Transactions of China Electrotechnical Society  2023, Vol. 38 Issue (13): 3388-3399    DOI: 10.19595/j.cnki.1000-6753.tces.220560
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Microgap Discharge Characteristics of Methane/Air under the Needle Plate Electrode
Wang Dangshu1, Deng Xuan1, Liu Shulin1, Yi Jia'an1, Wang Xinxia2
1. School of Electrical and Control Engineering Xi'an University of Science and Technology Xi'an 710075 China;;
2. School of Science Xi'an University of Science and Technology Xi'an 710075 China

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Abstract  Electrical equipment working in explosive environment should meet the requirements of intrinsic safety. Domestic and foreign scholars evaluate the intrinsic safety based on the voltage and current characteristic curves of the discharge measured by IEC-SSTA spark test device. However, the discharge mechanism and particle change law in case of equipment failure are not clear. In the design of intrinsically safe electrical equipment, there will be transition explosion-proof, which limits the improvement of intrinsically safe output power, resulting in that the output power of intrinsically safe equipment can only reach about 20 W. Therefore, based on the electrode structure of the program-controlled micro-nano discharge test device, the discharge physical model is established in the methane-air mixture environment. The discharge parameters such as axial electron density, various positive ion density and so on are obtained by two-dimensional modeling and simulation of the discharge through hydrodynamics, thus clarifying the discharge mechanism of plasma micro-gap.
Firstly, a two-dimensional discharge model is established based on the electrode structure, and the boundary conditions are set. Secondly, add the chemical reaction equation and set the conditions such as the cross section data involved in the reaction. Then, set the initial gas concentration and particle density. Finally, the simulation is carried out to obtain the data by changing the electrode distance and the concentration of methane-air mixture. According to the simulation, the influence of the change on the electron density, the number density of positive ions, the electric field strength and the average electron energy is analyzed, and the micro-gap discharge mechanism is analyzed from the microscopic level.
At the initial stage of discharge, the degree of electron collision ionization reaction between the electrons in the electrode gap and other gas molecules is relatively low, and the electron density is in a slow rising stage. As the energy gained by the electrons in the electric field increases, the collision ionization becomes more and more intense, and the electron density increases rapidly. With the development of motion, the probability of recombination of electrons and positive ions also increases, which slows the growth of electron density. In general, the electron density first increased to the peak and then decreased, and the discharge continued to develop and gradually entered a stable stage. Then change the pole spacing to get the conclusion that the pin-plate spacing not only affects the particle impact ionization degree, but also affects the discharge mechanism. When the needle plate spacing is greater than or equal to 10 μm, there is no field emission in the discharge process. After changing the concentration of methane, it can be concluded that the contribution rate of O2 and CH4 to the discharge is greater than that of N2, and the discharge is mainly the collision ionization of O2 and CH4. Finally, experimental verification is carried out. The experimental results are consistent with the simulation results, which verify the correctness of the theoretical analysis.
The following conclusions can be drawn from the simulation analysis: (1) During micro-gap discharge, sheath area will be formed at about 2 μm of the positive electrode and about 0.5 μm of the negative electrode. The positive ion sheath area near the cathode plate distorts the field intensity emission of the cathode plate. (2) When the methane concentration changes from 3.5% to 13.5%, the number density of $H_{2}^{+}$ is relatively stable about 4×1015 m-3, the number density of $O_{2}^{+}$ increases with increasing methane concentration when methane concentration is less than 8.5%. When the methane concentration is greater than or equal to 8.5%, the O2 collision ionization in the mixed gas reaches saturation and does not increase with the increase of methane concentration. (3) The change of electrode distance affects the electric field intensity between electrodes and the discharge mechanism.
Key wordsExplosive environment      needle plate electrode      hydrodynamic method      micro gap discharge     
Received: 08 April 2022     
PACS: O461  
  TM863  
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Wang Dangshu
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Wang Dangshu,Deng Xuan,Liu Shulin等. Microgap Discharge Characteristics of Methane/Air under the Needle Plate Electrode[J]. Transactions of China Electrotechnical Society, 2023, 38(13): 3388-3399.
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