Treatment of non-equilibrium phenomena in thermal plasma flows

Abstract

Thermal plasma flows provide a uniquely high specific enthalpy source that is well suited to transformation of matter, often via phase changes. As a consequence, numerous thermal-plasma-based processes have been developed to, for example, destroy pollutants, modify surfaces (e.g. cutting and welding), synthesize nanostructures and deposit functionalized nanostructured coatings. In many cases, departures from equilibrium (both thermal and chemical) occur in regions of such plasmas; for example, in electrode erosion phenomena or in the injection of a liquid into a plasma jet. This paper reviews the treatment of non-equilibrium phenomena in thermal plasma flows, in particular the methods of calculation of the composition and transport coefficients of non-equilibrium plasmas, which are required for modelling the above processes. The focus is on two-temperature plasmas, in which electrons and heavy species are at different temperatures. Methods of calculation of the composition of plasmas both in local chemical equilibrium (LCE) and out of LCE are presented. A comparison of the different methods shows large discrepancies, even assuming LCE. Two-temperature transport coefficients obtained from simplified expressions, from the modified Chapman–Enskog method and from the Stefan–Maxwell relations are presented, as well as examples focusing on the influence of plasma composition. Different methods of calculation of the collision integrals required in determining the transport coefficients are also reviewed. Particular attention is paid to diffusion, in particular to the combined diffusion coefficient method, which simplifies treatment of plasmas in LCE. The method of calculation of the reactive thermal conductivity and the influence of excited states on transport coefficients are also addressed in some detail.

Electrical potential difference during laser welding

“…We note here that the application of this relation does not necessarily require the species involved to have the same temperature (see e.g. [7] for an in-depth discussion) and use where Q Z is the partition function of the ionization state Z, n e , n Z and n Z+1 are the number densities of electrons, ions of charge state Z and ions of charge state Z+1, respectively, and W denotes the ionization energy from Z to Z+1. Furthermore, m e is the electron mass and h Planck's constant.…”

3D static and time-dependent modelling of a dc transferred arc twin torch system

“…The equations for the 2T thermal non equilibrium model are summarized as follows (see [1,10] for details). Mass conservation and Navier-Stokes equations are the same as LTE model while separate energy equations for the species are needed.…”

“…Unfortunately the results shown in [9] predict a higher voltage with respect to the values usually found experimentally, probably to due the LTE model underestimation of electrical conductivity in the cold regions. It is known that also at atmospheric pressure thermal and chemical non equilibrium may occur [10], leading to an overpopulation of electrons in cold regions, where LTE model predicts almost no ionization, and then to an increase of electrical conductivity. In a conventional transferred arc the non equilibrium effects are mainly located in the arc fringes [11] and have almost no influence on the arc voltage, limiting the necessity of 2T calculations.…”

Three-dimensional modelling of arc behaviour and gas shield quality in tandem gas–metal arc welding using anti-phase pulse synchronization

“…To take all the effects into account would require treating the transport and reactions of each plasma species separately. This would inevitably introduce inaccuracies because of the lack of available reaction data and uncertainties about appropriate calculation of thermodynamic and transport properties [27,28].…”