Numerical Simulation of the Behaviour of Sprays Produced by High Speed-Rotary Bells With External Charging

Recently, the authors have presented results on the simulation of electrostatically supported high speed rotary bells used in the automotive industry. Applying an extended version of FLUENT which includes the static electric field and the space charge effect due to the charged droplets, transfer efficiency and film thickness distribution of the coating processes of flat plates could be estimated to a high degree of accuracy. In the present contribution the obtained results are transferred to the case of high speed rotary bells with so-called external charging. This charging mechanism, using electrodes to produce a flux of free ions interacting with the paint droplets is mainly applied in the case of water based conductive paints. Hence, the existing program for direct charging atomisers has to be extended further, considering the following features: - calculation of the static local density of ion current and the corresponding space charge field - calculation of particle trajectories taking into account the transient charging within the flow domain In principle, the space charge field is calculated solving an additional conservation equation for the flow of ions. For the time dependent charging of the paint droplets existing models are applied using additional corrections factors based on experimental results of the local currents. Current results indicate a good agreement of the main features of measured and calculated static film thicknesses, e.g. the general geometry and the extention. This is also true for the transfer efficiency. Consequently, also the dynamic film thickness profiles, produced by the atomiser moving over a flat target, are very similar. As the next step, the program will be verified with respect to the sensitivity to parameter changes of the atomiser and more complex target geometries. Future applications of the program are not limited to the calculation of the coating process only. A potential task may be the development of improved atomizers.