Simulation Of The Streamer And Streamer-Leader Transition In SF₆/N₂ Mixture In Non-uniform Electric Field

The breakdown of strong negative gas in a short-gap inhomogeneous electric field can be caused by the streamer directly through the gap or a series of processes including streamer,streamer-leader transition,and stepped leader breakdown.The streamer-leader transition of strong negative gas includes the stem-leader mechanism and the precursor-leader mechanism,while the precursor-leader mechanism dominates.The stem leader mechanism is similar to the one in long-gap air,and the precursor leader mechanism is a unique one in strong negative gas.It is still difficult to directly test through experiments,in order to understand the characteristics of streamer and leader.Therefore,it provides the basis for interpreting the physical mechanisms of precursor in strong negative gases dominate by simulation.In this thesis,the short-gap needle-plate gap is used for study.In the single streamer discharge characteristics,a single streamer model combining plasma chemistry and fluid mechanics has been established,and a single stream injection was calculated in the case of two discharges where the steamer through gap and the streamer cannot penetrate.The distribution of particle concentration,electric field intensity,energy,and temperature in the channel provides the data basis for subsequent analysis of the streamer-leader conversion process.The result shows that the velocity of the streamer and the change of the amplitude of the external current show a synchronous trend.The electrons are mainly distributed at the streamer head,positive ions are distributed at the streamer head and channel,while negative ions are mainly distributed at the streamer channel.In the case of streamer through the gap,the temperature rises from 300K to 470K.When the streamer cannot penetrate the gap,the temperature rises from 300K to 305K,which is almost constant.It provides data basis for the analysis of the subsequent streamer-leader transition process.Streamer-stem leader transition mechanism believes that the energy of multiple streamer channels is injected into the same root channel to increase the temperature,leading to gas decomposition,causing a leader discharge.This thesis establishes a calculation model for the temperature rise of the streamer channel as a function of injection energy.Calculating the charge injection required to increase the temperature up to a decomposition temperature of 1500K for the SF₆/N₂ gas mixture is approximately 720pC.It is found that the required charge is much higher than the corona charge in this voltage,whose amount is23pC.The result proves that the streamer-stem leader transition mechanism is difficult to occur.Streamer-precursor leader transition mechanism believes that one channel inside the corona causes a positive and negative ion drift and separation phenomenon at the corona boundary,which enhances the electric field at the corona boundary and causes the leader discharge.Based on the existing streamer-precursor leader model,this thesis attempts to propose an electric field criterion.It is believed that when the electric field distortion enhancment reaches E_cr,the streamer-precursor leader transition occurs.Considering the corona charge and the distorted flow field corona boundary electric field is calculated by neglecting the space charge q_di with drifting corona charge.The result shows that to reach E_cr for the electric field distortion of the corona boundary the q_di required is 1pC when corona charge is taken into account.For neglecting the corona charge the q_di required is8pC.In the single streamer simulation results,the charge amount near the corona boundary of the streamer channel is 0.6pC,which is close to the q_di considering the corona charge condition and satisfies the conditions for the streamer-precursor leader transition.As a result,the streamer-precursor leader transition is more likely to occur,consistent with experimental observations in the literature.