【作者】 黎林村；

【导师】 夏维东；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2008， 博士

【摘要】 电弧等离子体由于具有高温、高焓、高化学活性等优点而获得了广泛的应用,但其体积小、能量高度集中、参数梯度大等特点制约了它在化学沉积、材料制备等领域的发展,这些领域迫切需要一种体积大、参数梯度小的热等离子体,以提高生产效率、改善产品质量。本文提出了利用磁旋转分散电弧产生大体积均匀等离子体的研究思路,设计制作了一套磁旋转电弧发生器装置。该发生器由80mm内径的管形石墨阳极与棒状石墨阴极组成,外磁场则由绕制在阳极外壁上的螺线管线圈产生。试验结果表明,利用强外磁场高速旋转的方法可以有效地促进收缩电弧分散。在完全分散状态下,弧室截面上的等离子体光强分布趋于均匀,电弧电压的波动幅度显著降低。利用高速CCD与ICCD连续拍摄了弧室截面上的电弧图像,发现了磁旋转电弧分散发展过程中的多种电弧形态:不断发展的螺旋结构、并联电弧、多阴极弧根、扩散型电弧根、完全扩散的电弧等离子体等。利用电弧图像与电弧电压、电流信号的同步采集,分析了电弧分散过程中的动态特性,研究了外磁场、电弧电流、工质气流量、电极热状况等对电弧形态、电压特性、弧根附着状态等的影响,对并联电弧的稳定、破裂等给出了定性分析。以商用CFD软件FLUENT为平台,开发并验证了模拟热电弧传热与流动的计算程序,进而对三种不同条件下的磁作用电弧等离子体进行了二维数值模拟研究:(1)数值研究了轴向外磁场、工质进气速度对完全分散电弧等离子体中传热与流动规律的影响。外磁场高速旋转电弧引起的离心力促使等离子体从发生器中轴线附近向阳极壁转移,引起该区域的压力降低,发生器喷口外的环境气体回流进入发生器内,电弧等离子体在轴向上回缩,而在径向上扩张。这种独特的分布形状是在喷口外拍摄磁旋转电弧等离子体时,其光强分布比较均匀的根本原因。(2)数值研究了发生器喷口外是空气环境时,空气回流对磁旋转氩电弧等离子体特性的影响。空气混合极大地提高了电弧电压,而进口氩气流量的增加可以有效地抑制空气回流,因而在发生器内,电弧电压反而是随着氩气流量的增加而降低的,与实验测量结果定性相符。(3)数值研究了磁控焊接电弧中的传热与流动规律。磁控电弧表现为中空钟罩形状,其阳极表面的温度、压力、电流密度呈现双峰分布。磁作用后,电弧加热阳极的范围更大,传递给阳极的热量也增强。然而,利用强外磁场并不能无限制地径向扩张电弧,外磁场强度超过临界值后,会转而引起电弧径向压缩。实验和数值模拟研究表明:由于高速旋转,电弧等离子体产生了强烈的周向与径向流动,促使电弧等离子体在周向与径向上分散,从而产生了大体积、参数相对均匀的等离子体,并维持其稳定。更多还原

【Abstract】 Direct-current arc plasmas at atmospheric pressure have many diverse industrial uses nowadays as their special properties of high temperature, enthalpy and chemical activity. The majority of arc plasmas presently in industrial use are usually of small volume, concentrated energy, and inhomogeneous parameters as a direct result of their inherent shrinkage. However, in some application areas, especially for the chemical synthesis, material processing/preparation and etc, a large-volume with uniformity in plasma parameters is one of the most important requirements that the arc plasma source should satisfy to advance the product quality and production efficiency. With this motivation, a large-scale magnetically rotating arc plasma generator is designed and utilized to obtain diffuse plasma in this study. The generator consists of an 80mm diameter graphite anode chamber and a concentric graphite cathode. A solenoid coil wrapping the anode is used to produce an AMF (axial magnetic field).Rotation induced by the AMF results in improved stability and drastic changes in plasma structure. With the increase in arc current and AMF strength, the arc plasma in the chamber is observed in various configurations, it temporally and spatially evolves from a contractive column to fully diffuse plasma cloud which fills the entire chamber cross section. The fully diffuse plasma is distributed throughout the electrode gap and no anode attachment can be seen in the chamber cross-section, the diffuse anode root is symmetrically distributed over the electrode. Furthermore, this diffuse plasma runs more steadily and its voltage fluctuation is significantly reduced. The diffuse plasma consumes more power but its emission is weaker, which implies that the volume of the fully diffuse plasma should have been greatly enlarged.An axisymmetric-swirl model based on the commercial CFD code FLUENT is also used to qualitatively discuss the AMF effects on the flow and heat transfer of the fully diffuse plasma inside an assumed magnetron arc torch. The AMF induces a low pressure region around the torch axis and a strong backflow at its spout. So as that the AMF plasma is significantly retracted axially and expanded radially and as a result, the arc shape seems to be a flag dancing windward. The plasma is of high temperature at the flagpole but cooling a little at the banner and this is responsible for the plasma intensity distribution on the chamber cross-section is more uniform.When the ambient gas outside the chamber is air, the air backflow induced by the AMF mixes with the argon inflow and significantly heightens the arc voltage. The air quantity depends on both the AMF strength and the inlet gas velocity, i.e., it increases with the former but decreases with the latter. An increase in the amount of the argon inflow restrains the quantity of the air backflow, and this should be responsible for a lower arc voltage in such an AMF torch when a larger gas inflow is used.Finally, we simulate the plasma flow inside an AMF welding argon arc and the heat transfer from the arc to its anode at the atmospheric pressure. Numerical results indicate that the imposed AMF induces most of the plasmas flowing by the arc mantel which leads to the appearance of a low-pressure region at the arc core. The AMF arc significantly retracts axially and presents a hollow bell shape. The characteristics of temperature, overpressure and current density distributions on the arc-anode interface are also discussed. Using a small AMF is contributive to expand the arc attachment on the anode, however, strengthening the AMF can’t excessively expand the arc radially but pinch it finally.更多还原