Demixing in free-burning arcs

Demixing in atmospheric-pressure free-burning arcs is investigated using a two-dimensional numerical model that incorporates the combined diffusion coefficient treatment of diffusion. Arcs in mixtures of ar on with helium, nitrogen, oxygen, and hydrogen are modeled. It is found that demixing almost always has a large influence on arc composition, with the greatest changes occurring in the argon-helium and argon-hydrogen arcs. The influence of three different demixing processes is assessed. Demixing due to frictional forces is found to dominate in the high-temperature regions of the arc, while demixing due to mole fraction gradients is important in the regions where dissociation of the molecular gases occurs. Demixing due to thermal diffusion has a smaller effect. The effects of demixing on arc temperature and flow velocity are usually small. The heat flux to the anode is significantly increased near the axis by demixing in argon-nitrogen and argon-hydrogen arcs. The predictions of the model are validated by comparison with spectroscopic measurements of arc composition in argon-nitrogen and argon-helium arcs.