Analysis of Heat Transfer to a Spectrum of Particles in Plasma Spray Coating Process

Abstract

Abstract In plasma spray coating process, ceramic/metallic particles are introduced in a flow of thermal plasma. The heat transfer from the plasma to the particles results in heating and melting of the particles. The molten particles then impinge on a substrate to be coated. The impacting particles spread on the substrate due to their high velocities forming a thin layer. Heat transfer from the particles to the substrate results in rapid solidification and formation of a thin protective coat on the substrate. The heat transfer to/from particles affects the quality of the coatings. It is therefore important to predict and control the particle heat transport. In this paper, we have considered a spectrum of particles moving in thermal plasma. The particle spectrum is represented by a regular array of identical particles. The particle longitudinal and lateral spacing is varied independently to study the effect of particle spacing on the flow and heat transport. The Reynolds number based on the particle diameter is in the intermediate regime (Re ∼ 20–100). The background gas, argon, is at atmospheric pressure and the flow is considered steady. The flow and transport is analyzed by a cylindrical-cell numerical model. An orthogonal adaptive grid is generated to body-fit the particle surface as well as the cylindrical outer boundary of the cell envelop. The grid near the surfaces of the particles is very fine and resembles spherical co-ordinates. The conservation equations for mass, momentum, and energy are solved in the plasma by a finite volume numerical method. The flow field and the temperature distribution are calculated in the plasma for various combinations of lateral and longitudinal particle spacings. The local variations of the viscous and pressure forces acting on the particle surface as well as the local Nusselt number variations are calculated. The overall Nusselt number and the drag force acting on each particle are determined. Results indicate that the flow and transport around a given particle is significantly influenced by the presence of the adjacent particles. The heat transport and drag force results for a particle in a spray are substantially different from those available for an isolated particle at the same Reynolds number. An increase in the lateral spacing between particles results in a decrease in the Nusselt number as well as the drag coefficient. In contrast, increasing longitudinal particle spacing leads to an increase in both the Nusselt number and the drag coefficient. At lateral spacings greater than about four diameters, the effect of side particles becomes negligible. However, the influence of upstream particles remains significant even at longitudinal particle spacing of five diameters. Simple correlations for the overall Nusselt number and drag coefficient have been proposed to incorporate the effect of inter-particle interactions.