Understanding the natural ventilation of buildings induced by the force of buoyancy is crucial if we are to design ventilation systems that provide the ventilation flow rates required to maintain comfortable and healthy internal conditions while using little energy. This thesis considers exchange flow, a natural ventilation flow pattern that may occur at a horizontal opening in the ceiling of a room containing air that is warmer than the outside air temperature. Exchange flows are characterised by the simultaneous flow of air into and out of a room through the same opening. The flow is unsteady but has been observed to pulsate at a regular frequency. If the flow rate in both directions through the opening is the same, the exchange is balanced. If there is a greater flow rate in one direction than the other, the flow is unbalanced. Whilst the flow rate of balanced exchange flow has previously been measured experimentally and described theoretically, prior to the work herein there has been no theoretical description of unbalanced exchange flow, only experimental measurements of the flow rate and the conditions under which it occurs. We have developed the first theoretical model of unbalanced exchange flow by viewing it as a superposition of a unidirectional flow through an opening and a balanced exchange flow across the opening. Despite the simplified nature of the model, the predictions of the model showed excellent agreement with all the available experimental data. Next we investigated the consequences of unbalanced exchange flow for the overnight purging of warm air from a room with a high-level and low-level ventilation opening. Existing design guidance for natural ventilation systems states that the airflow will be unidirectional, with flow in through the low-level opening and out through the high-level opening, and that there will be a finite time for all the warm air to be purged from the room. However, we showed that during the purging process unbalanced exchange flow will occur at the high-level opening, with the result that there is no longer a finite purging time. Finally, we considered the purging of warm air from a room by a balanced exchange flow. Whereas previous research has assumed that the interior of the room will stay at a uniform temperature, by modelling the flow descending from the opening as a turbulent plume we found that the air in the room stratified.
buoyancy-driven, exchange flow, flow, unbalanced, ventilation
Networked Digital Library of Theses and Dissertations