Numerical modeling on heat transfer of supercritical water in cooled horizontal circular tube

Abstract

As an efficient and energy-saving fluid, supercritical water (SCW) has great potential in power plants. In this paper, the cooling heat transfer of SCW in horizontal tube was investigated numerically. The two highly recommended models (Re-Normalization Group (RNG) k-ε model and Shear Stress Transport (SST) k-ω model) are validated against experiments and it is observed that the SST k-ω model is closer to the experimental data. The cooling heat transfer characteristics of the SCW in horizontal tubes are revealed and the effects of heat flux, mass flux, tube diameter and buoyancy on the heat transfer performance are investigated. Results demonstrated that the heat transfer coefficient (HTC) peaks when bulk enthalpy (Hb) slightly greater than the pseudo-critical enthalpy (Hpc). Heat flux affected SCW heat transfer at Hb>Hpc and had no significant effect at Hb<Hpc. Higher HTC and more uniform wall temperatures could be achieved by increasing the mass flux. Temperature and velocity stratification could be observed, and the wall temperature at the top was higher than the bottom due to the buoyancy effect. The secondary flow induced by buoyancy force disturbed the velocity field, drawing better heat transfer performance at the top surface. The HTC discrepancy between at the top and bottom could be well explained by the field synergy principle. Finally, two buoyancy criteria were compared and the results indicated that Ri=Gr/Re2 could better predict the buoyancy effect for SCW cooling flow in the whole enthalpy regimes. The conclusions can provide guidelines for further design of supercritical water heat exchangers.