短碳弧的有效二维模拟：阳极和阴极斑点
Validated two-dimensional modeling of short carbon arcs: anode and cathode spots
摘要：
为了研究短碳弧的特性，将自洽模型应用到CFD程序ANSYS-CFX中。该模型处理的热和电流在等离子体和电极中的耦合方式的传输，并占在腔室中的气体对流。模拟了电极表面的多个过程，包括空间电荷限制鞘层的形成、碳的烧蚀和沉积、辐射和电子的发射和吸收。模拟结果表明，电弧在阴极和阳极前表面附近收缩，导致电极斑点的形成。阴极斑点是一种众所周知的现象，其形成机制在其他地方也有报道。然而，在这项工作中发现的阳极斑点的形成机制之前没有报道。我们的结论是，斑点的形成是不相关的等离子体不稳定性，因为通常认为在收缩的放电列的情况下，而是发生由于在阳极的热平衡的高度非线性性质。我们还证明了这一属性与减少阳极传热模型。我们还表明，斑点的大小增加与电弧电流。在我们的实验中也证实了这种阳极斑点行为。由于阳极斑点的形成，在阳极附近出现大的碳气体密度梯度，这驱使一部分烧蚀的碳回到阳极的周边。因此，这可以降低总消融率。模拟结果还表明，电弧可以达到局部化学平衡（LCE）状态的柱区域，而局部热平衡（LTE）状态是不典型的实验条件下实现。这表明，在短碳弧的模拟中，考虑不同的电子和气体温度是很重要的。
英文摘要：
In order to study properties of short carbon arcs, a self-consistent model was implemented into a CFD code ANSYS-CFX. The model treats transport of heat and electric current in the plasma and the electrodes in a coupled manner and accounts for gas convection in the chamber. Multiple surface processes at the electrodes are modeled, including the formation of space-charge limited sheaths, ablation and deposition of carbon, emission and absorption of radiation and electrons. The simulations show that the arc is constricted near the cathode and the anode front surfaces leading to the formation of electrode spots. The cathode spot is a well-known phenomenon and mechanisms of its formation were reported elsewhere. However, the anode spot formation mechanism discovered in this work was not reported before. We conclude that the spot formation is not related to plasma instability, as commonly believed in case of constricted discharge columns, but rather occurs due to the highly nonlinear nature of heat balance in the anode. We additionally demonstrate this property with a reduced anode heat transfer model. We also show that the spot size increases with the arc current. This anode spot behavior was also confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, which drives a portion of the ablated carbon back to the anode at its periphery. This can consequently reduce the total ablation rate. Simulation results also show that the arc can reach local chemical equilibrium (LCE) state in the column region while the local thermal equilibrium (LTE) state is not typically achieved for experimental conditions. It shows that it is important to account for different electron and gas temperatures in the modeling of short carbon arcs.
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