Numerical simulations of droplet trajectories from an electrostatic rotary-bell atomizer

Abstract

Electrostatic spray coating (E-spray) is widely used for coating conductive substrates. The combination of a high-velocity focusing air, an imposed electric field and charged droplets, leads to higher transfer efficiency than that of conventional spray coating. In this thesis, a mathematical model of trajectories of droplets generated by a rotary-bell electrostatic atomizer is described which enables predictions of coating deposition rate patterns. A dilute spray assumption (i.e., no particle-particle interactions) allows modeling single droplet trajectories resulting from a balance of electrostatic force, drag and inertia. Atomization of liquid droplets is not modeled explicitly—instead an empirical correlation is used for the mean droplet size while individual droplet sizes and starting locations are determined using random distributions. The electrostatic field and droplet trajectories are strongly coupled and calculated iteratively with successive substitution and relaxation. Parametric studies on how bell voltage, bell rotation speed, and focusing air affect spray pattern, mass transfer efficiency, and droplet trajectories are presented. Simulated spray patterns are compared to those of physical experiments. For the parameter values considered in this thesis, most of the predicted sprays are hollow cones with negligible deposition near the center axis, deposition of a heavy ring surrounding the center and a tapering of thickness towards the outer edge of the substrate. In contrast, most of the experimental results display non-axisymmetric deposition patterns in the form of two lobes of coating. In addition, the experimental deposition patterns are relatively insensitive to any of the three primary parameters (i.e. bell rotation speed, bell voltage, or focusing air intensity). These results are not obtained with the simulations, which show a moderate trend between both uniformity and transfer efficiency, both of which are favored by high bell rotations speeds and lower voltages.