Abstract
Fouling is a great problem that significantly affects the continuous operation for large-scale radio-frequency (RF) thermal plasma synthesizing nanopowders. In order to eliminate or weaken the phenomenon, numerical simulations based on FLUENT software were founded to investigate the effect of operation parameters, including feeding style of central gas and sheath gas, on plasma torches. It is shown that the tangential feeding style of central gas brings serious negative axial velocity regions, which always forces the synthesized nanopowders to “back-mix”, and further leads to the fouling of the quartz tube. Moreover, it is shown that sheath gas should be tangentially fed into the plasma reactor to further eliminate the gas stream’s back-mixing. However, when this feeding style is applied, although the negative axial velocity region is decreased, the plasma gas and kinetic energy of the vapor phase near the wall of the plasma reactor are less and lower, respectively; as a result, that plasma flame is more difficult to be arced. A new plasma arcing method by way of feeding gun instead of torch wall was proposed and put in use. The fouling problem has been well solved and plasma arcing is well ensured, and as a result, the experiment on large-scale production of nanopowders can be carried out for 8 h without any interruption, and synthesized Si and Al2O3 nanopowders exhibit good dispersion and sphericity.
Highlights
With high enthalpy, high temperature, good heat, and electric conductivity, RF thermal plasma provides a unique environment for chemical reaction and nanopowder synthesis distinct from the ordinary solid, liquid, and gas phases, which makes it a versatile technique that is capable of controlling the morphology, size, and chemical composition of the nanopowders through the design of experimental devices and their operating parameters [1,2,3,4,5,6]
Since plasma technology is often used in powder treating, the parameters inside the plasma reactor need to be noted, including axial velocity fields, radial velocity fields, temperature fields, and so forth
Since it has been concluded above that tangential feeding of the sheath gas and axial feeding of the central gas are beneficial to decrease negative velocity regions, it is better to apply the inlet styles of Case B to eliminate the effect of negative velocity regions
Summary
High temperature, good heat, and electric conductivity, RF thermal plasma provides a unique environment for chemical reaction and nanopowder synthesis distinct from the ordinary solid, liquid, and gas phases, which makes it a versatile technique that is capable of controlling the morphology, size, and chemical composition of the nanopowders through the design of experimental devices and their operating parameters [1,2,3,4,5,6]. Improving the plasma technique and equipment thermal plasma synthesis of nanopowders was thoroughly studied and the nanopowders synthesized for large-scale production of nanopowders has become an urgent problem to be solved at present. Simulation of the plasma torch makes it visualized, clarifies the fluid fields inside the plasma torch, and numerical simulation has been widely used in momentum transfer [19,20], heat transfer captures these impacts, which probably brings exciting results [17,18]. Chemically reactive species in a nonequilibrium inductively coupled argon–hydrogen thermal plasma [29] applied a two-dimensional electromagnetic model based on FLUENT code to study the fluid and under pulse-modulated operation. Numerical models are developed to simulate the RF thermal plasma torch with different inlet styles of central gas and sheath gas.
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