【作者】 刘佳；

【导师】 郑志镇；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2017， 硕士

【摘要】 微等离子弧因其能量密度大、能量利用率高、设备成本低、对环境要求不高等特点而得到广泛应用。但因其具有极高的温度和温度梯度,一直以来缺乏可靠、有效的实验设备直接进行温度等特性的测量。而微等离子弧的温度等输出特性对生产指导具有重要意义。微等离子弧的数值模拟涉及到流动、传热以及电磁场的多物理场耦合,同时在阴极、等离子弧和阳极之间存在复杂的传热传质过程,在现今计算机技术的基础上,直接编程计算求解难度极大。因此,本文采用COMSOL Multiphysics 5.2有限元软件,在流体力学、电磁学与传热学的理论基础上,建立了脉冲直流微等离子弧二维轴对称的数值模型。计算域包括阴极、喷嘴、阳极工件以及微等离子弧,计算模型接近于微等离子弧的实际工作状态。本主要研究结果如下:(1)模拟得到脉冲直流微等离子弧各物理场的时变特性与空间分布。随着输入电流的变化,电势、电流密度、总热源均呈现出脉动性质。由于喷嘴的压缩作用,喷嘴内总热源、电流密度分布范围远小于微等离子弧的自由扩散区。同时发现,微等离子弧的主要热源组分为焦耳热;电子、重粒子的焓能转移在阴、阳极表面的影响效果明显。(2)在输入电流的上升沿,首先微等离子弧的电势达到峰值,击穿常温离子气形成等离子体,同时获得最大速度;然后电流密度与总热源达到峰值,等离子体温度急剧升高;最后,在输入电流达到最大值时,微等离子弧的电势、电流密度、总能源均以稳定的方式输出,温度与速度达到周期性的稳定状态。(3)模拟分析喷嘴孔径、喷嘴工件距、电流大小、离子气流量与阴极高度等主要参数对微等离子弧的影响。(4)通过数值模拟与实验测量的方式,比较直流恒流微等离子弧的弧柱区温度和电子数密度以及工件表面温度分布,对比结果十分吻合,验证了本文数值模型的可靠性。更多还原

【Abstract】 Micro-plasma arc(MPA)has been widely used for its large energy density,high energy efficiency,low cost equipment and low requirement of production environment.Its high temperature and temperature gradient lead to a lack of reliable,effective experimental equipment for direct measurement.However,the distributions of its temperature field and other physical fields have very important influence to guide the use of MPA.With the development of computer technology,the use of numerical simulation has become an important method to study the characteristics of MPA.The numerical simulation of the MPA involves the multi-physics coupling of the flow field,the temperature field and the electromagnetic field.At the same time,there is a complex heat and mass transfer process between the transmitting cathode,the plasma arc and the anode.As the program calculation is extremely difficult based on the existing computer software.Therefore,the COMSOL 5.2 Multiphysics software is used in this study,along with coupling to solve the electromagnetic field and fluid mechanics equation,established a two-dimensional axisymmetric transient numerical model of the MPA with pulsed direct current.The calculation includes the cathode,the nozzle,the anode workpiece and the MPA.The simulation results of the multi-physics numerical model are close to the experimental results.The main results are as follows:(1)The time domain characteristics and the spatial distributions of the physical fields of the pulsed direct current MPA are simulated.As the input current changes,the potential,the current density,and the total heat source exhibit with pulsating properties.The mechanical compression,hot compression and self-magnetic compression of the nozzle are important conditions for obtaining a stable MPA.Due to the compression of the nozzle,the total heat source and current density distribution within the nozzle is much smaller than the free diffusion region of the MPA.It is also found that the Joule heat is the main heat source component of the MPA.The enthalpy of electrons can be transferred on the surface of the cathode and anode,and the effect of enthalpy is obvious.(2)The breakdown effect of the plasma is simulated on the rising edge of the input current.After the potential between the cathode and anode reaches the peak,the ionization of the gas between the cathode and anode form the plasma at room temperature.When the movement velocity of plasma to reach the maximum,the current density and the total heat source also reach the maximum,these promotes the increasing of the temperature of the plasma.Whilst the input current reaches its maximum value,the potential,the current density and the total energy are output in a more stable way,the temperature and the velocity of the plasma reach the seasonal stable state.(3)The influence of the main parameters such as the nozzle diameter,the distance from the nozzle to workpiece,the arc current,ionized gas flow rate and the cathode height on the MPA are simulated.(4)Compared with the results of the numerical simulation and the experimental on the electron temperature,electron density of the arc column area from MPA,and the workpiece surface temperature distribution,it shows that the numerical simulation results are in good agreement with the experimental results,this verifies the reliability of the numerical model proposed in this paper.更多还原