Investigations on the steam condensation in a vertical tube bundle geometry

Abstract

The Passive Containment Cooling System (PCCS) is a safety feature intended to depressurize the containment using condensation phenomenon. PCCS consists of multiple condensing tube bundles. Steam condenses at the exterior of the tubes and the performance of PCCS is significantly affected by the arrangement of tube bundles. For instance, tubes at the inside of the bundle may have a lower heat transfer coefficient than those located at the periphery due to depletion of steam within the bundle. This is like a screen effect due to the surrounding tubes. Meanwhile it is known that condensation changes steam concentration distribution and consequently enhances steam diffusion process in the vicinity of the condensing region, which is called a suction effect. Because of these two effects, the average condensation heat transfer rate of a tube in the bundle could be higher or lower than that of a stand-alone single tube. In this paper, we investigated these screen and suction effects numerically using a CFD code, and experimentally. A hexagonal bundle of 12 tubes was set up and heat transfer coefficient of each tube was measured individually, and the aforementioned effects of screen and suction phenomena were observed. The average heat transfer coefficient of a tube in the bundle was found to be larger than that of the standalone single tube, which implies that the suction effect was larger than the screen effect. For numerical investigation, a CFD model was set up using the STAR-CCM+ code. To validate the CFD model, turbulence models and mesh tests were conducted and benchmarking simulations with the well-known correlations such as Uchida and Tagami’s were performed. In numerical investigation, we examined the bundle geometry and arrangement more thoroughly by changing the pitch to diameter ratio and arrangement shape, to propose an optimum tube bundle arrangement. The CFD results also confirmed the screen and suction effects. However, the magnitude of these effects were found somewhat lower in the numerical simulation than in experiment, which was thought probably due to the instrument wires packed around the bundle hindering the convection flow. © 2016 Association for Computing Machinery Inc. All Rights Reserved.