Emptying-filling boxes with non-Boussinesq plumes and fountains

In this paper the dynamics of the simultaneously filling and emptying of a box is studied theoretically and experimentally. Both situations, consisting of negatively buoyant (fountains) and positively buoyant (plumes) discharges, are considered. For the sake of generality, no assumption is made about the value of the density deficit (between the release and the ambient fluid), so this study deals with the so-called non-Boussinesq general case. Experiments are carried out with buoyant air-helium mixtures continuously released from the top (fountain) or bottom (plume) into air in a cylindrical tank with an open bottom boundary and a top vent of variable areas. At steady state, for both fountain and plume configurations, a buoyant layer of constant thickness and density forms under the tank ceiling. Based on mass and buoyancy conservation equations applied on the buoyant layer, theoretical models are proposed to estimate its depth and density at steady state. The theoretical models compare favorably with the experimental data. Subsequently, these models allow us to compare the plume and the fountain configurations for identical source conditions, box size, and vent area. Even if, in the majority of situations, the plume configuration allows a better mixing of the buoyant fluid with the environment, it is found that beyond a certain value of the fountain height, the fountain configuration becomes more efficient than the plume configuration for mixing phenomena.

Automated summary

This study examines the dynamics of filling and emptying a box simultaneously, considering both fountain and plume configurations. Experimental results show that in some cases, the fountain configuration is more efficient than the plume configuration.