This study was intended to examine sodium entrainment behavior in the case that a hole was formed on a tube wall in the steam generator of a fast breeder reactor and high pressure and high temperature water jetted out into sodium. Flow visualization experiments of an air jet in liquid were performed. The test vessel was 270 mm wide, 5 mm depth and 300 mm high. The air jet was blown vertically upward into stagnant liquid in the test vessel from a rectangular cross-section nozzle of 1 mm wide, 5 mm depth and 20 mm long which was located at the bottom of the test vessel. A flow state of the jet in the liquid was recorded with a high speed video camera at the fastest 150,000 frame/s. The test liquid was water and kerosene. Filament-like ears and wisps pulled out from the wavy interface were noticed on the interface between liquid and the air jet. The ears and the wisps were broken off and entrained into the air jet. The droplets broke up to small entrainments. This process seemed quite similar to the entrainment process in the annular dispersed flow in a pipe. Entrainment was initiated at a little bit downstream from the nozzle outlet. The entrainment inception point moved downstream as the air jet velocity increased. Axial directional entrainment velocity increased as the air jet velocity increased and the entrainment proceeded downstream. Transversal directional entrainment velocity was much slower than the axial directional entrainment velocity. The variation of the entrainment velocity in the transversal direction was not so prominent. The entrainments produced at the interface of the air jet moved to gather at the center portion of the air jet as those were accelerated.

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