Investigation on the flow and heat transfer of supercritical CO2 in a semicircular channel under cooling conditions

The turbulent flow and heat transfer characteristics of supercritical carbon dioxide (S-CO2) within a semicircular channel represent a critical aspect in the thermal-hydraulic design of printed circuit heat exchangers (PCHEs). This study employs large eddy simulation to analyze the unique flow and heat transfer phenomena occurring in a semicircular channel under cooling conditions. The effects of three distinct heat flux levels and three Reynolds numbers on the flow and heat transfer characteristics are systematically examined, considering a channel diameter of 2 mm to reflect microchannel dimensions typical of PCHE applications. Both forced and mixed convection regimes are incorporated into the simulation framework. The study particularly focuses on characterizing the circumferentially non-uniform heat transfer distribution, mechanisms responsible for heat transfer deterioration under cooling, and impact of channel orientation on thermal performance. The results reveal pronounced non-uniformity in wall temperature along the channel circumference, with marked localized temperature reduction near the corner region. However, increasing the Reynolds number significantly enhances heat transfer in this corner zone, mitigating the corner effect. Moreover, an increase in heat flux augments heat transfer—a behavior contradicting trends reported under heating conditions. Additionally, heat transfer deterioration attributed to buoyancy-induced effects occurs at the bottom wall, whereas buoyancy enhances heat transfer at the top and corner regions. Finally, in terms of average heat transfer efficiency, horizontally oriented channels outperform vertical configurations. These numerical insights provide a valuable foundation for optimizing the thermal-hydraulic design of pre-coolers.