Natural ventilation of an enclosure containing two buoyancy sources

This paper describes experiments and theoretical modelling of a naturally ventilated enclosure containing two point sources of buoyancy. Previous work on the flow and stratification that develop in a space owing to the presence of a single source of buoyancy has been extended to cover two sources of positive or negative buoyancy in an attempt to produce more realistic models of practical problems. Examples include naturally ventilated buildings, and industrial or medical processes involving dispersal of contaminants by air currents. The experimental technique involves the use of a water-filled enclosure with salt solution injected at two points. These sources of negative buoyancy form turbulent plumes within the enclosure. Two sources of unequal strength are found to produce a vertical density profile consisting of three distinct, fully mixed layers. The paper describes a theoretical model that successfully predicts the depths and densities of these layers. The positions of the interfaces between the three layers were found to be a function only of the effective area A* of the enclosure openings, the height of the enclosure H and the ratio of the strengths of the two sources of buoyancy B-1/B-2. The behaviour of a system with one source of positive buoyancy at the lower boundary and one of negative buoyancy at the upper boundary of the enclosure is also examined. This case has particular relevance to the prediction of thermal stratification in rooms with both cold and warm surfaces present. Again the height of the interfaces is dependent only on the geometry of the enclosure and the source strength ratio. Three distinct types of flow pattern are predicted and observed.