Numerical study on mechanisms underlying the heat transfer enhancement of upward supercritical CO2 flow at low Reynolds numbers near the pseudo-critical region through a microtube

The buoyancy and thermal acceleration effects of supercritical CO₂ 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 investigated mechanisms underlying the heat transfer enhancement of upward supercritical CO₂ flow through a microtube, 0.5 mm in diameter, at low Reynolds numbers. The heat transfer enhancement was closely associated with the appearance and disappearance of the M-shaped velocity profile. 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. When the M-shaped profile started disappearing due to the thermal acceleration in the core region, 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.