【作者】 查俊；

【导师】 夏维东；

【作者基本信息】 中国科学技术大学 ， 热能工程， 2013， 博士

【摘要】 电弧等离子体技术广泛应用于机械加工、冶金、化工、材料制备和环境保护等工业领域。由于其自收缩效应,电弧等离子体通常具有能量高度集中、体积小、参数梯度大的特点,而这也限制了电弧等离子体技术在工业中的应用,尤其在大面积的表面处理、薄膜制备和材料制备领域,这些特点对生产效率、产品的均匀性和稳定性都有不利的影响。磁旋转电弧可以对工质和弧室比较均匀的加热,提高电弧对工质的传热效率,并且能有效降低电极烧蚀,延长电极工作寿命。本文主要采用实验方法研究同轴电极(棒状阴极和圆筒阳极)等离子体发生器内磁旋转电弧的位形结构及特性、磁旋转电弧产生大面积均匀等离子体的机制、电弧分散的演化过程及分散电弧等离子体的特性,研究轴向进气、电弧电流、轴向磁场、电极材料、电极结构、发生器结构等条件对电弧位形结构、分散电弧等离子体参数的影响,主要研究内容和结果如下：随着电弧电流和轴向磁场的增加,电弧演化过程的位形结构依次表现为：螺旋电弧、局部分散的并联电弧伴随着阳极弧根的扩散以及最终完全分散的均匀等离子体。而阴极弧根则随着电弧对阴极的加热程度表现为从单点、多点到环状或在端面完全扩散。具体表现为：1)施加轴向磁场后,磁旋转电弧首先表现为典型的具有准稳定性螺旋型位形结构。由于电极对弧根粘滞阻力,螺旋弧柱处于“发展-破裂”的重复过程：随着电弧的旋转运动,电弧螺旋结构会得到发展；之后螺旋电弧会因弧柱对阳极壁面的击穿分流而破裂。电弧电压随之波动,其频率具有一定的稳定性；产生比较稳定的螺旋电弧的条件是较低的磁场、较低的电弧电流和较小的轴向气流。2)随着轴向磁场或和电弧电流的增加,收缩的电弧会局部分散。在氩气流量为3Nm3/h、200<I<400A,0.24<I·B<4.8A·T条件下(阳极弧室直径80mm),会出现多阳极弧根和并联双弧现象。多阳极弧根的形成机制是由于弧室内存在的分散的电弧等离子体在阳极壁面附近收缩而形成的。并联双弧是由两条部分分散的弧柱和对应的阳极弧根组成的；在其运行过程中,电弧电流可以在并联双弧之间相互转移；其中一个阳极弧根的消散标志了并联双弧的结束。3)随着轴向磁场和／或电弧电流的进一步增加,电弧分散面积增大直至充满整个弧室截面。其显著特点是：电弧温度降低,电场强度提高,电弧具有下降的V-I特性；平均电压随轴向磁场的增加而增加。分散电弧等离子体位形呈圆盘状,位于阴极侧面和阴极端面,而阴极侧面为弧柱的主要区域；电弧从阴极向阳极径向发展,轴向厚度先变窄,后变宽,电子温度逐渐降低。轴向磁场和电弧电流的增加以及轴向进气的减小会加快电弧的分散速度,提高电弧的分散程度。4)阴极弧根的位形主要与阴极温度相关,同时也与电弧分散程度相关。随着电弧旋转加热阴极,阴极温度逐渐升高,阴极端面的阴极弧根从单个驻留的收缩斑点发展到多个驻留的收缩斑点,斑点数目随轴向磁场上升而增加。其中收缩弧柱的弧根在各斑点旋转移动,而分散电弧则可以具有多阴极弧根。论文明确了磁旋转电弧产生大面积均匀稳定等离子体的机制是：轴向磁场驱动电弧高速旋转,与轴向进气的冷气流产生对流使收缩的电弧被分散,产生分散的电弧等离子体。等离子体高速旋转流动使其自身均匀和稳定,且维持扩散的阳极弧根在阳极弧室内角向均匀分布。在闭口式发生器中,轴向磁场增加到一定程度,可以造成磁旋转电弧逆阴极自磁场洛仑兹力方向移动,即电弧在轴向上后退。而且轴向磁场越大,电弧的后退距离越大。分析这种后退现象是由电弧旋转运动的加强引起的弧室内的流场改变形成的电弧轴向前后压差造成的；其后退平衡位置由电弧前后气压差以及阴极自磁场对其洛伦兹力共同作用决定。在较小电流(50-100A)下,随着电弧电流的增加,阴极弧根的电流密度、弧根半径、阴极表面温度增加；阴极表面电场强度减小。测量得到纯钨电极表面最高温度在3530-3790K之间,弧根附着处阴极表面温度近似呈指数分布：(Tmax为弧根中心处最高温度,r为距离阴极弧根中心最高温度处距离,A(I)在48.73-77.32K之间,H(I)在0.56～0.44mm之间)。阴极弧根电流密度和电场强度的分布也与之相关。利用测量的阴极温度分布,计算得到平均电流密度电弧在5.9～6.9×107A/m2,平均电场强度在9.17～7.26×107V/m之间,阴极弧根半径从0.51mm增加到0.68mm。在电弧电流50～100A,轴向磁感应强度0.01-0.02T条件下,平端面纯钨阴极附近的等离子体电子温度在16100-22500K之间,阴极电位降在19～23V之间；锥形阴极附近的等离子体电子温度要高于前者,在18000-26000K之间,阴极电位降要小于前者,在16-19V之间。两者的温度都随电弧电流的增加而增加,随轴向磁场的增加而减小；阴极电位降都随着电弧电流和轴向磁场的增加而减小。在形成扩散型阴极弧根时(100A,0.02T),等离子体电子温度显著降低,为1.61×104K；阴极电位降有所升高为20.54V,高于轴向磁场相同时I=50A,80A的收缩弧根的阴极电位降(20.24V,19.5V)。更多还原

【Abstract】 Arc plasma technology is widely used in industry and industrial processes, such as machining, metallurgy, the chemical industry, material preparation and environmental protection. The arc plasma is a highly-concentrated energy source, with small volume and large gradients of temperature and other parameters, due to its strong constriction. The application of arc plasmas is limited by these characteristics, especially in the areas of surface, film and material preparation. The characteristics have adverse effects on the production efficiency, and the uniformity and stability of the product.The magnetically rotating arc plasma can heat the working medium and the chamber uniformly and increase the heat transfer rate, and the electrode erosion can be reduced and the electrode lifetime increased. This article considers the configuration and characteristics, and the mechanism and evolution of the magnetically rotating arc, and the characteristics of the dispersed arc plasma in an arc plasma generator with coaxial electrodes (rod-like cathode and cylindrical anode). The effect on the magnetically rotating arc and the dispersed arc plasma of the axial gas flow, arc current, axial magnetic field (AMF), electrode material and the configuration of the electrode and the generator is also studied. The main results are as follows:As arc current and AMF increasing, the evolution of the arc configuration shows: a spiral arc, partly dispersed parallel arcs with diffusive anode arc root and fully uniform dispersed arc plasma successively. The modes of the cathode arc root are single spot, multiple spots and annulus or fully diffuse on the cathode end face successively as the heat to the cathode by the arc increasing. Specific content as follows:1) The magnetically rotating arc has a spiral structure that is quasi-stable with respect to the AMF. The arc column is in a process of repeated "development and fracture" due to the viscous resistance of the electrodes:the spiral structure would develop with the arc rotating. And then the spiral arc can be destroyed due to breakdown between the arc column and the anode. The arc voltage fluctuates with an approximately-constant frequency. Relatively-low values of the AMF, arc current and axial gas flow promote stability of the spiral arc.2) The constricted arc becomes partly dispersed with a diffuse anode arc root for increasing AMF and arc current. The multiple-anode arc roots and parallel dual-arcs occur for200<I<400A,0.24<I·B<4.8A T, and axial gas flow3Nm3/h in a80mm diameter generator. The multiple-anode arc roots are formed by partial shrinking of the dispersed arc plasma with diffusive arc attachment to the anode. Parallel dual-arcs consist of two partly diffusive arc columns and corresponding anode arc roots. The arc current keeps transferring between parallel arcs. The extinguishment of one of the anode arc roots means the end of the parallel dual-arcs.3) The arc disperses until it fills the whole cross-section of the chamber when the AMF and arc current are increased. The temperature of the arc plasma decreases, while the electric field intensity increases. The arc plasma has a decreasing volt-ampere characteristic. The average voltage increases with the AMF. The configuration of the dispersed arc plasma is disc-like, and mainly located at the side and the end-face of the cathode. The thickness of the arc plasma initially decreases and then increases, and the electron temperature decreases, from the cathode to the anode. The speed and level of the arc dispersion improve with arc current and AMF increasing and the axial gas flow decreasing.4) The configuration of the cathode arc root is mainly relative to the cathode temperature and the dispersion of the arc. The rotating arc keeps heating the cathode and the cathode temperature increases gradually. The configuration of the cathode arc root develops form a single fixed spot to multiple fixed spots. The number of the spots increases with AMF increase. The cathode arc root of the constrictive arc column moves between the spots while the dispersed arc plasma may have multiple cathode arc roots.The experimental results confirm that the dispersion of the magnetically rotating arc is a consequence of the high-speed rotation of the arc due to the AMF, and convection driven by the cold axial gas flow. The plasma is uniform and stable due to its high-speed rotation, and the diffuse anode arc root has a uniform distribution in the azimuthal direction.In the closed generator, the arc may move counter to the Lorentz force induced by the cathode self-magnetic field, means the arc draws back in the axial direction. The greater the AMF, the larger the distance of the arc counter-motion. The mechanism of this phenomenon is thought to be the difference between the pressures in front of and behind the arc in the axial direction, induced by the change of the flow field inside the chamber due to the increased rotation of the arc. The location of the arc is mainly determined by the balance between the pressure difference and the Lorentz force induced by the cathode self-magnetic field.The current density, radius of the cathode arc root and the cathode surface temperature increase and the electric field intensity decreases with increasing arc current (50-100A). The maximum temperature of the cathode surface is in the range3530-3790K. The cathode surface temperature has an approximately exponential distribution:(Tmax:the maximum temperature of the centre of the cathode arc root, r:the distance to the centre of the cathode arc root A(1):48.73-77.32K,H(Ⅰ):0.56-0.44mm). The current density and the electric field intensity have similar distributions. The average current density is5.9-6.9×107A/m2and the average electric field intensity is9.17-7.26×107V/m, depending on the distribution of the cathode surface temperature. The radius of the cathode arc root increases from0.51mm to0.68mm.In the range50-100A arc current and0.01-0.02T AMF, the electron temperature of the plasma near the flat end of a pure tungsten cathode is in the range16100-22500K and the range of the cathode fall voltage is19-23V. The range of the electron temperature of the plasma near the conical end of a pure tungsten cathode is larger, in the range18000-26000K and the cathode fall voltage decreases to16-19V. In both cases, the electron temperature increases with arc current and decreases with increasing AMF, and the cathode fall voltage decreases with increasing arc current and AMF. In particular, the electron temperature decreases significantly to16100K for the diffuse cathode arc root (100A,0.02T). The cathode fall voltage for the diffuse cathode arc root (20.54V) for I-100A, B=0.02T is greater than that for I=50,80A for the same AMF (20.24V,19.5V).更多还原