Aspects of the Fluid Mechanics of Night-Purging Multi-Storey Buildings

This research concerns the mathematical and physical modelling of the buoyancy-driven flow of warm air at night from a multi-storey atrium building by displacement ventilation. The process of clearing warm air from a building at night is known as ‘night-purging’. The primary aim of this research was to enhance understanding of these flows to thereby facilitate efficient design of passively night-purged buildings. The thesis begins with a review of research on natural ventilation and night-purging. Particular focus is given to the conditions for achieving classical displacement flow, in which there is an absence of mixing between warm and cool air, as this has been shown to be the most efficient means of removing warm air from a space. We identify that the dynamics of the plumes of warm air discharged from the storeys into the atrium play a crucial role in the development of the thermal stratification in the atrium. The majority of research on turbulent plumes has concerned plumes from horizontal sources, while ventilation openings are often oriented at some angle off-horizontal. We therefore investigate how varying the angle of orientation of the plume source (or ventilation opening) affects the dynamics of the plume to determine the implications for buildings ventilated via wall-mounted windows. This modelling reveals that, for a significant proportion of a typical night-purge in a single storey, a plume from a vertically oriented opening will not project away from the opening. Thus the simple plume model we have developed will not apply during the late stages of a night-purge. In order to develop a model of plumes which do not project away from the source, we investigate the limiting case of a plume from a vertically distributed source (such as a vent) with zero source momentum flux, such that the motion is entirely parallel to the source. To investigate the overall flow in the building, guided by the results of the plume modelling, we develop a simplified mathematical model to predict the purging of warm air in a generic two-storey atrium building. The pre-