Chemical nonequilibrium modelling of a free-burning nitrogen arc

Abstract

Thermal and chemical nonequilibrium modelling is performed to investigate the plasma characteristics of a nitrogen arc. The arc plasma is coupled with the electrodes self-consistently in the computational domain. The self-consistent effective binary diffusion coefficient approximation treatment of diffusion and a generalized form of Ohm’s law are incorporated in the model. It is found the electric field has to undergo a reversal in front of the anode to preserve current conservation due to the strong diffusion current density. No field reversal is found in front of the cathode. The detailed chemical reaction processes are analyzed to understand the species density behaviour. Along the cathode surface, the maximum values of current density and heat flux density occur at the intersection of the flat tip and the conical surface. The distributions of different components of the current density and heat flux density change with the arc current. On the anode side, the electric field and diffusion components contribute to the current density, and have opposite signs and similar magnitudes on the axis, leading to an off-axis maximum of the current density. The electron condensation heat makes the largest contribution to the total heat flux in the high current density region, and the heavy-species contribution from the plasma becomes important in the arc fringe. The high heat flux and current density at the anode show that the nitrogen arc possesses excellent energy source properties for arc welding and other processes that require workpiece melting.

Non-equilibrium modeling on the plasma–electrode interaction in an argon DC plasma torch

“…Because the spacecharge sheath is considered as an interface between the plasma and electrodes in this study, the space-charge sheath characteristics are revealed by the appropriate boundary conditions imposed at the electrode-plasma interface. This method has already been adopted and validated in the transferred arc simulations, such as short carbon arc and free burning arc [23][24][25][26][27][28]. The flux boundary conditions applied at the interface between the plasma and electrodes are as follows.…”

“…Many numerical methods have been applied to consider the plasma-electrode interactions in arc plasma simulations over the past several decades [18][19][20][21][22][23][24][25][26][27][28][29][30]. The early unified approach with account of arc-electrode interaction consists of an local thermodynamic equilibrium (LTE) bulk plasma and a cathode layer where the diffusion equation is solved to provide ionization layer features [18][19][20].…”

“…Then, the thermal non-equilibrium model was used to describe the bulk plasma and the spacecharge sheath was included in the model, which is called the 2T-ionization layer-sheath model [21,22]. Recent and more advanced models were based on the chemical and thermal nonequilibrium model, in which the ionization layer was included in the bulk plasma [23][24][25][26][27][28]. The space-charge sheath features are described by imposing appropriate boundary conditions at the electrode-plasma interface.…”

Numerical simulation of hydrogen arcjet thruster with coupled sheath model

“…Lun et al [17] developed a model for the vacuum-arc cathode-spot and plasma region to predict the performance of vacuum-arc thrusters operating roughly in the arc current range of 80-300 A. Lun et al [17] reported the conventional-based cathode-spot model with many simplifying assumptions predicted spot radius, surface temperature, electric field, current densities, plasma densities, and energy fluxes. The most recent research conducted in the area has been done by Wang and his co-workers, where the plasma flow feature [18] and species diffusion [19] in a low-power nitrogen-hydrogen arcjet and plasma characteristics of a nitrogen arc [20] were studied by simulation of non-equilibrium arcjets to better understand the arcjet physics. Shen et al [21] studied the starting-up process of arcjet thruster with arc voltage signals.…”

Numerical simulation of anode heat transfer of nitrogen arc utilizing two-temperature chemical non-equilibrium model*

“…[10,14] However, the plasma near electrodes obviously deviates from LTE [15][16][17] and LCE, [18,19] thus the thermal and chemical non-equilibrium model is required to study the anode heat transfer of nitrogen arc. In our previous paper, [20] a thermal and chemical nonequilibrium model has been developed for nitrogen arc, but we did not focus on the anode heat transfer characteristics. Moreover, in the previous studies, [21,22] the space charge sheath can significantly affect the plasma energy flux and species flux on the anode surface.…”