Numerical study on heat transfer and pressure drop characteristics of supercritical CO2 flow near critical point through microtubes

The buoyancy and thermal acceleration effects of sCO2 flow near the pseudo-critical region have been widely mentioned as the mechanism of heat transfer enhancement in the literature. However, most publications deal with turbulent flows and do not discuss the details of how these effects alter flow structure and enhance heat transfer. The present study numerically examined the mechanisms that enhance heat transfer in upward sCO2 flow through microtubes, specifically 0.5 mm in diameter at low Reynolds numbers and 0.847 mm in diameter at high Reynolds numbers. The heat transfer enhancement was closely associated with the appearance and disappearance of the M-shaped velocity profile for the low Reynolds numbers. When the M-shaped profile started forming by the buoyancy effect, the first local maximum of the heat transfer coefficient appeared as the thermal acceleration of the boundary layer entrained fluid from the wall region. The fluid entrainment carried thermal energy from the wall toward the core, thus enhancing the heat transfer. At high Reynolds numbers, the results showed that gravity has no significant impact on heat transfer characteristics, suggesting that buoyant forces do not play a crucial role in the heat transfer behavior of sCO2 flow. When the M-shaped profile started disappearing due to the thermal acceleration in the core region at low Reynolds number, the second maximum appeared in some cases due to abrupt turbulence developed by two forces in the opposite direction: one force dragging the local maximum velocity in the M-shaped profile and the other force accelerating the core region. As the second maximum is not observed due to high inertia force at the high Reynolds number, negative radial velocity at any location is significant for increasing the heat transfer coefficient. It also concluded with important remarks on pressure drop, surface roughness, the possibility of using LES simulations, and the possible modification of the RANS model.