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Simulation of the Electrostatic Spray Painting Process with High-Speed Rotary Bell Atomizers

: Domnick, J.; Scheibe, A.; Steigleder, T.; Weckerle, G.; Ye, Q.

Carnegie Mellon University:
ICLASS 2000. Eighth International Conference on Liquid Atomization & Spray Systems
Pasadena, Calif., 2000
International Conference on Liquid Atomization and Spray Systems (ICLASS) <8, 2000, Pasadena/Calif.>
Conference Paper
Fraunhofer IPA ()
rotary bell atomizer; Elektrostatik; Sprühen; lackieren

High-speed rotary bell atomizers are widely used in the painting industry for high quality applications, especially in the automotive industry. 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 atomisers is a fine and reproducible atomisation of a large variety of different paints, ranging from solvent-based material to highly non-Newtonian water-borne systems. Furthermore, a broad range of paint flow rates must be covered.
The present contribution summarises investigations aiming to completely model the spray painting process by means of CFD. A powerful commercial code, in the present case Fluent5.2, 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 were taken. Also, LDA measurements in front of the target were taken yielding comparative data of the air flow 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 that finally deposited on the target. The agreement was achieved using a droplet charge between 5 and 10% of the Rayleigh limit, which is confirmed by existing literature.
In long term, these results will serve as a part of a complete paint process simulation, including an estimate of the final colour and metallic effect. Additional models have already been developed, deriving paint film characteristics from the properties of the paint droplets depositing on the target.