Research On The Time-resolved Images Of Air-droplet Two-phase Electrical Discharge Development

Gas-liquid two-phase discharge has become an increasingly important topic in the field of plasma science and technology because of its important applications in water treatment,nanomaterial synthesis,material processing,sterilization and biomedical applications.In recent years,research on gas-liquid two-phase discharge has focused on two directions.One is discharge in water,which also includes discharge of pre-set bubble in water.The other is the discharge of liquid in the air,for example,a liquid-electrode discharge in the air,and a mist or aerosol discharge in the air.The lightning phenomenon in the cloud is the most common gas-liquid two-phase discharge phenomenon in nature.It is a severe discharge process involving tiny droplets in the cloud.So far,however,the role of tiny droplets in the discharge process,the mechanism of influence on the physical process of discharge,and the impact of plasma on tiny droplets are not clear.Therefore,it is necessary to explore the effect of tiny droplets on the discharge process.In this paper,we first explore the time-resolved images of a single droplet with a diameter of 0.3 mm discharged with a single nanosecond pulse voltage to explore the development of the discharge.By changing the electrode-droplet position relationship,droplet size,droplet-electrode distance,and breakdown voltage,it is further explored how the droplet-air interface affects the discharge process under different discharge conditions.Secondly,using a 1mm diameter droplet of the internal preset air,the breakdown mechanism with the gas-liquid-gas structure and the characteristics of the gas-liquid two-phase discharge with small-sized liquid phase boundaries are researched.The conclusions are as follows:(1)When a solid droplet of 0.3 mm-0.6 mm diameter is between the electrodes and the electrodes do not contact the droplet,the air plasma region at the early stage of the discharge is limited to a width of 0.05 mm(the same width as the electrode head),and connecting the two electrodes and droplet like a strip-shaped discharge channel.If the position of the electrodes is changed,the position of the strip-shaped discharge channel also changes,but it must be a "Surface-hugging discharge"(droplet surface).The air plasma region then diffuses into the ellipsoidal region with a long axis of 1.5-1.8 mm,and the duration of the light is as long as several tens of microseconds(the pulse width of the nanosecond pulse voltage is about 100 ns).In time-resolved images,the light durations of different regions are different.The images show that the duration of light near the droplet is always longer than the duration of light near the electrode.With the discharge,the droplet is squeezed and the distance between the electrodes and the droplet has a large effect on the velocity of droplet deformation.(2)The image shows that the discharge always develops along the surface of the droplet,and the droplet appears to have the effect of "attracting" the ionizing wave,causing more space charges to accumulate on the surface of the droplet.When the charging voltage is close to the threshold breakdown-voltage,the breakdown delay of the discharge with a droplet is significantly shorter than the breakdown delay of the discharge without droplet.The larger the droplet diameter(within a certain range),the shorter the breakdown delay(the difference is on the order of 10-8s),but the shorter the duration of the light(the difference is on the order of 10-5s).When in overvoltage breakdown,the duration of the discharge with a droplet is tens of microseconds longer than the duration without droplet.If under the same droplet size,the larger the charging voltage,the longer the light duration.(3)When the electrodes both insert into droplet,the droplet has a very small probability to swell(less than 5%)and a large probability of splashing.Droplet splash may be due to an electric double layer effect caused by the applied electric field and inducing a decrease in the interaction force at the interface between the liquid and the metal electrode.In addition,the thermal effect of the electrodes reduces the inter-attraction force between the water molecules.So that some droplets(one pixel size)directly splash out under the impact of the plasma.The probability of droplet expansion is less than 5 %,analyzing its discharge current waveform and it is more like the result of superposition of two-loop discharge currents.(4)When exploring the droplet discharge of the internal preset air,the time-resolved images of the discharge shows that,at least ensuring one electrode contacts the air inside the droplet,the air inside the droplet is likely to be ionized.If the electrodes both do not directly contact the air inside the droplet,even if the water shell thickness is less than 0.1 mm,the air inside the droplet seems never to be ionized.The discharge light emission outside the droplet will last for tens of microseconds;the duration of the light emission inside the droplet is less than 10 ?s.The reason of the difference may be complicated.It is possible that the air plasma consumes a large amount of kinetic energy and thermal energy when striking the inner surface of the droplet.The droplet absorbs the thermal energy of the particle and the droplet is expanding by the impact to obtain kinetic energy and potential energy.This process weakens the impact ionization process during the original gas discharge,thereby shortening the duration of the entire discharge process.(5)As long as the air inside the droplet is ionized,the droplet must "expanding" or "wounding"(the plasma breaks through the surface of the droplet).This is the result of plasma impact the internal surface of droplet.The droplet expanding process is much slower than the plasma directly breaks through the surface of the droplet.