Similarity Studies of Buoyancy Effects in Impinging Jets – Application to Sfr Core

After a circular jet has impinged a wall, a radial flow develops and spreads over the wall. This situation has been widely investigated through many theoretical or numerical studies describing spreading jet characteristics. One situation has been less studied because of an industrial importance, it concerns buoyancy effects. This phenomenon occurs in Sodium Cooled Fast Reactors (SFR) where the sodium core jets are impinging a structure called the Upper Core Structure (UCS) leading to a radial flow; it spreads in a colder sodium environment where some buoyancy effects in the radial jet can be observed. To get a better understanding of the involved physical phenomena, a simplified mock-up named PIGNIA has been designed, allowing studies of a single impinging jet for nozzle-to-plate distance H/d = 4, 6&10. A theoretical model based on simple assumptions for impinging jets is proposed. This model provides a satisfying prediction of the radial densimetric Froude number FrDrad based only on the inlet variables of the buoyant jet problem. Extended to SFR, the model is useful to prevent the condition under which the radial jet rises and leads to thermal fatigue in the vessel. This mock-up provides also an experimental database of the critical conditions under which the radial jet rises. This transition is obtained for a threshold value of around FrDrad similar or equal to 3, 5.