Unified numerical predictions of free burning arc properties in nitrogen

Ng, Whui Liang (2001) Unified numerical predictions of free burning arc properties in nitrogen. University of Southampton, School of Engineering Sciences, Doctoral Thesis.

Abstract

In this thesis, a unified self-consistent fluid model, which provides an integrated simulation of entire arc region but omitting the space charge effects in the sheath regions, is established to study the physical behaviour of a high intensity free burning arc in nitrogen, as in the arc phenomena associated with low voltage switching devices. The present unified model differs from the previous ones in that the pre-sheath layers and the arc column, including the contraction regions, are treated as a continuum in the computational domain. The study is based on physical equations describing chemically reacting, thermal non-equilibrium flow of a multi-component, fluid, in which the electrons and heavy particles are regarded as two co-existing continuum fluids characterized by two independent temperatures. The computation of the arc properties across entire arc region are performed in a unified manner. One of the major advantages of this approach is the elimination of complicated numerical processes associated with the requirement of different treatments of the sub-models across the entire arc. The effects of chemical reactions of all plasma constituents and convection due to the induced macroscopic fluid flow are explicitly included in the solution of the species continuity equations. The required transport coefficients are derived in the course of computation based on the Chaprnan-Enskog formulations. An approximate mixture method for the transport coefficients, including a self-consistent effective binary diffusion, are treated. With appropriate boundary conditions, self-consistent solutions to physical equations are derived using appropriate numerical algorithms via an operator splitting sequence.
Calculations show that thermal non-equilibrium has a dominant influence in determining the derived arc voltages, at least for low current arcs. The solutions of temperature field indicate that the conventional one-temperature fluid model could over-predict the arc temperature under some circumstances. The effects of convection due to the electromagnetic effects have substantial influence on chemical composition of the arc plasma, and hence on the predicted arc characteristics. The deviation from chemical equilibrium is likely because the chemical reactions cannot follow the rapid macroscopic motion of the species. The actual values of the transport properties derived from the numerical model in a non-equilibrium flow field may be very different from the one obtained from local thermodynamic equilibrium database tables.

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