Numerical Simulations of Paint Droplet Hydrodynamics and Distributions in an Airless Spray Coating Process

It is an advanced surface modification technology using paint droplet rotary spray coating in pipelines trenchless rehabilitation, which can improve surface anticorrosion properties. Due to the difference of collisional contact interactions of paint droplets and pipe walls, the interface strength of interior paint liner is changed as the paint droplets are deposited on the wall, which becomes the dominant source of the droplet adhesion during spray coating. This article aims to study the droplet distributions with changing rotation speed of rotary sprayer in an airless spray coating process. The contact interactions of collisions between paint droplets and the pipe wall were studied in an airless spray coating process, realizing in a high-strength paint protective liner using centrifugal spraying technology. The methodology was based on the discrete element method (EDEM) with JKR contact model, assuming paints as discrete phase. The calibration method of surface energy used in JKR contact model was proposed using a virtual repose angle test. The virtual accumulation angle was obtained from the paint droplet pile. The surface energy was determined according to the variations of accumulation angles. The droplet discharge velocity equation of sprayer holes was derived based on the motion equation of droplets under the action of the centrifugal field. The expression represented a relationship between paint droplet discharge velocity and sprayer rotation speed. Simulated results showed that the paint droplet accumulation angles changed from a moderating region, transitioning to an increasing region with increasing surface energies. The higher the surface energy, the larger the accumulation angle, resulting in a high height of the paint droplet pile. The threshold value was found from the intersection between the moderating region and increasing region. The value of surface energy was found to be 32.7 J/m2 from the statistical analyses of the repose angle virtual test. Simulations showed that the exit droplet velocity of sprayer holes increased with the increase of rotation speed and sprayer diameter. These droplet velocities were induced by centrifugal forces due to sprayer rotation. The larger the sprayer size, the higher the exit droplet velocity. The equation of exit droplet velocity was correlated with discharge coefficient. Results indicated that the paint droplet discharge coefficient of holes was nearly constant as a function of dimensionless Froude numbers. The characteristics of paint droplets were simulated with changing rotation speed in an airless coating process. The paint droplet numbers increased with times. The change of droplet numbers can be roughly divided into three stages: an initial stage, a transition stage and stable stage with times. Results indicated that the number of paint droplets on the pipe inner wall decreased with the increase of sprayer rotation speed, suggesting that a reasonable rotation speed was helpful to increase the number of paint droplets on the pipe inner wall. The paint droplet number was the largest at the rotation speed of 2 500 r/min with the large standard deviation and relative error. The standard deviation and relative error of paint droplet numbers were reduced with the increase of rotation speeds. It can be inferred that the rotation speed of the rotary sprayer is suggested not less than 5 000 r/min in order to obtain a uniformity of paint droplets in an airless paint droplet spray coating process. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.