Charging Level and Deposition of Droplets in Electrostatic Painting

The process of electrostatic painting has become a very important method of coating in a wide range of industrial applications including those used in the automobile industry. The general principle of spray coating is to deposit liquid droplets or solid powder particles on coated targets having various shapes. The electrostatic coating process consists of three main stages: droplet formation and charging, transportation and deposition. The complication of this process is caused by various factors, such as the physical properties of the material to be used, the appropriate electrical and mechanical conditions and the target surface to be coated, which affects significantly the deposition uniformity and the finish quality, especially when it contains some sharp edges and recessed areas. In this thesis, a numerical investigation of the charging level on a spherical droplet formed out of a cylindrical ligament in an external uniform electric field is presented. The droplet charge on a single ligament was predicted for different droplet sizes, ligament lengths, ligament diameters and electrode widths. The effect of these model parameters on the charge levels was found to be significant. A mathematical approximation for the charge magnitude as a function of the droplet radius to some exponent and ligament length is also formulated. The value of the radius exponent decreases dramatically with increasing the ligament length. Also, the estimated values of the droplet charge were compared for linear and circular arrays of ligaments, which show a great influence of the geometry of the sprayer on the charge levels. A very good agreement between the experimental and numerical results in the case of a circular array of ligaments, including a specified space charge, was obtained. All numerical simulations were performed using COMSOL, a Finite Element commercial software. A further study is carried out to investigate the deposition thickness profile on a stationary and moving flat target by incorporating a numerical simulation of the industrial electrostatic coating process via ANSYS, a CFD commercial software. A modified injection pattern was suggested to achieve a closer agreement with the experimental data. The injection pattern includes 15 bands of different particle sizes (i.e. polydispersed particles) and charge to mass ratios. A combination of different injection angles and fractions of mass flow rates was suggested in each size band. A very good agreement between the experimental and numerical deposition patterns was obtained in both cases of a stationary and moving target. Also, the deposition thickness profile was calculated on a target surface with a small perturbation at the center using ANSYS numerical model. Different model parameters of a perturbed surface, such as the size of the indentation or the protrusion and the radius of the corner were investigated in this study. The numerical results reveal a very low particle concentration inside the indentation, which is caused by the Faraday cage effect and it is strongly affected by the depth of the indentation, while the edge effect, which shows the high concentration of deposited particles at the corner, increases with decreasing the radius of curvature. The predicted deposition patterns were very consistent with the calculated values of the electric field for different surface perturbations