Experimental and simulation study of spatial distribution of human respiratory droplets under typical indoor air distribution patterns

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Experimental and simulation study of spatial distribution of human respiratory droplets under typical indoor air distribution patterns


Author: Li, Xiaoping
Title: Experimental and simulation study of spatial distribution of human respiratory droplets under typical indoor air distribution patterns
Degree: Ph.D.
Year: 2013
Subject: Indoor air pollution -- Health aspects.
Indoor air quality.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxvii, 231 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615888
URI: http://theses.lib.polyu.edu.hk/handle/200/6990
Abstract: The outbreaks of epidemic and pandemic viral infections, including severe acute respiratory syndrome (SARS) in 2003 and H1N1 during 2009 and 2010, create serious threats to human well-being both mentally and physically, and also result in huge economic losses throughout the world. It is reported that ventilation system processes strong relationships with the transmission and spread of some infectious diseases. Questions remain on whether the three typical air distribution methods, mixing ventilation (MV), under-floor air distribution (UFAD), and displacement ventilation (DV), can have significant differences in reducing the transmission of infectious diseases between occupants in a space. This study therefore aims to develop some fundamental understandings on infection control in indoor environments with different air distribution systems. Dispersion of exhaled droplets released from normal breathing and coughing is investigated with the intention to generalize the approaches in mitigating infectious diseases transmission for various breathing maneuvers. This work mainly consists of four parts: 1) an experimental study of distributions of artificially generated droplets in a DV ventilated full-scale chamber, 2) numerical estimations of droplet dispersion and co-occupant's exposure when human respiratory droplets are introduced into room air by nose-breathing or coughing at different intensities under MV, UFAD, and DV, 3) assessments of co-occupant's exposure level when the infected person coughs at different orientations to the co-occupant, including face-to-face, bending the head down, and turning the body around; and with two physical blockings, namely mouth covering and desk partition, under the three ventilation schemes, 4) numerical investigation on the performances of two types of personalized ventilation (PV) devices, i.e. a chair-based PV and a desk-mounted PV, on the co-occupant's exposure under MV and DV at different PV use conditions.
From the experimental study, it is found that the human related parameters, including breathing mode, breathing air temperature, and heat load of human body, play important roles on contaminant distribution and personal exposure for the exposed person. In order to ensure a lower exposure level, systematic and deliberate investigations must be conducted to fully assess the combined effects of these influencing parameters on contaminant distribution. The numerical study of nose-breathed droplets gives the same conclusion as the experiment that larger exhaled droplets under DV can travel farther horizontally and may pose higher risk for the co-occupant. However, lower inhaled fraction can be ensured for smaller droplets in stratified ventilation system since the upward air movement can transport the exhaled droplets to the space above the breathing level. The numerical study on coughed droplets reveals that there are two distinctive infection transmission stages, a first direct exposure stage initiated by the high momentum coughing jet and a second indirect exposure stage caused by indoor air movement. Sitting/standing in the traveling area of the coughing jet may result in higher direct exposure and higher total inhaled dose irrespective of the ventilation systems in place. Here, stratified ventilation does not help in lowering the first stage exposure. Coughing at a lower velocity, bending the head down, turning the body around, covering a cough, or blocking the coughed airflow by desk partition can all relieve the co-occupant from the direct exposure and consequently reduce the total inhaled dose. However, their effects in reducing personal exposure vary with ventilation systems. A preferred air distribution method in reducing infection transmission can not be easily identified, and detailed analysis is needed case by case. Overall, these personal and physical interventions have limited influences on the second stage exposure level under MV, while dramatic effects can be observed for stratified ventilation systems, especially for DV. The numerical simulation on personalized ventilation (PV) system concludes that the type of PV device plays an essential role in indoor infection control. A well-designed PV system can ensure better inhaled air quality for all kinds of PV operation conditions, especially under stratified ventilation system, while higher personal exposure may be induced if the PV supplying air enhances the mixing of exhaled droplets with room air. It is therefore important to conduct detailed analysis regarding to the effects of preferred PV system on infection transmission prior to implementation.

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