On the Interaction of Natural Convection and Mechanical Ventilation

Abstract

Motivated by the need to ventilate buildings and prevent the accumulation of contaminants and excess heat, this thesis explores the interaction of natural convection and mechanical ventilation in an enclosure where fluid is supplied at a low level and extracted at a high level. A turbulent buoyant plume rising from a localised source of buoyancy interacts with this upward ventilation flow to establish a steady-state stratification with a buoyant upper layer above a layer of the supply fluid. In this thesis, we use experimental observations of a model system to gain insight into the fundamental flow mechanics, which informs the development of models for the flow. In chapter 2, we identify a new over-ventilated regime where the total flux from natural convection is less than the ventilation flux, but there is still a stable two-layer stratification. We develop an energy balance model to predict the interface height in both the over- and under-ventilated regimes that is consistent with our experiments. Next, in chapter 3, we investigate the impact of changes to the buoyancy of the low level fluid supply and identify four new flow regimes depending on whether the buoyancy increases or decreases and the magnitude of the change. Small decreases in the buoyancy of the supply fluid cause the plume to intrude at the interface between the original upper and lower layers; large decreases lead to an intrusion below the original lower layer. Increasing the buoyancy of the supply fluid leads to mixing between the supply fluid and the fluid already in the space. Small increases only mix the lower layer, but large increases also mix the original upper layer. We classify these flow regimes and develop numerical models for the flow. In chapter 4, we explore the transient evolution between steady states after disturbing the ventilation flow or buoyancy source. These disturbances act over different time scales and can be complementary or act in opposition, leading to diverse transients. We classify the flow regime based on the eventual change in the interface height and the buoyancy of the upper layer and develop a numerical model for the flow evolution. Finally, in chapter 5, we summarise the findings of this thesis and discuss some areas for further investigation.