Numerical study on cooling heat transfer of turbulent supercritical CO2 in large horizontal tubes

This paper presents the results of computational investigations on cooling heat transfer of turbulent sCO2 in three horizontal tubes with diameter of 15.75mm, 20.00mm and 24.36mm using RANS turbulence models at a pressure of P=8.0MPa. Four models with good prediction performance demonstrated in literature (RNG k-ε model and three other low-Reynolds number k-ε models of LS, YS and AKN) have been validated against experimental measurements and to observe that results from the AKN model are closer to experimental data. Details of heat transfer behaviour of sCO2 cooled in horizontal tubes within this diameter range are revealed and the influence of heat flux, tube diameter and buoyancy on heat transfer performance have been discussed. Results demonstrate that at Tb>Tpc (pseudocritical temperature), sCO2 heat transfer performance is enhanced as the heat flux and tube diameter increase; whereas at Tb0.1, the buoyant force is enhanced, which in turn impairs the heat transfer near Tpc. This is a result contrary to past reports confined to small diameter tubes, which is mainly attributed to the accumulation of denser cold fluids at the bottom of the pipe when buoyancy effects are strong.