at combined AC-DC voltage provides a lower PD inception E-field strength. Conductive water droplets have lower PD inception values in comparison with non-conductive ones under AC-DC stress.

Experimental investigations on partial discharge characteristics of water droplets on polymeric insulating surfaces at AC, DC and combined AC-DC voltages

Non-Ceramic Insulators, NCIs, are widely replacing traditional porcelain or glass insulators, due to their low weight, high contamination resistance and hydrophobic surfaces. However, aging of the NCI's surface can gradually decrease the surface hydrophobicity, which leads to corona and even surface flashovers. Deformation of a water droplet affects the E-field distribution present on the insulating surface, making it difficult to estimate the droplet induced electric field enhancement on the insulator surface or to predict the partial discharge inception electric field strength. 
PD inception E-field strength of water droplets on hydrophobic surfaces under variation of a large set of parameters is measured, analyzed and discussed in this study. A systematic investigation on the behavior of water droplets at AC, DC and combined AC-DC stress is analyzed and compared, particularly in deformations and PD characteristics point of views.
The major parts of these investigations have also been used to validate the dynamic E-field simulation package recently developed by the Computational Electromagnetics Laboratory (TEMF) of TU Darmstadt. A preliminary good agreement between simulations and experiments has been obtained. However, further cooperation is still needed to complete the validation of new software package.
The maximum allowable E-field strength along a polymeric insulating surface of wettability class WC 1 for a single water droplet on a 
at surface has been estimated experimentally to be between 6 and 7.5 kV/cm (peak). The PD inception E-field strength was reduced to about 4.5 kV/cm (peak) for more water droplets on the surface like random
spray and dew droplets.
In spite of the benefits of polymeric insulators rather traditional glass and porcelain insulators, they are still in practice a relative new technology. The behavior of water droplets on the surface of polymeric insulators have been shown to be a key parameter to facilitate understandings of long-term and aging behavior of this type of insulators.

Experimental Investigations on Water Droplets on Polymeric Insulating Surfaces under the Impact of High Electric Fields

Investigation on corona performance of conductors using fabricated indoor corona cage

Friction Stir Welding is a suitable solid-state joining technology to connect dissimilar materials. To produce an effective joint, a phase of optimization is required which leads to the definition of process parameters such as pin geometry, tool rotational speed, rotation direction, welding speed, thickness of the sheets or tool tilt angle. The aim of this review is to present a complete and detailed frame of the main process parameters and their effect on the final performance of a friction stir welded joint in terms of mechanical properties and microstructure. Attention was focused in particular on the connection between different aluminum alloys. Moreover, the experimental results were correlated to the development and the applications of tools which can be effectively used in the design of the manufacturing process such as finite element analyses, artificial neural networks, and statistical studies. The review also aims to be a point of reference to identify the best combinations of process parameters based on the dissimilar aluminum to be joined.

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Effect of Process Parameters on Friction Stir Welded Joints between Dissimilar Aluminum Alloys: A Review

Corona inception voltage due to a water droplet on epoxy resin in an electrode gap is high under DC voltage compared to AC voltage. It is observed that, as the contact angle of the epoxy nanocomposite material becomes higher, the corona discharge inception voltage increases. The droplet movement is observed, using a high-speed camera, on application of the voltage. It is seen that a droplet moves toward the ground electrode under an AC or a negative DC voltage, whereas it moves toward the high-voltage electrode under a positive DC voltage. It is also observed that carbonization occurs near the ground electrode under AC and negative DC voltages, and near the high-voltage electrode under a positive DC voltage. During the evaporation of the water droplet (during arcing) on the surface of the insulating material under AC and DC voltages, carbonization of material occurs and is high both in pure epoxy resin and in nanocomposites with 5 wt% epoxy clay. The magnitude of the arcing current is nearly the same irrespective of the percentage of clay in the epoxy nanocomposites. The magnitude of discharge current flow is high under negative DC voltage compared to positive DC/AC voltages. The rise time of injected current pulses, at the time of corona inception and during arcing, under AC/DC voltages, is a few nanoseconds. Ultrahigh-frequency signals were emitted as a result of the corona discharge from the water droplet on epoxy nanocomposites and at the time of arcing between the droplets and the electrodes, both under AC and DC voltages, with its dominant frequency in the range 1–2 GHz. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Understanding discharges initiated by water droplet on epoxy nanocomposites under DC voltages adopting UHF technique

The friction stir welding (FSW) is widely used in the fabrication of Aluminium alloy and other non-ferrous alloy. It has good potential to be used in major industries such as automobiles, aerospace, shipbuilding and can be used in the joining of high strength alloys. The FSW process low distortion and heat affected zone (HAZ) with fine recrystallized microstructure which leads to better mechanical properties at the weld zone and produces great stability. In this study, the different FSW parameters such as weld speed, tool rotation speed, tool tilt angle, feed per min has been discussed. The different types of tool pin profile and shoulder have also been discussed and their impacts on mechanical and microstructural properties at welded joints. Among various welding parameters the rotational speed is the most influencing parameter in FSW. Increasing the rotational speed exhibits the increase at tensile strength and is supposed to improve the mechanical properties. The most affected tool pin profile would be considered to be tapered threaded cylindrical pin profile which makes the adequate mixing of material with better flow ability and provide the fine grains at nugget zone. Comparing the FSW with other arc welding processes, it shows a wide range of environmental benefits which are noticeable such as saving in consumable materials, decrease in consumption of filler material and reduction in grinding wastes. Harmful emissions created from arc welding causes a health hazard to the welder. For achieving the high joint-strength for aerospace aluminium alloys and high temperature sustainable metallic alloys, friction stir welding will be preferred.

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Influence of Welding parameters on Mechanical property during Friction Stir Welded joint on Aluminium Alloys: A Review

The Friction stir welding (FSW) is widely used to weld aluminium alloy for industrial applications such as shipbuilding, aerospace and automobile due to its lighter weight and high strength. The current work is carried out to examine the different process parameters to weld AA6063-T6 aluminum alloy with FSW process. The tool rotation speed, feed, plunging depth and dwell time are considered as the welding parameter. The three test coupons have been welded at three different rotational speeds. The tensile strength and hardness at the weld vary with the speed of rotation of the tool. The higher tensile strength is achieved at lower tool rotational speed. The tensile strength achieved at 1000rpm and 1400rpm is 191Mpa and 131rpm respectively. The higher strength is achieved due to the refinement of grains and recrystallization in stirred zone. The microstructure test confirms the presence of uniformly fine grains at weld area and equiaxed grains at HAZ region.

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Ajay Pratap Singh

Papers

Understanding discharges initiated by water droplet on epoxy nanocomposites under DC voltages adopting UHF technique

Influence of Welding parameters on Mechanical property during Friction Stir Welded joint on Aluminium Alloys: A Review

Welding Investigation on AA6063-T6 Aluminium alloy during Friction Stir Welding process