Mitigation of Cross-Contamination in Indoor Spaces in Light of Individual Thermal Preferences

Abstract

Protecting people against plausible cross-contamination in indoor spaces is one of the vital issues the world is facing nowadays, as the transmission of airborne infectious diseases is a critical threat to human health, and needs to be thoroughly mitigated. When infected, people constitute one of the main sources of such airborne contaminants via exhalation resulting from different respiratory activities. It is settled in literature that the typical total volume ventilation strategies used to dilute the indoor air contaminants’ concentration do not provide the required protection levels to occupants. Thus, new ventilation approaches should rely on air distribution and source control strategies, which include organizing the airflow pattern in a way to directly dilute the microenvironment of the occupant, and/or extracting the pollutants locally. Such source control strategies, also known as “occupant-centered” strategies, directly target the microclimate of occupants, and consist mainly of innovative localized ventilation like personalized ventilation (PV) and personalized exhaust (PE) devices. On one hand, PV systems provide occupants with protection and enhanced inhaled air quality by delivering clean conditioned air towards their upper-body part – namely the face. PV systems also provide users with the possibility of generating their own preferred microenvironment by adjusting the delivered airflow conditions to suit their personal preferences. On the other hand, PE system is proven in literature to control the transmission of infectious exhaled air by extracting it locally at the source. Being occupant-centered, the efficient operation of these strategies is affected by the occupancy density of the space. Thus, this research work aims towards investigating efficient ways to design and operate occupant-centered ventilation strategies that are adequate for two categories of indoor spaces: non-densely occupied office spaces and densely occupied classrooms, in pursuit of mitigating cross-contamination. Therefore, the work will be divided into two parts.

The first part considers a two-workstation office space equipped with a mixing ventilation system assisted by a typical computer-mounted PV system, where the occupants are given the freedom to control the PV-supplied jet to meet their personal thermal preferences. Such individual PV control may aggravate the possibility of cross-contamination between occupants if the PV user is infected. The aim is thus to investigate the effect of individual PV control on the transport of contaminants between the two PV users and assess the resulting cross-contamination. Two seating configurations are investigated: face-to-face or in tandem (i.e. back-to-face). The infected person is considered conducting two respiratory activities: nose-breathing and coughing. Two modes of PV operation are assessed: the steady PV operation mode (S-PV) and the intermittent PV operation mode (I-PV). For the S-PV operation, the PV users can control the PV-delivered flowrate to match their thermal needs. For the I-PV operation, the frequency of the supplied PV jet can be varied upon user preference. A transient 3-D computational fluid dynamics (CFD) model is experimentally validated and used to simulate the transport of contaminants and predict potential cross-contamination. Concerning the S-PV operation, the obtained results implied that a layout with face-to-face seating ensured lower cross-contamination between occupants in comparison to a tandem seating when they were controlling their PV according to their thermal comfort preferences. It was also concluded that individual PV control does not work in favor with the mitigation of contaminants’ dispersion; thus, the efficient integration of PV in multi-occupied office spaces requires assistive solutions (PV temperature control, PE integration, office partitions). As for the I-PV operation, results showed that when considering cross-contamination, the increase in the frequency of I-PV for users is generally undesirable, as it increases the turbulence and entrainment of contaminants into the delivered jet, which puts the healthy occupant at increased risk of exposure. It was thus recommended to operate the I-PV in the lower range of [0.3 - 0.5] Hz for enhanced protection level. Note that, similarly to the S-PV operation mode, it was found that the breathing of healthy occupants using I-PV can be disregarded when assessing the resulting breathable air quality.

The second part of the work introduces novel and practical integration of source control strategies in typical classrooms with close proximity of students. The aim is to decrease the risk of cross-contamination between students without the need for increased physical distancing. These strategies include the dilution of the microenvironment of students using PV and/or extracting local contaminated air using PE system. These systems are designed to be embedded in a “ductless” way in classroom chairs, for a practical and easy retrofitting in real life scenarios. Two different background ventilation systems are considered: displacement ventilation (DV) and downward piston ventilation (DPV). The aim is to provide students with the protection against cross-contamination while minimizing dead space areas in the classroom. Thus, the target is to design and set an adequate operational range for the proposed ventilation system, as well as assess its effectiveness in reducing the spread of exhaled contaminants from potential infected students. A transient 3-D CFD model is therefore developed and experimentally validated to predict the contaminants’ transport in the space. It was found that the implementation of the proposed chair-ventilation system at the high operation range (PV/PE operating at flowrate higher than 6 l/s) results in enhanced protection levels than the increased inter-distancing between students in DV-ventilated classrooms. Furthermore, the chair-embedded PE system assisting the DPV in densely occupied classrooms was a win-win strategy that caused a 51 % energy savings while maintaining the same protection levels to students.