Investigation and analysis of the electro-hydrodynamic (EHD) effects in asymmetric air gaps. The wire - cylinder case

Abstract

In this work, the electro-hydrodynamic (EHD) mechanism, in the case of a thin wire parallel to a conducting cylinder of significantly larger diameter, has been studied. The wire-cylinder geometry may be used in a wide range of applications as a corona discharge generating pair, due to its simple structure and the highly inhomogeneous electric field that can be generated under high voltage DC application, which, in turn, is capable of producing significant EHD flow in air. The physical phenomenon of the EHD effect has been studied through simulations, experiments and comparisons. The operational parameters, such as the electric field and potential distribution, in variously dimensioned electrode arrangements have been investigated, while their dependence on geometrical characteristics of the electrodes, such as the electrode length, the electrode gap and the emitter and collector electrode radii, has been examined. On this purpose, both computer modelling and experimental studies have been conducted. The electric field and potential distribution has been studied by implementing Finite Element Analysis (FEA). The simulation results have shown that the maximum electric field intensity (Emax) was mainly associated with the wire-cylinder radii ratio, while the distance between the electrodes strongly affected the distribution along the axis of the gap. Further analysis of the results has shown that Emax may be estimated by an empirical formula which was found to be satisfactory in all cases. An approximate technique for the determination of the unipolar saturation current limit, based on the analysis of the electric field lines has been proposed and the model has been verified through experiments, which have shown that the wire-cylinder unipolar corona discharge current is closely related to the specified theoretical limits. The experimental study of the EHD flow has shown that the corona discharge current is related to the applied voltage through a quadratic relationship following the well-known Townsend’s general model. The ionic wind velocity found to be an approximately linear function of the applied voltage and proportional to the square root of the discharge current, while on the other hand, a linear relationship between the generated thrust and the corona discharge current has been determined. In all cases, the electromechanical efficiency and the thrust efficiency, which is frequently used as an overall performance evaluation factor, have been derived.