Experimental investigation on cooling heat transfer and buoyancy effect of supercritical carbon dioxide in horizontal and vertical micro-channels

Abstract

This study experimentally investigated the cooling heat transfer characteristics of supercritical carbon dioxide (CO2) in circular horizontal and vertical (downward and upward) micro-channels (Dh = 1 mm). Measurements were performed for operating pressure ranging from 7.5 to 8.2 MPa, mass flow rate ranging from 1.323 to 2.567 kg/h, wall heat flux ranging from 14.05-39.34 kW/m2. The effects of mass flow rate, wall heat flux, operating pressure and flow direction on heat transfer coefficient and buoyancy effect were explored. The experimental results revealed that heat transfer intensification benefited from increased mass flow rate and decreased operating pressure. Wall heat flux exhibited relatively mild influence on heat transfer coefficient. A significant heat transfer enhancement in downward flow was discerned compared with horizontal flow. The buoyancy effect significantly influences the heat transfer characteristics during supercritical CO2 cooling process as thermal properties dramatically changes with the bulk temperature. The forced convection aligned region where the buoyancy effect was less dominated witnessed distinctly higher heat transfer coefficient in contrast with mixed convection flow. Furthermore, a comparative study was performed by comparing several existing correlations with experimental results of Nusselt number, and large deviations were reasonably observed due to limited applicability.