Numerical Investigation on Deteriorated Heat Transfer of Supercritical Water Flowing Upward in Tubes with Variable Cross-Sectional Geometries

To enhance the safety performance of heat exchanger systems operating at supercritical pressures, supercritical water (SCW) flowing upward in tubes with variable cross-sectional geometries are numerically investigated to understand the mechanism of mitigation effects on the DHT phenomenon. The effect of converging channel, diverging channel, and periodic geometries of various amplitudes on DHT are analysed to characterize their heat transfer performance. The results for the wall temperature distribution in smooth channels are compared with experimental data and good agreements are revealed. The Thermo-Hydraulic Performance Evaluation Criterion (PEC) is proposed to evaluate the performance of the studied geometries using dimensionless parameters Nu/Nu 0 , f/f 0 , and PEC= (Nu/Nu 0 )/(f/f 0 ) 1/3 . The convergent channel is seen to be the most important geometry in terms of DHT mitigation, whereas the divergent channels led to DHT aggravation. Mechanism analysis suggested that the rapid acceleration of SCW in the convergent section not only increases the Turbulent Kinetic Energy (TKE), but also impairs buoyancy-induced flow laminarization, leading to 60% increase in PEC