Numerical Study On Flow Characteristics Of Thermal Plasma

Thermal plasmas have the characteristics of high temperature,high enthalpy,and high chemical activity,and have been used in industry on a large scale,such as metallurgy,spraying,welding,material preparing,and so on.There are various flowfield organizations of thermal plasma in industrial applications,including flows inside the plasma torch,free jets,coflow jets,and counterflow jets outside the plasma torch.Understanding the flow characteristics of thermal plasma under the different flowfield organizations are significant for optimizing the design of plasma reactors and developing novel plasma reactors.Numerical solvers coupled with electromagnetic field,flow field,temperature field and species field have been developed based on the CFD software OpenFOAM in this paper.Furthermore,numerical simulations have been performed to study the flow characteristics of thermal plasma under the different flowfield organizations.The main work and conclusions are as follows,Firstly,quasi-direct numerical simulations have been employed to investigate the effects of current intensity and inlet flow rate on the flow field,temperature field and electromagnetic field in the plasma torch.Compared with the cold flow(without considering the influence of arc),numerical results in the plasma torch show that thermal expansions caused by the high current density in the cathode result in the disappearance of recirculation zone.However,the fluctuations with characteristic frequency 11.8 kHz are found in the torch for cases with the low current intensity(200 A)because the effects of thermal expansions are insufficient.Meanwhile,the fluctuations of velocity are observed in the cathode tip,and the fluctuations of temperature are observed in the region between thermal plasma and cold boundary layer,respectively.With the increase of current intensity,the velocity,temperature and potential in the plasma torch increase,the fluctuations phenomena disappear gradually,and the flows tend to be stable.In addition,the increase of flow rate in the inlet enhances the instability of plasma torch,in which the characteristic frequency and potential fluctuation amplitude are improved.Moreover,the arc length and velocity near the axis of the plasma torch increase with the increase of flow rate in the inlet.While the temperature of thermal plasma near the axis are hardly affected.Secondly,the effect of Reynolds number and turbulence intensity in the jet inlet on the flow and mixing characteristics of argon thermal plasma jet have been investigated by large eddy simulations.Detailed analyses are carried out from five aspects:mean flow field,instantaneous flow field,turbulence statistical characteristic,mixing layer thickness and the self-similarity.It is shown that laminar thermal plasma jets transform into turbulent jets when Reynolds number changes from 1148 to 1722,and the turbulence intensity changes from 2%to 10%.For the cases with Reynolds number 1148,increasing the turbulence intensity in the jet inlet will increase the turbulent transport mechanism near the mixing layer rather than in the jet axis,causing the faster development of turbulence.However,the characteristics of flow and mixing are not affected by turbulence intensity in the jet inlet for cases with high Reynolds number.The turbulent transport mechanism increases with the increase of Reynolds number in the jet inlet.It was also found that the mean axial velocity and mean temperature in the axis of the turbulent thermal plasma jet satisfy the self-similarity aspects downstream.In addition,decay constant K is 1.25,which is much smaller than that(5.7～6.1)of the turbulent cold gas jet and has nothing to do with the Reynolds number or turbulence intensity in the jet inlet.Third,the influences of velocity and width in the coflow argon jet inlet on the flow characteristics of laminar argon thermal plasma jet have been studied by large eddy simulations.Numerical results show that in comparison to the laminar argon thermal plasma jet flowing into cold air,the Kelvin-Helmholtz instability between thermal plasma jet and coflow argon jet is caused after the coflow argon jet is applied.As a result,a laminar jet transforms into turbulent jet.Increasing the width in the coflow argon jet inlet can create a dense argon atmosphere.On one hand,more coflow argon rather than ambient air is entrained into the upstream of thermal plasma jet.On the other hand,the specific enthalpy and specific heat of argon are much lower than those of air at the same temperature,causing the higher temperature in the upstream of argon thermal plasma jet.In addition,with the increase of velocity and width in the coflow argon jet inlet,the mixing characteristics between argon thermal plasma,coflow argon and ambient air are also improved.Meanwhile,the transition from laminar thermal plasma jets to turbulent thermal plasma jets can be advanced.At last,three-dimensional quasi-direct numerical simulations have been performed to investigate thermal plasma reactors with counterflow jet.The effects of momentum-flux ratio and distance between counterflow jet and thermal plasma jet on the flow characteristics are addressed.The numerical results show that the dimensionless location of stagnation layer is significantly affected by the momentumflux ratio,but it is independent on the distance.Specifically,the stagnation layer is closer to the outlet of plasma torch with the increase of momentum-flux ratio.Furthermore,the flow regimes of stagnation layer and the characteristics of thermal plasma jet are closely related to momentum flux ratio.The deflecting oscillation flow regimes are found in the reactor when the momentum flux ratio is low,which provokes large fluctuations of axial velocity inside the thermal plasma jet.However,for the cases with high momentum flux ratio,the flapping flow regimes are identified.The thermal plasma jets are very stable and axial velocity fluctuations mainly exist in the stagnation layer.