Thermal stratification in liquid metal pools under influence of penetrating colder jets

An experimental study is presented on the evolution of thermal stratification in a liquid gallium pool when a colder jet is introduced resulting in mixed convection currents. Different injection flow rates are considered for this experimental work with an objective to find the transition where flow fluctuations are able to overcome the restoring buoyant forces. At low flow rates, clearly stratified thermal regimes are observed and a diffuse planar front evolves during the injection experiments. However, at the higher end of the flow rates, thermal fluctuations accompany the stratified interface. This scenario is expected in some of the advanced liquid metal-cooled nuclear reactor designs where these fluctuations can lead to fatigue in adjacent solid structures and can be detrimental for reactor safety. The Gallium Thermal-hydraulic Experiment (GaTE) used for this study was designed with a moving magnet pump to inject gallium at controlled flow rates and a distributed temperature measurement system, based on the principles of Rayleigh back-scattering, to obtain high resolution spatio-temporal data. These capabilities allow the maneuvering and monitoring of the flux Richardson number (Ri(f)), which signifies the transition from planar to fluctuating thermally stratified front. Experimental results exhibit thermal fluctuations near the stratified interface for Ri(f) <= 1.3, which is close to the theoretically expected criteria i.e. Ri(f) = 1. In terms of the global Richardson number, these liquid gallium experiments show the transition criteria to be Ri < 8.