A Study On The Evolution Of The Spark Channel Induced By Nano-second Pulsed Positive Electrical Discharge In Water

High voltage electrical discharge in water involve s complex physical and chemical processes such as high intensity electric field,ultraviolet radiation,shock wave,ultrasonic wave and formation of various chemical species.It has been attracted intensive attention for its various potential applications such as environment,medical treatment,material preparation and engineering technology.Underwater spark discharge is an important mode of high voltage discharge in water.It is believed that the spark channel produced by spark discharge in water is in gaseo us phase.A variety of chemical active species are produced,and many chemical reactions are proceed in the spark channel.The physical parameter of the spark channel,such as the radius of the spark channel,the gas density,pressure,and the temperature in the spark channel,have a significant influence on the formation and distribution of various kinds of chemical species.So far there is a lack of experimental measurements of gas density,pressure,and temperature in the spark channel.In this study,the spatial and temporal evolution of spark channel induced by a nano-second pulsed positive electrical discharge in water was investigated.The time evolution of the radius of the spark channel and gas density in the spark channel is measured by using the shadow photography method.Furthermore,the evolution of gas pressure and temperature in the spark channel are evaluated.The main results are as follows:(1)The radius of spark channel increases first and then decreases with time,showing that the spark channel experiences an expansion and collapse process.The radius of spark channel reaches its maximum value(of about 1.2 mm)at about 180 ?s after the discharge.The radius of spark channel near the tip is greater than that of other locations.(2)The gas pressure in the spark channel is of about 5 atm at 2 ?s after discharge.It decreases first and then increases with time,reaching its minimum value(of about 1 atm)at about 180 ?s after the discharge.(3)The gas density in the spark channel is of about 600 kg/m3 at 2 ?s after discharge.It decreases first and then increases with time,reaching its minimum value(of about 450 kg/m3)at about 180 ?s after the discharge.The gas density near the tip is smaller than that of other locations.(4)The gas temperature in the spark channel is of about 1900 K at 2 ?s after discharge.It decreases first and then increases with time,reaching its minimum value(of about 500 K)at about 180 ?s after the discharge.(5)By comparing the results obtained under three water conductivity conditions,it is found that the water conductivity has no significant effect on the evolution of the spark discharge channel radius,gas density,pressure and temperature within the range of our studies.