3D CFD simulation of passive decay heat removal system under boiling conditions: Role of bubble sliding motion on inclined heated tubes

Abstract

In order to design advanced nuclear reactors with enhanced safety systems such as passive decay heat removal system (PDHRS), a new design of Isolation Condenser (IC) has been proposed. The effect of inclination of condenser tube on sliding bubble dynamics and associated heat transfer has been studied for seven angles of tube inclination α (with respect to vertical direction), in the range 0°≤α≤90°. For this purpose, two phase transient 3D CFD simulations using mixture model (based on Euler–Euler approach) have been performed. The model considers different mechanisms such as single phase natural convection, latent heat transfer due to evaporation, transient conduction due to disruption of thermal boundary layer and enhanced liquid convection due to bubble sliding motion (quenching). The transient vapor fraction (ϵG) contours and flow distribution enables to understand the mechanism of bubble formation and bubble sliding motion. The major heat transfer mechanism was found to be the liquid agitation caused by sliding bubbles on the tube surface. The heat transfer contribution due to evaporation was found to be very small because of highly sub cooled (∆Tsub=70K) liquid inside the tank. Results show that the heat transfer was found to be maximum for α=75° and minimum for α=30°. The bubble sliding length at the tube top was found to decrease and at the tube bottom was found to increase with an increase in the inclination angle (α). The enhanced transfer at α=75° ensures excellent thermal mixing and hence results in reduction in thermal stratification. This paper is in continuation of our earlier two papers which employed 21l (Gandhi et al., 2013a) and 200l (Gandhi et al., 2013b) PDHRS whereas this paper presents the simulation of 10 l and 10,000m3 PDHRS.