Bidirectional vortex stabilization of a supersonic inductively coupled plasma torch

Abstract

Radio-frequency (RF) inductively coupled plasma (ICP) torches using a supersonic nozzle have many industrial materials processing applications and have also been proposed as novel electrothermal plasma thrusters for space propulsion. The gas injection method in plasma torches plays an important role in both gas heating dynamics and overall discharge stabilization. Here, we investigate reverse vortex gas injection into a supersonic ICP torch for RF powers up to 1 kW, argon mass flow rates between 15-180 mg/s, and plasma torch pressures from 270 Pa to 50 kPa. In this configuration, gas is injected tangentially just upstream of the nozzle inlet. This produces a bidirectional vortex flow field where gas first spirals upwards along the outer edge of the plasma torch walls, before then reversing direction at the torch end and spiralling back down through the central plasma region towards the nozzle exit. Results are compared to a more conventional forward vortex configuration where gas is instead injected tangentially from the upstream end of the torch, and which forms a unidirectional vortex that spirals towards the downstream nozzle. While performance is similar for gas flows below 80 mg/s, we show that at higher mass flow rates both the effective torch stagnation temperature and thermal efficiency can be increased by almost 50% with reverse vortex injection. Considering that the measured RF antenna-plasma power transfer efficiency is similar for both configurations, this enhancement occurs because of the unique bidirectional vortex flow field which leads to reduced gas-wall heat losses and consequently an increased enthalpy flow leaving the torch.

Parametric study of a vortex-enhanced supersonic inductive plasma torch

“…For an RF power of 800 W and argon mass flow rates above 80 mg s −1 , the use of bidirectional vortex injection was observed to increase the torch stagnation temperature and thermal efficiency by almost 50% compared to conventional gas injection methods. Additionally, the bidirectional vortex was able to achieve a large gas specific enthalpy change of 1.5 MJ kg −1 even at high thermal efficiencies, whereas by comparison, the specific enthalpy change with conventional injection configurations was much lower and also peaked at lower thermal efficiencies [6]. This strong improvement in performance was correlated with the unique bidirectional vortex flow which leads to a reduction in heat losses at the bounding walls.…”

“…The plasma torch used is similar to that previously described [6]. In brief, it consists of a 7-turn copper coil wrapped around two concentric alumina tubes that connect to top and bottom aluminium end caps.…”

“…Depending on the specific application, the downstream end of the ICP tube can be terminated by a subsonic or supersonic nozzle. In this latter case, the gas flow chokes at the nozzle throat and is then accelerated in the diverging section of the nozzle to generate a high-speed supersonic gas flow [6].…”

“…In previous work [6], we used a supersonic RF ICP and investigated the effect of different feed gas injection configurations on the system stagnation temperature, thermal efficiency, and gas specific enthalpy change. In particular, a novel injection method referred to as a bidirectional vortex [37,38] was tested and compared with conventional injection configurations.…”

“…In this work, we perform a characterization of a vortexenhanced supersonic ICP torch and investigate the effect that different nozzle sizes have on the plasma and gas properties, as well as the overall device performance. In our previous work [6], a comparison of torch performance with conventional and bidirectional vortex gas injection configurations was performed for a single nozzle size. Our present work is differentiated from this by now only focusing on bidirectional vortex injection and explicitly investigating the effect of different nozzle sizes with such a gas injection method.…”