P. Atten

Macroscopic model of interaction between particles in an electrorheological fluid

Abstract The particle-particle interaction in an electrorheological fluid is investigated starting from the basic remark that, under DC conditions, the electric field distribution is controlled by the conduction properties of both solid particles and carrying liquid. An experiment on a large scale model sphere of very slightly conducting material facing a metallic plate is developed in particular by varying the even weaker but controlled conductivity of the surrounding liquid. The measurement of the attraction force between sphere and plate reveals two regimes of variation of the force as a function of the applied electric field. An approximate analysis taking into account the non-linear character of electrical conduction of the liquid at high electric fields leads to formula which fairly well account for the observed dependence on the electric field as well as on the ratio Γ of solid and liquid conductivities.

On a negative electrorheological effect

Abstract After recalling the primary role of the conduction properties of solid particles and suspending liquid in promoting the electrorheological effect, we examine the case of suspensions for which the conductivity of solid particles is much smaller than that of the carrying liquid. A negative effect is found, i.e. a decrease in apparent viscosity of the suspension under the action of the electric field. Observations indicate that this effect arises from the migration of the particles towards one electrode.

Numerical Simulation of the 2-D Gas Flow Modified by the Action of Charged Fine Particles in a Single-Wire ESP

A numerical model for simulating precipitation of submicrometer particles in a singlewire electrostatic precipitator is discussed in this paper. It includes all important phenomena affecting the process: electric field, space charge density, gas flow, including the secondary electrohydrodynamic flow caused by the corona discharge and charged particles, and particle transport. A simplified corona model assumes just one ionic species and neglects the ionization zone. The fully coupled model for the secondary EHD flow, considering the ion convection, has been implemented. The dust particles are charged by ionic bombardment and diffusion. The gas flow pattern is significantly modified by the secondary EHD flow, which depends on the particle concentration. As for fine particles the drift velocity is small and particles practically follow the gas streamlines, the particle concentration has a very strong effect on the precipitation efficiency.

Effect of electro-aero-dynamically induced secondary flow on transport of fine particles in an electrostatic precipitator

Abstract The turbulent motion which strongly affects the distribution of charged particles in an electrostatic precipitator is investigated using a laboratory model with barbed ionising electrodes. The flow visualisation reveals a vigorous well organised roll-like secondary flow. A simple 2-D numerical model confirms that a typical velocity of this secondary flow can be of the order of 1 m/s .

CORONA DISCHARGES IN HIGH PRESSURE AIR

Abstract The purpose of the present work was to investigate in a point-plane electrode assembly the processes of corona discharge in air as a function of gas pressure (P

Properties of streamers in transformer oil

The electrical characteristics of creeping discharges and single creeping streamers in transformer oil first are compared with those of streamers developing in the liquid bulk. The distribution of electric potential along the channel of a single negative creeping streamer is determined using a capacitive probe technique. Then the distribution of the space charge associated with each streamer channel is discussed and the electric field around the channels is estimated. A strong correlation between the mean potential gradient and the capacitance of the streamer channels is found. The different results and considerations tend to support the hypothesis of the same basic physical mechanism for both creeping discharges and streamers developing in the bulk.

Experimental investigation of capillary instability: results on jet stimulated by pressure modulations

We investigate the behaviour of a liquid jet stimulated by pressure disturbances using a photometric measurement of the jet shadow width. Two apparatuses involving lights of different nature are utilized and measurements are taken from the exit of the nozzle to drop breakoff for different operating conditions. Fourier analysis is applied to characterize the spatial evolution of the jet shape. In contrast to previous studies where only amplitudes of the Fourier modes are reported, phase shifts are also recovered for low and high initial perturbations. We show that the spatial reconstruction of the jet from the temporal Fourier analysis at different abscissae is in excellent agreement with the experimental profiles

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.