等离子体在科研和工业生产中有很多重要应用。在磁流体发电和等离子体鞘套地面模拟装置产生的气流中，温度通常只有3000K左右，因此往往将碱金属注入工作介质，利用碱金属的热电离来提高气体的电子数密度和电导率。国内外在磁流体发电中大量应用了碱金属作为电离种子，但是考虑到等离子体鞘套地面验证装置环境的不同及其多物理场耦合性，关于碱金属注入参数对工作介质的温度、速度和电子数密度等物理场影响的研究十分必要。

针对这一问题，本文从注入碱金属这一物理化学过程出发，抽象出问题模型。在自行设计的仿真模型基础上，用CFD软件进行三维数值模拟，预测出不同工作参数下的温度、组分浓度的分布；探索注入形式、粉末尺寸和流量、边界温度与注入速度等因素的影响规律。本文的主要内容包括以下几个部分：

（1） 本文介绍了碱金属载气与工作介质两股射流相交的运动规律；碱金属与高温气体之间相互作用规律，即离散相和连续相之间的动量、热量和质量传递过程；推导出各组分的控制方程和组分之间的反应速率计算方式。

（2） 根据射流运动规律，本文对比了不同的碱金属粉末的注入模型：垂直入射、斜入射、切向入射和旋转进气。在进行数值计算时，本文选择 模型作为湍流模型，根据粒子负载情况采用Discrete Phase Model描述了离散相和连续相之间的相互作用关系，同时也借助化学反应模型计算碱金属注入之后的汽化过程和电离反应结果，并根据工程中的实际情况设置了全方面的边界条件。

（3）进行了多组不同参数下的碱金属粉末注入过程的仿真，分析各个变量对于热力参数（温度、速度和压强等）和浓度分布的影响规律。对不同注入结构下的场分布结果进行比较，发现在实际应用中选择斜入射结构，同时主管道采用旋转进气的方式可能会有更高的出口电子数密度和更好的一致性。通过对比不同的碱金属粒径和注入量下的计算结果，发现颗粒尺寸越小，出口电子数密度和均匀程度都越高，所以在成本可以接受的范围内应该尽量减小颗粒尺寸；碱金属的注入量存在一个最佳值，对于以金属单质钾作为电离种子的情况，推荐注入量为主气流流量的2.5%。计算得到出口电子数密度随主气流温度和载气温度升高而增大的规律。最后也提出通过改变载气流量（射流速度）来控制混合强度的思路，给出了达到各种射流穿透深度所需的边界条件。

本文的研究内容是等离子体鞘套地面验证装置中重要的组成部分，随着对碱金属提高电子数密度研究的不断深入，本课题中提出的不同碱金属注入形式具有较大的应用价值；另一方面，文章中对碱金属的粒径、注入量、边界温度以及载气流量等参数影响的仿真工作为后续关于优化非平衡等离子体产生的研究提供了参考。

Plasma has many important applications in scientific research and industrial production. In the flow generated by the magnetic fluid power generation and the plasma sheath ground simulation device, the temperature is usually only about 3000K, so alkali metals are often injected into the working medium, and the thermal ionization of the alkali metals is used to improve the electron number density and electrical conductivity of the gas. A large number of alkali metals have been used as ionization seeds in both domestic and foreign magnetic fluid power generation. Considering the environment difference between the plasma sheath ground simulation device and MHD with also its multi-physics coupling, studies on the effects of alkali metal injection parameters on the physical fields such as temperature, velocity and electron number density of the working medium are very necessary.

 

Aiming at this problem, this article starts with the physical and chemical process of alkali metal injection and abstracts model of the problem. Based on a self-designed simulation model, CFD software is used to perform 3D numerical simulation to predict the distribution of temperature and component concentration under different operating parameters; it also explores the influencing factors of  injection form, powder size and flow, boundary temperature and injection speed. The main content of this article is described as below:

 

(1) This article introduces the motion law of the two jets intersection, which are the carrier gas of the alkali metal and the working medium; it also describes the interaction law between the alkali metal and the high-temperature gas, that is, the process of momentum, heat and mass transfer between the discrete phase and the continuous phase; and it finds out the governing equation of each component and the calculation method of the reaction rate between the components.

 

(2) According to the law of jet motion, different injection models of alkali metal powder are compared in this paper: vertical incidence, oblique incidence, tangential incidence, and rotating intake air. During the numerical calculation, the  model is selected as the turbulence model, according to the particle loading the Discrete Phase Model is used to describe the interaction between the discrete phase and the continuous phase. At the same time, the vaporization process and the ionization reaction after the injection of alkali metals are also calculated using the chemical reaction model, and this article sets all the boundary conditions according to the actual situation in the project.

 

(3) The simulation of alkali metal powder injection process under different groups of parameters was performed, and the influence of each variable on the thermal parameters (temperature, velocity, pressure, etc.) and concentration distribution is analyzed. Comparing the field distribution results under different injection models, it is found that selecting the oblique incidence structure while the main pipe adopting the rotary inlet may result in higher outlet electron number density and better distribution consistency in practical application. By comparing the calculation results of different alkali metal particle sizes and injection amounts, it is found that smaller particle size will make outlet electron number density and the degree of uniformity higher, so the particle size should be minimized as far as the cost is acceptable; there is an optimal value for the injection volume of alkali metal. For the case where metal elemental potassium is used as the ionization seed, the recommended injection volume is 2.5% of the main gas flow rate. After simulation we found that outlet electron number density increases with the increase of the main gas flow temperature and the carrier gas temperature. Finally, the idea of controlling the mixing intensity by changing the carrier gas flow rate (jet velocity) is also proposed. The boundary conditions of implementing different penetration depth are given.

 

The research content of this article is an important part of the plasma sheath ground simulation device. With the continuous deepening of the research on how alkali metals improves the electron number density, on one hand, the different alkali metal injection forms proposed in this subject have great application value; on the other hand, the simulation on the effects of parameters such as particle size, injection volume, boundary temperature, and carrier gas flow rate of the alkali metal in this paper provides reliable reference for subsequent research on optimizing non-equilibrium plasma generation.

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