Heat transfer characteristics of horizontal supercritical CO2 flow through a microtube at low Reynolds numbers and its comparison with vertical flows

Abstract

The heat transfer characteristics of supercritical CO2 flow at 8.0 MPa through a horizontal microtube, 0.509 mm in diameter, were experimentally investigated for Reynolds numbers less than 1000 at the tube inlet. The thermocouples were alternatively attached to the top and bottom of the horizontal tube by employing the small solder casts. Due to the buoyancy effect, the heat transfer rate at the tube bottom was measured to be higher than at the top. The discrepancy was more prominent near the pseudo-critical point while becoming smaller as the flow moved away from the pseudo-critical point. When a higher heat flux was applied at the wall, the Nusselt number was measured to be lower. However, when the Nusselt number was plotted against the dimensionless specific enthalpy evaluated at the film temperature, the heat transfer coefficient was independent of heat and mass fluxes, correlating with a single curve in the region away from the pseudo-critical point. Comparing horizontal flow with vertically upward and downward flows showed a significant buoyancy effect on the heat transfer characteristics at low Reynolds numbers. And the heat transfer rate in the horizontal configuration was considerably higher than those in the vertical configurations at the same operating conditions.