| Arc plasma is characterized by high temperature and high chemical activity,and widely used in industrial processes,such as thermal spraying,metallurgy,machinery processing and material preparation.Numerical simulation has long been a important method to research on arc plasma.In numerical simulation,the coupling of arc calculation with sheath calculation is always a tough job,since the arc and sheath is quite different in physical process,time and space scales,equilibrium state and so on.Considering the fact that nowadays arc simulation lacks a fully coupled model,in this paper we developed a model coupling electrode solid,near-electrode region and plasma column,which is based on previous individual arc simulation and sheath simulation.In presheath-arc interface,we introduce thermal perturbation layer in order to couple one dimensional model of sheath-presheath with typical two temperature arc model.In the modeling of this paper,Schottky correciton to thermal emission current of cathode is taken into account.Electron flux accelerated by the sheath layer brings in a lot of energy to the plasma column.Part of this great energy is passed to heavy particle through collision process.Using the two temperature electrode-sheath-arc model,we get results as follows:1.We calculated two cases corresponding to two experiments.The first arc discharge case includes a cathode with 60 degree cone angel.The distance between cathode and anode is 5mm,with current varies 100A?300A.Temperature distributions on cathode surface and arc column temperature,predicted by simulation,quite agree with experimental values.The second arc discharge case includes a round tip cathode rod;the distance between cathode and anode is 10mm;the total current varies in the range of 20-200A.The differentials between the arc voltages predicted by the model of this paper and by experiments are less than 2V.2.Axial-symmetric investigation of arc dischage in argon with a thermionic cathode and hollow anode chamber is carried out:the diameter of anode chamber is 20mm,the diameter of cathode rod is 5mm,the external axial magnetic field varies between 0.01-0.03T.Computational data reveals that:(a)there exists two modes of arc discharge:the spot mode,which has a obvious cathode surface temperature peak in the arc attachment center;the diffuse mode,which has a flat cathode surface temperature distribution and a larger arc attachment area.(b)The cathode surface temperatures of modeling results agree with previous experimental observation in diffuse mode;however in spot mode the experimental results is non-axisymmetrical.A further 3D simulation is obiviously needed to investigate the non-axisymmetrical features.(c)The numerical results reveal that,the cathode arc attachment has a trend to shrink with the increase of externl axial magnetic field.(d)the near-cathode layer voltage drop in spot mode is 1.4V lower than that of diffuse mode,while the plamsa column voltage drop of spot mode(9.98V)is much higher than that of diffuse mode(4.68V).(e)The plasma obviously shrinks in axial direction and expands in radial direction,with the configuration holding a flag shape.The plasma coloumn in front of cathode is more contractive in spot mode than in diffuse mode,making the electron and heavy particle temperature of plasma in spot mode higher.3.An application is presented for an atmospheric free burning argon arc with the arc currents of 50A-600A,and arc gap 5mm.The modeling results show some interesting points:(a)themal emission electrons cools the cathode surface while the back diffusion electron flux and ion flux heats it;(b)At cathode surface energy transport due to heat conduction of heavy particles and electrons is comparable to total heating flux,when current varies in the range of 50-600A;(c)the two dimensional character of the voltage drop of near-cathode layer becomes obvious with increase of current,for spot mode;(d)The near-cathode layer voltage drop(Utot)is larger in diffuse mode than in spot mode for a same current in this work,which is just the opposite in "decoupled" modeling.Using thermal equilibrium model we get results as follows:1.Coupling model without sheath is applied in the research of influence of external axial magnetic field on free burning argong arc(current 200A,external magnetic field 0-0.02T).The results show that:(a)With the AMF,the cathode arc attachment shrinks towards its tip.(b)Compared with fixed cathode arc attachment calculation,the shrinking cathode arc root of coupling model enhances the plasma centrifugal flow,inducing a more hollow area in front of anode surface.(c)The arc column near cathode is more contractive using coupling method,inducing a higher plamsa temperature in front of cathode tip.(d)The coulping method predicts a higher arc voltage drop and total anode heat,than results using fixed cathode arc attachment.2.3D steady-state simulation of twin-torch arc discharge is carried out(electrode distance 99mm,electrode axis intersection angle 90 degree,current 500A).The results show that:(a)The cathode jet and anode jet are bent to "bow shape" and the jet shape is a final balanced result of repellent and attractive Lorenz forces.(b)the moment of cathode plasma jet almost equals that of anode plasma jet.This makes the downstream aligned jet flow in the direction of 45 degree intersection angle with cathode/anode axis.(c)the joint region of cathode jet and anode jet has a effect of sucking surrounding gas,which accelerates the joint jet and enhances the total moment of plasma. |
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