| Arc plasma has the characteristics of high temperature,high enthalpy,and high chemical activity which have been widely used in many industrial applications such as plasma spraying,waste treatment,metal cutting,welding and so on.The boundary between the arc and the electrode has very complicated physical mechanisms.It is very difficult to measure in actual situations,and the numerical simulation is just an effective means to understand the arc discharge process.Based on the existing model of arc column,the near-cathode region and the near-anode region,including 2T and chemical non-equilibrium model of arc column,one-dimensional model of sheath/pre-sheath and Knudsen layer,and coupled model of sheath and arc column,we further analyzed the coupling relationship on important parameters such as current density,particle density,electron temperature,and energy flux between the near-electrode region and the arc column.The global coupling physical model including cathode,the near-cathode region,arc column,the near-anode region and anode is carried out,and the calculated results are compared with the results of experiment and the original model.The model is used to study the whole arc plasma discharge region and some meaningful results are obtained.1.Firstly,the coupled model of arc column and cathode is established.The 2T and chemical equilibrium model is adopted for the arc column,and one-dimensional modeling is adopted for the sheath and presheath in the near-cathode region.The heat conduction of electrons and heavy particles is considered in the energy balance of the near-cathode region.Secondly,in the coupled model of arc column and anode,the arc column which considers chemically non-equilibrium is directly coupled with the anode sheath.The different movement mechanisms of ion and electron for positive and negative anode sheath voltage are considered in the anode sheath model.Finally,on the basis of the above models,the model of arc column is further improved.The chemical non-equilibrium is considered in the whole plasma domains.The Stefan-Maxwell’s equation is used to get the diffusion fluxes of plasma components and current density of the generalized ohm’s law,which can be applied to arc plasma at any degree of ionization.At last,a global coupling physical model of cathode-arc column-anode is established by further deducing the relevant boundary conditions.2.Taking magnetic rotating arc plasma generator as the research object,the cathode root attachment behavior of the magnetic rotating arc plasma generator is studied experimentally:by controlling magnetic field,arc current,gas flow and the length and diameter of cathode rod,the "spot mode" and "diffuse mode" of cathode root spot are obtained.Low magnetic field is conducive to the formation of spot mode With the enhancement of the external magnetic field,the cathode arc root spot gradually increases and further evolves into the diffusion mode.A two-dimensional coupled cathode sheath model is used to analyze the athode root attachment behavior of magnetic rotating arc plasma generator,which is in line with the trend of the experiment.The analysis of numerical simulation results shows that:the energy flux on the cathode surface increases with the increase of magnetic field,which is the main factor that causes the change of the cathode arc root from spot mode to diffusion mode.In addition,a comparison between the coupled model and the decoupled model for free-burning argon arc are carried out in the range of current from 20 to 200 A at the diffuse mode.The simulation shows that:the energy(due to thermal conductions)delivered from the arc column to the near cathode layer is always positive in the coupled model,and it increases with increasing arc currents.Due to the above energy transfer,the voltage drop of the near cathode layer calculated by the coupled model is always lower than that calculated by the decoupled model.The cathodic root predicted by the coupled model is more diffused.3.A coupled anode sheath model is established to study the physical process in the near-anode region for an atmospheric free-burning argon arc(the distance between the cathode tip and anode surface is set to 5 mm,the total current is set to 50 and 150 A).The simulation shows that:(1)Both positive and negative anode sheath voltages are obtained.The distributions of electric potential in front of anode predicted by the model are in good agreement with experiment.(2)Under the mode of negative voltage drop anode sheath,the ohm current density in the near anode layer can be opposite to the diffusion current density.However,under the anode sheath mode of positive voltage drop,the diffusion current density of near anode layer is opposite to the total current density.(3)There is an obvious positive feedback effect between the anode sheath voltage drop and the diffusion current:the lower the diffusion current,the higher the anode positive voltage drop.On the contrary,the greater the diffusion current is,the greater the negative voltage drop of anode will be,leading to two stable states of positive voltage drop and negative voltage drop of anode sheath mode.(4)In the positive anode sheath mode,the energy transport of electrons plays major role in anode heating;while thermal conduction from the arc column plays a major role in anode heating in the negative anode sheath mode.(5)The anode sheath mode is significantly affected by the flow,and the high flow rate is more likely to generate a diffusion mode.4.A global coupling physical model including the near cathode region,arc column and the near anode region is established to study the whole arc plasma discharge region.The distance between the cathode tip and anode surface is set to 5 mm,the total current is set to 150 A.The simulation shows that:(1)The plasma is in a chemical(ionization)equilibrium state in the middle region of the arc column,and the ionization balance is broken near the electrode region.(2)Near the cathode,the electron density decreases rapidly due to the rapid increase of the non-equilibrium of ions on the cathode surface.The difference between the actual electron number density and the number density at ionization equilibrium is more than an order of magnitude,which leads to an increase in the net production of ions moving toward the cathode due to the electric field and density gradient.In the vicinity of the anode,with the decrease of electron temperature,although the actual electron density is higher than the saha equilibrium density,the actual electron density will also decrease due to the net recombination of ions in the region near the anode.(3)The electric field in the near-cathode region rapidly rises from 104 V/m to 106 V/m within 0.1 mm,resulting in considerable electrical energy stored in the near-cathode plasma layer.The electric field near the anode is one order of magnitude smaller than that near the cathode,so the energy stored in the near anode region is also about an order of magnitude smaller than the near cathode region.(4)At the cathode tip,the current density of back-diffusion plasma electrons(jbd)exceeds the ion current density(ji),and the current balance is mainly achieved by the back-diffusion electron current;when the distance from the cathode tip is more than 1 mm,the current balance mainly depends on the ion current.On the anode side,the electron current from the quasi-neutral plasma is two orders of magnitude larger than the ion current,and the anode current is mainly maintained by the electron current.5.At last,the three-dimensional 2T model is used to predict the distribution of temperature field,flow field and electromagnetic field inside the six-cathode plasma torch,and analyze the formation mechanism of high temperature diffusion arc inside the plasma torch,then compare it with the experimental results.The total current is 240A,total inlet flow rate 1 Nm3/h,the numerical simulation results show that:(1)Six cathode-jet flows converge to form a high-temperature diffusion arc in the central region,which is in good agreement with the observed arc images.(2)The uniformity of the high-temperature region of the diffusion arc is analyzed,and it can be found that when one of the cathode currents decreases to 50%,the high temperature region of the diffusion arc can still maintain good symmetry and uniformity.It is shown that the six-cathode diffuse arc device has good operation stability. |
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