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 T_b>T_pc (pseudocritical temperature), sCO₂ heat transfer performance is enhanced as the heat flux and tube diameter increase; whereas at T_b<T_pc, the heat flux and tube diameter almost do not affect the heat transfer performance. The buoyancy effect only generates slight enhancement for turbulent heat transfer from sCO₂ flowing in horizontal tubes with large diameters. However, as the values of Richardson number Ri that quantifies the buoyancy effects continue increasing within Ri>0.1, the buoyant force is enhanced, which in turn impairs the heat transfer near T_pc. 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.