Flow regime analysis of high-pressure transcritical fluids in microducts

Microduct flows are known for their inherent laminar regimes resulting from the characteristic small dimensions and low velocities. In this regard, direct numerical simulations are employed to investigate an innovative approach that harnesses the unique thermophysical properties of high-pressure transcritical fluids to achieve significantly higher rates of mixing and heat transfer in microduct geometries. The strategy is based on the sizeable changes in properties that supercritical fluids, at pressures and temperatures exceeding their critical value, undergo across the pseudo -boiling region. To this end, four different cases are considered, and systematically analyzed, in which the bulk pressure and temperature difference between walls are varied. The results obtained indicate that laminar flow prevails at low-pressure conditions, while flow regimes with turbulent characteristics can be achieved when operating at high-pressure conditions with a transversal temperature difference. The transition to the turbulence -like regime is assessed by quantifying variations in velocity and temperature profiles, accompanied by the observation of secondary flow motions. As a result, substantial increases in the Nusselt number of roughly 20x, indicative of enhanced heat transfer, are obtained at the hot wall in comparison to cases with same temperature differences at low pressure.