The formation of ultrafine metal particles through homogeneous condensation in a supersonic nozzle and the particle impaction in a supersonic flow at low pressure have been studied numerically. For a gas phase, the analysis of compressible Navier-Stokes equations was carried out by using Lower-Upper Symmetric Gauss-Seidel scheme. In addition, the Lagrangian trajectory model was used for a particle phase.
In the study of supersonic condensing flow, the gas-particle flow was assumed to be two-way coupled due to the vapor depletion and release of latent heat, and the mixture equations were introduced to solve the flow field. The gasdynamic characteristics of nozzle flows with homogeneous condensation have been clarified for the various reservoir conditions and nozzles. The particles are made up of molecules ranging from several tens to about 2000, which are similar to the particle size range formed in a ionized-cluster beam deposition method. Furthermore, the mean size and mean mass concentration of particles are correlated for each initial vapor mass fraction.
In the low pressure particle impaction study, the dilute gas-particle flow was assumed to be one-way coupled. The effects of freestream pressure, particle size, and carrier gas on the process of particle deposition were investigated. The local deposition characteristics were also demonstrated in terms of particle size and freestream pressure. As the freestream pressure decreases, the deposition efficiency curves become sharper and eventually correlated with a single curve as a function of a modified Stokes number. With an increase of particle size and/or pressure, particles are deposited rather uniformly on the impaction surface. On the other hand, the particles with low total deposition efficiencies can be deposited rather uniformly on the impaction plane when the effect of thermophoresis is considered.
