Three-Dimensional Steady State Numerical Simulation of Non-Transferred DC Arc Plasma Torch
Cheng Wenbo1, Wu Jin1, Wu Nan1, Huang Qiping1, Fan Junxin2
1. College of Electrical Engineering Naval University of Engineering Wuhan 430033 China; 2. School of Electrical Engineering Southeast University Nanjing 210096 China
Abstract：The non-transfered DC arc plasma torch is the core equipment in the radioactive waste plasma melting treatment system. The physical field parameters of the jet are not only an important index for the design and optimization of the plasma torch, but also the key to improve the system energy utilization rate and the quality of the vitreous products. Aiming at a plasma torch with cascade anode structure, based on the theory of magnetohydrodynamics, this paper uses the finite element software COMSOL to conduct three-dimensional steady-state numerical simulation of the phenomenon of nitrogen hot plasma jet flowing into the cold nitrogen domain, establishes a multi-physical field coupling model of electric-magnetic-flow-heat, calculates the physical field parameters of the jet, and compares the calculated results with relevant experimental data. The effect of changing the input gas flow rate on the performance of plasma torch under the condition of fixed current intensity is analyzed. The results show that the model can predict the actual situation well, and the obtained data of jet temperature field, velocity field, electrode surface temperature and current density distribution can provide reference for the design and optimization of plasma torch. Firstly, the 3D structure diagram of the plasma torch is accurately drawn in COMSOL, and the materials are defined in different areas. Secondly, based on the theory of magnetohydrodynamics, relevant assumptions and mathematical equations are listed, which include electromagnetic field, fluid and heat transfer. Thirdly, the boundary conditions of the model and the artificial conductivity of nitrogen plasma in different regions are introduced in detail, and some specific Settings are explained. Finally, the experimental platform is built, and the data obtained by a series of continuous experiments are compared with the simulation results. Comparing the experimental data with the simulation data, it is found that the calculated average voltage is very close to the actual measurement results but a little lower, and the error comes from the steady-state hypothesis. Thermal efficiency calculation is reliable; The temperature changes of the torch tube surface and section are very obvious, which indicates that the calculation accuracy can be improved by adding the "cooling water" calculation area to replace a class of boundary conditions in the model. The prediction accuracy of the datum point of jet temperature 3 410℃ is slightly higher than that of the two-dimensional model. By analyzing the simulation data, it is found that the pressure gradient and radial Lorentz force between the cold gas and the plasma are obvious. There are two maximum values of speed under all conditions. In the torch tube, the higher the gas flow rate, the lower the temperature, and the opposite is true outside the torch tube. The electrode boundary can be regarded as a "balanced discharge boundary heat source" to predict the electrode performance synchronously, and the temperature distribution data of the electrode is consistent with the ablation test results. The model can provide a reference for the design and optimization of DC arc plasma torches, and subsequent studies will be carried out on this basis, mainly focusing on the analysis of the influence of different variables on jet characteristics and thermal efficiency, including different types of working gases, intake modes, the combination of current intensity and gas flow conditions, and the combination of electrode structures with different shapes.
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2024, Vol. 39 
