1. IntroductionThere exists a very thin transition domain between the arc plasma and the workpiecein TIG welding (D.C., workpiece-positive). This domain is called as the anode boundary layer within which there are very steep temperature gradient and various transportphenomena, such as energy, momentum and mass t.ansfe.11>. The complicated physicalprocesses occurred in the anode boundary layer directly dominate the values and distribution of current density and heat flux on the workpiece surface, and have great effect onthe welding heat input, size and geometry of weld pool, the microstructure and propertiesof weld joint and so on. Thus, conducting deep investigation on the transport mechanismin the anode boundary layer is of great theoretical significance and engineering practicalimportance for welding process analysis and quality control.The anode boundary layer combines the welding arc and the weld pool together, andacts as a key factor in determining the welding thermal efficiency. UP to now, there isstill a lack of understanding of the physical phenomena in the anode boundary layer
.Some researchers focus on numerical analysis of fluid flow and heat transfer inside the weldpool, and consider the arc action to weld pool as only a boundary condition and assumethe Gaussian function-type distribution of current density and heat flux on the weld pool..,fac.I4'5>. On the other hand, other researchers put more emphasis on calculation andmeasurement of electromagnetic field, fluid and heat flow in the arc column itself, withoutaccounting for the anode boundary layer between the arc and the weld pooh'l'>. In thispaper, the physical transport phenomena in the anode boundary layer are studied for moreefficielltly employing arc plasma and precisely analyzing and controlling welding processes.2. Governing EquationsAs shown in Fig.1, the anode region is divided into three subzones: the anode boundary layer, the presheath and the sheath. Inthe boundary layer, the presence of the relatively cold anode is felt. The boundary layeris characterized by steep gradients of temperature and particle densities. The boundary layer thickness is in the order of 0.lmmwhich is much larger than the particle meanfree-path length. The main feature of theboundary layer is the ionized gas in the layermay be treated as a continuum and as a true plasma. Very close to the anode (in the orderof one electron mean-free-path), the usual continuum approach is no longer valid. Theionized gas in the presheath and in the sheath may not be treated as a continuum becausethe total thickness of the presheath and the sheath is equal to one electron mean-free-path.In the presheath, the neutrality of the plasma is maintained. In the Debye sheath, theneutrality is broken and sharp potential drop occurs. The sheath is formed immediately infront of the anode accommodating the transition from electrical conduction in the plasmato metallic conduction in the anode. The thickness of the sheath is in the order of theDebye length. In this paper, a model for the anode boundary layer is firstly introduced.Due to its small thickness, the anode region may be treated as a one-dimensional region.For the purpose of solving the conservation equations, the plasma parameters are assumedto vary only in the direction perpendicular to the anode surface. In the boundary layer, thecoatinuum approach and the condition of charge neutrality are valid. Since the electrontemperature (Te) may be higher than the heavy particles (atoms and ions) temperature(Z), electrons and heavy particles are regarded as two separate fluids coexisting in theplasma.In a three-component system (electrons, atoms and ions), the electron flux may bewritten aswhere ne is the-electron number density3 6e is the electron drift velocityl be is the electron mobility) E is the electric field strength, kB is the Bolzmann constant, and e is theelementary charge. The electron flux is driven by the potelltial gradient (E = -- V gb),
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