Propagation and entrainment of buoyancy-driven flows in a narrow horizontal space and implications for buoyant contaminant transport under natural ventilation

Contaminant transport induced by buoyancy-driven flows is crucial for proper ventilation design in buildings. Compared to conventional single-zone spaces, considerably less research has been conducted on the dynamics of buoyant contaminated flows in narrow horizontal spaces with large length-to-width ratios. Experiments were performed to explore the characteristics of natural convection produced by a brine source in a narrow horizontal tank immersed in fresh water. The source was located at the center of the tank ceiling; hence, the buoyancy-driven flow consisted of a plume region and a symmetrical horizontal current. A light attenuation technique was used to measure the density distribution and vertical profile of the horizontal velocity of the current. The current density was vertically stratified, but declined very slowly in the horizontal direction. The head of the horizontal current advanced toward the two ends of the tank at a constant velocity that depended only on the source buoyancy flux. The average thickness of the horizontal current was correlated to the tank height and the source Froude number. In our experiments, the interface between the tail of the horizontal current and the fresh water was very stable, and nearly no mass exchange through the interface was observed. However, mixing between the fresh water and brine occurred in both the plume region and the rearward side of the horizontal current head. Both the total entrainment and the proportion of the entrainment occurring in the head were determined. The latter increased with the source Froude number.