Influence of droplet charge on spray painting with a high-speed rotary bell

Abstract

Numerical simulations of the spray coating process with an electrostatically supported high-speed rotary bell with internal charging were carried out, focusing on the effects of droplet charge on the spray formation, the film thickness distribution on the target and the transfer efficiency. From simulation results of the electrohydrodynamics (EHD) atomization process close to the bell edge, droplet size-to-charge relationships and initial droplet velocities were derived and used in the present coating simulation. Three different droplet charge models were applied and the resulting electric fields were analysed. It was found that the droplet size-to-charge relationship plays an important role for the spray-painting performance. Despite a relatively low influence of the charging model on the transfer efficiencies (TE), varying between 89 % and 94 % at an experimental TE of 94 %, significantly different film thickness distributions on the target were found. The best agreement between measured and simulated film thickness distributions was obtained using the proportionality between charge and droplet volume as indicated by the EHD-breakup simulations. Furthermore, effects of the initial droplet positions and velocities were analysed, comparing simulative and experimental results with respect to film thickness distributions on the target and droplet velocity profiles downstream the bell.