Turabbhai Mohammadi, Murtaza
(2024)
Inter-room transmission of pollutants in street canyons.
PhD thesis, University of Nottingham.
Abstract
Clean air is a fundamental requirement for health and wellbeing and has been incorporated as one of the 15 Sustainable Development Goals set by the World Health Organisation. Epidemiological studies have highlighted the role of polluted air and the flow of contaminants via airborne routes in spreading diseases and causing ailments. Episodic scenarios in the past, such as gas leaks, fires, epidemics etc., have further shed light on routes of airborne transmission of pollutants and pathogens, which can endanger the health of urban dwellers. More recently, the SARS pandemic in 2002 and the COVID-19 pandemic in 2020 have pointed out the airborne spread of pathogens within the built environment which can compromise human safety. It is thus necessary to understand the routes of pollutant transmission in buildings and urban habitats before any strategic intervention can be introduced to limit their spread.
Within the purview of outdoor routes of transmission, past studies have investigated the role of wind conditions and building clusters on the spread of pollutants between various indoor apartments across one or more buildings. Most of these studies have been restricted to the investigation of highrise buildings and isolated building cases. While this has certainly revealed the role of building geometry on inter-apartment transmissions, the impact of street configuration remains to be explored within the context of the cross-transmission of pollutants. Streets represent a common and important urban feature, ubiquitous to any city. It is crucial to comprehend the inter-apartment transmission within this urban setting to effectively mitigate the spread of pollutants.
In addition, the majority of simulation-based studies on indoor ventilation have failed to validate their computational model using relevant experimental data. Often relying on spatially different experimental models due to limitations in replicating the setup. The simulated results in this case may not reflect the actual flow characteristics and consequent pollutant dispersion.
To bridge the identified literature gaps, the work undertaken in this project has two main objectives. The first is to generate an experimental dataset, which can be used to validate a computational model of airflow inside a street canyon integrated with indoor spaces. The second objective is to identify the routes of inter-apartment pollutant transmission in street canyons and the impact of contextual parameters.
For the first objective, a wind tunnel experiment was conducted to identify the pressure trends inside the apartments (or rooms) located at the centre of a street canyon. The results indicated a lower negative pressure coefficient (C_p) inside the upstream rooms and higher airflow through them. Contrastingly, the downstream rooms have higher negative C_p values accompanied by lower airflow. Subsequently, a Computational Fluid Dynamics (CFD) model was developed to simulate airflow through this indoor-outdoor coupled model of a street canyon. The simulation utilised the steady-state Reynolds-Averaged Navier-Stokes (RANS) approach and was validated against experimental data.
The latter part of the study is directed towards the assessment of wind-driven pollutant dispersion mechanisms and the impact of natural ventilation strategies. The parametric assessment involved the simulation of nine window opening modes, emulating different ventilation scenarios. Normalised pollutant concentration was examined in 12 unique apartments located in the street, each equipped with a point source of pollutant emission. Three routes of inter-unit transmission routes were identified from the results: vertical, across-the-street and along-the-street routes, arranged in decreasing severity. The concentration of adventitious pollutants within a room was contingent upon both the air change rate and the room's relative position within the street. Increasing the air change rate proved effective in diminishing cross-contamination, while the central placement of rooms in the street exacerbated contamination. Conversely, rooms positioned at the street's edge possessed a greater potential for contagion, capable of affecting rooms farther down the street, albeit in smaller quantities. The formation of recirculating vortices in the front and, in the wake of buildings played a pivotal role in the vertical and across-the-street transmission, whereas the interaction between the canyon and the edge vortices propelled along-the-street transmission.
Supplementary investigations were carried out to evaluate the sensitivity of pollutant concentration towards the street configuration and ambient wind conditions, encompassing street aspect ratio, street length, wind speed and flow direction. The insights derived from this study can empower urban planners and architects to approach building and neighbourhood design with a heightened awareness of air quality considerations and pollutant transmission dynamics.
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