Experimental and numerical investigations on the operational characteristics of a hollow electrode plasma torch with an exit nozzle and reverse polarity discharge structure

A plasma torch consisting of a rear hollow electrode, a cylindrical front electrode and a nozzle, all of which has a diameter of 10 mm, was designed and its operational characteristics in the electrical connection of reverse polarity were investigated experimentally and numerically. Assuming that the cathode spot is formed on the end part of the front and the arc plasma inside the torch is laminar flow, numerical analysis was carried out, which agreed well with the experimental results. For example, both results showed that the arc voltages and thermal efficiencies of the torch were increasing linearly when decreasing the arc current or increasing the flow rate of G1 gas, which is introduced between the rear and front electrodes. However, G2 gas injected between the front electrode and exit nozzle has little effect on the arc voltages and thermal efficiencies of the torch. This indicates that when the cathodic arc roots are anchoring on the end of the front electrode due to the electrical connection of reverse polarity, G2 gas primarily affects their axial movement into the nozzle section rather than arc length or temperature fields inside the electrodes, playing a role in stabilizing the plasma jet. As a result, the proposed plasma torch can produce a stable N₂ plasma jet with the thermal efficiency of the torch higher than 70% and the arc voltages higher than 250 V at the arc currents lower than 70 A and total gas flow rates of 50 L/min.