Investigation of the short argon arc with hot anode. I. Numerical simulations of non-equilibrium effects in the near-electrode regions

Abstract

Atmospheric pressure arcs have recently found application in the production of nanoparticles. Distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play important role in operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable with the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on the current density. The model was extensively verified and validated against previous simulation results and experimental data. Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In case of strong anode cooling (water-cooled anode) when anode is cold, temperature and plasma density gradients increase with current density resulting in decrease of the anode voltage (absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with current density, leading to drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission -and the opposite trend in the VAC is observed. The results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions.Parametric studies of short atmospheric pressure argon arc with tungsten electrodes were performed for various current densities and inter-electrode gap sizes. Non-equilibrium effects in the near-electrode regions were studied. Anodes with and without water cooling were considered. Effect of electron emission on current-voltage characteristic of the near-anode layer was investigated. Analytical formulas for scaling of non-equilibrium regions widths and Volt-Ampere's characteristics of these regions and the whole arc are given in the accompanying paper 44 .The organization of the paper is as follows. In Section II governing equations and boundary conditions for plasma and electrodes are presented. Section III describes numerical procedure of solving the governing equations. Results of simulations including validation of the model and parametric studies of the arc are presented and discussed in Section IV. Conclusions of this work are summarized in Section V.

Synthesis of nanoparticles in carbon arc: measurements and modeling

“…The plasma model was benchmarked in Ref. [42] by comparison to the results of previous numerical studies [41] and in Ref. 43 by comparison to analytical solutions.…”

Alternative model of space-charge-limited thermionic current flow through a plasma

“…Emission from the anode is not considered since it would get reflected promptly back to the anode in the inverse mode. (In classical and SCL modes, emission from the anode can enter the plasma and play a significant role discussed by Levko [38] and Khrabry et al [39]. )…”

Investigation of the short argon arc with hot anode. II. Analytical model

“…In this section, system of transport equations describing species transport and heat transfer in the arc is presented. Full set of governing transport equations for the arc was already given in the first paper of this series 7 and in Ref. [22] with proper description and derivation.…”

“…Parametric studies of the atmospheric pressure argon arc for various current densities and interelectrode gap sizes were performed in Ref. [7]. It was shown, in particular, that the different arc regions are rather autonomic even in case of short arcs (weakly depend on the arc length) and can be considered separately.…”

“…It was shown, in particular, that the different arc regions are rather autonomic even in case of short arcs (weakly depend on the arc length) and can be considered separately. Based on the results of the simulations performed and presented in the first paper 7 , the current study reports self-consistent analytic models of the near-electrode regions and the arc column combined into a unified self-consistent analytical model of the whole arc. Non-equilibrium processes in plasma and effects of near-electrode space-charge sheaths are taken into account.…”