Transient transmission of droplets and aerosols in a ventilation system with ceiling fans

Different droplet and aerosol particles are generated in various respiratory activities, and their transmissions behave differently under various indoor airflow patterns. Since the judicious use of fans can reduce aerosol transmission by dispersing nearfield aerosol concentration in steady-state tracer gas experiments, this study further investigates the effect of ceiling fans on the transient droplet and aerosol transmission in the coughing process. Both large-eddy simulation (LES) and experiments, including particle image velocimetry (PIV) technique, concentration measurements, and video recordings with the high-speed camera were conducted. The results showed that the operation of ceiling fans changed the particle trajectory downwards up to 0.4 m above the floor surface. Since the airflow from ceiling fans could settle down the large particles within a short period and dilute the concentration of small-diameter particles, the aggregated concentrations in the breathing zone were reduced by 87%. Compared to the supply air from diffusers, the ceiling fan airflow had a more significant effect on the droplet and aerosol transmissions and achieved better protection for the manikin located underneath from the cough exposure. The ceiling fans strongly affected the indoor airflow pattern and also showed a potential to reduce the exposure risk to horizontally directed coughs in the transient process.