Parametric study of a vortex-enhanced supersonic inductive plasma torch

Abstract

The feed gas injection configuration in radio-frequency (RF) inductively coupled plasma (ICP) torches plays a critical role in discharge stability, gas heating, and device thermal management: particularly if a supersonic nozzle is used to subsequently accelerate the hot gas. A novel injection configuration is the bidirectional vortex, which segments the internal ICP flow field into two counter-propagating vortices that can significantly enhance gas heating and reduce heat losses. The diameter of the interface between the vortices (known as the mantle) is expected to be an important dimensional parameter affecting torch operation, especially relative to the nozzle size. In this work, we investigate the effect of nozzle throat diameter on the behaviour and performance of a vortex-enhanced supersonic ICP torch. The system is operated at RF powers and argon mass flow rates between 200–1000 W and 0–400 mg s−1 respectively, and different nozzle diameters ranging from 1.5 to 4 mm are explored. Because of the high-temperature environment, and to prevent disruption of the vortex flow fields, non-invasive diagnostics are used to measure the gas temperature and plasma density, and to infer the torch thermal efficiency and achievable gas specific enthalpy change. The maximum temperature is between 8500–9500 K with the 1.5 mm nozzle giving the highest temperature for a given power and mass flow rate, while plasma densities vary between 1020–1021 m−3 depending on the operating conditions. The thermal efficiency increases from 29% for the 1.5 mm nozzle to just above 70% for the 4 mm nozzle with a similar maximum specific enthalpy of around 1.5 MJ kg−1. These results demonstrate the important coupling between torch properties, and how system optimization can lead to tailored performance of potential interest to several ground and space-based applications.