Optical visualization of heat transfer in supercritical carbon dioxide under near-critical, liquid-like, and gas-like conditions

Heat transfer in supercritical carbon dioxide (sCO₂) is experimentally visualized, and a measurement method is proposed for evaluating the transport phenomena in near-critical, liquid-like, and gas-like conditions. There are various uses for sCO₂ in engineering applications, such as abstraction, material processing, and soil remediation. However, the heat and mass transfer under supercritical conditions have not been fully revealed, and innovative measurement techniques with higher spatial and temporal resolutions are required. This study focuses on the evaluation of heat transfer in sCO₂ using a high-speed phase-shifting interferometer. The density distribution of sCO₂ under different temperature and pressure conditions is successfully visualized with the proposed interferometer. Characteristics of the density field patterns are observed near the critical point and in liquid-like and gas-like conditions. It is demonstrated that the sensitivity of the density (i.e., refractive index) to temperature changes is different for each condition. The transient heat transfer under gas-like condition is evaluated by the interferometer, and numerical simulations with 3D model are performed to evaluate the experimental results. Finally, the interference fringes pattern obtained by the interferometer is shown to be qualitatively in good agreement with the numerical temperature field change. Additionally, transient variations of optical path length difference obtained in the experiment, which means apparent temperature distribution, were compared with numerical simulations. Experimental results are quantitatively in good agreement with the numerical results under a thermal diffusivity of order 10⁻⁸ m²/s, confirming the feasibility of the proposed measurement technique for the transient heat transfer in sCO₂.