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 (sCO₂) Brayton-cycle need a thorough understanding to heat transfer of sCO₂. As a continuation of our experiments, numerical studies are performed to explore the mechanisms behind different heat transfer behaviors of sCO₂ occurred at different mass fluxes. Seven turbulence models are assessed against the test data, and the v2f model and SST k-ω 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 (λ) 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 sCO₂, but this is seldom concerned in former research.