Radiative characteristics and heat transfer regime transitions in CO2-based thermal plasma jets

This experimental study examines the radiative heat transfer of CO₂-operated thermal plasma plumes based on axial measurements of radiative intensity. Narrow-Angle Radiometry (NAR) and Infrared Thermography (IR) were employed to characterize radiative intensity and plume morphology from a 300 kW el plasma generator (PG) under varying arc current and CO₂ flow conditions. Peak intensities at 5 mm from the nozzle exceeded 100 kW/m 2 ·sr. Increasing arc current displaces the localized near-nozzle high-intensity emission zone axially while the NAR probes remain at fixed positions, such that the recorded intensities decrease not due to reduced plume energy but due to the shift in peak emission location. IR imaging confirmed that despite this decrease in line-of-sight radiative intensity, the plume remained thermally energetic, with detector counts from a tungsten reference rod placed downstream within the plasma plume increasing systematically with specific gas enthalpy, establishing a clear transition from radiation-dominated near-field heating to convection-dominated downstream energy transport. IR imaging shows a radial constriction of the plume referred to as necking, with neck diameters decreasing from 19 mm at 150 A to 6 mm at 300 A. An idealized spectral radiative transfer analysis is additionally presented, demonstrating how the measured NAR intensities can be used to examine possible gas temperature fields and CO₂ dissociation states consistent with the experimental observations. The results provide a well-defined experimental benchmark for future modelling of CO₂ plasma heat transfer in high-temperature industrial processes such as rotary kilns.