On the structure of an electrostatic spray of monodisperse droplets

Abstract

An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse primary droplets, with the remainder in a shroud of satellites. Droplet size, axial velocity, and concentration were measured throughout the spray by phase Doppler anemometry (PDA). The complementary use of these measurements permitted the determination of the electric field via the spray momentum equation. It was found that droplets are ejected from the jet at a relatively high velocity in a region characterized by a very intense electric field. They maintain this velocity farther downstream because of inertia, even though the field is precipitously decreasing, and ultimately decelerate under the action of the drag force and a progressively weaker electrostatic force. Velocity and concentration fields were shown to be self-similar. Comparison between the external field, due to the potential difference applied between the electrodes, and the space charge field shows that the droplet axial motion is driven primarily by the external field, whereas the droplet radial motion and, consequently, the jet lateral spreading, is controlled primarily by the space charge field. The latter is typically at least one order of magnitude smaller than the external one, except at off-axis locations near the breakup region of the spray, where the two fields can be comparable. The droplet charge distribution was also determined via the spray momentum equation and the simultaneous measurements of droplet size and velocity in a region where droplets experience negligible acceleration. The charge distribution was found to be narrow, with a ratio of standard deviation over mean of 0.15.