Entrance effect on supercritical water heat transfer in horizontal tubes: Enhanced heat transfer performance and new correlation development

The unique physical properties of supercritical fluids result in a pronounced entrance effect on heat transfer. This study systematically investigates the influence of the entrance effect on heat transfer characteristics of supercritical water in a horizontal tube using both experimental and simulation methods. Based on variations in boundary conditions, the heating process of supercritical water is divided into three stages: the Thermal Establishment Stage, the Axially Asymptotic Developed Stage (AADS), and the Thermal Removal Stage. Each stage is clearly defined, and its heat transfer characteristics are analyzed. The Thermal Establishment Stage, influenced by the entrance effect, exhibits superior heat transfer performance. In horizontal tubes, buoyancy-induced thermal stratification and secondary flow significantly extend the range of the entrance effect and intensify its impact. The influence of the entrance effect extends over 150 times the tube diameter, potentially increasing the overall heat transfer coefficient by >30 %. Higher heat flux, lower mass flux, and more pronounced changes in physical properties enhance the entrance effect. Based on experimental data, a heat transfer correlation is developed that excludes wall temperature parameters or those dependent on wall temperature, while effectively capturing the influence of the entrance effect. This study provides valuable insights into utilizing the entrance effect to mitigate heat transfer deterioration and improve heat exchanger performance.