COMPUTATIONAL FLUID DYNAMICS ANALYSIS OF THE ELECTROSTATIC SPRAY PAINTING PROCESS WITH A ROTATING BELL CUP

摘要

The use of electrostatic fields in the spray painting process is a common technique utilized to increase the transfer efficiency of the process and the quality of the applied coating. The electrostatic field is usually generated by applying a voltage differential between the spray gun and the grounded target surface so that the electric potential will drive the charged droplets toward the target. In modern booths, the coatings are often applied via rotating cup-type spray guns mounted on automated robotic arms that move along preprogrammed trajectories. In such systems, the paint droplets are generated by the atomization of the thin film of paint that covers the surface of the rotating cup when it hits the serrated cup rim. The so-formed droplets are charged with the same potential polarity as the spray gun via induction or corona charging and are pushed by the electrostatic force toward the grounded target surface. In addition to the electrostatic force, a strong airflow coaxial with the spray gun, referred to as "shaping air," helps convection of the droplets toward the target. This process typically occurs in a painting booth where, for safety reasons and according to regulations, a strong crossflow of air-conditioned air exists. The interaction of these three forces as well as the particle size determines the path that the droplets will follow and ultimately the location of their deposition. While the general characteristics of the electrostatic spray painting process are well understood, there is a lack of detailed physical analysis that is essential for the optimization of the process. The present work makes use of CFD methods to analyze the interaction of the flow field with the electrostatic field and the effect that their coupling has on the paint droplets. While the flow field and the electrostatic field are computed using an Eulerian approach through the use of a finite volume formulation, the droplet trajectory is computed using a Lagrangian scheme that tracks each particle trajectory by integrating its equation of motion. The paint droplets have an initial size distribution determined by experiment, and their initial specific electrostatic charge is calculated following empirical correlations and basic principles. The coupling between the flow field and the electrostatic field is given by the computed particle trajectories since the motion of the particle is affected both by the flow field and the electrostatic field and, in turn, the electrostatic field is affected by the particle motion via the space charge. Flow turbulence effects are computed using the realizable k — e model, and particle turbulent dispersion is computed using a random-walk approach. The CFD commercial code Fluent was used to perform these simulations since the off-the-shelf code is capable of simulating the flow field and the electrostatic field as well as the particle trajectories. In addition to these standard models, specific subroutines were written to compute the interaction of the droplets with the electrostatic field via the space charge. A graphic user interface was developed in order to make the setup of these simulations faster and more consistent so that a high degree of repeatability can be achieved. The present paper is a summary of the ongoing work being performed in this area at the Lebanon Ansys office.