Thermal investigation of sCO2 flow cooling ability in a thin cylindrical tube

In the context of cleaner, safer, and more cost-effective machining processes, supercritical carbon dioxide (sCO₂) has emerged as a promising alternative to conventional cutting fluids, particularly for drilling titanium alloys such as Ti-6Al-4V. The dual role of sCO₂, simultaneously providing localized cooling and assisting chip evacuation, underlines its potential to extend tool life, reduce cutting forces, and improve overall process sustainability. Nevertheless, the optimal operating conditions for sCO₂ jets in drilling remain undefined, and the direct influence of jet morphology on heat transfer mechanisms is not yet fully clarified. The present study intends to contribute to this understanding through an experimental investigation of the thermal performance of sCO₂ jets. Results show that the jet structure is essentially dependent on the upstream conditions. Due to the complicated jet structure under these conditions, the nozzle-to-plate distance is an important parameter, and the evolution of the supersonic structure in function of the nozzle-to-plate distance is evaluated. The cooling behavior is found to be directly related to the structure of the underexpanded jet and thus to the upstream parameters and the impinging distance. To investigate this phenomenon, high-speed Schlieren imaging was carried out under varying initial pressures and temperatures. The visualizations provided detailed insights into the jet structure and revealed the presence of three successive regions: laminar, transitional, and turbulent. Infrared images are also used to evaluate the jet cooling ability in a hot cylindrical tube for varying upstream conditions. Building on these results, future work will focus on the influence of the expansion distance on the jet cooling efficiency.