Investigation On Motion And Melting Behavior Of Particles In RF Plasma

Radio frequency plasma has unique advantages such as high temperature,high enthalpy,no electrode pollution,and low flow speed,making it highly advantageous in the preparation of micro and nano materials.The original powder particles are injected into the RF thermal plasma,absorbing heat until they completely melt into liquid phase.Due to the surface tension of the liquid,the droplets become spherical;Subsequently,when they fly out of the high temperature zone of the plasma,the droplets quench and condense,ultimately becoming spherical powder particles.This process is called the spheroidization process of powder particles.The injection conditions of powder are the key step in the spheroidization process,which playing an important role in the melting process of powder particles and determining the final sphericity,particle size distribution,and flowability of the powder.In this paper,the mathematical physics model of radio frequency thermal plasma and the mathematical physics model of particle plasma interaction are established,and the basic characteristics of radio frequency thermal plasma are studied.The influence of motion trajectory and initial velocity on the powder melting effect is attempted to elucidate the physical process of powder particles occurring in plasma,providing theoretical and technical guidance for the spheroidization process of powder particles.In terms of plasma,we use a three-dimensional numerical model to simulate the distribution of plasma temperature and velocity.In this model,Maxwell equations are used to describe the electromagnetic field distribution of the plasma,and the fluid mechanics equations are used to describe the flow of the plasma.In the simulation,the threedimensional asymmetric effect caused by the spiral winding of the coil and the turbulence effect in the flow process are considered.Research has found that the high temperature zone of the plasma is mainly distributed in the coil coverage area,with a maximum temperature of around 10000 K;The peak velocity of the plasma is about 40 m/s,and its position distribution is more downstream compared to the high-temperature zone.There is a significant deviation towards the tube wall at the tail flame,and the temperature and velocity of the plasma exhibit an asymmetric phenomenon,which is consistent with experimental observations.In terms of powder particles in plasma,the Discrete Phase Model(DPM)is used to simulate the force and heat transfer process of it.We investigated the influence of initial particle size range on the spheroidization process of cerium oxide powder: In the hightemperature region of the plasma,the particle size of small particle(10～22.1 μm)rapidly decreases until disappears;Medium sized particles(22.1～34.1 μm)can fly out of the plasma torch,but the final particle size does not strictly increase with the increase of the initial particle size,and exhibits irregular fluctuations.This indicates that the powder within this range can be refluxed into the high-temperature zone of the plasma for secondary or even multiple heating;The fluctuation of the final particle size of larger particle size(34.1～60μm)disappears,because although this part of the powder may also experience reflux,it is difficult for them to enter the high-temperature zone of the plasma,so the impact of reflux on the final particle size of the powder is relatively small.The melting process of two powder particles with the same injection conditions in plasma and the effect of different motion trajectories on the particle size of the powder particles were investigated,it was found that due to the asymmetric distribution of the plasma physical field,the particle size of the powder moving along the high temperature region of the plasma decreased more significantly,which is related to their effective melting in the high temperature region.Powder injection speeds also affects the heating and flight history of particles in the RF thermal plasma.There was an optimal initial velocity range for particle injection.By analyzing the force exerted on a single powder particle and the temperature field distribution of the plasma,it can be found that the initial velocity is too fast to allow the powder sufficient time to absorb heat,and too small to allow the powder to obtain a greater radial velocity to diffuse out of the high temperature region of the plasma,the heating effect is also poor.Therefore,there is an optimal initial speed range,which can make most powders have similar heat absorption effects,and the distribution of the final particle size will be more uniform and concentrated.