Charging of droplets with a plasma curtain discharge

R. SOSA; D. GRONDONA; A. MARQUEZ; H. KELLY; G.ARTANA

Colonia de sacramento

Encuentro; XI Reunión sobre Recientes Avances de Física de Fluidos y sus Aplicaciones; 2010

Droplets produced by the breakup of a liquid jet can be charged by different mechanisms (i.e. induction, injection, flow electrification,..). We concentrate our efforts here in analyzing the charging of the droplets downstream the region of jet breakup. The mechanism that is best suited for this purpose is the one that takes place as a consequence of the impact on the flying droplets of non neutral particles (electrons and ions) produced by an atmospheric discharge. In the past most efforts using this approach have been dedicated to develop systems considering a corona discharge (ac or dc). In this kind of discharge the ionization of the air is promoted by using electrodes of small curvature radii. This process occurs only at close proximity of its surface. The presence of charged species screens the electric field and in general the region between the electrodes remains dark. There charged species drift from one electrode to the other under the influence of a rather constant electric field. It is in this region that droplets are normally forced to flow to achieve a significant electric charge. Recently we have developed a system that enables to produce a discharge (Plasma Curtain) where ionization can take place in a region that largely exceeds the typical one of coronas. This kind of discharge is based on the combination of an ac dielectric barrier discharge with a dc corona discharge in a three electrode system. It can be sustained for large time periods and typical interelectrode air gaps are of the order of 20 mm and electrode length of 10 cm in the transversal direction. The discharge is composed of a train of streamers, with a repetition frequency in the range 50–200 kHz, and carrying an average current in the range 0.1–0.4 mA. Main goal of this article is to analyze the authority of this kind of device to charge train of water droplets passing through the interelectrode gap. To facilitate this analysis the water jet is generated with a small capillary that is excited mechanically with a fixed frequency to produce mono sized droplets.