Transport processes across a horizontal vent due to density and pressure differences

Abstract A very important flow and transport circumstance that arises in practical problems such as enclosure fires is that of heat and mass transfer across a horizontal vent. Such vents exist in enclosed regions like rooms and energy storage and ventilation systems. It is important to understand the basic nature of the transport processes that arise because of finite, non-zero, density and pressure differences that usually exist across such vents. The flow is driven by these two mechanisms and very complicated flow patterns arise, depending on the governing variables in the problem. A dominant pressure effect results in a unidirectional flow, whereas significant buoyancy effects lead to a bidirectional flow exchange. The heat and mass transfer associated with the flow is similarly strongly influenced by the flow regime. There is a strong need for flow visualization to determine if a unidirectional or a bidirectional flow exists across the vent and to study the basic characteristics of the transport processes involved. This paper presents a study of this heat and mass transfer problem employing water and air as the fluid media for two different experimental systems. Pure and saline water are used in the first case to obtain the unstably stratified circumstance with a pressure difference across the vent. Air at different temperature levels is used in the second case. A laser sheet, with smoke, is used for visualization in air and a shadowgraph for water. Results are obtained on the flow direction under different conditions and the corresponding transport rates. Since transient effects are important in many cases, video recordings are employed to obtain the frequency of observed oscillations in the transport and for determining the transition from one regime to the other. Quantitative data on the transport processes are also obtained.