Direct numerical simulations of cooling flow and heat transfer in supercritical CO₂ Brayton cycle coupled with solar energy

Direct numerical simulation was used to study supercritical carbon dioxide's flow and heat transfer mechanisms under cooling conditions in the supercritical CO₂ Brayton Cycle coupled with solar energy. This paper aims to investigate the effects of property variation, buoyancy, and thermal deceleration under different flow conditions. Momentum balance analysis is used to quantify changes caused by different mechanisms and to describe the flow and turbulence evolution characteristics. In forced convection, due to the dominant thermal deceleration effect, the pressure gradient decreases across the entire cross-section, resulting in the deceleration of the fluid. In upward flow, buoyancy plays a dominant role and acts as a resistance, causing the pressure gradient to increase significantly. As a result, when the core fluid accelerates, the velocity of the near-wall fluid slows significantly, enhancing heat transfer, and the Nusselt number increases by up to 33.21% compared to forced convection. In downward flow, buoyancy is strong, the pressure gradient is negative, and the flow is entirely driven by buoyancy. External and structural buoyancy effects contribute to flow laminarization, leading to heat transfer deterioration. With increasing heat flux, Nu decreases more significantly, with a maximum reduction of 62.94%, and begins to recover after reaching its lowest point. Importantly, in the recovery stage, the structural buoyancy effect triggers turbulence recovery. The recovery of shear production of turbulent kinetic energy is later than that of buoyancy production and can only occur with a strong M−shaped velocity distortion. The FIK identity decomposition shows that the heat transfer deterioration is related to flow laminarization and inhomogeneous effects. Heat transfer recovery is purely due to the increase in turbulence components.