Experimental and computational investigations of gas entrainment in SFR due to rotation of partially submerged pump shaft

•Overall behavior of the rotating flow is investigated through experiments on a Taylor-Couette model as well as by CFD simulations.•Two different mechanisms of gas entrainment, viz., shear mechanism at free surface and vortex breakdown at shaft surface are identified.•From the gas entrainment point of view, four regimes of the rotating flow are identified.•The vortex depth with respect to Rotational Reynolds number is studied from experiments and CFD simulations.•This study will help in developing mitigation devices to avoid gas entrainment. Gas entrainment in cold pool of fast reactors, induced by partially submerged pump shafts is studied by experimental and computational simulations. The model is a partially filled annular tank with a rotating shaft at the centre. The rotational Reynolds number (Re) has been varied and the resulting gas entrainment due to various mechanisms has been characterized. It is found that the fluid dynamics in the annular region can be classified into four regimes that occur with increasing Reynolds number. In the first regime encountered at low Re, there is no risk of gas entrainment. In the second regime, mild gas entrainment takes place due to the shearing at the free surface between two opposite vortices. In the third regime, gas entrainment is almost absent due to weakening of flow mechanism of the second regime. In the fourth regime, vortex break down is observed at the rotating shaft leading to large scale gas entrainment.