A study on density stratification by mechanical extraction displacement ventilation

The purpose of this study is to investigate the stratified flow driven by mechanical extraction displacement ventilation and compare the effects of the supply source and the extraction sink on the flow in the ventilated space. The extraction sink effect, which has rarely been addressed in the previous research, is discussed in this paper. This study investigates the flow stratification, the thickness of the intermediate stratified layer and their relationships with the suction, buoyancy and inertia forces in the space. The salt-bath technique was employed to conduct experiments simulating mechanical extraction displacement flow by using an acrylic reduced-scale model. According to the connection opening area on the partition, experiments were categorized into two series, denoted as Ex(I) and Ex(II). Experimental results show that as the extraction flow rate increases, the distance between the plume source and the interface height increases and the reduced gravity of the dense layer decreases as predicted by the two-layer stratification model. The stratification stability highly depends on the magnitude of the force ratio in the ventilated space. Similar to the previous research, the inflow inertia force has a clear influence on the formation of the intermediate stratified layer. The strong suction force in this study seems to aid the flow stratification and diminish the intermediate stratified layer thickness. The linear fit relationship between the intermediate stratified layer thickness and the force ratio gives a close result to the previous study. Similar to the previous study on displacement ventilation, the density in the dense layer is observed to be uniform when the extraction flow rate is small. The density distribution along a horizontal level with a certain non-zero gradient in the dense layer is clearly identified when the flow rate is high and the location is near the extraction sink, an observation that is very different from the previous study on displacement ventilation.