Numerical investigation on deteriorated heat transfer of supercritical water flowing upward in tubes with variable cross-sectional geometries

To mitigate Deteriorated Heat Transfer (DHT) of Supercritical Water (SCW) upward flow, tubes with variable cross-sectional geometries are numerically investigated. Three types of variable cross-sectional geometries: converging channel, diverging channel, and periodic geometries, are modelled. Results of wall temperature in the smooth channel are compared with experimental data, and good agreements are obtained. The wall temperature effects of convergent and divergent channels of various proportions and periodic geometries of various amplitudes are investigated. Thermo-Hydraulic Performance Evaluation Criterion (PEC) is proposed to evaluate the performance of the studied geometries using dimensionless parameters Nu/Nu₀, f/f₀, and PEC = (Nu/Nu₀)/(f/f₀)^1/3. The results indicated that the convergent channels suppress and delay DHT downstream, while opposite effects are observed for divergent channels. Periodic geometries, which have alternating convergent and divergent sections, suppress DHT with minimal pressure drop. 1st temperature peak is reduced by 200 K with 1 mm of outlet radius reduction for the convergent channel. Average PEC is enhanced by 60% with 0.5 mm A of periodic geometry. It is discovered that the convergent section can suppress DHT, and the mitigation effect is proportional to the size of convergence. It is concluded that, since the DHT caused by buoyancy-induced flow laminarization has flattened the cross-sectional velocity profile, the convergent section reverts the laminarization to the normal velocity profile by the nozzle acceleration effect. Nozzle acceleration also increases Turbulent Kinetic Energy (TKE) in the boundary layer which further enhances turbulent heat transfer.