Investigation of external heat transfer characteristics in a partially submerged vertical heat exchanger tube bundle

Abstract

A passive residual heat removal heat exchanger (PRHR HX) is commonly employed in nuclear reactors to dissipate decay heat from the reactor core during design extension conditions. In the later stages of such accidents, the coolant level in a storage tank may drop, leading to partial exposure of the heat transfer tubes. To investigate the effect of reduced coolant levels on shell-side heat transfer performance in a vertical tube bundle, an experimental setup was designed, featuring four coolant levels: full, 4/5, 2/3, and 1/2. Saturated steam was introduced to the inner side of the tubes, while distilled water served as the coolant on the outer shell side. High-speed imaging and precise thermal measurements enabled the observation and analysis of three distinct heat transfer regions: nucleate pool boiling, liquid film evaporation, and steam convection. A new method was proposed to calculate heat transfer coefficients, accounting for the complex phenomena in the liquid film evaporation region at low coolant levels. The results indicate that shell-side heat transfer performance does not degrade linearly with decreasing coolant levels. At the 4/5 and 2/3 coolant levels, intensified thermal disturbances from water jet impacts and liquid film falling improved heat transfer. Even at the 1/2 coolant level, the heat removal capacity remained over 75 % of that at full submergence, due to effective phase-change heat transfer in the liquid film region. This study concludes that PRHR HX systems can maintain effective heat removal even at reduced coolant levels, driven by film evaporation and droplet impingement mechanisms. These findings offer valuable insights for the thermal design and safety analysis of passive cooling systems in advanced nuclear power plants.