Jean-Luc Reboud

Study of Dust Removal by Standing-Wave Electric Curtain for Application to Solar Cells on Mars

Multiphase electric curtains generate traveling waves which can lift and convey charged particles, whereas single-phase electric curtains create a standing wave. However, this paper confirms that, in certain conditions, such a standing-wave curtain can expel the deposited powder as well. Indeed, we present results of experiments performed in atmospheric air and in carbon dioxide with electrodes coated with an insulating material for different powders under various pressures down to that existing on Mars (p cong 7 mbar). Under high-enough pressure in air (p = 1 bar), a part of the powders is put into motion when raising the applied voltage below the ionization threshold. Above the discharge threshold, the deposited powder can be completely expelled from the stressed zone by the dielectric barrier discharges (DBDs) occurring in the gas just over the surface of the insulating layer. This proves that the charging of particles by collection of gaseous ions and electrons produced by the DBDs is involved in the lift and in the removal of powders. The powder removal becomes more difficult when p is lowered. For gas pressure around 7 mbar, a good powder removal requires a distance between axes of adjacent electrodes lower than 1 mm. The dust removal efficiency also depends on the size of the particles and on the contact between the particles and the substrate. With Mars analog dust being spread out with a brush, the removal of the so-produced agglomerated particles is often satisfactory. When injecting and dispersing the same powder into the vessel under reduced pressure, the layer resulting from particle sedimentation can be removed. However, when the injected tribocharged particles are driven directly onto the standing-wave curtain, the resulting dust layer remains unperturbed by the action of electric field and DBDs. Very likely, this is due to the intimate contact between the particles and the substrate. These observations are discussed in the light of the different forces acting on particles.

Corona Discharge and Electrostatic Precipitation in Carbon Dioxide Under Reduced Pressure Simulating Mars Atmosphere

In this paper, the possibility of using electrostatic precipitation (ESP) to clean the gas above solar panels of modules on the surface of planet Mars is investigated. Results are presented on corona discharge in carbon dioxide gas under reduced pressure ranging from 5 to 10 mbar with different electrode configurations. The corona-discharge inception voltage and the threshold of back discharge have been measured for three electrode configurations. The charging of suspended particles of micrometer size in the gas by unipolar ions is examined. Under the considered reduced pressure, diffusion charging very likely dominates over field charging. The drift velocity of charged particles is then estimated and is found to be not drastically lower than in industrial precipitators for fine particles despite the much lower electric field which can be applied under reduced pressure. Finally, the results of a laboratory experiment examining the dust deposit onto photovoltaic cells are presented. It appears that ESP reduces the rate of a Mars analog dust deposit and might be used in order to increase the lifetime of solar panels during Mars missions.

Electrocoalescence of water drops in oil shear flow: Development of an experimental set up

Present research aims at determining the conditions leading to electrocoalescence of water drops in oil flows and at characterizing the interplay between fluid dynamics and electric field in the mechanism. We describe here the build-up of an experimental set-up designed to investigate the critical coalescence conditions in the case of two free water droplets in an oil shear flow under the action of an applied electric field. Drop pairs will be injected in a Poiseuille flow to study, by optical means, their relative motion, deformation and possible coalescence. Important part of this preliminary work concerns the injection of controlled charge free drop pairs using a newly developed EHD actuation technique. Second paper [1] deals with theoretical and numerical analyses in the same configuration.

Field induced coalescence of two free water drops in a viscous dielectric fluid

The basic process of coalescence of droplets in a flowing water-in-oil emulsion under the action of an electric field is considered. The coalescence probability depends on the ratio of time of close proximity of droplets and time of decrease of spacing down to drops contact. For two free drops aligned with the field, the dynamical problem consists in the deformation of the drops, their motion and the thinning of the oil film between the drops. For very small droplets, assuming a negligible interface deformation, a very small initial spacing and a high value of viscosity ratio leads to an order of magnitude estimate of the time required for the drops to achieve contact. Numerical simulations confirm that this time is roughly inversely proportional to the maximum initial electrostatic pressure pe0 at the facing interfaces and point up an influence of the electric Bond number defined as the ratio of electric and capillary forces.

Drop-on-demand extraction from a water meniscus by a high field pulse

As a part of a study of electrocoalescence of water droplets in oil, the controlled generation of small drops (diameter ~ 100 mum) is considered. The technique used consists in applying a voltage pulse promoting the deformation of a meniscus at the end of a capillary tube through the action of electric forces. For pulses of short enough duration, the transient deformation can lead to the ejection of a small drop electrically neutral. The experimental results of water drops extraction in oil are presented. Using capillary tubes of outer tip diameter varying from 0.5 mm to 1 mm, it is possible to obtain in a reproducible way drops of diameter ranging from 50 mum to 200 mum. For a given meniscus shape, the diameter of the extracted droplet depends on the voltage amplitude V and on the pulse duration Deltat. Order of magnitude considerations on the meniscus deformation process suggest that the main parameter which determines the size of the generated droplet is the product V2 Deltat. The experimental results support this guess for pulse durations low enough so that there is no electrical field (and, therefore, no surface charge) during the last stage of meniscus elongation and break-up. The possible use of this technique of drop-on-demand generation is discussed, taking into account the transient oil flow around the meniscus which most often brings the droplet off the system axis.

Field induced disruption of a planar water-oil interface influenced by a close metallic sphere

The problem of instability of a horizontal interface between water and an insulating fluid, electrically influenced by a metallic sphere located just above it, is considered here with its relevance to the basic electrocoalescence phenomenon of close water droplets suspended in an insulating medium. Results are presented of preliminary experiments performed using visualisation and shadowscopy techniques. The evolution of the interface shape is characterised for various applied step voltages and for different values of the initial spacing s0 between sphere and interface. An order of magnitude analysis leads to good estimates of the characteristic parameters and time scales.