ENGINEERING AND CFD

Spray painting is the most common application technique in coating technology. Typical atomizers used in spray coating industries are such as High-speed rotary bell and spray guns with compressed air. High-speed rotary bell atomizers provide an excellent paint film quality as well as high transfer efficiencies (approx. 90%) due to electrostatic support. Small and medium-sized enterprises (SME) continue, however, to use compressed air atomizers, although they no longer meet today's requirements from an economic and environmental point of view. It is very important to understand the atomization mechanisms of these two kinds of atomizers, in order to improve the paint quality, to reduce the overspray and to optimize the coating process.   

Atomizer development is usually being carried out mainly empirical with experimental methods. Although the liquid break-up process can be observed by using high-speed cameras, it is difficult to investigate disintegration processes quantitatively, e.g. drop sizes and velocities directly at the bell edge.

In an ongoing research project, a smart spray painting process has been launched coordinated by the Institute of Industrial Manufacturing and Management (Institut für Industrielle Fertigung und Fabrikbetrieb/IFF) of the University of Stuttgart and the Fraunhofer Institute for Manufacturing Engineering and Automation, Stuttgart (Fraunhofer-Institut für Produktionstechnik und Automatisierung/IPA). The project mainly focuses in detail on liquid atomization using high-speed rotary bell and HVLP-spray gun, droplet trajectory and impact processes by means of numerical simulation and experiments.

Numerical studies of atomization of viscous liquids using the above mentioned two atomizers, focusing on the combination of the liquid breakup and droplet transport processes, were carried out. Coupled Volume of Fluid (VOF) and Lagrangian particle tracking approaches were applied. Because of modern high-performance computing technologies available at the HLRS in Stuttgart, such investigation are made possible, namely: the simulation of liquid break-up near the bell and micro droplets impingement.

Figure 1 shows the wetting process on the bell cup, the time of 20 ms for the complete wetting is identical to the experimental observation. Figure 2 shows the airflow field around the bell cup. The simulation results from Fig. 1 and Fig. 2 deliver the necessary boundary conditions for the further calculation of liquid disintegration process.

The corresponding results of primary breakup simulation using a high-speed rotary bell are shown in Figure 3. At high paint flow rate and low bell velocity, a jet disintegration mode: It can be observed that long ligaments are formed at the bell edge undergoing a further breakup process downstream. At lower liquid flowrate with higher bell speed, the length of ligaments is significantly reduced. Liquid breakup occurs mostly directly at the bell edge. The experimental observations (Fig. 3 right) validate the simulation results.

Atomization simulation using a HVLP spray gun was also carried out. A snapshot of liquid-gas boundary with a value of the liquid volume fraction of 0.5 is shown in Fig. 4/left, from which the intact liquid length as well as the detailed velocity around the paint liquid (Fig. 4/right) can be analyzed. Transfer of liquid VOF-lumps to droplets was calculated. It can be seen that droplets undergo breakup mainly in the region z < 10mm.