Experimental study on unstable free surface vortices and gas entrainment onset conditions

• Gas entrainment onset conditions were experimentally investigated in cold water.
• Qualitative criteria were adopted to classify the observed free surface vortices.
• Correlations of the critical heights for different transition zones were obtained.
• Scaling factors for sodium test cases are discussed for gas entrainment occurrence.

Free surface vortices formation has been, for many years, a relevant issue in many engineering applications with hydraulic intakes, since strong whirlpools introduce swirl flow at the intake causing entrainment of floating matters and gases (air or other gases). The Gas Entrainment (GE) phenomenon has also been an important topic in nuclear industry in the last years, due to the possibility of free vortex formation at the surface, especially in sodium-cooled fast reactors (SFRs). This phenomenon may result in unlikely positive reactivity insertion accident, affecting the safety performances of the reactor itself. The GETS experimental facility (Gas Entrainment Test Section) has been built in the thermal–hydraulics laboratory of the DIAEE in order to study free surface vortices occurrence. The main purpose of this facility is to understand the influence of different parameters on free surface vortices formation and evolution. Experimental tests and preliminary observations, carried out, with cold water as working fluid, different water level in the tank, circulating flow rate and outlet tube diameter, are presented as occurrence maps in this paper. The observed vortices have been qualitatively classified in different “formation stages” and the most important parameters that influence the physical phenomenon have been identified through a dimensionless analysis. Empirical correlations based on dimensionless parameters of the transition boundaries between stages, including the onset of gas entrainment, have been developed and discussed, and very preliminary considerations on the sodium–water similitude, based on the Froude and Weber numbers, are introduced.