【作者】 胡明；

【导师】 万树德； 马锦秀；

【作者基本信息】 中国科学技术大学 ， 等离子体物理， 2014， 博士

【摘要】 电弧等离子体由于具有高温、高焓、高化学活性等优点在机械加工领域、材料科学领域、冶金工业领域、环保领域、化工领域以及环保领域等方面获得了广泛的应用。但是由于电弧的分流现象和电流的轴向分量与电弧电流产生的自磁场的周向分量相互作用产生的Lorentz力而引起的弧柱自收缩效应使得电弧发生器通常表现为射流不稳定以及等离子体参数不均匀等特性,从而在一定程度上制约了电弧等离子体技术在现代工业应用中的发展前景。比如,在等离子体喷涂应用中迫切需要解决等离子体炬不稳定性问题,在废物处理方面需要均匀性好、大体积的等离子体发生器。因此,针对不同的应用领域设计不同的发生器,以提高生产效率、改善产品质量,是现阶段一项重要的任务。本文一方面根据外部磁场与电弧等离子体的相互作用性设计了磁旋转的直流电弧等离子体炬,实验研究了电弧弧根的旋转频率、电弧的运动形态等随外部磁场的变化；另一方面,本文运用泰克公司的双通道示波器记录了电弧等离子体放电过程中的电弧电压变化,分析了不同实验条件下的电弧电压及其波动与工作参数(电弧电流、工质气体流量、供气方式以及外部磁场等)之间的关系,通过快速傅立叶变换技术,运用FFT算法将电弧电压信号从时域变换到频域,从原始信号中提取出能够反映电弧脉动行为的频率分量；同时,测量了电弧射流的光谱信息,获得了外部磁场对电弧射流中等离子体参数的影响,探讨了外部磁场作用下电弧等离子体局域热力学平衡的判定。最后,本文根据电弧等离子体射流波动的特性,自行设计研制了电弧射流波动诊断系统,并对电弧等离子体射流波动进行了研究,通过对比发现,射流脉动和电弧脉动之间存在密切的关系。运用高速摄像技术对电弧通道截面的电弧图像进行捕捉,分析了电弧弧根的旋转频率、电弧的运动形态等随外部磁场的变化。利用电弧图像与电弧电压信号的同步采集,分析了电弧旋转过程中的动态特性,利用经典的信号处理工具,如统计分析、快速傅立叶变换等分析了电弧电压及其波动随电弧电流、工质气体的流量、供气的方式、外部磁场的变化。在无磁场的条件下,电弧电压信号的频域分析结果显示除了300Hz的低频脉动(由整理电源的纹波分量引起),不存在任何高频分量(1-15Kz),而外部磁场的作用改变了电弧脉动的频率,因为外部磁场和等离子体的相互作用产生螺旋不稳定性,甚至产生高频分量,但是在合适的磁场作用下即使产生螺旋不稳定性,数值也很小,可以忽略,而且300Hz的低频脉动也被削弱。通过光谱诊断技术对电弧射流中的等离子体参数进行了细致的研究,由于电弧的波动对光谱诊断有重要的影响,因此我们仅对工作在Steady模式和Takeover模式下射流的电子密度和电子激发温度进行了诊断,深入分析了外部磁场对电弧射流的特征谱线的影响,估算出外部磁场对电弧射流中等离子体参数的影响,最后还对外部磁场对LTE的影响进行了分析。电弧等离子体射流中的脉动是等离子体射流的典型物理现象之一,并且这些脉动特性直接决定了等离子体工艺过程处理的性能。实验研究表明,电弧分流、整流电源的纹波分量以及等离子体射流的流体动力学过程引起的湍流和射流对环境气体的卷吸是导致射流能量波动的主要原因,而射流能量的波动决定了材料在射流中的加速与加热也是不平衡的,有可能对工艺的质量产生很大的影响。为此,本文针对射流光强信号自行设计制作了专门的射流波动诊断系统,主要是通过光电倍增管实现电弧射流光强转换为电压信号,并通过与电弧电压信号比较得出电弧射流脉动和电弧电压之间的关系。基于以上的研究,由于阳极弧根的旋转,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,产生了参数相对均匀的等离子体,但是根据磁场和其他外部条件(供气方式)相互作用的方式不同,我们获得性能不同的电弧等离子体为不同的工业服务。在电弧等离子体的放电通道中,等离子体的传热和流动特别的复杂,特别是添加磁场和旋气以后,要想弄清楚不同工作模式的具体机理,有待进一步的实验研究。更多还原

【Abstract】 Due to the high temperature, high enthalpy, high chemical activity and other advantages, dc arc plasma torch has been widely applied in the field of mechanical processing, materials science, metallurgy industry, chemical industry and environmental protection, etc. However, the arc column shrinking because of the shunting arc and the Lorentz force which produced by the arc current and self-induced magnetic field, which makes the arc generator always showing the plasma jet unstable, the non-uniform of plasma parameters and other characteristics. And that impede the development prospects of arc plasma technology in modern industrial applications at a certain extent. For example, in the application of plasma spray urgent need to solve the problem of instability of the plasma torch, and in the application of the waste disposal needs a uniformity and large volume plasma generator. Thus, for different applications with different design generator to increase productivity, improve product quality, is an important task at this stage.In this paper, on the one hand, through the interaction between the external magnetic field and the arc plasma, we design the rotating magnetic DC arc plasma torch. The arc with the experimental and we study the change of the arc rotational frequency, the arc movement patterns and so on with the external magnetic field. On the other hand, we use the dual-channel oscilloscope to record the change of the arc voltage during the arc plasma discharge, and analyze relationship between arc voltage and its fluctuations under different experimental conditions with the operating parameters of the plasma torch (arc current, refrigerant gas flow, way relationship and supply an external magnetic field, etc). By the technology of the fast Fourier transform (FFT), we use the FFT algorithm to change the arc voltage signal from the time domain to the frequency domain, and we extract the frequency component which can reflect the arc pulsating behavior from the original signal. Meanwhile, we the measure the spectral information of the arc plasma jet, get the plasma parameters in the arc plasma jet effected by the external magnetic field, and discusses the determination of plasma arc local thermodynamic equilibrium under the external magnetic field. Finally, according to the characteristics of arc plasma jet fluctuations, we design and develop the arc jet fluctuations diagnostic system, and study the fluctuations in the arc plasma jet. By comparison, we find that there is a close relationship between the arc jet arc jet fluctuations and the arc arc jet fluctuations.We use the high-speed camera technology to capture the arc motion picture in the cross-section of the arc discharge channel, and analyze change of the arc root rotational frequency, the arc of movement patterns and so on with the external magnetic field. We collect Image the arc voltage signals and the arc picture synchronously, and analyze the arc dynamic characteristics during rotation. Using classical signal processing tools, such as statistical analysis, fast Fourier transform analyze the change of the arc voltage and its fluctuation with arc current, the working gas flow rate, the way of gas supply, and the external magnetic field. In the absence of magnetic field, the frequency domain of the arc voltage signal analysis result show that there is no high-frequency components (1-15Kz) in addition to low frequency pulsation of300Hz (caused by the ripple component of the eliminator supply). The external magnetic field changes the frequency of the arc fluctuations, because the interaction between the external magnetic field and the arc plasma cause the helical instability and even high-frequency component. However, a suitable magnetic field which caused helical instability is small and can be ignored, and weaken the low frequency of300Hz.By spectroscopic diagnostic techniques, we study the plasma parameters in the plasma jet deeply. Due to the arc fluctuations effect the spectral diagnostics significantly, we only diagnose the electron density and electron temperature working under Steady mode and Takeover mode. We analyze the influence of characteristic spectral line by the external magnetic field, estimate the influence of the plasma parameters in the arc plasma jet by the external magnetic field, and finally analyze the influence of LTE by the external magnetic field.Pulsating in the arc plasma jet is one of the typical physical phenomena in the plasma jet, and these pulsating characteristics will determine the performance of the plasma treatment process. Experimental studies have shown that arc shuntting, ripple component of rectifier power supply, and the turbulence caused by fluid dynamic processes of the plasma jet and environment gas entrained from the plasma jet are the main reason leading to the jet energy fluctuations. Fluctuation of the jet energy determines the acceleration and the heating of the material in the plasma jet is also uneven, it is possible to produce a great impact on the quality of the process. therefore, in this paper, contraposing optical intensity signal, we design and produce a special jet fluctuation diagnostic systems, which is primarily to change the optical intensity signal into the voltage signal by a photomultiplier, and obtain the relation between the plasma jet pulsation and the arc voltage fluctuation the by comparison with the arc voltage signal. This diagnostic system is not only simple in structure, convenient operation, but also lower production costs.Based on these studies, because of the arc rotation, the arc with rotating anode, the arc plasma produce a strong circumferential and radial flow, prompting the arc plasma dispersed in circumferential and radial, generating relatively uniform plasma parameters. However, depending on the interaction between the magnetic field and other external conditions (the way gas supply), we get different properties plasma torch to services different arc plasma industrial. In the arc plasma discharge channel, the plasma heat transfer and flow particularly complicated, especially after adding vortex gas and external magnetic field, in order to figure out specific mechanisms under different operation modes, pending further experimental studies.更多还原