J.M. Cabaleiro

Electrical Double Layer's Development Analysis: Application to Flow Electrification in Power Transformers

A charge zone known as the electrical double layer exists at a solid-liquid interface. The liquid flow induces a phenomenon called flow electrification: it generates a streaming current (caused by charge convection) and a rise in the solids potential (if it is insulated from the ground). These potentials may reach values high enough to produce electrical discharges and cause accidents. Although this phenomenon was identified a long time ago, its physical description remains unknown (i.e., production and displacement of charges, equilibrium, etc.). We have modeled flow electrification phenomena occurring when transformer oil flows through a rectangular pressboard duct. The results of a parametric study made with this model are presented in this paper. The ducts geometry and the materials were selected to compare some of the numerical results to experimental ones. The facility used to obtain these experimental results was developed some years ago as a part of the research program of Electricite de France and the University of Poitiers. Another facility will be designed in the near future with the aim of reproducing surface electrical discharges. The results of this parametric study will be useful for its design.

Flow electrification process: the physicochemical corroding model revisited

Even if the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments (Cabaleiro, 2008) made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analyze of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreements with the experiments, especially, for high laminar Reynolds numbers.

Flow electrification process: The physicochemical corroding model revisited

Though the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analysis of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreement with the experiments, especially, for high laminar Reynolds numbers.

Single camera time-resolved 3D tomographic reconstruction of a pulsed gas jet

Experimental characterization of micro-jets is challenging because of the small dimensions of the micro-nozzle. In this study, we propose a new technique to visualize the instantaneous 3D structure of a pulsed gas micro-jet. Using phase-averaging of Schlieren visualizations obtained with a high-speed camera and 3D reconstruction through a filtered back projection algorithm, it is possible to follow the high-speed dynamics of the pulsed jet. The experimental technique is illustrated by a 3D reconstruction of a pulsed helium micro-jet. The technique is simple yet very useful. To our knowledge, it is the only experimental method to analyze the instantaneous 3D structure and high frequency dynamics of pulsed micro-jets.Graphical Abstract

Interaction between a laminar starting immersed micro-jet and a parallel wall

In the present work, we study the starting transient of an immersed micro-jet in close vicinity to a solid wall parallel to its axis. The experiments concern laminar jets (Re < 200) issuing from a 100 μm internal tip diameter glass micro-pipette. The effect of the confinement was studied placing the micro-pipette at different distances from the wall. The characterization of the jet was carried out by visualizations on which the morphology of the vortex head and trajectories was analyzed. Numerical simulations were used as a complementary tool for the analysis. The jet remains stable for very long distances away from the tip allowing for a similarity analysis. The self-similar behavior of the starting jet has been studied in terms of the frontline position with time. A symmetric and a wall dominated regime could be identified. The starting jet in the wall type regime, and in the symmetric regime as well, develops a self-similar behavior that has a relative rapid loss of memory of the preceding condition of...

A polymer chip-integrable piezoelectric micropump with low backpressure dependence

We describe a piezoelectric micropump constructed in polymers with conventional machining methods. The micropump is self-contained and can be built as an independent device or as an on-chip module within laminated microfluidic chips. We demonstrate on-chip integrability by the fabrication and testing of an active micromixer with two pumps. Average flow rates from sub-μl min−1 to 300 μl min−1 can be obtained with low influence from the backpressure up to approximately 10 kPa. The micropump design allows potential use in low-cost disposable polymeric Lab on a Chip devices.

Numerical study of Electrical double layer development: Analysis of the charge genesis

This work presents a numerical simulation of the Electric Double Layer (EDL) development process at a solid/liquid interface for adsorption and corrosion models. First, the study is conducted for static EDL development (without liquid flow) until it reaches a static equilibrium. Afterwards, the EDL is perturbed by a laminar liquid flow leading to flow electrification phenomena (dynamic study). The charge conservation equations of the liquid species have been implemented in an industrial code. A parametric study was performed to consider different chemical reaction scenarios and different models.

Axis-switching of a micro-jet

In this study, it is shown that free microjets can undergo complex transitions similar to large-scale free jets despite relatively low Reynolds numbers. Using an original experimental method allowing for the 3D reconstruction of the instantaneous spatial organization of the microjet, the axis-switching of a micro-jet is observed for the first time. This is the first experimental evidence of such complex phenomena for free micro-jets. Combining these experimental results with Direct Numerical Simulations it is shown that the mechanism responsible for the axis-switching is the deformation of a micro-vortex ring due to induction by the corner vortices, as it occurs in large scale non-circular jets.

Analysis of flow electrification parameters of power transformer oils

As a part of a research program at EDF, University of Poitiers has developed a prototype of a sensor dedicated to the monitoring of oil flow electrification through transformer pressboard-made channels. This phenomenon can be responsible for destructive breakdowns when the resulting charge accumulation leads to high electrical fields. In order to identify transformers running electrostatic hazards, an experimental setup been has developed to measure the generation and the accumulation on pressboard surface of electric charges with regard to different oil flow conditions (velocity, temperature). Various oils have been characterized with the same reference pressboard: new oils and oils coming from operating transformers (healthy or suspected to develop the phenomenon). The analysis of the measured parameters makes it possible to derive some criteria with regard to the electrostatic hazard probability in transformers.

Transient electrical double layer dynamics in a quiescent fluid

From the moment a liquid is put in contact with a solid, physico-chemical reactions occur at their interface. These reactions have been modeled over the years by preferential adsorption or corrosion. Whatever the model, they lead to a charge distribution called electrical double layer (EDL), formed by a charge layer at the solid wall, and a diffuse charge layer of opposite sign in the liquid. In this work we focus on the formation of the EDL when a plane wall is put in contact with a quiescent fluid. The governing equations and a semi-analytical solution are presented. A simpler solution can be found by assuming the instantaneous formation of the charge density profile. The semi-analytical solution presented in this work is compared to the former at different reaction rates leading to the conclusion that for fast enough wall reactions, the instantaneous charge density profile cannot be assumed.