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

Abstract

In the thermal plasma¯ash 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 signi®cant 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 ef®ciency. Eulerian approach is employed in this research and the corresponding convection±diffusion governing equation is established for the cluster concentration ®eld. The environment pressure for the plasma jet is set to be 50 Torr and the working gas is argon plasma. The velocity and temperature ®elds are simulated ®rstly and then the cluster transportation process is modeled within the established¯ow ®eld 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 ®eld and the radial distribution of deposition¯ux 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 signi®cantly suppressed by the thermophoretical force, and the effect of thermophoresis plays a dominant role than that of diffusion thus almost uniform deposition ef®ciency is achieved for clusters of different sizes. q

Turbulent thermophoresis effect on particle transport processes

“…The numerical results of thermal deposition of aerosol particles on a flat body indicate that the deposition flux decreases with increasing surface temperature and increases nonlinearly with increasing thermophoretic coefficients [14]. Modeling clusters deposition in the thermal plasma flash evaporation process shows that the concentration boundary layer is significantly suppressed by the thermophoretic force, and that the thermophoresis plays a dominate role than that of diffusion giving rise to an uniform deposition efficiency for different cluster sizes (1-10 nm) [15]. An experimental measurement of the thermophoretic deposition of submicron particles on the smooth wall of a circular rod and a simple approximation of a two-dimensional method for predicting the thermophoretic effects on submicron particles has been introduced [16].…”