Neutralization of Electrosprays by Corona Discharge

Resum

The electrospray is a technique to atomize electrically conducting liquids by using electric forces to overcome surface tension. The droplets of an electrospray carry a high electric charge. This may be advantageous to prevent coalescence or to guide the spray with an electric field, but causes serious problems when the spray has to be focused or has to flow near metallic surfaces (to which the charged droplets are attracted), or when the droplets evaporate and Coulomb explosions ensue. A solution to this problem is to neutralize the droplets with a corona discharge. Even though electrospraying and corona discharge processes are known well separately, their combination leads to complex situations that are not yet fully understood. The problem of the neutralization of a dilute electrospray by a corona discharge of polarity opposite to that of the spray droplets in the gap between two parallel electrodes charged to different electric potentials, is formulated and numerically analyzed in this thesis. An Eulerian description of the spray is combined with a transport equation for the corona ions, a Poisson equation for the electric potential, and mass and momentum conservation equations for the gas. Two configurations of the corona are considered. In one of them the corona is generated outside the gap and the corona ions are injected through an orifice opposite to the inlet orifice of the spray. The corona current that can be injected by these means is limited by the space charge of the ions already present in the gap. To better understand the neutralization process and the influencing variables, this study is made first for a simple one-dimensional model and then extended into a more realistic two-dimensional model. The maximum corona current increases with the voltage applied between the electrodes, and the minimum voltage required to achieve full neutralization increases with the flow rate of the electrospray. In the other configuration the corona is generated around a point electrode on the axis of the spray. The ionic current is not limited in this case. However, partially discharged droplets are attracted to the point electrode and deposit on it if neutralization is not completed before the droplets reach the electrode.