CFD modeling of condensation inside emergency condensers of passive heat removal systems

Future nuclear reactor concepts are frequently equipped with a passive emergency cooling system which removes decay heat from the reactor core in case of any emergency accidents. The emergency cooling system considered here consists of slightly inclined horizontal pipes which are immersed in a tank of subcooled water. Under normal operating conditions, pipes are filled with water and no heat transfer exists between primary and secondary side of the emergency condenser. In case of an accident the level of water in the core decreases, afterward steam enters the tubes on the primary side of the emergency condensers and because of the heat transfer from the subcooled water around the pipe to the steam, steam condensation occurs inside the pipes. Therefore, the emergency condenser acts as a strong heat sink which is responsible for a quick depressurization of the reactor core. The focus of the current paper is on CFD modeling of the whole condensation process inside the inclined pipe and validation of the results with the data obtained from experiments performed in TOPFLOW facility of HZDR for a single condensation pipe at operating conditions close to the reality, i.e. at high pressure and high saturated steam temperature.

During the condensation process, different flow morphologies may occur inside the pipe. The process is initiated due to the heat flux from the pipe’s wall to the steam. Because of the phase change, a thin layer of liquid film is generated near the wall leading to annular flow. The generated liquid film stays in direct contact with steam which is on the saturation temperature and cause direct contact condensation at the interface of the steam and the liquid. Because of the gravity force, the laminar liquid film is falling, gathering at the lower part of the pipe and finally, a stratified flow occurs. Furthermore, by enhancing the condensation rate, different flow morphologies such as stratified wavy flow, slug flow, plug flow and bubbly flow occur inside the pipe. CFD modeling of combined wall condensation and direct contact condensation inside the inclined pipes and effects of the liquid film on the heat transfer coefficient is the major focus of the current paper. In the end, the CFD modeling results are validated with the experimental data.