Three-Dimensional, Time-Dependent Model of DC Plasma Torches

Summary form only given. A key aspect of the behavior of non-transferred DC arc plasma torch is the stochastic aspect of arc formation inside the nozzle. This phenomenon results in instabilities in the jet flow that are enhanced by the turbulent mixing with the ambient gas when the plasma jet issues in the quiescent surrounding gas. In addition, the wear of electrodes contributes to make the flow time-dependent. The main applications of such plasma torches include synthesis of advanced materials, recycling or destruction of hazardous waste materials, production of hydrogen gas from biomass, metallurgy and plasma spraying. The latter is a well-established method for manufacturing protective coatings and free-standing shapes of a wide range of alloys and ceramics. However, the fluctuations of arc voltage and thermal energy input resulting from the continuous movement of the anode arc root introduce an undesirable element in the spray process as it affects the acceleration and heating of the particles injected in the gas flow. A reliable model that predicts the dynamic behavior of the arc in terms of system geometry and process parameters can help to supply an insight into the time-performance of the process and optimize the spraying conditions. This model must involve the interaction between electric, magnetic, fluid dynamics and thermal phenomena. It is still a topical and challenging subject that has received a renewed attention during the last years. This talk addresses the modeling of the time-dependent conversion from electrical to thermal energy in the plasma-forming gas and of the resulting fluctuating plasma jet flowing in air. The 3-D transient model of the arc column and its attachment on the anode wall is based on the simultaneous solutions of the conservation equations of mass, momentum, energy and current and the electromagnetism equations. It makes it possible to predict the motion of the anode attachment root on the anode surface as well as the heat lo- d input to the anode surface at the spot location. The heat flux to anode is afterwards used as an input data for a 1-D enthalpy formulation model of anode heating that predicts the time-evolution of the thickness of the liquid layer on the surface at the anode spot and the quantity of vaporized material. The transient output data of the arc model are also used as input data for the computations of the time-dependent flow fields outside the nozzle and behavior of the powdered material injected in the flow.