Simulation of Fluid Flow and Heat Transfer Including Phase Change during the Impact of Semi-Molten Particles S. Alavi Graduate Student Ferdowsi University of Mashhad Mashhad, Iran sina.alavi@stu-mail.um.ac.ir M. Passandideh-Fard Associate Professor Ferdowsi University of Mashhad Mashhad, Iran mpfard@um.ac.ir J. Mostaghimi Professor University of Toronto Toronto, Ontario, Canada mostag@mie.utoronto.ca ABSTRACT A numerical method is proposed in this paper which is capable of modeling the impact and deposition of semi- molten particles. By attributing a high viscosity to the solid region, simulating the motion of solid cores including solidification/melting is made possible inside the particle. The Navier-Stokes equations are solved in combination with the Volume-of-Fluid (VOF) technique to track the free surface of the particles. In addition, the heat transfer including phase change is modeled using the enthalpy method. The viscous terms are treated implicitly to relax the restrictions associated with small computational time-steps in explicit formulations. Several case studies with operating conditions of a typical thermal spray process are simulated. These cases include the impact of molten and semi-molten nickel particles in an atmospheric plasma spray process (APS). The effects of various parameters such as particle solid-core diameter, initial velocity and temperature are investigated. The simulations show that the size of the solid core has an important effect on the amount of particle splashing during the impact. KEYWORDS: Semi-molten particles, Thermal spray processes, Volume-of-Fluid (VOF), High viscosity method, Solidification and heat Transfer. 1. INTRODUCTION Thermal spraying technology includes the coating of surfaces using different techniques to serve in several industries such as aeronautics and space industries, automotive industries, chemical and electronic industries, etc. In these techniques, molten particles are generated from raw material which is fed into a thermal spray torch, in the form of wire, rod or powder. For example in plasma spraying, a high-temperature plasma flame melts and accelerates particles, usually provided as powder, to form the coating on the substrate. The properties and microstructure of the coatings strongly depend on the phenomena happening during the particle flight time between the injection of the particles into the flame and their impact on the substrate [1]. Depending on the size and properties of the powder particles and the plasma operating conditions, such as temperature and velocity, the particles may be completely molten, completely solid or semi-molten at the impact. The presence of semi-molten particles highly increases the porosity of the final coating; as a result, the existence of these particles in a coating process is undesirable [2]. A numerical model capable of simulating this complex phenomenon, therefore, is essential in understanding various characteristics of a coating related to the impact and deposition of partially molten particles. A literature review of the research in this field explains this fact. The history of the simulation of droplet impacts began in the late sixties when Harlow and Shannon [3] first employed the Marker-and-Cell (MAC) method in cylindrical coordinates to model the splash of droplets. Implementing heat transfer along with the fluid flow simulation during the impact was performed later by many researchers [4-7]. Brackbill et al. [8] proposed the Continuum Surface Force (CSF) model that highly improved the application of surface tension forces on the free surface. Many other aspects of the droplet impact phenomenon were studied by other researchers [9-12]. Wetting effects, droplet recoiling, convection effects, contact resistance, and droplet impacts with different velocities and temperatures on various substrates are among the studied subjects. Later, 3D numerical simulations made possible the modeling of asymmetric phenomena like splashing and Proceedings of the ASME 2012 Summer Heat Transfer Conference HT2012 July 8-12, 2012, Rio Grande, Puerto Rico HT2012-58252 1 Copyright © 2012 by ASME