On charging and breakup of paints using a high-speed rotary bell atomizer with internal charging system

Experimental and numerical studies on charge transport and liquid atomization with the consideration of electrohydrodynamics (EHD) have been performed for a high-speed rotary bell atomizer with internal charging system. The VOF-to-DPM hybrid model in the commercial CFD code ANSYS Fluent is used to investigate and analyse the liquid breakup. Furthermore, the charge conservation equation is solved not only considering flow convection but also the significant ion drift convection due to the presence of the strong electric field. We introduce the so-called apparent ion mobility coefficient that depends on the permittivity and conductivity of liquid, the film thickness on the bell surface and the electric field strength. With this model we can calculate the charge migration from the electrode, in this case the bell surface, to the liquid film and the droplets. Simulation results show an inhomogeneous charge distribution perpendicular to the bell surface, namely charge accumulating mainly on the film surface. Breakup simulations are carried out using a Newtonian liquid and a real paint. The electric body forces have been included in the Navier-Stokes equations. Effects of EHD on the liquid breakup are analysed. The relationship of droplet charge to droplet diameter is obtained, from which the total current is predicted that compared well with experiments. Simulation results deliver useful information for an improved understanding of the relevant physical processes.