Comparative analysis of turbulent effects on thermal plasma characteristics inside the plasma torches with rod- and well-type cathodes

Abstract

The thermal plasma characteristics inside the two nontransferred plasma torches with rod type cathode (RTC) and well type cathode (WTC) are analyzed in conjunction with turbulent effects on them in the atmospheric-pressure conditions. A control volume method and a modified SIMPLER (semi-implicit pressure linked equations revised) algorithm are used for solving the governing equations, i.e., conservation equations of mass, momentum, and energy together with a current continuity equation for arc discharge. A cold flow analysis is introduced to find the cathode spot position in the WTC torch, and both the laminar and turbulent models are employed to gain a physical insight into the turbulent effects on the thermal plasma characteristics produced inside the two torches. The numerical analysis for an RTC torch shows that the slightly different values of plasma temperature and velocity between the laminar and turbulent calculations are resulted and the radial temperature profiles are constricted at the axis with increasing the gas flow rate, and that the large turbulent viscosities appear mostly near the anode wall. These calculated results indicate that the turbulent effects on the thermal plasma characteristics are very weak in the whole discharge region inside the RTC torch. On the other hand, the calculated results of the two numerical simulations for a WTC torch present that the significantly different values of plasma characteristics between the two models appear in the whole torch region and the plasma temperatures decrease with increasing the gas flow rate because the relatively strong turbulent effects are prevailing in the entire interior region of the WTC torch. From the comparisons of plasma net powers calculated and measured in this work, the turbulent modeling turns out to give the more accurately calculated results close to the measured ones compared with the laminar one, especially for the torch with WTC. This is because the turbulent effects are considerably strong in the WTC torch and subsequently govern the thermal plasma characteristics inside it.