Abstract
In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions.
Highlights
Ar–H2 mixtures are widely used in experimental devices and industrial processes, such as for example in plasma spraying, because they provide to particles injected in the plasma jet, on the one hand, a sufficiently high momentum through the argon gas, and on the other hand, a high heat transfer through the hydrogen gas [1]
Hydrogen affects fundamental mechanisms like diffusion and recombination while this is less relevant under atmospheric conditions at equilibrium
The investigation of argon–hydrogen plasma gas mixtures at low pressure showed that already small hydrogen contents can change the plasma jet characteristics considerably
Summary
Ar–H2 mixtures are widely used in experimental devices and industrial processes, such as for example in plasma spraying, because they provide to particles injected in the plasma jet, on the one hand, a sufficiently high momentum through the argon gas, and on the other hand, a high heat transfer through the hydrogen gas [1]. Plasma Chemistry and Plasma Processing (2021) 41:109–132 hydrogen permits to increase the voltage and the plasma power since the plasma arc in the torch adjusts itself to a size which minimizes the heat flux from the core to the cooled anode wall. This arc constriction is controlled mainly by the thermal conductivity of the plasma gas and by the amount of hydrogen admixture. The low electron densities imply a disposition to thermodynamic non-equilibrium Under such conditions, hydrogen affects fundamental mechanisms like diffusion and recombination while this is less relevant under atmospheric conditions at equilibrium
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