Human expiratory droplets can be pathogen carriers for various airborne infectious diseases. However, due to the discrete and polydispersive nature, investigations of their indoor transport characteristics have been challenged by various modeling and experimental difficulties. In this research, transport characteristics of expiratory droplets were investigated by treating the droplets or droplet nuclei as discrete matters. Motion tracks were modeled by a multiphase approach where the turbulent effects were treated by using stochastic methods. Experiments were performed in test chambers to obtain the turbulence characteristics of the bulk airflow using particle image velocimetry (PIV) techniques. The dispersions of polydispersed test droplets were measured in-situ by the Interferometric...[ Read more ]

Human expiratory droplets can be pathogen carriers for various airborne infectious diseases. However, due to the discrete and polydispersive nature, investigations of their indoor transport characteristics have been challenged by various modeling and experimental difficulties. In this research, transport characteristics of expiratory droplets were investigated by treating the droplets or droplet nuclei as discrete matters. Motion tracks were modeled by a multiphase approach where the turbulent effects were treated by using stochastic methods. Experiments were performed in test chambers to obtain the turbulence characteristics of the bulk airflow using particle image velocimetry (PIV) techniques. The dispersions of polydispersed test droplets were measured in-situ by the Interferometric Mie imaging (IMI) method combined with light-scattering aerosol spectrometers. The applicability of the numerical model was tested by comparing the numerical data with the experimental results.

Results revealed the size-specific transport characteristics of expiratory droplets. Droplets or droplet nuclei with an initial size up to 45μm showed airborne transmittable behavior while those with initial sizes of 87.5μm or above settled quickly due to heavy gravitational effect. This is inconsistent with the current definition in infection control guidelines, in which airborne pathogen carriers are regarded as droplet nuclei of size 1-5μm. Smaller droplets and droplet nuclei were more favorable for lateral dispersions as characterized by the overall dispersion coefficient. The ventilation flow pattern also showed significant effects on the dispersion of expiratory droplet nuclei. The bulk airflow was the major driving mechanism for lateral dispersions, which was about an order of magnitude stronger than that by turbulent dispersion. The vertical settling time was a limiting factor for the final dispersion distances. The exhaust air vent significantly enhanced lateral dispersion towards its direction, indicating the needs for careful design of air vent locations. This study demonstrated the applicability of the current methodology without employing the perfectly mixed assumption and continuous phase surrogates.