针-板空气间隙流注放电起始过程的三维PIC/MCC仿真研究

流注放电作为自然界中闪电传播的预电离机制、高压输变线路间长空间间隙放电的重要初始阶段,在工业领域存在诸多潜在应用,近年来引起人们越来越多的关注.流注放电具有典型的多尺度、非线性的放电特征,实验观测中多呈现出分叉等不规则结构.为了研究其微观结构特性和行为特征,本文采用三维粒子仿真模型(PIC/MCC),着重研究了流注从针型正电极的起始和发展过程.模型采用了可变自适应网格、可变粒子权重以及并行计算等技术,有效地降低了三维粒子仿真的计算时间.通过调节针型电极上的施加电压幅值、改变气体组分及调整电极形状尺寸等,研究了放电参数变化对流注放电的分叉结构、半径等行为的影响.模拟结果表明:随着电压的升高,流注的半径及分叉数目增加;对比不同气体组分(纯氧以及不同比例氮氧混合气体),发现其对流注的分叉数目影响较为显著;针型电极结构直接影响了流注的起始时间和形貌.

Three-dimensional PIC/MCC numerical study on the initial process of streamer discharge in a needle-plate electrode in atmospheric air

Streamer, which usually appears at the initial stage of atmospheric pressure air discharge, acts as a precursor of lightning. It also occurs as large discharges (called sprites) in upper atmosphere, far above the thundercloud. The streamer discharge has many potential applications in industry, such as gas or water cleaning, ozone generation, assisted combustion, etc. The streamer discharge is difficult to investigate both experimentally and computationally, because of its non-linear and multi-scale characteristics. Various studies on streamer discharge have been carried out, and some progress has been made. However, some things remain to be further understood, i.e., the law of particles motion and the factors influencing streamer discharge. In this paper, we use a pre-established three-dimensional (3D) particle model (PIC/MCC) to study streamer discharge with a needle-plate electrode in air. To simplify the condition, we only use nitrogen-oxygen mixture to represent dry air, regardless of other components such as CO₂, H₂O gases, etc. In this model, we take photoionization, attachment and detachment processes into account. The adaptive mesh refinement and adaptive particle weight techniques are used in the code. In order to facilitate the simulation, we artificially put a Gaussian seed right on the top of the needle electrode. We adjust some computational parameters to analyze how the streamer discharge starts and evolves from the needle electrode. Many factors can influence streamer discharge during its evolution, from among which we choose three important parameters:voltage amplitude, gas component, and the radius of curvature of the needle electrode tip, to study the generation and evolution of streamer discharge, and focus on inception cloud, streamer branches, and electric fields. The simulation results show that the radius of inception cloud increases with the increase of voltage amplitude, and the diameter of steamer channel and the number of branches also increase with voltage increasing. We choose 4 kV as a proper simulation voltage for next two parts of simulations. By comparing the results obtained in the cases of different gas components (pure oxygen and different ratios of nitrogen-oxygen mixtures), we discover that the nitrogen-oxygen mixture ratio significantly affects the total number of streamer branches. With 0.1% oxygen, discharge grows irregularly with small protrusions on streamers. In the pure oxygen case, streamer seems to have much more thin branches than in other cases. Needle geometry directly changes the inception cloud of the streamer and its morphology, especially when the tip becomes blunter. In this circumstance, electric field strength around the electrode decreases, and inception cloud can be barely seen. Instead, a single-channel streamer discharge develops right toward the plate electrode, later this single-channel streamer splits into branches.