Effect of recirculation zones on the ventilation of a public washroom

Abstract

Air-borne transmission can pose a major risk of infection spread in enclosed spaces. Venting the air out using exhaust fans and ducts is a common approach to mitigate the risk. In this work, we study the air flow set up by an exhaust fan in a typical shared washroom that can be a potential hot spot for COVID-19 transmission. The primary focus is on the regions of recirculating flow that can harbor infectious aerosol for much longer than the well-ventilated parts of the room. Computational fluid dynamics is used to obtain the steady state air flow field, and Lagrangian tracking of particles gives the spatial and temporal distribution of infectious aerosol in the domain. It is found that the washbasin located next to the door is in a prominent recirculation zone, and particles injected in this region take much longer to be evacuated. The ventilation rate is found to be governed by the air residence time in the recirculation zone, and it is much higher than the timescale based on fully mixed reactor model of the room. Increasing the fan flow rate can reduce the ventilation time, but cannot eliminate the recirculation zones in the washroom.

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

“…It assumes that the infectious aerosol is uniformly distributed in the air contained in the room, and that the air in every part of the room is replaced at the same rate. This estimate can be grossly inadequate, as shown in a recent study (Sinha et al 2021).…”

“…CFD simulations of this geometry are presented in a recent work (Sinha et al 2021), and a snapshot of the results are shown in Fig. 2.…”

“…The larger droplets tend to settle on surfaces, which can then be sanitized by spray disinfection. On the other hand, small droplets and aerosol can persist in the air trapped in the recirculating flow, potentially exposing subsequent users of the same space (Sinha et al 2021). The details of the CFD simulations are presented in the previous work (Sinha et al 2021).…”

“…Of particular interest are the recirculation zones formed at the corners of the room and around obstacles like furniture (Narayanan and Yang 2021). The air in such recirculation zones remain trapped for a longer duration than the average residence time given by the volumetric ACH estimate (Sinha et al 2021). This could lead to higher exposure of person seated in recirculation zones to the infectious particles floating in the air.…”

Natural Ventilation and Aerosol Particles Dispersion Indoors

“…Pourfattah et al [26] conducted a state-of-the-art review on the challenges in modelling and simulation of minimal particle dispersion such as airborne viruses or aerosols. Furthermore, researchers investigated the effect of natural ventilation and air circulation on various different scenarios of indoor airborne virus transmission [10,[27][28][29][30], including the effect of air purifiers [10]. Park et al 2021 [31] proposed natural ventilation strategies to prevent coronavirus transmission inside a school building.…”