流注放电低温等离子体中电子输运系数计算的蒙特卡罗模型

Monte Carlo simulations of electron transport coefficients in low temperature streamer discharge plasmas

流体或者粒子-流体混合数值仿真是研究流注放电基本物理机制的常用手段,而精确的电子输运系数是保证其仿真正确性的必要前提.鉴于现有电子输运系数求解工具存在一定缺陷,本文开发了采用蒙特卡罗方法求解低温等离子体中电子输运系数的仿真工具,测试表明其准确性和精确度均较高.研究了氮氧气体混合比及大气压下三体碰撞吸附对电子输运系数的影响.氮气中流注放电仿真表明,流体仿真中采用本模型改进后的电子输运系数可显著改善流注通道内部的等离子体参数分布.

Streamer is usually present at the initial stage of atmospheric pressure air discharge, which occurs in nature as a precursor to lightning, transient luminous events in upper atmosphere and has much potential applications in industry,such as the treatment of polluted gases/liquids, assisted combustion, plasma enhanced deposition etc. Streamer is a multi-scale problem both in time and in space, which brings much difficulty to the conventional diagnostic approaches.In past decades, fluid or particle-fluid hybrid models have been frequently used for understanding the mechanisms of streamer discharges because of their high efficiencies of calculations. Accuracies of the electron transport coefficients（including drift/diffusion coefficient, ionization/attachment coefficient, electron mean energy and extra） play a key role in ensuring the correctness of the fluid or hybrid simulations. As far as we know, BOLSIG＋ and MAGBOLTZ are two typical tools for obtaining the electron transport coefficients and have been widely utilized in previous models. BOLSIG＋uses ＂two-term＂ approximation which is not sufficient for some molecular gases, MAGBOLTZ cannot calculate the bulk transport coefficients： these data are required for some models. METHES is an additional tool for computing electron transport coefficients, however, specific platform is required which is not very user-friendly. As sorts of drawbacks exist in currently available calculating tools, in the paper, a Monte Carlo model is developed for computing the electron transport coefficients in gases, the model is flexible to choose any type of gas mixture and its accuracy has been validated by comparing with BOLSIG＋ and METHES. Furthermore, the influences of N2-O2 mixture and three-body attachment process in high gas pressures on the transport coefficient are investigated. It is worth mentioning that three-body attachment process can significantly change the electron transport properties at a relatively low reduced electric field.Thus, specific attention must be paid to the transport coefficients if simulation is performed at a high pressure. In addition, differences between the bulk and flux coefficients are analyzed which are not distinguished in some previous models. Finally, we further validate the present Monte Carlo model by performing simulation of streamer discharge in atmospheric N2, which shows that the improved electron transport coefficient from our Monte Carlo model can improve the simulated plasma properties, in particular at the interior of the streamer channel. The existence of divergence at the tip of the streamer channel might be due to our local field approximation; if a density gradient term is included in the impact ionization term and local electron energy approximation of the electron transport coefficients is used, the accuracy of the fluid can be improved further.