Effects of rib geometries and property variations on heat transfer to supercritical water in internally ribbed tubes

Flow and heat transfer of supercritical water (SCW), which experiences dramatic thermophysical property variations and is regarded as Variable Property Fluid (VPF), was numerically studied in vertical upward internally ribbed tubes (IRT). The Shear-Stress Transport k-ω model was employed to solve the turbulent flow and conjugate heat transfer. After validating the model against experimental data from IRT with different geometries, effects of rib geometries on heat transfer to SCW were studied at a pressure of 25 MPa, mass velocity between 600 and 1000 kg/(m2s) and heat flux between 200 and 470 kW/m2. Impact of property variations in IRT is analyzed by comparing SCW flow and Constant Property Fluid (CPF) flow. For the CPF case, IRT always has superior performance over smooth tubes. As for SCW flow, rib geometries play only a small role in both average and local heat transfer under conditions of forced convection. While in mixed convection rib geometries have a significant effect on heat transfer by suppressing the buoyancy effect along the whole circumference to different extents, and by helping SCW recover its large cooling capacity. Internal ribs can more efficiently improve heat transfer to VPF when compared with CPF at the same Reynolds number.