DROPLET DYNAMICS IN THERMAL SPRAYING COATING PROCESSES

Abstract

Modeling of highly deformable free surface problems presents a great challenge to numerical analysts and computational scientists. Two basic approaches have been used to simulate this kind of problem. One is based on the interface track method in which a deforming finite element with adaptive grid scheme has been employed so that the free surface can be tracked explicitly using the Lagrangian algorithms. Although this method can achieve high resolution at the interface but it is difficult to handle the large deformation and free surface breakup problems. Another approached is based on the volume tracking method, such as volume of fluid (VOF) scheme, in which a finite difference technique with a fixed grid system is used. The VOF method suffers from difficulty in calculating the curvature of free surface from volume fraction. However, the method is easy to use and implement. Beside the VOF method level set methods is also in this category and is widely used to simulate the bubble formation and impact. It can accurately compute the free surface flows with surface tension at complex topology. In plasma spraying, micro-droplet soldering and inkjet printer applications, droplet inflight and impact are important. During simulation, inflight spherical droplet without deformation is usually assumed and its heat transfer and solidification are calculated using either a zero- or a onedimensional model. During droplet impact simulation, a spherical droplet with constant velocity and uniform temperature is usually assumed as initial geometry and condition. The droplet-gas interaction is neglected. In the experiments, melt flow inside the particle will influence the shear stress on the droplet surface, and it will significantly change the drag force of the droplet. Also, surface tension force will cause droplet oscillation, all phenomena are important and have neglected in the modeling. In this paper, a numerical model based on the level set method is developed to simulate the droplet deformation during inflight and spreading after impact. The level set function will be used to track the deformation of the droplet free surfaces. The capability of the model on accurately and efficiently modeling the droplet deformation and oscillation will be demonstrated, and the model will be used to study the melt flow inside the droplet, and deformation of free surface. Particles with different morphologies, especially, hollow particles, are widely used in plasma spraying. For example, YSZ hollow particles are used in thermal barrier coating, which will produce coatings have lower thermal conductivity. The effects of droplet size and morphology on the deformation have been investigated.