Dispersion of Expiratory Droplets in a General Hospital Ward with Ceiling Mixing Type Mechanical Ventilation System

Abstract

This study investigated the dispersion characteristics of polydispersed droplets in a general hospital ward equipped with ceiling-mixing type ventilation system. Injections of water test droplets containing non-volatile content were produced. The injections simulate human coughs with a similar droplet size distribution (peak size at 12 μ m) and airflow rate (0.4 L/s). The dispersion of test droplets was measured in-situ by interferometric Mie imaging (IMI) method combined with an aerosol spectrometer. A multiphase numerical model was employed to simulate the droplet dispersion tracks to provide additional transient position tracking data. Results show that the small size group of droplets or droplet nuclei (initial size ≤45 μ m) behaved airborne transmittable as some nuclei stayed airborne for more than 360 s. The dispersions were strongly affected by the ventilation airflow pattern. The expiratory droplets exhibited a two-stage lateral dispersion behavior, in which rapid dispersion was found in the early “initial dispersion” stage and then the dispersion became much slower in the subsequent “stable” stage. The exhaust air vents significantly enhanced lateral dispersions towards their direction. Droplets in the large size group (initial size = 87.5 μ m and 137.5 μ m) were subjected to heavy gravitational effect and stayed airborne for less than 30 s. Results indicate that the location of exhaust air vents has significant impact on the dispersion pattern of expiratory droplets. It should be carefully considered in designing ventilation systems for health-care settings.