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• Experimental platform established for sCO 2 mini-channel flow local visualization. • Quantitative procedures established for sCO 2 boundary in field info extraction. • Rapid density decreases and small t differences (0.01 K) under sCO 2 local heating. • Grouped nu trends found for the first time during local boundary visualization. This study is focused on measuring the transient boundary heat transfer in supercritical CO 2 mini-channel flows under local heat application. A pixelated-array masked phase-shifting interferometer system and quantitative data procedures have been established for sCO 2 boundary flow. The experiments have been conducted at a Reynolds number level of 104, spanning across both subcritical and supercritical regions (5.0 ∼ 7.9 MPa). High resolution local density and temperature have been extracted from the raw interferograms by developing a digital phase-shifting algorithm. The temporal and spatial evolutions of Nu show good agreement with classic correlations. The effects of differential pressures (10.87 ∼ 52.59 Pa) and heat fluxes (2014, 5500, 14,057 W/m2), along with their associated heat transfer behaviors, have been quantitatively examined under transient scenarios using the current method. The results show: (1) although the Re number is high, such small thermal perturbations can be clearly visualized in the entire window, which shows the capability of field extraction of the current method proposed for complicated supercritical boundary flows; (2) rapid density decrease (1.0 kg/m3) occurs locally, while small temperature differences (0.01 K) are observed, which also lead to local Nu variations under different operation conditions; (3) detailed convective patterns and time evolution of Nu curve groups identified for subcritical, near-critical and supercritical states. [ABSTRACT FROM AUTHOR]

The boundary heat transfer dynamics of supercritical carbon dioxide (CO 2) is vital to its application in solar heater, extraction, refrigeration, heat pumps, power generations, nuclear systems and many other occasions. In this study, an experiment system using pixelated-array masked phase-shifting interferometer has been established for the visualization of temperature of supercritical CO 2. The laser diagnostic process has been operated with a carefully designed visualization chamber to give clear transient results of the density and temperature fields for the first time. Those field information has been used to calculate the heat transfer characteristic parameters of the supercritical CO 2 fluid in different boundary conditions. The current quantitative visualization analysis by the utilization of pixelated methods is shown capable of giving transient analysis of detailed field evolutions. The results show that (1) small temperature differences are generated in supercritical boundary due to the typical large thermal capacity; (2) supercritical convection boundary is with much quicker thermal quenching than subcritical states, while the subcritical states boundary parameter show localized characteristics while supercritical cases show fluctuation phenomena; (3) the boundary fluctuations heat transfer field exist for supercritical states and the fluctuations affect the overall heat transfer behaviour and the field evolution in supercritical states. [ABSTRACT FROM AUTHOR]