Study On The Characteristics Of Multi-electrode DC Arc Plasma

Arc plasma is widely used in hazardous waste treatment,thermal spraying,metal welding,auxiliary combustion,and other fields due to its advantages of high energy density,various working gases,and wide pressure range.However,because of selfmagnetic compression and thermal pinch effect,the characteristic parameters of arc plasma（temperature,pressure,density,etc.）have a large gradient,and the radial shrinkage of arc column is within a few millimeters,which greatly limits its application in materials preparation,powder spheroidization and other fields requiring large thermal plasma size.In this paper,the discharge characteristics of multi-arc plasma are studied,the temperature field of multi-arc plasma and the flow field distribution of multi-arc plasma jet are diagnosed,and the spheroidization of silicon dioxide particles by multi-arc plasma is studied by using a self-designed multi-electrode DC arc plasma generator.The specific work content and results are as follows:（1）The multi-electrode DC arc plasma generator was developed independently,and the spatial distribution of large-size arc plasma was controlled by adjusting the electrode arrangement.The discharge process of multi-arc plasma was obtained by using high speed camera and synchronous acquisition of arc voltage,and the typical discharge state and dynamic behavior of multi-arc plasma under different electrode arrangement were analyzed.The results show that with the increase of arc current,the cathodic arc mainly exhibits a contracting state,while the anodic arc transitions from a contracting state to a diffusive state.When the current is low,the arc is mainly distributed near the edge of the discharge area of the multi-arc generator.With the increase of current,the arc tends to extend to the center of the discharge area.In the multi-arc plasma generator with electrode configuration of adjacent electrodes with opposite flow distribution and electrode configuration of opposite electrodes with swirl flow distribution,the arcs extend to the center of discharge area and form a high temperature region in the center,and the fluctuation of multi-arc discharge is violent.In this kind of multi-arc generator,the electrode ablation is slight,and the anodic ablation is more obvious than the cathodic ablation.Furthermore,the anodic ablation is located on the copper cone surface of electrode tip,while the cathodic ablation mainly occurs on the surface of electrode tip embedded tungsten rod.（2）Multi-arc voltage signal is analyzed by data statistics,fast Fourier transform,correlation analysis and short time Fourier transform.The results show that the arc voltage of multi-arc plasma discharge is positively correlated with the arc current.The low frequency parts（0.05～20 kHz）of the arc voltage signal in the frequency domain are mainly caused by the restrike of arc-to-arc,the change of lap sequence of anode arc（or cathode arc）and its adjacent cathode arc（or anode arc）,and the extension of arc to the center of discharge region.The correlation analysis of multi-arc voltage signal shows that the correlation coefficient of arc voltage signal changes regularly,but this change has no fixed period.（3）By analyzing the Fowler-Milne method and the relative intensity method,an improved temperature field diagnosis method combining the two methods is proposed.Firstly,the linear relation between the gray value of the arc discharge image and the intensity of the arc discharge emission spectrum is obtained by the Fowler-Milne method.Then,the linear relation is brought into the relative intensity method and combined with the multi-arc discharge image,the temperature field of multi-arc plasma is solved.The emission spectrum information of the center of the multi-arc plasma discharge region with four kinds of electrodes was collected by emission spectrometer.The electron excitation temperature of the plasma was found to be 8×103～1.2×104 K by Boltzmann slope method,and the electron density of the plasma was found to be 1022 m^-3 by Stark broadening method.The multi-arc plasma is judged to be in local thermodynamic equilibrium state.（4）The interaction between particles and multi-arc plasma is analyzed and the flow field in multi-arc plasma jet is diagnosed by particle tracer velocimetry method.Threshold segmentation and distance transformation algorithms were selected to identify the particles after discharge,and the position coordinates of the particles were obtained.Then,the multi-frame particle tracking algorithm is improved based on the nearest neighbor and path smoothing ideas,and the flow field distribution in the multiarc plasma jet region is obtained.The results show that the velocity of particles in multiarc plasma is mainly 1～7 m/s.The flow field distribution of the detected particles is consistent with the actual observation results,indicating that the multi-frame particle target tracking algorithm is suitable for particle flow field diagnosis.（5）The spheroidization of SiO₂ particles was characterized by scanning electron microscope,X-ray diffraction,particle size distributor and other techniques.The results show that the spheroidization rate of SiO₂ particles is positively correlated with arc current,and SiO₂ particles will change from crystallized state to amorphous state after spheroidization.The amorphous transformation of SiO₂ is a process of slow transform at first and rapid transformation to amorphous state when reaching the critical value.The average particle size of SiO₂ particles increased after plasma treatment.