The simulation of the electrostatic spray painting process with high-speed rotary bell atomizers. Pt.II: External charging

The present contribution summarizes investigations aiming to completely model the electrostatically supported spray painting process with high-speed rotary bells by means of CFD. In this part II, so-called external charging atomizers, where high voltage is applied to emitting electrode needles, are considered. Here, charging of the droplets takes place due to free ions produced from corona discharge at the electrodes. Part I dealt with direct charging atomizers, where potential is applied directly to the rotating bell. The commercial CFD-code Fluent has been extended to account for the electrostatic field and the space charge effect due to the ions. Here, a model for the time-dependent and inhomogeneous field charging of the droplets was applied. Furthermore, the direct interaction between the ion current and the flow field, i.e., the so-called ion wind, could be calculated. As input conditions, the airflow from the shaping air orifices and measured droplet sizes close to the bell edge using Fraunhofer diffraction were taken. In general, numerical and experimental results are in good agreement, confirming the applicability of the chosen physical approach. This is especially true for the final film thickness on the target and the transfer efficiency, i.e., the amount of paint solids that finally deposited on the target. In the near future the calculations must be extended to true unsteady simulations including the dynamic meshing procedure.