Numerical study on flow structure and heat transfer of supercritical CO2 in tubes with different inclination angles

To reveal how buoyancy affects heat transfer of supercritical carbon dioxide (sCO(2)), the flow and heat transfer characteristics of sCO(2) in circular tubes with an inner diameter of 19 mm at different inclination angles were numerically investigated. The calculation results of RNG k-c, RKE k-c, SST k-omega and SST k-omega using a variable turbulent Prandtl number model (TWL model) have been compared with experimental data. The TWL model is chosen for numerical calculations because of its significant advantage in predicting wall temperature. Both forced convection (Richardson number Ri < 0.1, weak buoyancy) and mixed convection (Ri > 0.1, strong buoyancy) of sCO(2) are calculated. Result shows that the position of the velocity peak moves upward, then downward as the flow changes from vertical upward (inclination angle alpha = 0 degrees) to horizontal flow (alpha = 90 degrees) due to the secondary flow caused by radial buoyancy and the increase in velocity magnitude in the near top wall region caused by axial buoyancy. Under strong buoyancy effects, as alpha increases from 0 degrees to 90 degrees, the heat transfer deterioration (HTD) becomes severe when alpha < 15 degrees, then alleviated and finally severe again when alpha approaches 90 degrees due to the change of the heat transfer mechanism. This work deepens our understanding of sCO(2) heat transfer and may provide reference for the design of heat exchangers in nuclear reactors.