Numerical investigation of different heat transfer behaviors of supercritical CO2 in a large vertical tube

The design and optimization of key heat exchange components in supercritical CO2 (sCO(2)) Brayton-cycle need a thorough understanding to heat transfer of sCO(2). As a continuation of our experiments, numerical studies are performed to explore the mechanisms behind different heat transfer behaviors of sCO(2) occurred at different mass fluxes. Seven turbulence models are assessed against the test data, and the v2f model and SST k-omega model are recommended for low and normal mass flux cases, respectively. A novel analysis approach is proposed by treating heat transfer of SCFs as a coupling of heat conduction of boundary layer, pseudo-phase-change heat transfer of large specific heat (c(p)) fluid and convective heat transfer of turbulence core. For low mass flux cases, the special heat transfer enhancement (HTE) in low fluid enthalpy (h(b)) region is mainly caused by strong buoyancy effect, which thins the thickness of viscous sub-layer and promotes turbulent kinetic energy (k). But for normal mass flux case, heat transfer deterioration (HTD) occurs due to decreasing fluid thermal conductivity (lambda) of viscous sub-layer and suppressing turbulence via buoyancy. The buffer layer plays a bridge for heat transfer from viscous sub-layer to external turbulence region. Meanwhile, a noteworthy phenomenon is that, the heat conduction process of boundary layer shows a strong relevance with the evolution of heat transfer behaviors, and has a great effect on overall heat transfer of sCO(2), but this is seldom concerned in former research. (C) 2019 Elsevier Ltd. All rights reserved.