The study focuses on investigating the phenomenon of gas entrainment in the cold pool of fast
reactors, specifically caused by pump shafts that are partially submerged. Both experimental and
computational simulations are utilized to examine this process. The experimental model consists of a
partially filled annular tank with a centrally located rotating shaft. By varying the rotational Reynolds
number (Re), the researchers examine the resulting gas entrainment and characterize the different
mechanisms involved.
The findings reveal that the fluid dynamics within the annular region can be categorized into four
distinct regimes, each occurring as the Reynolds number increases. In the first regime, which occurs
at low Re, there is no risk of gas entrainment. Moving into the second regime, mild gas entrainment
occurs because of the shearing at the free surface between two opposing vortices. The third regime
shows a significant reduction in gas entrainment due to the weakening of the flow mechanism
observed in the second regime. Finally, the fourth regime exhibits vortex breakdown at the rotating
shaft, leading to a substantial-scale gas entrainment.