Computational Studies of Thermal-Plasma-Induced Turbulence on Nanopowder Generation and Sustained Arc Discharge

Thermal-plasma-induced turbulence (TPIT) which appears and affects thermal plasma applications is discussed based on computational studies. A demonstrative simulation was performed for Si nanopowder generation in a turbulent-like flow induced by an Ar thermal plasma jet. The model described nanoparticles' collective growth by nucleation, condensation, and coagulation and transport by convection, diffusion, and thermophoresis in microsecond to millisecond time scales. The computation results showed that the plasma jet formed nonuniform thermofluid fields and induced multiscale vortices not only near but also far from the plasma jet, and the nanopowder collectively grew up and diffused outside the plasma region. The larger size regions coincided with smaller number density regions, which indicated that simultaneous coagulation decreasing particle number played an important role. A TPIT in a sustained SF₆ arc discharge was also simulated by a simulation code named PLASTIPC (Plasma All-Speed Turbulence with Implicit Pressure Code) which was capable of capturing plasma-induced hydrodynamic instability leading to turbulence transition, treating subsonic and supersonic flows at all-speed range, and achieving numerically stable computation with a large increment of time steps even in such an extremely complex thermofluid system where hot plasma and cold nonionized gas co-exist. The computation results showed that the SF₆ arc plasma was constricted with high temperature over 20,000 K, but the ambient nonionized gas had low temperature below 1,000 K. The arc plasma region exhibited small Mach numbers indicating an incompressible and subsonic flow. The ambient nonionized gas formed a compressible flow in Mach number > 0.3 with island-like supersonic flow zones in Mach number > 1.0. The intermittent backflows and forward flows produced a turbulent-like state.