Modeling and simulation of rotary bell spray atomizers in automotive paint shops

Doctoral thesis, 2013

The paint and surface treatment processes in automotive paint shops are characterized by multiphase and free surface flows, multiphysics interactions, multiscale phenomena, and large moving geometries. The current version of the software for simulation of spray painting developed at the Fraunhofer-Chalmers Centre relies on measurements of droplet size distributions and velocity profiles below the applicator that can be used as input to the simulations. This thesis discusses techniques that can be used to reduce the need for costly and complicated measurements by performing detailed simulations instead. Surface tension plays an important role during breakup as it acts to stabilize the droplets. On the small scales of droplets from 1-100 μm in diameter it is a strong force yet localized to the interface between the droplet and the surrounding medium. It is therefore crucial to have control over the interface and to this end a novel method for reconstructing the interface of the droplet is proposed. The method relies on approximation by Radial Basis Functions using a technique that enables the omission of small length scale structures in order to obtain a smooth representation that is suitable for numerical discretization. Droplet size distributions have been simulated with the Taylor Analogy Breakup model. A modification taking the large viscosity of the paint into account is introduced to the model and it is applied to the case of rotary bell spray painting commonly used in automotive industry. Results show that the model is able to capture the overall shape of the size distributions, as well as the local spatial dependencies on the size distributions where large droplets are typically found further away from the center. By gaining a better understanding of the physical conditions close to the paint applicator the need for costly and complicated measurements is decreased. Having access to tools for fast and efficient simulation of the spray painting processes would be advantageous, since such tools can contribute to reduce the time required for introduction of new models, reduce the cycle time, reduce the environmental impact, and increase quality.