Author: Li, Xiujie
Title: A study of pollutant dispersion mechanism in medical environments using particle image velocimetry (PIV) and computational fluid dynamics (CFD)
Advisors: Mak, Cheuk Ming (BEEE)
Degree: Ph.D.
Year: 2023
Subject: Communicable diseases—Transmission
Airborne infection
Fluid dynamics
Particle image velocimetry
Dental clinics
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xxvi, 183 pages : color illustrations
Language: English
Abstract: Following the global outbreak of severe acute respiratory syndrome (SARS) in 2003, several respiratory infectious disease epidemics have emerged, including avian flu in 2005, swine influenza in 2009, and the novel coronavirus disease 2019 (COVID-19). The pandemics have substantially impacted human health and the provision of medical services. However, prior studies regarding pollutant dispersion were focused on limited scenarios and steady-state conditions. Medical institutions conducting aerosol-generating procedures have particular characteristics that render them susceptible to virus transmission. Although dental clinics have been widely regarded as one of the most vulnerable institutions, limited studies were conducted on virus-laden droplet transmission during dental services. In addition, the fluctuation characteristics of airborne transmission in transient exposure events have rarely been studied. Therefore, reducing transient cross-infection risks and protecting human health have attracted much more attention in crowded spaces and dental clinics. The thesis systematically investigated indoor pollutant dispersion mechanisms using particle image velocimetry (PIV) and computational fluid dynamics (CFD) methods. Three sub-works will be conducted to achieve the research objectives: (a) PIV experiments about the atomization mechanism of the dental aerosol-generating procedure and spatial-temporal distribution of emitted droplets; (b) CFD simulations about the effect of environmental conditions on the transmission mechanism of emitted droplets and cross-infection risk assessment; (c) large-eddy simulation (LES) about the fluctuation characteristics of airborne transmission in transient exposure events and the recommended inhalation monitor points.
PIV experiments were conducted to investigate the atomization mechanism of the dental aerosol-generating procedure from three aspects: the spatial-temporal distribution, diameter, and velocity distribution of emitted droplets. Based on the recorded velocity vector and trajectory map, the high contamination region would be within 1 m from the oral cavity. Two primary sources of inhalable particles in dental surgery environments were the particles trapped in the high-strain region and the evaporation-induced droplet nuclei. Owing to the long airborne lifetime, the fallow time (FT) between dental patients' appointments should be instituted to reduce the cross-infection risks. The minimum required FT in the 6 air changes per hour (ACH) falls within the time range of 27 to 35 minutes, while the cooperation of high-volume evacuation (HVE) can helpfully result in a reduction of FT by 3–11 minutes. In addition, the performance of HVE on emitted droplets was also visualized in the PIV experiment. This study provided the initial evidence to support the hypothesis that HVE has a moderate effect on small, high-velocity droplets.
Systematic CFD simulations were conducted to examine how changes in environmental and psychological conditions (such as variation of ventilation location, ventilation rate, relative humidity (RH), and patient’s breathing rate due to dental anxiety) can affect the transmission and evaporation of emitted droplets during dental services. The droplet velocity and diameter distribution recorded in the prior PIV experiments were defined as initial CFD boundary conditions. The results revealed that the diameter threshold for the droplet deposition and suspension was 60 𝜇𝑚, and 85% of droplets would be deposited near the dental treatment region. The patient’s torso, face, and floor (dental chair) can be accounted for around 63%, 11%, and 8.5% of deposited droplets, respectively. Increasing the ventilation rate from 5 to 8 ACH resulted in a 1.5% increase in the fraction of escaped droplets. 50% RH in dental environments was recommended to prevent droplets’ fast evaporation and potential mold. Variations in the patient’s breathing rate due to dental anxiety had little effect on the final fate and proportion of emitted droplets. In addition, the infection risk level of different dental atomization procedures: vibration ultrasonic scaling (vUS), and rotation high-speed drilling (rHSD), were numerically compared in the same environmental conditions. Cross-infection risk during high-speed drilling can generally be regarded as “higher” than ultrasonic scaling.
High-resolution LES was conducted to predict the fluctuation characteristics of airborne transmission in transient exposure events. Three representative physical distances (0.35 m, 1.0 m, and 1.5 m) between two occupants were examined. The findings showed that the exposure index during transient exposure events fluctuates over time, particularly within the first 1/ACH hour of exposure in areas where occupants are in close proximity. This variability poses significant uncertainty regarding the spatial and temporal progression of the cross-infection risks. Transient exposure events were predominantly caused by direct airborne transmission. Considering the randomness, discreteness, localization, and high-risk characteristics of direct airborne transmission, the localized method that directly interferes with respiratory flows would be more effective than general dilution ventilation. Because of the temporal changes in contaminant concentrations within the identified breathing zone during transient exposure events, occupants’ exposure risk analysis should take into account the characteristics of turbulence intensity and concentration fluctuations. The point at the center between the chin and lower lip was recommended as the more appropriate inhalation monitor point.
The findings can give policymakers insights into the impact of environmental conditions on the virus-laden droplets dispersion, serving as guidance to decrease the cross-infection risk in dental clinics. In addition, the research will provide references to the development of targeted mitigation measures, protecting human beings’ health and well-being.
Rights: All rights reserved
Access: open access

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