The simulation of the electrostatic spray painting process with high-speed rotary bell atomizers. Part I: Direct charging

Abstract

High-speed rotary bell atomizers are widely used in the painting industry for high quality applications. They provide a highly uniform film thickness with reasonable transfer efficiency due to the additional electrostatic field supporting the droplet transport towards the target. A basic requirement for this type of paint atomizer is a fine and reproducible atomization of a large variety of different paints, ranging from solvent-based materials to highly non-Newtonian water-borne systems. Furthermore, a broad range of paint flow rates must be covered. The present contribution summarizes investigations aiming to completely model the electrostatically supported spray painting process by means of CFD. In part I, so-called direct charging atomizers, where high voltage is applied directly to the rotating bell, are considered. Here, charging of the droplets takes place at the bell edge and corona effects can be neglected. A powerful commercial code, in the present case Fluent in its current releases, has been extended to account for the electrostatic field and the space charge effect due to the charged paint droplets. As input conditions, the air flow from the shaping air orifices and measured droplet sizes and velocities close to the bell edge using phase-Doppler anemometry and Fraunhofer diffraction were taken. Also, LDA measurements in front of the target were performed, yielding comparative data of the airflow field. In general, numerical and experimental results are in good agreement. This is especially true for the final film thickness on the target and the transfer efficiency, i.e. the amount of paint solids finally deposited on the target. The agreement was achieved using a droplet charge of 5% of the droplet size dependent Rayleigh limit. These results serve as a basis for a complete painting process simulation for complex work pieces, e.g. whole car bodies, in the future. This task, however, can only be successfully completed performing unsteady calculations with moving atomizers along given robot paths. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.