Field Experiments to Identify and Eliminate Recirculation Zones to Improve Indoor Ventilation: Comparison with CFD

Abstract

Ventilation of shared indoor spaces is crucial for mitigating air-borne infection spread among its occupants. Replacing the air in a room with fresh air is key to minimize the concentration of potentially infectious aerosol generated in the room. Recirculating air flow present at corners and around obstacles can trap air and infectious aerosol. This can significantly delay their evacuation by the ventilation system. Knowing the location and extent of such recirculation zones is, therefore, important. In this work, we present flow visualization experiments to identify recirculation zones in an enclosed space. It is based on the deflection of the smoke streak generated by an incense stick. We use particle image velocimetry (PIV) postprocessing to quantify the deflection of the smoke streak and use it as an indicator of the direction of local air flow. Positive deflection, defined as the deflection towards the exit location, is associated with primary flow present in well-ventilated regions of the room. On the other hand, negative deflection indicates reversed flow in recirculation zones, where the smoke streak is defined away from the exit location. The technique is applied to a public shared washroom, where the toilet seat is found to be in a well-ventilated region, while the washbasin is in a large recirculation zone. We compare the experimental point measurements with flow field solution obtained using computational fluid dynamics (CFD). We also explore geometry modifications as a strategy to eliminate the recirculation zone over the washbasin.

Virus-laden droplet nuclei in vortical structures associated with recirculation zones in indoor environments: A possible airborne transmission of SARS-CoV-2

“…1 b. These vortical structures have been reported in vertical airflow dispersion systems as Mixing Ventilation (MV) and Displacement Ventilation (DV) studied by Liu et al (2022a) and horizontal airflow extraction schemes by Sinha et al (2021 , 2022 ).

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“…Then Sinha et al (2021) concluded that increasing the flow rate does not eliminate the formation of recirculation regions, and they recommended that such regions should be identified in the room. Furthermore, Sinha et al (2022) suggested the elimination of the recirculation zones to improve ventilation by geometry modifications. Crowley et al (2022) performed an experimental and numerical study of non-evaporating droplet (10 to 200 μm) dispersion by flow nasal in a room (3 × 2 × 2 m) under MV, according to the configuration of inlet and outlet ports.…”

Evaluation of an emergency room in operation during the COVID-19 pandemic: diagnoses and recommendations concerning environmental factors

“…This may indicate that the room lacks a good air mix, as mentioned by CDC (2023). Such recirculating air flow, generally in the corners of the room and around furniture, can trap air and infectious aerosol, delaying its evacuation through the exhaust system (Sinha et al, 2022). This situation can be harmful to both the patient and the healthcare service providers.…”

Improving Indoor Air Ventilation by a Ceiling Fan to Mitigate Aerosols Transmission

“…However, the virus’s airborne transmission during inhalation is still a significant concern in indoor environments, for example, workplaces, offices, elevators, schools, and colleges (Sinha et al. 2022 ; Sen 2021 ; Liu et al. 2021 ).…”