Mixing of thermally stratified layers by a vertical turbulent buoyant jet in a cylindrical pool was analyzed to reveal the fundamental mechanisms which cause transition to stratification in enclosures. A hot buoyant jet injected at the center of the enclosure bottom interacts with the thermally stratified ambient, and circulating flows are formed. Besides the Froude number, two aspect ratios, namely, pool height to jet diameter and pool height to pool diameter, influence the non-dimensional velocity and temperature fields. The jet Reynolds number determines only the time scale. The circulating flow rate for the low Froude number is large, so that the ambient fluid stratifies and the pool surface temperature is relatively uniform. For low aspect ratios and high Froude numbers, the circulating flow rate is reduced and a double circulating cell was observed. In this case, the pool surface temperature just above the jet is high, but the surface far from the jet is kept at a lower temperature for a long time. The transitions between these primary flow regimes can be mapped as functions of Froude number and aspect ratios.
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