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Abstract. For the future development of Generation IV nuclear reactors, both safety and economic targets have to be achieved. In order to increase, at the same time, the power density generation and the safety features, a huge R&D effort is still required. Referring especially to Liquid Metal Cooled Fast Reactors, much attention is placed on Gas Entrainment (GE) phenomena, which could cause unlikely positive reactivity insertion accident. The GETS experimental facility (Gas Entrainment Test Section), especially aimed at studying the free surface vortices occurrence, has been built in the thermal-hydraulics laboratory of the DIAEE. The main purpose of this facility is to identify the most important parameters affecting the whirlpools formation and evolution. Experimental tests and preliminary observations have been performed. Different vortex behaviours related to different experimental conditions have been identified and presented in the present paper. 2D occurrence maps as function of different dimensionless groups (Reynolds, Froude and Weber numbers and H* = H/d ratio) have been defined. In the present paper, the results of a first experimental campaign, carried out with tap water, are discussed.

Particle Image Velocimetry (PIV) measurements have been carried out in order to analyze the structure of free surface vortices in a promoting geometry with two tangential inlets. Velocity fields associated to the free surface vortex have been obtained at different horizontal planes and Reynolds numbers. Average velocity fields have been calculated and tangential velocity profiles have been compared at different vortex stages and measurement planes. The results show that tangential flow is uniform along the vortex axis and it scales well with the average exit velocity. The tangential velocity profiles, in comparison to the potential behavior, show discrepancies especially at large distances from the vortex axis. Vorticity fields and circulation profiles have been also derived from the measured velocity fields and discussed. The circulation profiles increase along the vortex radius even at large distances from the vortex axis, so that the potential solution is not applicable at all. The comparison of tangential velocity and circulation profiles between promoted and free vortices, the last presented in a previous paper, shows that the tangential motion in a driven vortex is more intense and predominant over the sink effect (radial motion), except very close to the tank bottom, as in a forced configuration (i.e. rotating cylindrical tank).

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