Abstract
This study presents an analysis of a three-dimensional unsteady two-temperature simulation of atmospheric pressure direct current electric arc inside a commercial cascaded-anode plasma spray torch; it coupled the arc model with the torch electrodes and used an open-source computational fluid dynamics software (code_saturne). The previously published models of plasma spray torch either deal with conventional plasma torches or assume local thermodynamic equilibrium in cascaded-anode plasma torches. The paper presents the computation of the two-temperature argon plasma properties, compares two enthalpy formulations that differ in association of the ionization part of enthalpy and finally demonstrates the influence of the radiation heat loss data by comparingthe results for two different literature sources. It is the first to compare different enthalpy formulations in the context of plasma torch and discuss the differences in terms of the enthalpy gains and losses. It also explains why an unphysical simulation artifact of electron temperature lower than the heavy species temperature can occur in simulated plasma flow. The solution, then, consists in associating the ionization part of enthalpy to electrons and selecting the appropriate source of the data of radiation heat loss. However, negligible thermal non-equilibrium persists even in the hot core of electric arc, which ensures that the heavy species are heated up by collisions with electrons. The flexibility of the open-source software allows all the necessary modifications and adjustments to achieve satisfactory simulation results. Thus, the paper could be considered as a manual for development of a plasma spray torch model.
Highlights
Numerical simulations assuming departures from local thermodynamic equilibrium (LTE), especially close to the electrodes, are relevant so that two-temperature (2T) models should be applied to explore the processes inside a nontransferred arc direct current (DC) plasma torch under atmospheric pressure
The study deals with the commercial plasma spray torch SinplexProTM manufactured by Oerlikon Metco shown in figure 1
If Te < Th the exchange term is assumed to be zero. As it will be shown in the following, with some configurations of the 2T model, the predicted electron temperature can be lower than the heavy species temperature over a significant area of the simulated plasma flow, which was not observed experimentally (Murphy 2002)
Summary
The core of the plasma flow generated in such plasma torches can be considered as thermal plasma due to its high temperature, operation at atmospheric pressure and, a sufficient collision rate between plasma species. In Trelles et al (2007) the importance and results of consideration of thermal non-equilibrium and its superiority over the LTE assumption were demonstrated in the context of conventional plasma torch simulation. The study (Liang and Groll 2018) of a conventional plasma torch with non-equilibrium plasma coupled with electrodes demonstrated the importance of taking into account the cathode sheath model in general and the cathode sheath voltage drop in particular for the accuracy of the torch model predictions.
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