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A two-temperature (2T) thermal non-equilibrium model is developed to address the thermal nonequilibrium phenomenon that inevitably exists in the reverse-polarity plasma torch (RPT) and applied to numerically investigate the plasma flow characteristics inside and outside the RPT. Then, a detailed comparison of the results of the 2T model with those of the local thermal equilibrium (LTE) model is presented. Furthermore, the temperature of the plasma jet generated by a RPT and the RPT's voltage are experimentally measured to compare and validate the result obtained by different models. The differences of the measured excitation temperature and the arc voltage between the 2T model and experimental measurement are less than 13% and 8%, respectively, in all operating cases, validating the effectiveness of the 2T model. The LTE model overestimates the velocity and temperature distribution of the RPT and its plasma jet, showing that thermal non-equilibrium phenomena cannot be neglected in the numerical modelling of the RPT. Unlike other common hot cathode plasma torches, the thermal non-equilibrium phenomenon is found even in the arc core of the RPT, due to the strong cooling effect caused by the big gas flow rate.

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The reverse-polarity plasma torch (RPT) is a promising plasma source for material processing, e.g. plasma atomization for spherical powder and plasma synthesis for nanostructured carbon, due to its high temperature plasma jet and thermal efficiency. However, its performance highly depends on the working stability of the RPT, which is undermined by the large-scale shunting. To improve the working stability of the RPT, a novel method for predicting the occurrence of the large-scale shunting is proposed for optimizing the RPT’s internal structure and operation conditions. The method is based on the thermal non-equilibrium modelling of the RPT to obtain the thickness of the cold boundary layer (CBL) and the breakdown voltage. Then, the occurrence of the large-scale shunting is predicted by comparing the breakdown voltage with the voltage difference between the electrode surface and the arc column. Based on the proposed prediction method, three different shapes of the front electrode (cathode) corresponding to different thickness of the cold boundary layer (CBL) were manufactured. Experimental and numerical study on the effect of the electrode geometry, the arc current and the gas flow rate on the working stability of the RPT and the thickness of the CBL were conducted. Results showed the quantitative correlations between the CBL thickness and the instability of the RPT and verified that the proposed numerical method is useful for optimizing the design and operation of the plasma torch with minimizing the large-scale shunting instabilities.