Author

Wuhai He

Date of Award

6-1993

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

Dr. Malcolm H.I. Baird

Co-Supervisor

Dr. Jen-Shih Chang

Abstract

Electrohydrodynamics (EHD) is an interdisciplinary engineering science which has been known for more than a century and has been more intensively developed in the last two decades with the concern of the interaction between electric fields and fluids. The investigations of EHD droplet formation, dispersion and mass transfer in a viscous dielectric liquid are the main subjects of study for mass transfer enhancement in liquid-liquid extraction operations by an imposed DC electric field.

The droplet formation and dispersion of water in a high viscosity mineral oil have been studied at the conditions of low droplet Reynolds numbers with and without the presence of an electric field. The droplets formed at a hollow electrode with uniform size and spherical shape have been observed; the droplet size reduction and mutual repulsion can be controlled by the applied voltages. The droplet velocity near the hollow electrode is also significantly enhanced in the presence of the non-uniform DC electric field. The model for EHD droplet formation has been derived with the inclusion of electric force component based on the force balance upon droplet detachment from the hollow electrode.

Mass transfer of benzoic acid from droplets in this two-phase liquid-liquid system has been studied with and without the presence of an imposed DC non-uniform electric field. Total mass transfer efficiency can be significantly enhanced in the presence of the applied electric field. The results obtained from droplet mass transfer investigation indicate that the mass transfer mechanisms of these electrically charged droplets during the droplet formation and its subsequent free fall are similar to those of droplets in the absence of the electric field and these results can be correlated using the existing mass transfer models. The increased specific interfacial area and the accelerated drop initial velocity in the presence of electric field are the main contribution to the enhanced efficiency of mass transfer in the present investigation.

In parallel to the experimental investigations, the numerical modelling of electric field profiles inside the experimental cell has been performed based on the axial symmetric approximation and with the appropriate coordinate transformations. A finite difference algorithm with the control volume approach has been used to conduct numerical calculations. The numerical results obtained from the electric field profiles and predictions of the EHD force components on the droplet can provide much useful information to support the results obtained from the experimental investigation.

The mechanisms of electric field induced current flow and electrification of droplets during EHD droplet formation and dispersion have been postulated and discussed based on the observed current-voltage characteristics of the system, the electric charge acquired by the droplet and the results from numerical electric field modelling.

The mechanism of EHD droplet formation and dispersion from hollow electrode is proposed based on the modified Vonnegut & Neubauer model where an effective dielectric constant of a liquid-liquid two phase system has been defined. The electrically charged droplets formed at hollow electrode under high electric field intensity would all reach their maximum charge limit which determines and controls the uniform size of dispersed multi-droplets. Then the EHD force generated at high electric field intensity would contribute to the dispersion and acceleration of these electrically charged droplets. Thus the initial velocities of these droplets are significantly enhanced.

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