Research On Process And Mechanism Of Underwater Microsecond Pulsed Discharges Under Nonuniform Electric Fields

Underwater pulsed discharges have numerous physical and chemical effecs,e.g.active particles production,ultraviolet light emission,electrohydraulic effect and so on,which have been widely used in various applications such as waste water treatment,material recycling,extracorporeal lithotripsy,oil well stimulation,etc.On the other hand,liquid water with high permittivity and impulse breakdown strength,is very sutable for the energy storage dielectric of pulsed power system facilities,for example water swtich,water capacitor and water resistor.In order to promote the application of water dielectric in pulsed power technology,further researches on the mechanism and the breakdown characteristic of underwater pulsed discharge are required.The previous studies mainly focus on nanosecond underwater discharges,however,few attentions have been paid to microsecond and millisecond underwater discharges.Besides,the underwater pulsed discharges involve the interaction between multiple media?plasma,gas and liquid?,of which the mechanism is rather sophisticated.There are none general and comprehensive theory of underwater pulsed discharges estabulished so far.Therefore,the experimental and theoretical studies on the process and mechanism of microsecond underwater pulsed discharges under nonuniform electric fields will be carried out in this dissertation.Firstly,the development process and morphology evolution of underwater streamers under microsecond pulses are studied comprehensively.The detail physical processes and the intrinsic mechanisms of streamer initiation,propagation,branching and collapse are elucidated.The influences of applied voltage and electrode configuration on the development and morphology of underwater streamers are analyzed deeply.The initial process of underwater streamer inclues the formation of low-density shadowed region,initial bubble and initial streamer.Enhanced Joule heating under intensive electric fields is thought to be the dominant intrinsic dynamic in the streamer initiation process.There are three progation modes for underwater streamers:positive subsonic mode,in which positive streamers develop in the form of bubble cluster with constant propagation speed?average propagation speed is 22.9⁷6.6 m/s?;positive supersonic mode,in which positive streamers develop in the form of tree-like structure with strong luminosity and instantaneous propagation speed up to 30 km/s;negative subsonic mode,in which negative streamers develop in form of tree-like structure with numorous filements and periodic propagation speed?average propagation speed is 98.4³67.8 m/s?.As the applied voltage rises,the light emissions is enhanced and the propagation speed inceases almost linearly for the subsonic streamers.The positive subsonic streamers will have sharper shape and higher transition possibility to the positive supersonic streamers,at the same time the negative subsonic streamers will have more branches at higher applied voltage.The propagation and morphology of the subsonic streamers show an abnormal voltage polarity effect that the positive streamers have higher propagation speed as well as thicker root compared with the negative streamers.The voltage polarity effect is related to the local space charge aggregation caused by the huge mobility defference between electron and ion.Secondly,the microsecond underwater spark discharge and its electrohydraulic effect are researched in the dissertation.The development processes of underwater arc channel and gas bubble are observed.The influence of deposited energy on shock wave intensitiy is discussed.A calculation method for the deposited energy and the average arc resistance in consideration of the time-varying characteristic of arc impedence is deduced.The relationships between the deposited energy and the average arc resistance with the shock wave intensity are discussed.The influences of the pre-breakdown time delay,the initial arc shape and the streamer development mode on the shock wave intensity are analyzed.Between any two points with current of zero,the deposited energy can be approximated to the integral of the instantaneous power of arc,when the derivative of arc inductance with respect to time is much less than the arc resistance.The deposited energy and the average arc resistance reaches their peaks in the first half current oscillation period,and then the deposited energy falls off gradually but the average arc resistance fluctuates.The peak shock wave pressure P_m and the deposited energy in the first half current oscillation period E_R1 are found to follow the equation 0.36?E_R1^0.4 in the experiments.Increasing the arc resistance is beneficial to the energy deposition and the improvement of the shock wave intensity.The longer is the pre-breakdown time delay,the lower is the residual energy of capacitor and the weaker is the shock wave intensity.The longer is the initial arc length,the more is the deposited energy and the stronger is the shock wave intensity.The streamer development mode can make influence on the shock wave intensity by deciding the pre-breakdown time delay and the initial arc shape.Finally,the microsecond underwater breakdown and its polarity effect are studied in this dissertation.The influences of gap distance and pulse duration are investigated.A breakdown voltage prediction model which considers the influence of breakdown mode transition,and a novel classification theory of polarity effect based on the polarity effect variation points are proposed.The breakdown mode of positive polarity will be transited when the gap distance or the pulse duration is changed,which will eventually cause the varitian of the polarity effect.There are three polarity effect variation points for the breakdown characteristic curves:two for the breakdown voltage characteristic curve and one for the pre-breakdown time delay characteristic curve.According to the location of polarity effect variation points and the behaviors of polarity effect,four distinct regions can be devided:region I,in which the breakdown voltage of positive polarity is lower but the pre-breakdown time delay of positive polarity is longer;region II,in which the breakdown voltage of positive polarity is higher and the pre-breakdown time delay of positive polarity is longer;region III,in which the breakdown voltage of positive polarity is higher but the pre-breakdown time delay of positive polarity is shorter;region IV,in which the breakdown voltage of positive polarity is lower and the pre-breakdown time delay of positive polarity is shorter.