Modelling of condensation inside an inclined pipe

Designs of future nuclear boiling water reactor concepts are usually equipped with a so-called emergency cooling system which is passively driven to remove heat from the core to the outside in case of an accident. The emergency cooling system consists of a bunch of slightly inclined horizontal pipes which are immersed in a tank of subcooled water. At normal operation conditions, the pipes are filled with water and no heat transfer to the secondary side of the condenser exists. In case of an accident during which the water level in the core is decreasing, steam enters the emergency pipes and due to the cold water around the pipe, the steam condenses at the inner wall of the pipes. Therefore, the emergency condenser removes the decay heat from the reactor core. In the current PANAS-project all the involved thermal hydraulic components are studied intensively. The focus of the current paper is on CFD modeling of the emergency condenser and validation of the models with data obtained from experiments performed at the TOPFLOW facility at a single condensation pipe at operating conditions close to reality, i.e. at high pressure and saturated steam.
In this paper, the inflow of the pipe is assumed as pure steam. Due to wall condensation a thin liquid film is generated near the wall leading to annular flow. The generated liquid film stays in direct contact with steam which is on saturation temperature causing direct contact condensation at the interface between steam and 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. Combining wall condensation, direct contact condensation and effects of the liquid film on the heat transfer coefficient is a major focus of this paper. Finally, the results of the simulations are validated with the experimental data.