Experiments on gas entrainment phenomena due to free surface vortex induced by flow passing beside stagnation region

Highlights: • Gas entrainment evaluation method based on a vortex model. • Evaluation of circulation and downward velocity gradient. • Experimental data to develop the evaluation method. - Abstract: Argon cover gas entrainment into sodium coolant is a significant thermal hydraulic issue in the design of sodium-cooled fast reactors. This paper reports the results of an experiment on surface vortex-type gas entrainment, which occurs in a shear flow area where flow passes besides the stagnation region. In the experiment, the relationship between the free surface dimple shape and the velocity distribution around the free surface vortex was simultaneously grasped under several horizontal and suction velocity conditions by a combination of visualization and particle image velocimetry (PIV) measurements. The circulation and the vertical velocity gradient were also evaluated from the obtained velocity distributions at a plane just below the free surface and the middle plane between the free surface and suction nozzle. Quantitative relationships between the circulation, vertical velocity gradient, and gas core length were successfully obtained in time-trends as fundamental data to develop the evaluation method of gas entrainment. Furthermore, it was confirmed that the evaluation method based on a vortex model was an effective way to evaluate gas entrainment.