Quantitative visualization on boundary heat transfer of supercritical CO2 mini-channel through-flow (Re=104) via pixelated phase-shifting interferometry

This study is focused on measuring the transient boundary heat transfer in supercritical CO2 mini-channel flows under local heat application. A pixelated-array masked phase-shifting interferometer system and quantitative data procedures have been established for sCO2 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.