Research On The Transient Temperature Evolution Of Positive Initial Leader Channel In Long Air Gap Discharges

The characteristic of positive leader in long air gap discharge is the basis for the switching overvoltage insulation coordination of ultra-high voltage power systems and lightning shielding design of high objects on the ground.The leader discharge includes two stages:initial and continuous development.The initial stage is in a non-local thermal equilibrium state.Obtaining the thermal characteristics of the initial leader channel is critical for revealing the physical mechanism and whole process modeling of leader discharge.Preliminary studies on the thermodynamic geometric characteristics of the stem in the initial stage of leader discharge have been carried out.However,the temperature spatiotemporal evolution of the initial leader channel has not been obtained,and the relevant simulation models have not been verified.In addition,the quantitative schlieren systems driven by constant power light source have made great progress in the measurement of thermodynamic parameters of continuous leader channels,but it encounters the problem of insufficient spatiotemporal resolution in the parameters measurement of initial leader channels in a non-local thermal equilibrium state.This paper focuses on three aspects of the transient temperature measurement method,experimental measurement,and theoretical simulation,and conducts a systematic study on the channel temperature evolution law in the initial stage of positive leader discharge,which has important scientific significance and engineering application value for revealing the physical mechanism of leader discharge and supporting the equipment insulation coordination and the design of lightning shielding.Based on the transient temperature measurement theory of quantitative schlieren method,the quantitative relationship among the sensitivity,light intensity,and spatiotemporal resolution of quantitative schlieren system was deduced.A method of using a pulsed current source to drive a LED chip to generate transient strong light to improve the spatiotemporal resolution of quantitative schlieren system was proposed.A pulse-driven quantitative schlieren system for transient temperature measurement of non-local thermal equilibrium discharge channels was developed,with temporal and spatial resolutions of 0.37μs and 31μm/pixel,respectively.Compared with the highest index of general quantitative schlieren system driven by a constant power light source,the temporal and spatial resolutions were increased by 1.7 times and 2.2 times,respectively,which solved the continuous measurement problem of channel transient temperature in long air gap discharge.The experimental observations of the positive leader discharge in the 1.0 m rod-plate gap were carried out,and the data of discharge current,quantitative schlieren image,and channel optical image were obtained.The temperature distribution characteristics of stem were obtained by schlieren images:after the first streamer discharge occurred,the initial thermodynamic shape of the stem was a trapezoidal cylinder,and the diameters of its root and head were 0.4～0.6 mm and 0.6～1.0 mm,respectively,and the height of stem was0.8～1.2 mm.The temperature of the stem decreased along the axial direction with the increase of the distance from root,and the root temperature was about 2000 K,which was bell-shaped and symmetrically distributed in the radial direction.The temperature evolution law of stem was obtained:the stem temperature rose rapidly to the maximum value with occurrence of the first streamer discharge.Due to the difference in the charge of first streamer discharge,there were three situations for the temperature evolution of stem in dark period:first,when the translational kinetic energy deposition and dissipation rate were equal,the temperature was stable;second,when the dissipation rate was slow,the temperature gradually rose;third,when the dissipation rate was faster,the temperature gradually decreased,and the axial temperature of stem root with the highest temperature was stable at1000～1200 K before secondary streamer occurred.It was found that a thermal thin channel grew in front of the stem during the dark period,and the temperature in the thermal thin channel was between 400 and 800 K.The radial diameter of the thermal thin channel was about 0.2 mm,and its development speed was about 0.1 mm/μs,which was comparable to the ion migration velocity in the channel.The macroscopic formation mechanism of the thermal thin channel is elucidated:under the action of an electric field,positive ions collide with neutral particles to generate energy transfer,which in turn increases the temperature of the channel,and eventually forms the thermal thin channel.The spatiotemporal distribution characteristics of gas temperature in unstable leader channels were obtained by quantitative schlieren images after the occurrence of secondary streamer discharge,and the macroscopic evolution mechanism was elucidated.The secondary streamer occurred at the head of the thermal thin channel,and the free electrons entered the electrode through the thermal thin channel and primary streamer stem.The collisions between electrons and neutral particles produced energy transfer,which made the radial size of the thermal thin channel expand rapidly,and gas temperature at the root of stem rose to 2000 K rapidly,resulting in the occurrence of unstable leader discharge.The axial length of the initial leader channel was the sum of stem and thermal thin channel.With the continuous injection of unstable leader current,the axial distribution of channel temperature was uniform and the amplitude was about 5000 K.The channel extended along the axial direction and radially expanded,causing the convection loss and a drop in gas temperature.The discharge was difficult to maintain and entered the relaxation process,and a thermal thin channel grew again.At this stage,the temperature of the channel continued to drop due to the heat conduction loss,and finally stabilized at about 1600 K until the next discharge occurred.Based on the one-dimensional thermo-hydrodynamic model of leader discharge inception,the rates of different forms of energy deposition,dissipation and particle generation,consumption in the discharge channel were obtained through simulation,and the input parameters of model such as leader channel initial radius and discharge current were determined through experimental observations.The particle transport,energy transfer,and chemical reaction processes were numerically simulated,and the evolution mechanism of gas temperature in the initial leader channel was further elucidated from the microscopic point of view:After the first streamer occurred,the quenching of the excited state particles generated a large amount of energy and the stem temperature rose to the maximum value rapidly.In the dark period,the channel electric field continued to increase,and the ion current heating process with continuously increasing power existed in the streamer stem,which acted together with the vibration-translational relaxation in the translational kinetic energy deposition process.The above processes were in equilibrium with the energy dissipation process of heat conduction,and the temperature evolution of stem showed three situations of rising,stable,and decreasing.After the inception of secondary streamer discharge,the quenching process of excited state particles occurred again in the channel.At the same time,the thermal dissociation process produced a large number of O atoms and N atoms and generated a large amount of thermal dissociation energy,which made the temperature of stem continue to rise,resulting in the inception of unstable leader discharge.The association ionization of N and O became the dominant mechanism for electron generation,and it was in equilibrium with the electron-ion recombination process to maintain the electron density above 10¹⁵ cm^-3.With the rapid radial expansion,the convection loss became the dominant mechanism of energy dissipation,and the temperature began to drop.The rate of association ionization decreased and gradually became smaller than the recombination process.When the temperature dropped below 2000 K,the unstable leader discharge aborted and entered the relaxation process.