Non-equilibrium modelling of an oxygen-plasma cutting torch

Abstract

A two-temperature, axi-symmetric, chemical non-equilibrium model has been developed for an oxygen-plasma cutting torch in two dimensions to obtain distributions of different plasma quantities inside the torch. Apart from mass, momentum and potential conservation equations, separate energy balance equations are considered for electrons and heavy particles. The κ–ε model has been used to account for turbulence. Non-equilibrium properties required for fluid dynamic simulations are obtained from a non-equilibrium property code that includes chemical non-equilibrium. The results show distributions of temperature, velocity, pressure, potential, current density and different species densities inside the plasma torch for an arc current of 200 A. Plasma pressure inside the torch varies from several atmospheres to near-atmospheric pressure. It has been observed that the electron and the heavy particle temperatures differ less near the axis of the torch and appreciably near the wall. Interesting features, observed for other investigated quantities, found consistent with the recent experimental observations are discussed.

On the Double-Arcing Phenomenon in a Cutting Arc Torch

“…11, right) shows that the signal has a very strong component at 150 Hz, which is the fundamental frequency of the power source ripple. This experimental result shows that the NLTE plasma inside the nozzle of a cutting torch results strongly affected by the arc voltage ripple, and thus it is far from the steady state, as it is usually assumed in the literature (e.g., Gonzalez-Aguilar et al, 1999;Freton et al, 2002;2003;Ghorui et al, 2007;Zhou et al, 2009, Guo et al, 2010. Such dynamics could explain the mechanism of formation of the non-destructive double arcing; as it is shown with detail in Fig.…”

“…To do this, the following assumptions were made: 1) a linear variation of the pressure (p) and V p on z, 2) a constant value for T e in the axial direction (an average along the nozzle length) at a radial position close to the nozzle wall. Assumptions 1) and 2) have been shown to be valid in cutting torches Freton et al, 2002;Ghorui et al, 2007). In any case, the obtained solution is almost insensitive to the particular chosen variations of p since the dependence of the integral in equation (4) Fig.…”

“…However, most of the developed numerical simulations are based on the plasma local thermodynamic equilibrium (LTE) assumption (Freton et al, 2002(Freton et al, , 2003Gonzalez-Aguilar et al, 1999), in spite of the fact that substantial deviations from LTE should occur at the arc boundary inside the torch, where the electron density is presumably much lower than that prescribed by the Griem´s criterion for LTE equilibrium (Boulos et al, 1994); and where very high temperature gradients may be present over the last few electron Debye lengths from the nozzle wall. Only recently, a non-local thermodynamic equilibrium (NLTE) modelling of a 200 A oxygen-plasma cutting torch was presented (Ghorui et al, 2007). In this work, it was shown that the electron temperature remained high near the nozzle wall and hence well decoupled from the heavy particle temperature.…”

On the influence of the nozzle length on the arc properties in a cutting torch

“…Although the effects of non-LTE have been studied in cutting torches [5,7] a theory based on LTE can in general give satisfactory temperature predictions when compared with experiments [1].…”

On the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon

“…Assumptions (i) and (ii) have been shown to be valid in cutting torches. 15,16 In any case, the obtained solution is almost insensitive to the particular chosen variations of p since the dependence of the integral in Eq. (3) with n n is very weak (leading to a dependence of the kind n n 1/8 ).…”