Improved models for natural displacement ventilation
File(s)
Author(s)
Fiuza Dosil, Daniel
Type
Thesis
Abstract
The research conducted in this thesis investigates some of the fundamental physics of natural ventilation, with the goal of providing potential solutions to enhance the ventilation of smoke in simple dwellings that arise from the burning of unclean fuels.
The investigation focuses on the draining of buoyant fluid through circular horizontal openings. We find that during unidirectional displacement ventilation, the draining for sufficiently ‘thin’ layers will change of draining mechanisms, a new mechanism that we explore in detail. We also show that the hydrostatic driving head in draining flows driven by buoyancy may be influenced in some cases by the geometry of the opening and the flow behaviour within. The theoretical models of the ‘emptying box’ and ‘emptying filling box’ (Linden et al., 1990) are modified to account for these flow behaviours. The modified models are compared with experimental data obtained in the laboratory, which show to predict much better the draining and ventilation dynamics of enclosed geometries driven by buoyancy. The theoretical work provides a more accurate framework for designers and architects to design naturally ventilated dwellings under displacement ventilation. These results also provide a better understanding of the discharge coefficient Cd.
Regarding potential solutions to enhance ventilation, this thesis shows that a chimney stack (no matter its length) is ineffective to drain ‘thin’ buoyant layers during displacement ventilation, and that they will only enhance ventilation for sufficiently deep layers. Another solution is found, that shows to effectively enhance the ventilation of ‘thin’ buoyant layers where chimneys are ineffective. The solution is to simply align the buoyancy source with the ventilation opening to enhance the ventilation outflow. A new driving mechanism that we named ‘plume induced ventilation’ is explored, and the theoretical model of the modified ‘emptying filling box’ shows a general good agreement with the experiments. This practical solution complements the results of the chimney stacks, and it is a good alternative for enhancing ventilation in the regime where chimney stacks are ineffective.
The investigation focuses on the draining of buoyant fluid through circular horizontal openings. We find that during unidirectional displacement ventilation, the draining for sufficiently ‘thin’ layers will change of draining mechanisms, a new mechanism that we explore in detail. We also show that the hydrostatic driving head in draining flows driven by buoyancy may be influenced in some cases by the geometry of the opening and the flow behaviour within. The theoretical models of the ‘emptying box’ and ‘emptying filling box’ (Linden et al., 1990) are modified to account for these flow behaviours. The modified models are compared with experimental data obtained in the laboratory, which show to predict much better the draining and ventilation dynamics of enclosed geometries driven by buoyancy. The theoretical work provides a more accurate framework for designers and architects to design naturally ventilated dwellings under displacement ventilation. These results also provide a better understanding of the discharge coefficient Cd.
Regarding potential solutions to enhance ventilation, this thesis shows that a chimney stack (no matter its length) is ineffective to drain ‘thin’ buoyant layers during displacement ventilation, and that they will only enhance ventilation for sufficiently deep layers. Another solution is found, that shows to effectively enhance the ventilation of ‘thin’ buoyant layers where chimneys are ineffective. The solution is to simply align the buoyancy source with the ventilation opening to enhance the ventilation outflow. A new driving mechanism that we named ‘plume induced ventilation’ is explored, and the theoretical model of the modified ‘emptying filling box’ shows a general good agreement with the experiments. This practical solution complements the results of the chimney stacks, and it is a good alternative for enhancing ventilation in the regime where chimney stacks are ineffective.
Version
Open Access
Date Issued
2021-05
Date Awarded
2021-11
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Burridge, Henry
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)