Visualization of transient boundary heat transfer of supercritical CO2 through-flow in mini-channel under top heating

Abstract

The top-heated heat transfer dynamics of supercritical CO₂ (sCO₂) are critical for applications like power cycles, nuclear reactor cooling, duct heat exchangers, solar energy, and spacecraft thermal management. This study focuses on the experimental measurement of the transient boundary heat transfer behavior of sCO₂ inside a mini-channel under local heat flux from above. The flow starts from the pre-existing turbulent dynamics, followed by local heating from upper walls. A pixelated phase-shifting interferometer was employed to observed the test object, enabling quantitative analysis of the thermal boundary layer influenced by the combination of buoyancy effects and local top heating. The results reveal that: (1) the density stratification (∼0.7 kg/m³), predominantly formed in the upper region as the low-density fluid heatedly flows upward, significantly suppresses the generation of convention; (2) increased heat flux enhances the density shifts and buoyancy, accelerating lighter/hotter fluid perturbations and broadening the spectrum of secondary flows across the entire observed window; (3) relatively larger density shifts but smaller temperature gradients are observed under supercritical conditions (∼0.9 kg/m³, ∼0.006 K) compared to subcritical ones (∼0.25 kg/m³, ∼1.1 K), yielding more pronounces secondary flows and thermal stratification along the midline of the vertical axis.