Flow structures associated with CSRDM shroud tube and control rod assembly: A combined experimental and simulation study

Abstract

This paper presents numerical investigation of flow behavior near a submerged porous shroud tube, a protective covering over Control Safety Rod Driving Mechanism (CSRDM) inside liquid sodium coolant pool of a Fast Breeder Reactor (FBR) in synergy with corresponding experimental findings to explicate possible gas entrainment mechanism inside the liquid pool. Fluid entering the shroud tube whose top end is just below the free surface causes swelling of free surface near the axial outlet of the shroud tube that vastly affects the flow behavior nearby and liquid-fall type gas entrainment inside the coolant pool. To numerically capture the effects of various flow rate and free surface height on the flow behavior near the shroud tube, an approximate single phase realizable k−ε turbulence flow model has been implemented. The study is performed assuming Froude number similarity for Reynolds number ranging from 1.3 × 103 to 6.5 × 103 and corresponding Froude number ranging from 0.266 to 1.332. Both computational and experimental studies have been carried out with water as the working fluid. Geometry of hump profile, obtained experimentally, has been incorporated as input to the computation to avoid large computational time associated with free surface modeling. With such a coupled approach, a strong agreement is obtained between numerical prediction and experimental Schlieren and dye visualization results. The simulation study is subsequently used to report the radial and axial velocity and turbulence intensity distribution in the liquid pool. The nature of impinging jet-like velocity profile from the axial exit of the shroud tube and corresponding boundary layer-like growth near the air-water interface is compared with that of the free wall jet, impinging wall jet and impinging confined wall jet. The free surface boundary layer growth follows power law similar to that of the wall jet in low Froude number regime (Fr < 0.666). At higher Froude number, the axial flow along the shroud tube dominates over the radial free surface flow. The power law exponent of the free surface boundary layer growth is lower compared to that of the wall jet due to the lower viscous effect from the adjacent air medium and axial flow due to the hump entrainment.