Effect of recirculation zones on the ventilation of a public washroom

TL;DR

This study uses computational fluid dynamics to analyze how recirculation zones in a public washroom affect aerosol retention and ventilation efficiency, highlighting the impact on infection risk and the limitations of increasing fan flow.

Contribution

It provides detailed insights into the flow patterns and aerosol dynamics in washrooms, emphasizing the role of recirculation zones in infection transmission risk.

Findings

Recirculation zones significantly prolong aerosol residence time.

The washbasin near the door is a key recirculation hotspot.

Increasing fan flow reduces, but does not eliminate, recirculation zones.

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 give 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 time scale 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.