Determination of positive anode sheath in anodic carbon arc for synthesis of nanomaterials

. In the atmospheric pressure anodic carbon arc, ablation of the anode serves as a feedstock of carbon for production of nanomaterials. It is known that the ablation of the graphite anode in this arc can have two distinctive modes with low and high ablation rates. The transition between these modes is governed by the power deposition at the arc attachment to the anode and depends on the gap between the anode and the cathode electrodes. Probe measurements combined with optical emission spectroscopy (OES) are used to analyze the voltage drop between the arc electrodes. These measurements corroborated previous predictions of a positive anode sheath (i.e. electron attracting sheath) in this arc, which appears in both low and high ablation modes. However, the positive anode sheath was determined to be ~3-8 V, significantly larger than ~0.5 V predicted by previous models. Thus, there are apparently other physical mechanisms not considered by these models that force the anode sheath to be electron attracting in both ablation regimes. Another key result is a relatively low electron temperature (~ 0.6 eV) obtained from OES using a collisional radiative model. This result partially explains a higher arc voltage (~ 20 V) required to sustain the arc current of 50-70 A than predicted by existing simulations of this discharge.

Modeling of Free-Burning Arc and Effects of Boundary Conditions on the Anode Temperature Field

A two dimensional numerical model of the free-burning arc and its interaction with anode are given. The commercial CFD code FLUENT is used to model the plasma and the solid anode part. The anode sheath is considered, as well as the heat transfer mechanism of the anode surface. The second boundary condition of heat conduction is introduced to give a more reasonable cooling boundary, so that the temperature distribution in the anode plate is more realistic. Through iteration calculation of steady MHD equations, the temperature field, pressure field and velocity field profiles are given. The result is of significant use to analysis thermal and control the electric power in the arc industrial applications.