Experimental and Numerical Analyses of a Hollow Electrode Plasma Torch with Inter-electrodes and Reversed Polarity Discharges

As a novel heat source available for high voltage - high power operation at high thermal efficiency, a hollow electrode plasma torch with inter-electrodes was designed in reverse polarity and tested to provide basic performance data. From the regression analysis of the test results, first, a V -I characteristic equation for the designed torch was obtained as V = 2364L^0.61535˙^0.104753I^‑0.0636 for arc current I, N₂ mass flow rate m˙ and torch length L. As described by this V-I characteristic equation, arc voltages of the designed torch were observed to primarily depend on torch length while the influence of arc current was relatively limited. These effects of inter-electrodes insertion were confirmed by numerical simulations, which also clarified the influences of inter-electrode diameter on the arc voltage and the thermal efficiency of the proposed type plasma torch. Due to the proportional increase of arc voltages to the torch length, the high voltage - high power conditions were obtained by increasing the number of inter-electrode, which leads to the generation of high enthalpy plasma jet at high thermal efficiency. For example, a hollow electrode plasma torch with three inter-electrodes (L = 0.68 M) recorded the power level of 525 kW at the arc voltage of 880 V during the reversed polarity discharge, providing a plasma jet with high averaged specific enthalpy of 15.8 MJ/kg at thermal efficiency higher than 65%.