Numerical investigation on local heat transfer characteristics of S-CO2 in horizontal semicircular microtube

•SST k-omega model can predict the heat transfer of S-CO2 in horizontal microtube.•Interactions of Cp and buoyancy influence the local heat transfer.•Buoyancy play leading role in the heat transfer deterioration.•Hysteresis exists in the effects of buoyancy on density stratification.•Secondary flow changes the local heat transfer mechanisms profoundly. The nuclear reactor system with supercritical carbon dioxide (S-CO2) as working fluid has good prospects in generation IV reactors. The printed circuit heat exchanger (PCHE) is a promising candidate for recuperator in S-CO2 Brayton cycle. However, the dramatic variations of S-CO2 thermophysical properties and the geometrical factors of PCHE microchannels make S-CO2 heat transfer mechanisms complex. Grasping the S-CO2 heat transfer mechanisms provides references and theoretical supports for the design of PCHE. In this paper, localized heat transfer characteristics of S-CO2 heated in a horizontal semicircular microtube were investigated numerically. The mathematical and physical models in simulating the heat transfer characteristics of supercritical fluid were validated by experimental data. The effects of pressure, mass flux, heat flux and tube geometry on S-CO2 local heat transfer characteristics near the pseudocritical temperature were studied. The results demonstrate that the dramatic variations of thermophysical properties in pseudocritical region have significant effects on heat transfer mechanisms. The distributions of specific heat (Cp) dominate the heat transfer characteristics when the effects of buoyancy are neglectable. In low mass flux or high heat flux cases, the buoyancy effects strongly influence the heat transfer mechanisms by changing the sectional parameters distributions and intensifying the secondary flow, resulting in deterioration of localized heat transfer capacity.