Study On Pre-breakdown Process Of Cathode Discharge In Liquid

Plasma technology can be widely used in sewage sterilization purification,hydrogen production and degradation of organic dyes,etc.,but the generation mechanism of discharge plasma in liquid is a key problem to be solved urgently,this topic focuses on the space-time evolution process of cathode discharge pre-breakdown in liquid,and the main research work is as follows:（1）Through thermodynamic analysis of bubble generation in the liquid and the analysis of bubble discharge conditions,the theoretical analysis model of bubble sheath formation and change process（pre-breakdown process of discharge）around the cathode in the liquid is established,and the calculation formula of resistance,voltage,current and electric field of bubble sheath around the cathode under DC excitation is deduced.（2）Software Matlab is used to simulate the process of bubble expansion,the change of resistance with radius and the change of voltage and surface electric field cause by the change of resistance,as well as the characteristics of current changing with time and the analysis of influencing factors.The simulation results show that the resistance,voltage and surface electric field increase with the increase of bubble radius,but the current decreases with the bubble expansion process,and when the conductivity is determined,the higher the applied voltage,the faster the current decreases with time.（3）ICCD high-speed camera is used to optical diagnose the pre-breakdown process of cathode discharge in liquid,and spatial resolution images of transient bubble changes are obtained.Experiments shown that the bubbles form at first under joule heat but are tiny,as the applied voltage to the big,the bubble expands to a certain extent and then begins to discharge at 460 V,sheath layer of bubbles appear some bubbles breakdown discharge phenomenon around the cathode,when the bubble sheath is completely broken down,bright clusters of continuous discharge phenomena appear around the cathode.（4）The time evolution of the discharge current is diagnosed electrically by using a high-frequency current detector.It is observed in the experiment that the discharge current decreased with the increase of bubble radius,and when the conductivity is 2m S·cm^-1,the current drop rates corresponding to the successive increase of 10 V from 540 V to 590 V are0.18 A/s、0.29 A/s、6.5 A/s、10.3 A/s、10.5 A/s、15 A/s.The experimental results show that the higher the applied voltage is,the faster the discharge current decreases with time,which is consistent with the theoretical analysis.