Droplet Breakup in Automotive Spray Painting

Sammanfattning: Paint and surface treatment processes in a car paint shop are to a largeextent automated and performed by robots. Having access to tools thatincorporate the flexibility of robotic path planning with fast and efficientsimulation of the processes is important to reduce the time required forintroduction of new car models, reduce the environmental impact and increasethe quality. The current version of the software for simulation of spraypainting developed at the Fraunhofer-Chalmers Centre relies on measureddroplet size distributions that can be used as input to the simulations. Thisthesis discusses techniques that can be used to simulate the droplet sizedistributions and therefore reduce the need for costly and complicatedmeasurements.Surface tension plays an important role during breakup as it acts to stabilizethe droplets. On the small scales of droplets from 1-100 µm indiameter it is a strong force yet localized to the interface between thedroplet and the surrounding medium. It is therefore crucial to have controlover the interface and to this end a novel method for reconstructing theinterface of the droplet is described. The method relies on approximation byRadial Basis Functions using a technique that enables the omission of smalllength scale structures in order to obtain a smooth representation that issuitable for numerical discretization.Droplet size distributions have been simulated with the Taylor Analogy Breakup(TAB) model with promising results. A modification taking into account thelarge viscosity of the paint is introduced and the parameters of the model aretuned to the case of rotary bell spray painting commonly used in automotiveindustry. Results show that the model is able to capture the overall shape ofthe size distributions and that it captures the effect of the bell rotationspeed, shifting the distributions toward larger or smaller droplets.