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High-speed rotary bell atomizers are widely used in automated painting processes. They provide excellent paint film quality as well as high transfer efficiency due to electrostatic support and the possibility to control droplet size and droplet transport separately. However, the influence of complex material properties on atomization and the resulting droplet size distribution is still an open research topic.
The present contribution deals with numerical and experimental studies of the primary liquid breakup process using a high-speed rotary bell atomizer. The investigation focuses on the disintegration process of the paint liquid in the near-bell region. As inlet conditions for breakup simulations, the properties of the liquid film at the bell edge, i.e. film thickness, velocities and apparent viscosity, resulting from our previous work, were applied. The paint liquid used in studies can be characterized by a shear-thinning (pseudoplastic) behavior, measured by both, rotational and capillary viscometer.
A hybrid multiphase model that links the Volume of Fluid (VOF) model and the Discrete Phase Model (DPM) is implemented in CFD software Ansys Fluent. In the simulation, the VOF model tracks the liquid-gas interface to describe the flow of the liquid film on the bell and the liquid disintegration at the bell edge. In the near-field, different liquid disintegration structures mainly depending on bell speed and paint flow rate are found. These structures are also observed in experimental investigations using a high-speed camera. Under the given conditions, the developed Hybrid Model converts the liquid lumps resulting from the VOF-Solver into point masses that can be further tracked using the DPM model. Moreover, droplet properties such as volumes, equivalent diameters, velocities and positions can be determined with the DPM statistics. The resulting quantitative simulation data, i.e. drop velocities and droplet size distributions, are compared with the experimental results applying Laser Doppler Anemometry (LDA) and the laser diffraction instruments.
