Experimental investigation on heat transfer of an impinging jet of supercritical CO₂ and comparison with liquid and gaseous CO₂

In this study, a CO₂ circulation loop system is constructed to investigate the heat transfer of an impinging jet under various CO₂ states, including liquid, gaseous, and supercritical states (sCO₂). In order to evaluate the quantitative effect of the CO₂ states on heat transfer, mass flow rate, temperature and pressure of the jet and chamber, and heat flux of the impinging plate are systematically varied. In subcritical states, the Nusselt number exhibits consistent behavior along the radial direction from the stagnation point, irrespective of variations in the heat flux. The dimensionless Nusselt number (function of Reynolds and Prandtl numbers) of subcritical CO₂ shows that heat transfer predictions are possible within an average error of 9.1% compared to previous research. In contrast, the heat transfer with sCO₂ exhibits a different behavior, especially near the pseudo-critical temperature. Despite evaluating the jet, wall, and bulk properties alongside the dimensionless Nusselt number, they do not converge to a single line. Nevertheless, the dimensionless Nusselt number increases by a maximum of 264.7% compared to subcritical CO₂. Our findings offer valuable insights for designing and optimizing heat transfer systems using sCO₂, which are critical for advancements in power cycles and nuclear fusion/fission technologies.