Experimental study on supercritical CO2 heat transfer and field comparisons inside mini-channel under local heat flux from bottom and top walls

Abstract

This study experimentally investigated and compared the transient boundary heat transfer behavior of supercritical CO₂ (sCO₂) in a mini-channel subjected to localized heating from the top and bottom walls. A pixelated phase-shifting interferometer was employed to capture the transient field data, enabling the extraction of density field for quantitative analysis the thermal boundary layer affected by the combination of buoyancy effects and local heating. It has been found that: (1) In top-heated cases, the heated zone contracts due to secondary flow directed towards the upper wall, but is expanded in the bottom-heated cases as the buoyancy-driven fluid rises from the lower wall into the main-flow; (2) Rapid thermal variations strongly intensifies the bottom convective mixing, with increased Nu value, but slower and weaker in the top-heated cases; Local thermal stratification patterns and thermal stagnation deteriorate the heat transfer, yielding a lower Nu level; (3) Increasing thermal load amplifies local stratification due to thermal acceleration, as evidenced by the density-temperature shifts of 1.2 kg/m³, 0.017 K for q = 8.79 kW/m², compared to 0.6 kg/m³, 0.006 K for q = 0.55 kW/m². Secondary flow redistributes thermal energy, reversing a substantial portion of initially downward-flowing sCO₂ into upward motion.