Novel personalized chair-ventilation design integrated with displacement ventilation for cross-contamination mitigation in classrooms
Graphical abstract
Introduction
Airborne infectious diseases are a serious threat to human health, causing human morbidity and mortality worldwide [1]. In crowded places, it is challenging to control airborne disease transmission as it can easily spread when people spend long periods of time together in close proximity [2]. Therefore, the indoor “in-person” life must come with strategies ensuring safety of people, specifically in highly occupied spaces such as school and university classrooms – the places where investment in the future of the community occurs. The main acknowledged measures that limit the spread of viruses involve maintaining physical distancing (in the range of 2-m) [3] and improving ventilation indoors by increasing the ventilation rate to a sufficient level [1,4,5].
Nonetheless, when considering highly occupied classrooms applications, the above-mentioned solutions seem impractical to implement. In real life scenarios, it is not quite viable or effective to keep a 2-m distance between students in a classroom, as this result in the reduction of students’ number and the increase of wasted “dead space” areas. Increasing ventilation rates has been suggested according to various standards and studies [4,6,7] to improve indoor air quality. However, airborne infection control was not accounted for in highly occupied spaces where occupants experience close proximity to one another [8]. In such crowded spaces, the increase in ventilation rates will decrease the amount of contaminants in the macroclimate but will not prohibit cross-contamination between occupants. Meanwhile, increasing the ventilation rate results in a higher burden of energy consumption, which opposes the pressing need to have energy-efficient buildings [9]. Thus, more attention should be focused on examining “occupant – centered” strategies that could effectively remove contaminants from the microclimate of occupants to help minimize cross-contamination between them.
The interest in occupant-centered ventilation strategies has been growing over the last decade: these techniques control the microclimate of the occupant, improving the ventilation efficiency for individual occupants and providing protection and thermal comfort [2,10,11]. One established localized ventilation system consists of supplying clean cool air to the breathing zone (BZ) of occupants – renowned as personalized ventilation (PV) [[11], [12], [13], [14], [15]]. The PV provides good levels of breathable air quality while ensuring protection from transmission of contaminant [16,17]. Different designs of PV air terminal devices have been tested in literature: desk-mounted PV [12,18,19], chair-mounted PV [[20], [21], [22], [23]], ceiling-mounted PV [24,25], headset incorporated PV [26,27] etc. Li et al. [28] assessed the performance of both desk-mounted and chair-based PV systems in terms of respiratory contaminants' dispersion between two occupants. They reported that both PV systems were able to reduce the exposure levels. Melikov et al. [22] studied the performance of PV with seat headrest-mounted air supply terminal devices. They found a large improvement of the inhaled air quality and a decreased risk of airborne cross-infection when the seat headrest-mounted PV was used. Nonetheless, when implemented in classrooms for occupants seated at close proximity one in front of the other, the PV jet has the possibility to entrain the exhaled contaminants from an infected person present in the room towards the back, endangering thereby the exposed occupants [14]. This dictates the implementation of another localized ventilation strategy to exhaust the exhaled air as closely as possible from the infected person. Thus, a localized exhaust device – namely personalized exhaust (PE) – was introduced as an assistive ventilation strategy to PV for the control of the transmission of infectious air by extracting local contaminated air [[29], [30], [31]]. Yang et al. [32] evaluated the performance of an integrated PV-PE system in minimizing the spread of contaminated air exhaled by infected occupants efficiently. The exposure of the healthy occupant to contaminants was reported to decrease when the PE system was assisting the PV system by extracting the contaminated air outside of the space, highlighting its advantageous role in mitigating the release of exhaled contaminated air to the room air. Another study by Junjing et al. [30] investigated the contaminant removal effectiveness of a PV-PE system installed on the chair above the occupant's shoulder level. The system was found to enhance the inhaled air quality of occupants with respect to the PV system alone.
However, the retrofitting of such ventilation systems in classrooms requires extensive change in the air-ducting system to supply the fresh cool air to students, and exhaust the microclimate air from the students’ surrounding towards the outside. This is not practical in highly occupied classrooms, as it is restricted by the available free space areas. Adding to the ductwork problems, the PV and PE system ducting may shield the sight of students in some cases. Thus, there is a pressing need to come up with an innovative and practical application of such systems that optimizes space use while providing protection in terms of acceptable breathable air quality and mitigation of cross-contamination between students. Such occupant – centered application should be ductless, and designed in a simple manner for an effective real-life implementation in typical highly occupied classrooms.
Hence, this work proposes an innovative and practical ventilation design to provide acceptable protection levels for students in classrooms with minimal dead space areas. The aim 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, while taking into consideration the system's practical implementation. To achieve the work objectives, a 3-D Computational Fluid Dynamics (CFD) model is developed to predict the contaminants' transport in the space. This model is experimentally validated through a designed system including a prototype of the proposed chair-ventilation design. The different operating conditions of the PV/PE systems are thus investigated to depict and analyze the protective effects resulting from their implementation. To assess the effectiveness of the proposed chair-ventilation system, a “no chair-ventilation” case is studied and the exposure level is compared to that obtained from the proposed system operation, and the resulting reduction in the exposure level is reported. Furthermore, the practicality of the proposed ventilation system is compared to a reference case named “large distancing” case, where students were separated by the “safe distance” of 2-m and were not using the proposed ventilation system. The resulting exposure level is thus compared to that obtained from the chair-ventilation implementation. This accentuates the proposed system's value in efficiently using the space and increasing the occupancy density while still providing protection for students.
Section snippets
Description of the proposed system
This study investigates the implementation of a novel ventilation system for high-density classrooms that can be a practical solution for the mitigation of contaminants' transport between students, while considering optimizing the space use. The system should practically fit into the space without the need for extensive ductwork. Thus, the chair can be the ideal target for add-on equipment to implement the occupant – centered ventilation strategy (including PV and PE) [23,33]: The back of the
Numerical methodology
Numerical modeling is required to solve for the complex time-dependent physics occurring in the classroom, such as the turbulent respiratory jet and its interaction with the room ventilation field (PV, PE and DV induced flow fields), as well as the resulting transport of exhaled contaminants in the space upon the use of the proposed ventilation strategy. Subsequently, the commercial software ANSYS Fluent (version 19.2 [60]) was used to develop a 3-D CFD model simulating the dispersion of the
Experimental methodology
The accurate prediction of the flow field behavior and contaminants’ dispersion in the space were validated by conducting experiments in a climatic chamber (2.7 m × 2.7 m × 2.6 m) conditioned by a DV system and the proposed chair-ventilation system. The DV system supplied 50 l/s, while maintaining a supply air velocity of 0.3 m/s and temperature of 18 °C (i.e. maintaining the same conditions of the DV system operation). Two thermal manikins were used in the experimental room: a thermal manikin
Results and discussion
The effectiveness of proposed chair-ventilation system in mitigating cross-contamination between students was investigated using a CFD simulation model. The students were considered seated in a typical configuration: tandem seating with a separating distance of 0.4 m. The student sitting in the middle was considered infected, exhaling contaminants from the nose. The CFD model was first experimentally validated for extreme cases of PV/PE operation. Then, a parametric study considering the entire
Limitations of the proposed ventilation system
This work highlights the effectiveness of the proposed ventilation system in protecting students against cross-contamination without the need for large dead space areas separating them. The system managed to provide high protection levels that are comparable to those reached when large distancing between students is adopted. The DV background ventilation system has been studied for classroom applications in different literature studies [41,42]. However, when considering large classrooms with
Conclusion
In this work, a novel and practical chair-ventilation design is integrated with a DV system to provide protection for students against expelled contaminants in classrooms, with minimal dead space areas. The chair-ventilation design consisted of a ductless PV embedded in the back of the chair, supplying air towards the person sitting in the back, and a PE located near breathing level, above the chair backseat, to extract possible exhaled contaminants and provide a shield or “air-curtain” that
CRediT authorship contribution statement
Elvire Katramiz: Writing – original draft, Validation, Software, Methodology, Formal analysis, Conceptualization, Visualization. Nesreen Ghaddar: Conceptualization, Formal analysis, Funding acquisition, Methodology, Resources, Supervision, Writing – review & editing. Kamel Ghali: Writing – review & editing, Supervision, Resources, Funding acquisition, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to acknowledge the financial support of the Munib and Angela Masri Institute of Energy and Natural Resources at the American University of Beirut grant award 103973. In addition, the American University of Beirut PhD scholarship to Ms. Katramiz is highly acknowledged.
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