| High voltage pulsed discharge in water can produce strong ultraviolet radiation,shock wave,and a large number of active free radicals.Therefore,underwater discharge plasma technology has been widely used in wastewater treatment,rock breaking,and underwater sound sources.Since the limitations of measurement and diagnosis technology and theoretical model,it is difficult to describe the process of discharge initiation and streamer development by existing observation methods,which brings difficulties to the accurate control of discharge in practical application.To solve the above problems,the hydrodynamic model of long and short nanosecond pulse discharge in water is established by using finite element numerical simulation method.The initiation,development,and branching process of discharge streamer are analyzed and studied.The rationality of the model is verified through experiments.The main research contents and conclusions are summarized as follows:(1)Under different conditions,the initiation mechanism of the streamer in the pulse discharge process in water is discussed.With the help of electrostriction theory,the relevant fluid dynamics model is constructed;and the initial stage of the short-nanosecond pulse discharge streamer is simulated and analyzed.It is found that the electrostriction-field ionization compound theory is more suitable for the initiation process of short-nanosecond pulse discharge streamer in water.(2)Based on the existing literature,a numerical model of the development process of a short nanosecond pulse discharge streamer is established.The numerical simulation of the discharge process in water with the condition of 130k V amplitude voltage,5ns rising edge,and 1mm electrode gap is carried out.The results show that the electric field strength near the tip of the needle electrode is more than 10MV/cm in the initial stage of the streamer.The head radius of the development process is equivalent to the radius of curvature of the needle tip,and the average electron density is about 1×1024m-3.Moreover,the simulation calculation of different discharge conditions is carried out to discuss the factors affecting the characteristics of discharge streamer in water.The average velocity of the development of the streamer under different conditions is obtained and compared with the experimental data.The results are in good agreement with the experimental data.(3)Micro inhomogeneity disturbance is added to the model to investigate the effects of random factors such as inhomogeneous initial state,small impurity and carrier density fluctuation on streamer bifurcation behavior.The results show that:under 130 k V amplitude voltage,the random factors distort the distribution of needle tip electric field to some extent,and the location of branching behavior is closely related to the random factor region.The macro factor and micro factor are closely related to the micro factors.The prime determines the propagation and branching behavior of the streamer.(4)The experimental platform of long nanosecond pulse high voltage discharge in water under 20k V Voltage was built.The high-speed camera is used to capture the discharge image,and the canny operator is used to process the image to restore the details of the streamer development process.The random disturbance factor is introduced into the simulation model of long nanosecond pulse discharge to improve the integrity of the model.The streamer development morphology obtained is compared with the experimental results,and the two morphologies are in good agreement.In this paper,a mathematical model for the systematic analysis of the characteristics of long and short nanosecond pulse discharge is established by combining theoretical analysis,numerical simulation,and experiment.Also,the discharge of needle plate in water is established.It can provide a reference for further understanding of the process of streamer initiation,propagation,and branching. |
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