Experimental investigation on heat transfer characteristics of supercritical CO2 flowing upward and downward through a microtube at low Reynolds numbers

Both upward and downward flows of supercritical CO2 at 8.0 MPa through a microtube, 0.509 mm in diameter, have experimentally been investigated with Reynolds number at the tube inlet less than 1,000. The tube wall temperatures were measured with a new method that employs small solder casts, significantly reducing the thermal resistance between the thermocouples and the tube wall. For upward flow, a single local peak of the Nusselt number was observed for low heat flux as the pseudo-critical temperature was reached at a point where the fluid temperature was between the wall and film temperatures. As heat flux increased, however, the second peak started to appear. For downward flows, a single local peak of the Nusselt number was observed for low flow rates, while three local peaks characterized the heat transfer process for high flow rates. At each peak location, the sudden decrease of the enthalpy at the film temperature was observed, implying that fluid mixing takes place between the wall boundary and the core. Noting that the only parameter differentiating two different heat transfer characteristics of upward and downward flows is the direction of the buoyancy force, we conclude that the buoyancy effect plays a major role in the heat transfer characteristics of vertical laminar flows. Finally, we discuss the physical mechanisms underlying the differences between upward and downward flows and correlate the peaks of the Nusselt number with the modified Richardson number.