Droplet Breakup in Automotive Spray Painting

Licentiate thesis, 2011

Paint and surface treatment processes in a car paint shop are to a large extent automated and performed by robots. Having access to tools that incorporate the flexibility of robotic path planning with fast and efficient simulation of the processes is important to reduce the time required for introduction of new car models, reduce the environmental impact and increase the quality. The current version of the software for simulation of spray painting developed at the Fraunhofer-Chalmers Centre relies on measured droplet size distributions that can be used as input to the simulations. This thesis discusses techniques that can be used to simulate the droplet size distributions and therefore reduce the need for costly and complicated measurements. 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 described. 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 (TAB) model with promising results. A modification taking into account the large viscosity of the paint is introduced and the parameters of the model are tuned 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 and that it captures the effect of the bell rotation speed, shifting the distributions toward larger or smaller droplets.