Simple nonequilibrium model of collective growth and transport of metal nanomist in a thermal plasma process

The early stage of iron nanopowder fabrication using an argon thermal plasma jet is numerically demonstrated especially focusing on simultaneous growth and transport of iron nanomist around the plasma jet. A simple model is developed to describe the nonequilibrium processes of the nanomist’s collective growth through homogeneous nucleation, heterogeneous condensation and coagulation among the composing nanodroplets, as well as transport by convection, diffusion and thermophoresis. An original solver is also developed to express a turbulent-like plasma flow with multi-scale eddies and to capture discontinuous profiles with steep gradients in a nanomist distribution. A thermal plasma jet entraining cold non-ionized gas, a natural fluid-dynamic feature, is successfully simulated. The collective growth and transport of iron nanomist which is converted from iron vapor transported with the plasma flow are also clarified. A large number of small nanodroplets are generated especially around the plasma fringe. The regions where nanodroplets have large sizes almost coincide with those where nanodroplets exhibit low number densities because the nanomist also grows by coagulation.