Effect of pressure on turbulence and heat transfer under compressed and supercritical coolant flow conditions

The heat removal characteristics of water under supercritical and compressed fluid conditions are of direct relevance to the thermal-hydraulic optimization of nuclear reactor rod bundles. In this study, numerical simulations of upward water flow through a heated 2×2 rod bundle were performed using STAR-CCM+ at pressures of 23 MPa and 25 MPa, with systematically varied heat and mass fluxes. The SST k-ω (Menter) and realizable k-ε turbulence models were applied with All y+ wall treatment and validated against the experimental data of H.Y. Gu et al. The SST k-ω model demonstrated superior near-wall prediction accuracy, establishing its suitability for supercritical flow simulations, while both models performed comparably in resolving bulk flow temperature. At constant heat flux (300 kW/m 2 ), the heat transfer coefficient increased with both mass flux and pressure. At constant mass flux (228 kg/m 2 s), it decreased with increasing heat flux but increased with increasing pressure, confirming the combined governing role of the heat-to-mass flux ratio and an independent pressure effect attributable to pseudo-critical temperature shifting. Contrary to conventional expectations, elevated turbulent kinetic energy under supercritical conditions did not correspond to enhanced heat transfer. Superior heat transfer performance was instead associated with lower turbulence levels under compressed fluid conditions, a behavior attributed to the severe degradation of thermal conductivity, specific heat, and density near the pseudo-critical temperature, which suppresses turbulent transport effectiveness. TKE variations were more pronounced at 25 MPa than at 23 MPa with increasing mass flux, and overall TKE behavior was governed by a complex interaction of pressure, heat flux, mass flux, and axial position.