Experimental investigation of high-pressure transcritical CO2 in microchannels for enhanced heat transfer at low Reynolds numbers

Abstract

Microchannel heat exchangers are inherently constrained by laminar flow regimes resulting from the low Reynolds numbers attainable in microconfined geometries, which severely limit convective transport and overall thermal performance. In this work, high-pressure transcritical carbon dioxide is used as the working fluid to investigate how large thermophysical property gradients across the pseudo-boiling region influence microscale transport. By exploiting these gradients, localized flow instabilities are triggered, promoting turbulent-like transport within microscale channels. In this regard, although recent investigations have largely relied on system-level performance quantification, the present study employs high-resolution fluorescent scalar diagnostics to provide direct visual evidence of the microscopic flow mechanisms responsible for the substantial heat-transfer enhancements observed under transcritical conditions. To enable these measurements while simultaneously satisfying the stringent thermal, mechanical, and optical requirements imposed by high-pressure operation, a custom hybrid copper–Ostemer microfluidic platform was developed. The flow visualizations reveal a sudden and intense breakdown of the core flow into highly disturbed fluid packets as the fluid traverses the pseudo-boiling region, providing direct evidence of property-gradient-induced turbulence. These observations establish the physical mechanism underlying the measured thermohydraulic performance, which reaches up to a twenty-fold enhancement in the Nusselt number (Nub≳20) while maintaining notable thermodynamic efficiencies (ηth>0.8) near the critical point. Importantly, the visualized chaotic flow disturbances act as an effective mixing mechanism that enhances thermal transport across the channel width without incurring the severe friction penalties typically associated with shear-driven turbulence. Collectively, these findings challenge the conventional assumption that microchannel flows remain fundamentally laminar and establish a new pathway toward high-flux thermal management in space-constrained applications.