Pierre Atten

The Electrohydrodynamic Origin of Turbulence in Electrostatic Precipitators

The functioning of an electrostatic precipitator in the light of previous studies on various regimes of electroconvection in both parallel and divergent electric fields is examined. Coupling between velocity and charge density fluctuations for both ions and charged particles is discussed. It is shown that for a certain diameter range of the particles, their nondimensional mobility parameter takes values similar to those characterizing ions in liquids. An experimental simulation using insulating liquids is proposed.

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.

Oscillatory and steady convection in dielectric liquid layers subjected to unipolar injection and temperature gradient

The effect of a temperature variation of both dielectric constant and ionic mobility on the stability of a horizontal dielectric liquid layer subjected to an electric field, weak unipolar injection, and heating from below is analyzed. Regions of steady and oscillatory convective instability are delineated by means of a Galerkin method as well as by direct computer integration of the thermoelectrohydrodynamic equations. A heuristic description of all relevant physical phenomena involved in the problem is also given.

THE ROLE OF CONDUCTION IN ELECTRORHEOLOGICAL FLUIDS : FROM INTERACTION BETWEEN PARTICLES TO STRUCTURATION OF SUSPENSIONS

Abstract After a brief description of the electrorheological (ER) fluids and of their properties, we recall the basic physical processes responsible for the electrically induced interactions between particles. We then give a short review of recent work on ER effect in the case of D.C. applied fields where the conduction properties of the components play the major role. The conduction model for estimating the attraction force between two spheres is presented along with some experimental results. Finally we shortly address problems raised by the ER fluids such as the prediction of their static yield stress and of their apparent viscosity.

Role of EHD Motion in the Electrical Conduction of Liquids in a Blade-Plane Geometry

A blade-plane electrode assembly is used to study electrical conduction in divergent fields and, more generally, to investigate high field conduction of dielectric liquids. The stationary current-voltage characteristics show two regimes of conduction: a quasi-ohmic regime and a V¿ one, V being the applied voltage and 3 < ¿ < 6. The latter regime corresponds to dominance of charge injection by the blade, as revealed by the field distribution obtained by the Kerr technique. A liquid motion is induced by the Coulomb force which convects charge carriers and results in a decrease of the first transit time following the application of a voltage step. The transient injection currents exhibit a dependence upon the elapsed time during which no voltage is applied, and the possible origin of this phenomenon is discussed. Measurement of the liquid velocity by laser Doppler anemometry also exhibits two different laws of variation with applied voltage consistent with the two current regimes. In the steady-state conditions of the injection-dominant regime, the square of the measured velocity component w2 varies as the electrical power input IV(I being the current). This is tentatively interpreted by the viscous dominated character of the induced motion.

EHD convection in a dielectric liquid subjected to unipolar injection: Coaxial wire/cylinder geometry☆

Abstract The injection of charge carriers into a dielectric liquid lying between a wire and a coaxial cylinder generally induces a motion of the liquid. The characteristics of the convection and its effect on the passage of electrical current are examined with reference to the similar electro-convection which exists in a planar layer of liquid. After recalling the main results obtained in the latter geometry for both weak and space charge limited injections, a rough analysis is developed in the case of strong injection in an annulus. It gives some expressions for the typical convective velocities. The experimental determination of the variation laws of the current and of the typical velocity apparently reveals the occurrence of two conduction regimes. The analogies and differences with the characteristics of the regimes of convection in a planar layer are indicated and discussed.

Numerical Modeling of Finite Amplitude Convection of Liquids Subjected to Unipolar Injection

The unipolar injection stability problem is characterized by a hysteresis loop between nonlinear and linear stability criteria. The problem of determining the corresponding finite amplitude convective solution is investigated for rolls in the weak injection case, taking into account the peculiarities of the charge conservation equation and the possible nonstationary character of the flow. The results of three different approaches (finite difference method, approximate analytical solution, and particle-type method) are presented and discussed.

Behaviour of conductive particles in corona-dominated electric fields

The authors present a theoretical analysis and an experimental study of the behavior of spherical and cylindrical conductive particles in contact with a metallic electrode, and affected by a corona-dominated electric field in atmospheric air. An accurate mathematical model of the conductive particle movement is introduced, taking into account the variation of the electric charge of the small object in a mono-ionized electric field, and the action of air convection produced by unipolar injection. Several experiments have been performed, using point-plane and wire-cylinder electrodes to quantify the contributions of the electric field, space charge density, and shape, size and mass of the particles. The results are compared to those for electric fields without space charge.<<ETX>>

The electroviscous effect and its explanation I — The electrohydrodynamic origin; Study under unipolar D.C. injection

Abstract The electroviscous effect in isotropic liquids, which was related by some authors to the motion accompanying the passage of an electric current, is examined here in the light of recent developments concerning the conduction properties of dielectric liquids, the Coulomb force induced instabilities and electroconvection. It is proposed that electrically induced secondary motion is responsible for the increase in apparent viscosity when subjecting a Poiseuille or Couette flow to an electric field. This general explanation is illustrated by experiments performed for the particular case of pure unipolar injection under D.C. voltage. The effect, which can be very strong, is qualitatively justified and further supported by flow visualizations which reveal various textures in a certain voltage range.

Electrohydrodynamic stability of liquids subjected to unipolar injection: non linear phenomena

Abstract The phenomena related to fluid motion induced by the Coulomb force exerted on the space charges by the electric field are investigated in the case of unipolar injection in an insulating liquid. After a brief recall of theoretical and experimental work on the linear hydrodynamic stability problem, the non-linear behaviour is investigated. Theory predicts the occurence of a non-linear criterium lower than the linear one and therefore of a hysteresis loop associated with discontinuities of the steady current and liquid velocity at both criteria. Furthermore it it gives a justification for the appearance of hexagonal convective cells at critical voltage. Experiments performed under space charge limited conditions very satisfactorily agree with the theoretical description except for the critical values; this minor discrepancy is discussed.