Modeling of clusters deposition under the effect of thermophoresis during thermal plasma flash evaporation process

In the thermal plasma flash evaporation process, due to the high quenching rate of the impinging plasma jet and the steep temperature gradient in front of the substrate, thermophoretic force is expected to have a significant effect on the cluster transportation process. This paper aims at investigating the cluster transportation and deposition processes by the method of numerical simulation and presents the qualitative description of the effects of thermophoresis on the deposition efficiency. Eulerian approach is employed in this research and the corresponding convection-diffusion governing equation is established for the cluster concentration field. The environment pressure for the plasma jet is set to be 50 Ton and the working gas is argon plasma. The velocity and temperature fields are simulated firstly and then the cluster transportation process is modeled within the established flow field but the computational domain is only limited to a narrow region in front of the substrate. It is assumed that clusters of uniform size are generated in a certain region within the boundary layer and it is treated as the source term in the cluster transportation equation. The results of cluster concentration field and the radial distribution of deposition flux are achieved for the clusters in the size range 1-10 nm, respectively. Results are also given for the comparative cases without considering thermophoresis effects. It is found that the concentration boundary layer is significantly suppressed by the thermophoretical force, and the effect of thermophoresis plays a dominant role than that of diffusion thus almost uniform deposition efficiency is achieved for clusters of different sizes. (C) 2001 Elsevier Science Ltd. All rights reserved.